Understanding Weather

Checking out the weather report is a daily activity for many of us. At the very least, we want to know how hot or cold it will get, or whether there will be rain or snow. Some of us live in places where severe weather can produce dangerous conditions -- flooding, tornados, blizzards or fogs, so we pay attention to weather related warnings. And these days, weather reports are getting more sophisticated, with pollen counts and particulates for allergy sufferers, and UV indices to tell you whether you need sunscreen.

Since weather affects us so constantly, I put together this page to help understand the science of weather. I am not much interested in weather forecasting, which is a very technical subject. This information is much more basic, about why weather "happens", what's going on in the atmosphere, what weather-related terms we see on TV really mean, how to read weather maps. It's mostly practical information, from a not very technical perspective.

Weather and Climate

Before we begin, let's differentiate between weather and climate. Weather is the state of your local atmosphere at any given time, in terms of such measurements as temperature, wind speed, air pressure, precipitation, etc. Weather is very specific - it's about a particular place at a particular time. It varies on a relatively small scale - for example, it could be raining in your area, while it's dry 10 miles away. It could be 72 degrees near your home, but only 65 degrees a few miles away. You could have a thunderstorm at 6 p.m. and have the sky clear by midnight.

So when we're talking about weather, we are talking about a relatively small area and a very specific time. Moving to a different area, or going forward in time quickly changes the weather. On the other hand, climate is about long term averages. It concerns the same things as weather -- measurements like temperature, pressure, rainfall, precipitation -- but these measurements are averaged over a long period. If you say "the average high temperature for Boston in April is 56 degrees", then you are talking about climate. In order to report that average temperature, someone must have measured the high temperature each day in April, and then averaged those highs. Further, it's not enough to do that for one year, because any given year could be hotter or colder than average. So they must have measured high temperatures each day in April for several years, in order to calculate a multi-year average. In fact, in many places, such temperature records go back a century or more. These 100+ year records are used to calculate averages for temperatures, rainfall, weather patterns, etc., and these long terms averages constitute the climate.

It's important to remember that weather can be very variable, but climate is not. You could hit a high of 80 degrees on April 4th in Chicago one year, but in another year, the high on the same date might barely reach the freezing point at 32 degrees. Obviously, one particular April 4th was warmer than another, but this shows nothing other than a year-to-year variability. It doesn't even mean that the whole month of April was hotter, or the whole year was hotter. In order to make any long term comparisons, in order to show any trends, you absolutely need multi-year climate data.

The Earth's Atmosphere

Since weather is the condition of the atmosphere above a certain location, at a certain time of day or night, let's consider the atmosphere in more detail for a bit. The Earth's atmosphere extends from ground surface to the edge of interplanetary space. Most of this atmosphere is contained in a narrow band, about 7-10 kilometers high, which is known as the troposphere. About 80% of the mass of the atmosphere is contained within this thin band. Although 7-10 kilometers (23,000 - 32,000 feet) may not seem like a "thin" band, but it really is, if you consider how far the Earth's atmosphere extends. Technically, the Earth's atmosphere reaches half way to the moon (about 180,000 km) -- you have to go about that far before the density of atoms in the atmosphere equals the density of atoms typical of interplanetary space. Much of it is even visible to the naked eye. Astronauts in space can see the geocorona, which looks like a hazy band surrounding the Earth, extending to about 100,000 km above the Earth.

Of course, the upper atmosphere hundreds of miles above the Earth is unbreathable and almost empty. In fact, anything over 100 km is considered space, and if you go there, you are technically considered an astronaut by the World Air Sports Federation (this was the definition of space used for the X-Prize). The International Space Station (ISS) orbits at about 350 km. Low earth orbit, used by a huge number of satellites, extends to about 2000 km at most. These regions are commonly referred to as "space" by most people, but they are still part of the Earth's atmosphere. There is enough air up there that satellites slow down over time due to air friction, their orbits decay, and they ultimately fall back to the Earth. The ISS needs to be boosted every few months to a higher orbit, or it would also fall back to Earth. The Hubble telescope orbits at 595 km, and although it is more stable than the ISS, its orbit will also decay and fall back to Earth eventually. You have to go as far as geostationary or geosynchronous orbits (about 35,000 km) before the friction of the atmosphere (communications satellites are often in such orbits) becomes a smaller concern than gravitational perturbations. But this is still within the atmosphere.

For the discussion of weather, however, we do not usually need to consider such high altitudes. 80% of the Earth's atmosphere is contained within the troposphere, a zone which extends from the surface of the Earth to about 10 km (it varies with latitude -- about 7 km over the poles, and about 17 km over the equator). 99% of the atmosphere is contained within the troposphere and the next zone outward - the stratosphere. The stratosphere extends from the boundary of the troposphere (known as the tropopause) to about 50 - 55 km from the surface of the Earth. Since 99% of the Earth's atmosphere is inside the boundary of the stratosphere (known as the stratopause), this is where weather happens. Outside this region, the air is not dense enough to display phenomena which are energetic enough to affect the weather.

If the Earth's atmosphere were in complete equilibrium, we would have no "weather". Conditions would be unchanging - there would be no day or night, no seasons, no rainfall, nothing. The source of changes in weather is changes in some other condition, some other variable. That variable is largely the Sun, although other factors also play a role. The spin of the Earth about its axis produces day and night, which means that energy input from the Sun on any given area of the Earth's surface varies cyclically -- reaching a peak during the day, and falling at night. This day-night cycle is a major source of weather, but it is modified and added to by many other cycles and factors as well. Some of these other factors vary by time of day or year, while others vary by location on the Earth's surface. In either case, variations are what fuel changes in the atmosphere, which we call weather.

What factors cause the energy input of some local area on the Earth to change? Here are some:

Latitude

How far you are from the equator determines the angle of incidence of the Sun's rays at your location. This is extremely important in determining how much energy you receive from the Sun.

The diagram at the right explains how this happens. Note that the diagram is not to scale, it shows the Sun as much smaller than the Earth, but that makes no difference to the explanation.

The Sun is roughly spherical. It radiates energy in all directions. A very small portion of this energy is intercepted by the Earth. If we assume that the Sun radiates energy equally in all directions, we can imagine its surface (which radiates the energy) as being divided into patches, measured by degrees of solid angle (usually expressed in steradians). Since the Earth is very far away from the Sun, and very small, it intercepts direct light from a very small patch of Sun.

Notice the qualifier "direct", as in "direct light". This is important because the situation described is a simplification. In reality, the surface of the Sun emits light in all directions; therefore the Earth receives light from all parts of the Sun that are facing the Earth at a given time, not just a single patch which is closest to the Earth. However, the density or intensity of this light is greatest when it is direct, that is, when a ray of light perpendicular to the Sun's surface intersects the Earth. So the relationship still holds - the more the direct sunlight falls upon some area of the Earth, the greater is the energy that area receives.

For this reason, regions near the Earth's equator are warmest, while areas away from the equator get progressively colder, because they get less direct sunlight. This creates bands or zones on the Earth's surface, with the hottest zones at the equator and the coldest zones at the poles. A temperature gradient is thus created, with high temperatures near the equator and cold temperatures at the poles. This temperature gradient drives the movement of air, which we perceive as winds.

This variation is constant in time, meaning it does not change by time of the year. Latitude 50 North will always receive less insolation than latitude 5 North, no matter what season of the year. It is simply a variation by location, that is, dependent upon the latitude location on Earth.

Latitude is very important in setting up the permanent winds on Earth. We can divide the Earth (from North to South) into several well-marked zones. The band near the equator (about 5 °N to 5 °S) is called the doldrums. It's the hottest part of the Earth, since the equator receives the most direct sunlight every year. On both sides of the equator are the tropics. These stretch roughly from the doldrums to the Tropic of Cancer (23.5 °N) in the northern hemisphere, and to the Tropic of Capricorn (23.5 °S) in the southern hemisphere. The tropics have a "tropical" climate - hot in the summers, mild in the winters. Beyond the tropics are the sub-tropical zones, which stretch from the Tropic of Cancer (23.5 °N) to the Arctic Circle (66.6 °N) in the northern hemisphere, and from the Tropic of Capricorn (23.5 °S) to the Antarctic Circle (66.6 °S) in the southern hemisphere. The subtropics usually have mild summers and cold winters. Beyond the subtropics lie the polar zones, from the Arctic Circle (66.6 °N) to the North Pole (90 °N) in the northern hemisphere, and from the Antarctic Circle (66.6 °S) to the South Pole (90 °S) in the southern hemisphere. These are the coldest regions on Earth.

Although there are many variations between different locations within the same zone (due to other differences, such as altitude, nearness to the sea, etc. which are described below), the zones do broadly reflect the kind of climates found within. As mentioned earlier, they set up the patterns of the permanent winds - the trade winds, westerlies, polar winds. These permanent winds have a very strong effect on climate, and you can read about them in more detail on this page.

Season

The Earth's axis is not perpendicular to the plane of the Earth's orbit around the Sun; it is in fact tilted. The angle of tilt varies over time, but at present it is approximately 23.5 degrees. Because the Earth revolves around the Sun, during the course of a full orbit around the Sun, each of Earth's hemispheres is at times tilted towards the Sun (summer) and at other times tilted away from the Sun (Winter).

The periods of maximum tilt are the solstices. In the year 2010, Summer solstice is on June 21st at 11:38 AM (GMT). At this time, the Earth's axis will be maximally tilted towards the Sun, which corresponds to summer and the longest day of the year in the northern hemisphere. Winter solstice in 2010 will be on Dec 21st at 11:38 PM (GMT), which corresponds to winter and the shortest day of the year for the northern hemisphere.

As can be seen in the accompanying diagram, a similar effect to the latitude differential described above happens during summer and winter. During summers, since the northern hemisphere is tilted towards the Sun, it receives more direct sunlight, leading to higher temperatures. During winters, since the northern hemisphere is tilted away from the Sun, it receives less direct sunlight, leading to colder temperatures. The effect is reversed in the southern hemisphere. Summer solstice in the northern hemisphere corresponds to winter solstice in the southern hemisphere, and vice versa.

This seasonal effect can dramatically change weather patterns, and not just in terms of temperatures. The change in temperature patterns across the globe shifts the high and low pressure areas of the atmosphere, which can lead to seasonal changes in winds. Indirectly, they can also affect precipitation, if for example, a winter wind which blows from land to land switches to a summer wind, which blows from sea to land. Wind blowing from the sea contains more moisture, which can lead to rain or snow.

It's important to remember that while we think of seasons as a yearly phenomena, these changes are gradual and are happening constantly. Between the extremes of summer and winter solstice, each day the pattern changes gradually, the day becomes shorter or longer, depending upon whether the area is approaching summer or winter. While such small daily changes may seem miniscule when considered in terms of degrees of inclination or tilt, over the large surface of the Earth they correspond to significant shifts in the temperature zones.

It's easy to calculate the magnitude of these daily changes. Since the Earth's axis is inclined at 23.5 degrees, on summer solstice, latitude 23.5 North (the Tropic of Cancer) is directly underneath the Sun (meaning, the Sun is directly overhead at noon on summer solstice day, if you happen to be at latitude 23.5 North on that day). Similarly, on winter solstice day, latitude 23.5 South (the Tropic of Capricorn) is directly underneath the Sun. So in the 6 months between the summer and winter solstices, the Sun changes its apparent position by 23.5 + 23.5 = 47 degrees in the sky.

If we assume the Earth's radius to be 6400 km, then 47 degrees of latitude correspond to 47/360 = 5350 km of the Earth's surface. This means that the Earth's sun-directly-overhead-at-noon point migrates 5350 km north and south every 6 months. This is approximately 5350/180 = 29 km per day, or about 18 miles. As you can see, while it didn't seem much when we were simply looking at angles, if you translate that into actual miles on the Earth's surface, it is fairly significant. A cold or warm front moving 18 miles in a day would definitely be noticed by us. So these changes are important not just on a seasonal basis, but also in affecting our day-to-day weather.

Altitude

The higher you go, the thinner the air gets. Dense air has a greater capacity to absorb and retain heat than thin air, so this is one reason why the temperature is colder at higher altitudes. However, this is insignificant compared to another effect, which is the cooling of air as it expands. According to the ideal gas law, the temperature of air is inversely proportional to its temperature, all else being the same. This is because as air expands under low pressure, it does work in expanding, and loses energy as work done. Since the thermal conductivity of air is very low, it doesn't gain much heat from its surroundings, so the cooling is mostly adiabetic, and well approximated by the gas law. The presence of water vapor upsets this relationship a bit, but not by a whole lot. This is the main reason why it's much colder at higher altitudes than it is at sea level.

Therefore places which are near sea level and have thick, dense air are hotter than places at the same latitude which are at higher elevations. This is why the summit of Mount Kilimanjaro is covered with ice, even though it's located almost directly on the equator (about 3 °S).

There is a separate section here which talks about altitude-dependent atmospheric pressure changes in more detail. These changes are very important in determining the local climate of an area.

Land and Oceans

Land and oceans are heated differentially by the Sun. Land has a smaller thermal capacity than water. This has several interesting effects. First, it means that the same amount of solar heat will raise the temperature of land much more than it will raise the temperature of water. Therefore, during a given day land at the same latitude as water will become much hotter than the water. Since they are at the same latitude, they have received roughly the same amount of solar energy, and absorbed roughly the same amount of energy (actually, the water absorbs a bit more). But because of the difference in thermal capacities, land becomes much hotter than water with the same amount of energy. In terms of local winds, this might mean that the wind direction is from the land towards the water during the day (since air moves from higher temperature and low pressures towards colder temperatures and high pressures).

Secondly, the greater heating or cooling of land leads to greater temperature differentials. The rate of heat gain or loss of an object depends upon the temperature differential between that object and its environment. For example, if you heat a pot of water to boiling (100 °C), and then remove it from the stove and let it cool at room temperature, the water will lose its first 10 °C much faster than its last 10 °C. If room temperature is 20 °C, then the water will drop from 100 °C to 90 °C very quickly, but it will go from 30 °C to 20 °C much more slowly. This is because the temperature differential between the water and room temperature is much higher when the water is at 100 °C than when it is at 30 °C. Since land heats up more during the day, the temperature differential is higher, therefore land cools very rapidly as well. Water cools much more slowly, because the temperature differential is lower.

We can think of it this way: land has rapid heating/cooling cycles with each day/night cycle. A large body of water, on the other hand, has much slower cycles. In fact, the water cannot lose all the heat it acquired during a summer day overnight, so it starts the next day slightly warmer than it was the previous morning. So as summer progresses, large bodies of water get progressively warmer, and they maintain this heat through the night hours, when the land cools down. For this reason, oceans don't have diurnal peaks and troughs in their temperature like the land; instead, they have seasonal peaks and troughs in their temperature.

These things produce very significant effects on weather patterns. The general direction of the effect is towards the moderation of temperatures. Since the water heats more slowly but retains heat longer than land (and cools more slowly but retains coldness longer than land), the presence of oceans tends to moderate the climate of nearby land masses. At the same latitude, an area will be much hotter in the summer and colder in the winter if it's far away from the sea. Nearness to the sea will moderate temperatures, making it both less hot in the summer and less cold in the winter.

Even smaller bodies of water such as lakes can have a moderating effect on temperatures. Check the weather map of the midwest US, and on many days you'll see that the temperature at the lake front in Chicago is higher or lower than out in the suburbs (by a few degrees), simply because Lake Michigan cools the lake shore during the summers, and warms it during the winters. Smaller bodies of water can also produce local diurnal winds, such as a breeze from lake to shore in the mornings, and a breeze from shore to lake in the evenings. Again, this has to do with the differential heating of land as compared to water during the day.

Topography

The physical relief of land areas has much to do with weather. There can be many reasons for this. One is simply altitude - mountainous areas will be cooler than areas at the same latitude which are nearer to sea level. But in addition, variation of the terrain can influence wind patterns and therefore the weather.

One example is mountains as a barrier to wind flow. If a mountain range interrupts prevailing winds, air is forced upwards to pass over the mountains. As it moves upward, it cools down. Since the water carrying capacity of air diminishes as it cools, this results in precipitation on the windward side of the mountains. Conversely, once the air has risen over the mountains it has lost moisture and is now cold and dry. Therefore, the leeward side of the mountains will be in "shadow" and receive much less rainfall than if there had been no mountains along the way.

This effect can be seen almost anywhere in the world where there are mountains that interrupt some seasonal wind flow. It is very dramatic in the Himalayas in India, where the monsoon winds from the south meet the Himalayas. On the windward side, in the foothills of the Terai, there is very heavy rainfall. Cherrapunji in the Indian state of Meghalaya has historically been the wettest place on Earth (450 inches of rain on average per year), as the monsoon winds from the Bay of Bengal hit the Khosi hills and are forced to rise and shed water. Conversely, the Tibetan plateau, on the leeward side of the Himalayas is very dry, with less than 18 inches of rain/snow per year.

There are other effects of topography as well. Flat land which is uninterrupted by hills or mountains allows wind to build up over long stretches. This is why the midwest and plains states in the US are generally quite windy. Land which is more uneven breaks up lower level winds, so wind speeds are slower and winds are not as sustained. If a large area of flat lands then borders a hill or mountain range, these high winds can get channeled into valleys between the hills, and reach even higher velocities. You can see this effect on a much smaller scale even with man-made structures. Streets form canyons between skyscrapers in downtown areas of major cities, and wind is channeled through these "canyons", reaching much higher speeds than out in the suburbs. If you've walked through downtown Chicago or downtown Manhattan, you may have experienced this yourself.

Low lying troughs, on the other hand, may have days when the air stagnates and does not move, since it is blocked by higher elevations surrounding the trough.

Ocean Currents

Water, like air, is a fluid medium, which can move from one place to another under temperature differentials. Just as there are winds in the atmosphere, there are water currents in the oceans, which carry warm water or cold water from one place to another, sometimes for thousands of miles.

One well-known example of such a current is the Gulf Stream, which carries warm water from the Caribbean to near the shores of northern Europe. The Gulf Stream is largely responsible for the migration of populations into Europe after the last ice age. Without the Gulf Stream, Europe would probably be a sparsely populated wasteland.

Consider London, which in terms of latitude is slightly farther north than Calgary in Canada. The average January low temperature of Calgary is 8 °F, but the average January low temperature of London is 41 °F. This is a huge difference, and the Gulf Stream is responsible. While latitudes comparable to England and northern Europe are almost tundra-like across Canada or Asia, they are quite warm and habitable in Europe. In Canada or Asia, the growing season is very short this far north, and native cultures traditionally depend upon hunting, since agriculture is insufficient to provide the necessary calories. But in Europe, there is extensive farming, which can support much larger population densities. The Gulf Stream has made it possible; it is a critical part of Europe's habitability.

Ocean currents are one of the most important contributors to climate, but the topic is fairly complex. I have written a brief explanation here, which you should really read before going ahead.

How to Read a Weather Map

Now that we've talked in general about the factors influencing weather, let's look at a weather map and see if we can understand what it tells us. Weather maps are often shown on TV weathercasts, or printed in newspapers. You can also find them on the internet. Maps on the TV and in newspapers are often highly simplified, so they may contain only partial information.

To understand how weather maps are generated, we need to know how information for these maps is collected. Weather information is collected at hundreds or thousands of individual weather stations spread across the country, and also located on ocean buoys near the coasts. Each station is quite small, and the vast majority of them are automated and unmanned. They contain instruments which record basic information such as temperature, dew point, wind speed and direction, cloud cover, any precipitation, and atmospheric pressure. They relay this information periodically to weather centers, such as the one run by NOAA (National Oceanic and Atmospheric Administration), where the information is compiled into a map.

Other information may then be added, such as satellite imagery, Doppler radar, UV index, pollen counts, etc., and this extra information may also be added to the weather maps, or made separately available. All this information is collated and usually posted to the web, where meteorologists at TV stations or newspapers can download it, and generate a weather forecast.

You can look at these maps at the National Weather Service site (part of NOAA), or through the Weather Machine. Usually, the maps are updated every 3 hours. You'll see the maps listed as "U.S. Surface Analysis", and individual maps will be linked as "00Z | 03Z | 06Z | 09Z | 12Z | 15Z | 18Z | 21Z". The "Z" stands for "Zulu" for "Zulu Time". This is the same as Coordinated Universal Time (UTC) or Greenwich Mean Time (GMT). There are minor differences between these time standards, but for the purpose of this discussion they can be considered identical. Since the maps are generated every 3 hours, they are listed as "0Z" (midnight, Zulu Time), "3Z" (3 AM, Zulu Time), etc. Note that Zulu Time uses a 24 hour clock.

The maps are available in color or B&W (for fax machines which can't print color). Here is a typical color map from the National Weather Service.

I haven't reduced the map size here - this is literally how large the maps are. As you can see, they are a mess of numbers and symbols, and aren't easy to read. There are also higher resolution maps available at the National Weather Service site.

The map is marked "1800Z SURFACE ANALYSIS, MAY 23 2010". This means the data was collected at 6:00 PM on May 23rd, 2010. The map was issued at 1925Z, meaning 7:25 PM. This lag of about an hour and a half is normal, since it takes this long to collate the maps. Remember, all times are UTC.

The numbers represent the records of various weather stations spread across the country. The map above shows only a very small fraction of the data - there are many more weather stations than shown. You can see finer grained data if you select one of the regional maps instead of the entire US map.

Each cluster of numbers (in different colors - red green), along with the blue arrows and other symbols represent the records of a single weather station. Here's a figure showing what these numbers mean.

If you zoom into a region of the map (zoomed into central Illinois in the figure above), you can see the records of a weather station. Here's some of the information from the weather station:

Finally, you will notice various monochrome and color lines over the map. These show fronts and related information. The dark lines are isobars, connecting areas of equal pressures. The lines often form concentric loops. Loops indicate centers of high or low pressure. In the center of the loops, you'll see the letters "L" or "H" representing high or low pressure.

There are often several high/low pressure centers spread across the US and Canada, as you can see in the map. These differences in pressure cause winds to flow, from areas of high pressure to areas of low pressure. Because of the Coriolis Effect, wind flow follows a curved path, which is explained in much more detail here.

During the course of a day, or several days, a mass of air can stagnate over some part of the land or sea, and gradually warm or cool down, acquiring the temperature of the land or sea below. Air masses typically have a roughly uniform temperature and humidity throughout. Air masses over Canada are often made of cold air, while masses over the Gulf of Mexico may be warm and moist air, etc. Differences in pressure cause the winds to flow, and therefore these masses of cold and warm air are not static, but move along with the winds in the direction in which the winds are blowing.

The edge of the large air mass is called the "front". If the air mass is warmer than the air around it, the edge of this air mass is a warm front. If the air mass is colder than the air around it, it's edge is a cold front. Cold fronts are shown in blue, while warm fronts are shown in red. Symbols projecting on one side of the line representing the front show the direction of movement of the front. There are two symbols - warm fronts use half circles, cold fronts use triangles. Different symbols are chosen because the map must be readable when printed in B&W.

A line with alternate half-circles and triangles (projecting in opposite directions) represents a stationary front. This means that the air mass is not moving - there are not enough winds to move it. A purple line with alternating half-circles and triangles (pointing in the same direction) represents an occluded front. An occluded front happens when both a warm front and a cold front are moving in the same direction, but the cold front is moving faster, and overtakes the warm front. Some special weather effects may happen at occluded fronts, which we'll talk about later.

A brown line with empty half circles pointing the same way demarcates the edge of a dry air mass, which has very little water vapor. You can see on of these on the map above, about midway across the country. Air in the eastern half is more moist, at the time of this map.

A red line with breaks represents a squall line - a line of thunderstorms. The breaks represent areas of breaks in the storms. Storms very often do in fact occur in lines rather than patches, along the edge of a moving front. If you see animated Doppler radar maps on the TV, you'll often see lines of storms moving through.

Dotted brown lines represent troughs - generally long, thin areas of low pressure. On these maps, they also show the outflow boundaries of air cells. Solid lines represent isobars, as mentioned above. They also can represent ridges, or tropical waves, depending upon the map. A tropical wave is an area of maximum curvature of winds, specially in tropical areas, as winds circle around in a cyclonic fashion.

How to read a METAR Report

METAR is the most popular format for transmitting weather information. It was originally used by airports transmitting weather information to pilots, and is standardized by the Internation Civil Aviation Organization (ICAO). These days it is widely used by meteorologists in collecting and aggregating weather information from airports and permanent weather stations.

Most weather services on the internet provide a raw METAR feed in addition to a properly formatted weather report. Here is a typical METAR report from Chicago, taken on July 7, 2012 at 5:30 PM:

METAR KMDW 072151Z 05013KT 10SM FEW040 BKN250 28/19 A2997 RMK AO2 SLP135 T02780189

Here's a breakdown of what these terms mean:

Some METAR reports may be more complicated than this, depending upon what kind of weather activity is going on. This METAR report was taken on a relatively sunny, warm day, with no precipitation or other weather activity. A full description of all the codes used in a METAR report can be found in the Federal Meteorological Handbook Number 1, which can be accessed here. Here is a web based METAR translator.

High and Low Pressure Centers

Typically, there will be several high and low pressure centers across the US and Canada. They can be identified by isobars, which travel in loops around the centers. Usually in the center of the innermost loop, there is a large "L" or "H" signifying whether it's a low pressure center or a high pressure center. Isobars are lines which join areas of equal pressure. By convention, isobars are usually drawn at 4 millibar intervals. If you look carefully, you can see a number printed right across the line of each isobar, representing the pressure of that isobar.

Isobars that are spaced far apart indicate that the change of pressure is gradual around that center. Small pressure differentials produce calm conditions. If isobars are spaced very close together, it means that sharp pressure differentials exist, so those areas are likely to be windy.

Typically, air moves around a low or high pressure center, in a circle around its edges. Wind directions are mostly parallel to the isobar lines. Because of the Coriolis Effect, and because air moves towards low pressure centers while it moves away from high pressure centers, the circulation around a low pressure center is anticlockwise, while circulation around a high pressure center is clockwise.

In the same way that isobars are lines that connect areas of equal air pressure, some maps display isotherms, which are lines connecting areas of equal temperatures. Isotherms are very dense near fronts, where there is a rapid transition of temperatures. Similarly, other maps may have lines connecting areas of equal dew points. These lines are called isodrosotherms. They show margins between moist and dry air masses, and are therefore important in predicting precipitation.

The map at the upper right shows an overlay of isotherms and isobars. Isotherms are represented by the color coding, with different bands of temperatures represented by different colors. Isobars are represented by black lines. The numbers over the isobars indicate the pressure in millibars.

The map at the lower right shows isodrosotherms - lines connecting locations with equal dew points. As you can see, the eastern half of the country has air with a lot of moisture, while the western half has relatively dry air. The area just east of the Rockies shows a great density of isodrosotherms, indicating a sharp transition. There is a dry line in this region, separating the humid eastern air from the much dryer western air. This is a fairly normal state for the US. Moist air is unstable (since humid air is lighter than dry air, and naturally rises to the top). This can cause rain and storms very easily along the front, if the dry air pushes eastwards, since dry air (being heavier) would sink to the bottom and accelerate the rise of the moist air.

Dew points are a good way to tell where the fronts are located. Sharp transitions in dew points (where the green changes to the blue) are front locations in this map.

A comparison of the isotherms and isobars can help predict the weather. If the temperature boundaries (isotherms) are perpendicular to the pressure boundaries (isobars), then it's likely to get warmer or colder soon. This is because winds follow the paths of the isobars, and since the isobars are perpendicular to the isotherms, they are blowing air across temperature transitions. If, on the other hand, isobars and isotherms are roughly parallel (as they are in the map at the upper right), then it's unlikely that temperatures will be changing very rapidly.

Knowing where the air is coming from can tell you whether your location is likely to get warmer or colder. In order to do that, recall that air circles around high or low pressure centers, in paths that run parallel to the isobars. Air circles high pressure centers in a clockwise direction, and circles low pressure centers in an anticlockwise direction. So if you are near a warm or cold cell, check to see if you can expect air from that cell to reach you, keeping in mind that air has to blow across isotherms for temperatures to change significantly.

Remember that none of these lines - isobars, isotherms or isodrosotherms give you an accurate indication of areas between the lines. For example, you could have two isotherms, one for a temperature of 78 °F and the next one for a temperature of 82 °F, but you could have a spot on the map between the two lines which is at 86 °F. This is because all these lines are based on smoothed data, and don't show details on a fine scale. The number of weather stations is extremely high, and even detailed maps don't show all of them. Maps representing large areas, such as the entire US, are heavily smoothed. Data from thousands of stations is dropped out in order to simplify the map and reduce clutter. This means that on a local scale, conditions might be very different from what the map would indicate. Always check to see if there are more detailed maps available if you are interested in a specific location.

When Fronts Meet

When two fronts with different temperatures meet, usually clouds are formed and it may rain. The types of clouds and the intensity and type of rain vary depending upon which front approaches the other.

The figures to the right show the typical results. When a cold front approaches a warm air mass, the cold air burrows beneath the warm air, since cold air is denser and heavier.

This causes a large scale movement of air, since the cold air moves in at ground level, so the whole column of warm air is lifted up, right from its base. It has to rise above the entire thickness of the cold air, so the rise is quite substantial and relatively rapid. Of course, the warm air previously at ground level is the hottest air in the hot air mass, since it's close to the ground which radiates heat. So it was rising slowly anyway, even before the cold front arrived.

This large scale upward push on the entire warm air column produces cumulonimbus clouds, which are very tall clouds, rising from close to ground level, all the way into the upper atmosphere. These clouds are also known as thunderheads. They contain strong updrafts, since air rising due to the temperature gradient is reinforced by an upward push created by the cold air mass.

They often lead to intense rain, which might not last for very long, but is severe while it lasts. The strong updrafts may produce hail, as rapidly rising air loses its moisture, which condenses to ice.

In contrast, when a warm front moves towards a cold front, the effect is usually different. Since warm air naturally rises above cold air, there is no great pressure to push the entire air column upwards. The warm air slowly overlays the cold air, as it moves forward. This happens at different altitudes, throughout the air column. The cold air is gradually pushed in a direction away from the movement of the warm air.

This "layered" rising of the warm air about the cold air mass produces sheets of clouds, that are not very tall, but quite extensive horizontally. These sheet like clouds are known as stratus clouds, and depending upon their altitude, they may be (lowest to highest) nimbostratus, altostratus, or cirrostratus. At the very top, some wispy cirrus clouds may also be produced, very high in the atmosphere.

Rain produced by this sort of meeting of fronts is usually slow but steady. Each of the layers of stratus clouds formed can produce rain, so rain is often episodic, continuing throughout the day. It's usually light - to - moderate in intensity, since the clouds are releasing moisture slowly as they gradually rise above the cold air mass.

Movement of these fronts can sometimes generate signs that are traditionally seen as portents of good or bad weather. For example, there is a well-known rhyme that goes "red sky at night, shepherd's delight - red sky in the morning, shepherd's warning." While such folk wisdom is by no means infallible, it can still make sense. A red sky is caused by dust in the atmosphere. Dust typically rises high in the atmosphere only when the air is dry (moisture makes dust stick together and it won't rise) and there is some wind turbulence. Both of these are a sign of a high pressure center. If you see red sky at night, it must be to your west (since the sun sets in the west). Since weather systems in temperate climates move from west to east, that red sky and dust is moving towards you, which means you can expect dry air the next day, or no rain. Similarly, if you see red sky in the morning, it must be towards the east, which means the dry air is already past you, and there is probably a mass of moister air moving towards you. This could mean rain.

Dew Points

Weather reports typically quote dew points along with the temperature. This is a measure of the humidity of the air. Water is present in the atmosphere in the form of water vapor. Air has a certain "carrying capacity" for water vapor. If the amount of water vapor in a certain volume of air exceeds the carrying capacity, the water vapor will start to condense and precipitate out, as droplets of water. The carrying capacity of air depends upon its temperature. The warmer the air, the more water vapor it can carry before the vapor starts to condense out.

How to calculate relative humidity from dew point. The calculation assumes standard atmospheric pressure, and may therefore be only approximate if the actual pressure deviates greatly from standard pressure.

The amount of water vapor in the air is termed the humidity. Humidity can be expressed in two ways - absolute and relative. Absolute humidity, as the name implies, is a measure of the amount of water vapor per unit volume of moist air. Usually it is expressed as mass of water vapor per cubic meter of air. Relative humidity is the amount of water vapor in the air compared to the air's maximum carrying capacity at that temperature. It is expressed as a percentage. So if relative humidity is listed as "75%", it means that the air contains 75% of the maximum amount of moisture it could hold, at a given temperature. It is essential to specify the temperature when mentioning relative humidity, because the carrying capacity of air changes with temperature.

Consider air at 50 °F with a relative humidity of 60%, over a landmass. During the day, the Sun heats up the land, which in turn radiates heat into the atmosphere and heats up the atmosphere. As a result, the temperature of the air rises from 50 °F to 70 °F. There has been no rain, so there is no loss of moisture from the air. There are no lakes or rivers nearby, so there has been no gain in moisture either, from evaporation. So the absolute humidity of the air has not changed, meaning, the absolute amount of water vapor held by the air has not changed. However, the relative humidity has changed because the air became warmer. So while the same air was at 60% relative humidity when it was at 50 °F, its relative humidity decreases at 70 °F, because air at 70 °F can potentially carry a lot more moisture than air at 50 °F. The air now feels dryer.

For weather reporting, relative humidity is more important than the absolute humidity. This is because our perception of humidity is based on relative humidity. Humans control their body temperature by producing sweat, which evaporates and cools the skin. The ease with which sweat can evaporate depends directly upon the relative humidity. The higher the relative humidity, the harder it is for sweat to evaporate, and we feel this as heat (because the skin can't lose heat since sweat isn't evaporating as fast) and a muggy or sticky feeling (because the skin is moist, since the sweat isn't evaporating as fast). This is why "hot" is much more bearable than "hot and humid".

The same applies to cold weather as well. Cold, moist air feels clammy and produces a more intense sensation of cold. A temperature of 40 °F in a dry place like the midwest doesn't feel nearly as cold as the same temperature in a wet place like New Orleans. Moisture tends to intensify both the feeling of heat and cold, to humans.

Weather reports typically mention dew points as a substitute for both absolute and relative humidity. The dew point is simply the temperature at which the air would become saturated, that is, incapable of holding more moisture. For example, assume that the temperature is 70 °F and the relative humidity is 60%. If the temperature were lowered, the carrying capacity of the air would decrease, and the relative humidity would rise. At some low temperature, the relative humidity would increase to 100%, meaning that the air would be fully saturated. At this point, you would start to see dew (droplets of water) condensing out on objects. This is the dew point. If the weather report says that the temperature is 70 °F and the dew point is 56 °F, it means that there is enough moisture in the air that if you were to lower the air temperature to 56 °F, the air would become fully saturated and dew would start to form.

The box above shows how to calculate relative humidity from the reported temperature and dew point. For the consumer, the relative humidity is a more intuitive measure than dew point, since it tells you how "comfortable" you will feel, whether you will sweat or need moisturizer. However, for the meteorologist, dew points are more meaningful, since they condense information about both humidity and temperature in an easily understandable form. They tell you how close the air is to condensation, which leads to rain and snow and all sorts of interesting weather effects, which is their business to predict. Isodrosotherms (lines which connect places with equal dew points on a map) are very useful in detecting where fronts are located.