The Earth is covered by an atmosphere of gases, which exert a pressure on objects situated within this gaseous envelope. The pressure is due to the mass of the gases, which is attracted to the Earth under the influence of gravity (and therefore, it is due to the weight of the gases).
We can think of the atmosphere as a column of gases rising from surface level to the very edges of the atmosphere, in space. Depending upon the altitude at which an object is located in this column, it feels pressure due to the weight of gases above it. The higher the altitude, the shorter the column of air above the object, and therefore the less weight (and pressure) it experiences.
This relationship is not linear. The bulk of the atmosphere is near the surface of the Earth; this is where atmospheric density is highest. In the high reaches of the atmosphere, density is very low, and therefore the weight of atmosphere above a given altitude falls rapidly the higher it gets. This is understandable, since gases are highly compressible, and the weight of gases compresses them at the bottom more than near the top.
About 80% of the Earth's atmosphere is contained within the troposphere, the part of the atmosphere that is closest to Earth. The extent of the troposphere varies by latitude - it is thickest over the equator (about 17 kilometers) and thinnest at the poles (about 7 kilometers). This uneven distribution can be explained by the rotation of the Earth, which exerts a centrifugal force that varies directly with latitude - being highest at the equator and zero at the poles. Centrifugal force creates a bulging of the atmosphere over the equator. It shapes more solid objects too - the Earth itself has an equatorial bulge for the same reason. Over temperate latitudes, including the continental US, the thickness of the troposphere is about 9-12 kilometers, varying somewhat by day and seasonal weather conditions.
The next atmospheric zone above the troposphere is the stratosphere, which extends to an altitude of about 51-55 km. About 99% of the Earth's atmosphere is contained within this limit. The remaining 1% extends very far out into space. In fact, you would need to go half way to the moon (about 180,000 km) before the density of air equals the density of gases in outer space - that is, the point at which the atmosphere is no longer distinguishable. There is one other boundary of note, about 100 km above the surface, known as the turbopause. Below this level, turbulence in the atmosphere keeps its gases relatively homogenized, so they contain roughly the same ratio of gases (21% oxygen, 78% nitrogen, 1% other gases). Above the turbopause, only the lighter gases such as hydrogen and helium are found.
The graph above shows the thinning of the atmosphere with altitude. Even at the low altitude of 1000 meters (3000 feet), the greater part of the atmosphere is already below you. Less than a third of the atmosphere (by weight) remains above you, the rest is below. The steepness of the curve does decrease about 2000 meters, and from then on the slope is much more constant.
Since the topography of the surface of the Earth is very uneven (with mountains, plateaus, plain, depressions), we need to define a standard altitude with which to compare, when we talk about atmospheric pressures. This standard altitude is "sea level", which is the average level of the oceans of Earth. We say "average level of the oceans" because even the oceans are not uniform. There are many local level variations in the oceans. Some are easy to understand (such as waves and tides near the coastal regions), others are not so obvious (such as bulging of large expanses of water due to expansion driven by solar heating). However, the range of these variations is small, and we have established an average which we consider standard sea level.
Standard Sea Level Pressure
The standard atmospheric pressure at sea level is 1 atmosphere. As you can see, the atmosphere itself can be used as a unit of measurement, so long as we define the conditions under which the unitary measurement was made. The conditions are "at sea level". As you will see in later sections, even this isn't constant. Aside from the question of "how high is sea level", which we know isn't constant, but which we can at least average, there is also the question of local and seasonal variations. The same spot on the surface of the Earth at sea level may have a certain pressure on one day, but a very different pressure on another day, due to local weather patterns. These things are described in more detail in later sections, but at this time it suffices to say that saying "sea level" isn't really enough, not even if you specify where on Earth the sea level is located.
For these reasons, we have to fix the number more firmly. Sea level pressure is therefore fixed as a pressure of 101325 pascals, and this is what we mean when we say "1 atmosphere". The relationship between these two units is a matter of definition - we have defined 1 atmosphere to be exactly 101325 pascals. This number was obtained from some standard measurement at sea level on a calm day. It no longer matters where the measurement was originally taken, or the particulars, just that 101325 pascals is the average you would expect under calm conditions at sea level.
The "pascal" is an SI unit, equal to a force of 1 newton on an area of 1 square meter. The pascal is therefore defined as:
1 pa = 1 N/m2
Other units are also used in measuring atmospheric pressure. So atmospheric pressure at sea level is:
|1 standard atmosphere||by definition|
|101325 pascals||by definition|
|101.325 kilo pascals||1 kilo pascal = 1000 pascals|
|1.01325 bars||1 bar = 100,000 pascals|
|1013.25 millibars||1 millibar = 1/1000 bar|
|760 Torrs||by definition|
|29.9247 inches of mercury||weight of a mercury column|
|14.696 psi||measurement in pounds per square inch|
Here's some additional explanation for these units.
The standard atmosphere was defined at sea level, under certain conditions. Initially, atmospheric pressure was measured by mercury barometers, in which a column of mercury reaches equilibrium with the atmosphere so that the weight of some standard column of mercury (say a column with a cross sectional area of one square centimeter) weighs exactly as much as the entire column of air at that location (of an identical cross section). Thus barometric pressure was reported in "inches of mercury".
As the world standardized to a metric standard, "inches" became obsolete, and were converted to metric millimeters. People noticed that the height of a column of mercury in the barometer at a pressure of 1 atmosphere was almost exactly 760 mm (760.087 mm). So another unit was born - the Torr, which is defined as exactly 1/760th of standard atmospheric pressure at sea level.
Pounds per square inch, is of course, a standard way of measuring pressure in various mechanical applications. It's simply a force equivalent to 1 pound of mass under standard Earth gravity, applied to 1 square inch of surface. It's not used very much for describing the atmosphere, but the conversion is easy from any other unit. High pressure compressors and high pressure gas cylinders often list tolerances in pounds per square inch.
So far, the pascal is the best unit for measuring pressures, since it's an SI unit, making calculations very easy. However, there is a final twist. Meteorologists don't like using big numbers, like 101325 pascals for 1 atmosphere. Further, most of their recording instruments don't record atmospheric pressures with that degree of precision. So they invented a new unit - the bar, which is defined as 100,000 pascals. This is very close to the standard atmospheric pressure, and therefore handy when talking about atmospheric pressure and weather. While it's not an SI unit, converting to SI units is simply a matter of moving one decimal place, so it's handy in that sense too.
Typically, weather charts report pressures in millibars, which are 1/1000th of a bar. So standard pressure at sea level would be 1013.25 millibars. This would be usually measured as 1013 millibars or at most 1013.2 millibars, since that is the usual standard of precision for weather instruments.
Variation with Altitude
Variation with altitude was mentioned briefly at the beginning of this document. Let's consider this in more detail. There are two factors determining the pressure at higher altitudes. One is gravity, which compresses the air at the base of the air column, so that air is densest near the surface of the Earth and thins out as you go higher. The second is temperature, which also falls with altitude. Thin air is expanded air. Since the expansion is mostly adiabetic (no net heat gain or loss), the temperature falls quite dramatically with height. Lower temperatures have a direct effect on pressure - the lower the temperature, the lower the pressure, all else being constant. Therefore the temperature lapse rate - the rate at which temperature falls as you ascend higher into the atmosphere, is essential in calculating the atmospheric pressure at a given height.
Based on these two variables, equations can be derived from the ideal gas law which provide an indication of air pressures at specific heights. This is known as the barometric formula, which actually consists of two equations. The reason for two equations rather than one is because the temperature does not fall uniformly as we go up. Within the troposphere, the temperature continues to fall from surface temperature to as low as -55 °C to -75 °C at the tropopause, about 7 - 17 kilometers up. However, within the next layer of the atmosphere - the stratosphere - temperature rises steadily, until it's about 3 °C at the stratopause, about 50 km high. In the next layer of the atmosphere - the mesosphere - temperature falls again, until it reaches about -85 °C at the top of the mesosphere at the mesopause, about 80 km high. This is the coldest part of the atmosphere. Beyond the mesopause, the atmospheric is too thin for "atmospheric temperature" to be a meaningful term.
So as you can see, there are two kinds of conditions when we consider the vertical column of air. There are zones with a positive or negative lapse rate, where the temperature is either rising or falling. And there are places where there is a zero lapse rate, in the zones intermediate between rising and falling temperature zones. This is why two formulas are needed to calculate atmospheric pressure, depending upon the altitude.
The exact formulas are not very complex, but too technical to mention in this article. You can read more about them here.
The figure on the right shows how atmospheric pressure varies with altitude through the atmosphere. Compare the shape of this curve with the graph at the top, which shows how much of the atmosphere is below a certain height. Although similar, the graphs are not identical (forget the matter of the revered left/right flipping of the x-axis for the moment).
The steepest part of the graph is near the bottom, where pressure decreases rapidly with altitude for the first couple of thousand meters above sea level. After that, the curve slowly becomes less steep, reaching an inflection near the edge of the troposphere, at the tropopause. At the upper limit of the stratosphere (the stratopause), the pressure has diminished to a very tiny value (the bulk of the air is below this point), and after that the pressure diminishes only very gradually through the mesosphere, thermosphere and exosphere.
Breathing becomes difficult at high altitudes because of the low density of the air. There is not sufficient oxygen in the atmosphere to oxygenate the blood adequately. Oxygenation of blood depends upon the partial pressure of oxygen in the atmosphere. As atmospheric pressure decreases, so does the partial pressure of oxygen. At 3000 meters (high altitude ski resorts, for example), both atmospheric pressure and partial pressure of oxygen are about 70% of sea level values. At 5000 meters (16,400 feet), they are at 50% of sea level values. On the top of Mount Everest, it's about 29% of sea level. Altitudes this high are not conducive to life. Some people have done ascents of Mount Everest without the use of bottled oxygen supplements, but they can only stay at such altitudes for a few hours. These are people who are very well adapted to high altitudes. Most people would not survive at that altitude without supplemental oxygen.
For short durations, humans compensate for high altitudes by increasing the respiratory rate (hyperventilation). During a longer period of acclimatization, physiological changes occur. The number of red blood cells in the blood increases (polycythemia). Other changes in the body's acid-base balance can also happen. Rapid ascents to high altitudes, or lack of acclimatization can lead to serious illness or death.
Variation with Atmospheric Conditions
Atmospheric pressure changes on a daily or even hourly basis, as well as seasonally. This is due to the normal day/night cycle of solar insolation, and also due to local weather patterns. High and low pressure centers are created in different regions of the Earth, because of differential solar heating of the surface. These centers are not always static - they can move due to winds.
Typically, low pressure centers form over warm regions. The air above the hot land or ocean heats up due to the transfer of heat from the surface below. Hot air expands, which decreases its density. This is accompanied by a decrease in pressure. The warm air rises due to buoyancy. Cooler air rushes in from the sides, to replace the air that is rising.
Similarly, cold areas can develop high pressure centers. The cold surface cools the air above it, which makes the air denser, and the pressure increases. Typically, there is an outflow of air from a high pressure center, towards surrounding areas where the pressure is not so high. These airflows eventually dissipate both high and low pressure centers, but it can take time, and such centers can last for several days. New high and low pressure centers are being created continually.
High pressure centers are usually associated with calm weather and no precipitation. Low pressure centers can produce bad weather, since warm air rises, and upon reaching high altitudes, it cools down and sheds its moisture as rain. However, not every low pressure center will cause bad weather, and not every high pressure center is immune to bad weather. There are many other variables to consider aside from air pressure. For a detailed look at the causes and patterns of weather, check out this page.
Local weather based variations can be quite extreme. The highest atmospheric pressure ever recorded was 1094 millibars, at Agata in the USSR (now Russia), on December 31st, 1968. This was caused by a long period of very cold weather, with temperatures between -40 °C and -58 °C at the time. The lowest pressures occur in the eyes of hurricanes and typhoons. The lowest pressure ever recorded was 870 millibars, from Typhoon Tip in the west Pacific, on October 12, 1979.
Although these variations are large, they pale compared to the variations caused by altitude. Even at an altitude of 2000 meters (less than Denver, CO), atmospheric pressure due to altitude would be about 795 millibars, a much greater decrease than the most extreme weather conditions. Even on a smaller scale, local topography can be quite variable even a few miles away. This is particularly true in hilly areas, but even in relatively flat areas there may be gentle slopes that increase or decrease the altitude significantly over a few miles.
For this reason, when drawing weather maps, absolute pressure values are not of much use. If you compare the absolute pressure of two nearby locations, you don't really know if the difference is due to altitude, or because a cold front is moving through. Winds and the movement of cold and warm fronts are major factors in determining the weather of a location. However, essential information about determining the presence and movement of such pressure centers and fronts would be obscured if we simply took absolute pressure readings, since the altitude-based pressure differential would hide the weather-related changes. Therefore, weather maps typically correct atmospheric pressure readings to sea level. In other words, the atmospheric pressure listed for a location is not the real pressure as measured by a barometer placed at that spot, but rather what it would be if that location was suddenly elevated or depressed to sea level.
The image on the right is a typical weather map showing isotherms and isobars. Isobars are lines connecting areas of equal atmospheric pressure. They are shown as black lines in the picture. Isotherms are lines connecting areas of equal temperatures. They are shown with the color coding in the map, with each color representing a temperature range and the edges of each color representing the isotherms. You can read more about weather maps and isotherms and isobars here. Note that these isobars spread over the entire country. For example, the 1016 millibar isobar runs through the center of the western half of the country. It intersects the cold blue patch (labeled "H"), which is obviously located over the Colorado Rockies. Obviously, the absolute pressure up in those mountains couldn't possibly be that high. As mentioned above, at 2000 meters altitude the air pressure is less than 800 millibars. Most of these mountains are much taller than 2000 meters, so the air pressure would be even lower. Local weather conditions could never raise air pressures from 800 millibars to 1016 millibars; it is too extreme a difference.
This shows that the map is using corrected pressures, not absolute pressures. As you can see at the top of the map, that's what it says - "Sfc Temp (F) / Sea Level Pressure (mb)" - meaning, surface temperature in degrees Fahrenheit, and pressure in millibars corrected to sea level. This is the only way you can actually draw those isobars and locate the high and low pressure centers.
You can read more about the atmosphere and weather patterns here. Or if you'd like to read how variations in atmospheric pressure produce winds, check this page. Also, there is a page here which talks about how ocean currents affect weather patterns.