By Robert Hazen, Ph.D., George Mason University
The Earth’s envelope of atmosphere contains gases, hundreds of kilometers in thickness, which surround the planet. At first glance, it might seem odd to speak of an atmospheric cycle; after all, all the atmosphere appears to be one continuous mass of gas, of air, rather than several discrete repositories. However, the atmosphere can be divided into separate air masses.
Properties of Air Masses
At any one time, the atmosphere can be divided into separate air masses. Each of these air masses is more or less uniform in its properties, and each air mass can be thought of as a separate air reservoir, which has distinctive properties from the adjacent air masses.
At times, there are adjacent air masses that occupy different regions close to the ground. For example, one can have a low-pressure system in the Midwest while there is an adjacent high-pressure system in New England, and those would be two adjacent air masses.
It’s also possible that air masses could represent layers of the atmosphere. For example, the troposphere, near the ground, and the higher-up stratosphere can be two masses of air with separate properties, and therefore, behave as separate reservoirs of air. In order to understand the properties of air masses, we need to define two closely related terms: the weather and the climate.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
A Matter of Definitions
Weather is the state of the atmosphere at a given time and place. At any given time, at any given place, one can define the weather; and it’s influenced by lots of factors that are variable from day to day, not to mention from season to season. There certainly are places where the weather can be fairly consistent.
If one goes to the desert Southwest, they can expect hot, dry weather. If they go to Santa Barbara in the summertime, they can expect somewhat cooler but also quite dry weather most of the time. But in New England, the weather can vary day to day and hour to hour—and minute to minute, it would seem—as different fronts come in, and different air masses collide, and different amounts of humidity and cloudiness prevail in the region.
Climate, on the other hand, is a very long-term average of weather for a given region. A regional climate can be hot or cold, wet or dry, and the weather on a given day could be quite different from that average, long-term climate. Yet, we do see long-term patterns that persist for decades, for centuries, even for thousands of years.
On the other hand, for reasons we don’t quite understand, it appears that, at times, climate can shift quite rapidly. An area that was hot and dry can become much cooler and wetter, or vice versa, and that’s a great area of research. Why do climates change the way they do?
Learn more about the difference between heat and temperature.
Temperature on the Ground Level
There are five variables that define the state of the atmosphere, and these are the key to understanding both weather and climate: temperature, air pressure, humidity, cloudiness, and prevailing winds. The local weather report typically covers all five of these variables. Here we will cover two.
First is the temperature: the temperature is reported in daily weather predictions, and that refers to the temperature at ground level. The range of temperature is huge on the surface of the Earth.
The highest average daily temperature is 94 degrees Fahrenheit at the Danakil Depression in Ethiopia. The coldest average temperature is minus 70 degrees Fahrenheit at Plateau Station in Antarctica.
Temperature always varies strongly with altitude above the ground, and indeed, this fact is used to define the major layers of the atmosphere.
Everything Is Under Pressure
The second variable is the atmospheric pressure. Pressure is defined as a force acting on an area: a force per unit area. Atmospheric pressure represents the weight of air that’s above us. We have a column of air pushing down on all of us, squeezing us from all sides, and at sea level, that’s at about 14.7 pounds per square inch.
Air pressure decreases quite significantly with altitude because air is compressed by its own weight. The higher one is, the less air there is above them, and the less weight there is to push down. If one goes up to a height of 5.5 kilometers—and that’s far below where commercial jetliners fly—they find that the air pressure is about half of the value at sea level. If they went up to 32 kilometers, which is much higher than commercial jetliners, the pressure is only one-hundredth of that at sea level.
Learn more about the symmetry of nature.
Variations in Air Pressure
Pressure also varies laterally; that is, there are regions, air masses, that tend to be at higher pressure or at lower pressure. The reason this is true is quite simple. Air tends to pile up, just like any fluid would. If one has a circulating air mass going around, at the center of that circulating air mass, the air is being pulled outward by centrifugal force, so there’s a low-pressure area; but regions at the boundaries of that experience high pressure, as the air piles up towards the outside of one of these rotating masses of air.
There’s another phenomenon: air in low-pressure systems tends to rise up, and when one rises up air into the cooler regions, they tend to get increased clouds because the water starts condensing out as it gets colder and colder in the air. So low-pressure areas tend to be more cloudy areas; indeed, they tend to be much wetter with more rain. By the same token, high-pressure areas tend to feature warmer, dryer air.
Common Questions about How Air Masses Affect the Weather
Weather is defined as the state of the atmosphere at any given time and place. When air masses collide, they can change this state and, in turn, the weather.
There are five variables that define the state of the atmosphere, and these are the key to understanding both weather and climate: temperature, air pressure, humidity, cloudiness, and prevailing winds.
Pressure is defined as a force acting on an area. Atmospheric pressure represents the weight of air that’s above us. We have a column of air pushing down on all of us, squeezing us from all sides, and at sea level, that’s at about 14.7 pounds per square inch.