The Atmosphere and the Weather

Vertical Structure of the Atmosphere

Temperature

Often, the Earth's atmosphere is divided into several different layers that are defined according to the typical change in air temperature. (See Image Below).

Layers of the atmosphere based on temperature

Atmospheric layers defined by structure of air temperature.

Troposphere:

• Located from the surface of the Earth to approximately 11 kilometers in altitude. This is an average, as the depth of the troposphere varies with time and location, range 6 - 16 km. Troposphere literally means the "turning or changing sphere" in reference to the fact that this lowest layer of the atmosphere is characterized by strong vertical mixing of air. The upper boundary of the vertical mixing is at the top of the troposphere at the troposphere-tropopause boundary.
• Essentially all weather and clouds occur in the troposphere or this overturing layer.
• The troposphere contains about 75% of the total mass of the atmosphere.
• Typically, the maximum air temperature occurs near the Earth's surface and drops with increasing height at an average rate of 6.5°C per 1000 meters (or 3.6°F per 1000 ft). The reason the temperature usually decreases with increasing height is because the atmosphere is mainly heated from below by the underlying ground surface. The gas molecules in the atmosphere are generally transparent for visible radiation coming from the Sun, i.e., if it is not cloudy, most of the Sun's radiation passes right through the atmosphere, and is absorbed by the ground surface, heating the ground up. Air in contact with the warm ground is heated from below, therefore, the air temperature typically lowers as you move up away from the warm ground.
• However, it is not all that uncommon for there to be a shallow layer of air where the air temperature actually increases with increasing altitude. These layers are called temperature inversions and can be very important in weather forecasting. One situation where temperature inversions frequently form is just above the ground surface during long winter nights caused by the air contacting the cold ground surface. This type of inversion will persist into the early morning until the sun is able to heat the ground surface (See Figure X).
• Figure X also indicates the typical changes in air temperature that take place over the course of a day. The largest change in air temperature happens just above the ground surface with much less change higher up. The reason is that the ground surface temperature responds more rapidly to changes in radiation than the air. During the day, the ground surface warms rapidly by absorbing radiation from the sun, which heats the overlying air from the bottom up. At night, the ground surface cools rapidly by emitting (or giving off) radiation energy, which cools the overlying air.

Tropopause:

• Isothermal (constant temperature) layer that exists above the troposphere. On average, it is found between about 11 and 20 kilometers. At these levels the air temperature is much colder than that found near the ground surface.
• Separates the troposphere from the stratosphere.
• This change in vertical structure of air temperature (from cooling with increasing altitude in the troposphere to remaining constant with increasing altitude in the tropopause), acts like a "lid" on rising air motion. This "lid" can prevent air from moving vertically up or down through this troposphere-tropopause boundary. The reason will be better explained in later lectures. Because it is difficult for tropospheric air to rise into the tropopause region, clouds are typically confined below the tropopause region (See Figure Y), since clouds form in air moving vertically upward. This is why we say most clouds and weather are confined to the troposphere.
• Layer in the atmosphere where the jet stream exists.

Stratosphere:

• Extends from 20 to 48 kilometers above the surface (average location).
• Temperature increases with altitude because ozone gas molecules, present in this layer, absorb ultraviolet sunlight creating heat energy.
• The layer of higher ozone concentrations, which reaches a maximum between 20 and 30 km above sea level, is also called the ozone layer. Ozone in the stratosphere protects life from harmful exposure to the sun's ultraviolet radiation. NOTE: Even though we refer to an "ozone layer", keep in mind that ozone molecules account for a very small percentage of all air molecules in the stratosphere. Even within the ozone layer, ozone is still a trace constituent.

Density

Air density can be defined as the number of air molecules per unit volume (number density). Near sea level there are about 2.7x10¹⁹ molecules per cm³(cubic centimeter) or 4.4x10²⁰ molecules per inch³(cubic inch). Air molecules are held near the earth by gravity. In other words, air has weight. Weigh an empty bag, then fill it with air, it now weighs more. In addition gases, like air, are easily compressed, i.e., squeeze a gas together and its number density increases. In other words, we say gases are compressible because they can easily be squeezed into a smaller volume. Solids and liquids on the other hand are not easily compressed.

The weight of all of the air above a given point in the atmosphere squeezes air molecules closer together, which causes their numbers in a given volume to increase (increase in number density). The more air above a level (and hence the more weight of air above a level), the greater the squeezing effect (or compression).

Since air density is the number of air molecules in a given space (volume), air density is typically greatest at the surface or sea level (where it is squeezed by the weight of the entire atmosphere above) and decreases as we move up in the atmosphere because the weight of air above becomes less and hence there is less of a squeezing effect (See Figure Z).

Pressure

Atmospheric air pressure results from the Earth's gravitational pull on the overlying air. Without gravity holding the atmosphere just above the ground surface, air molecules would spread out, and the gas pressure would be close to zero.

The weight of the atmosphere acts as a force upon the underlying surface of the Earth. The amount of force excerted over an area of surface is called atmospheric pressure or air pressure. Near sea level, the average air pressure is about 14.7 pounds per square inch. In this class we will use the unit millibars(mb) to specify air pressure. At sea level the average air pressure is 1013 mb. Another way to think of this is that the total weight of all the air above sea levels weighs enough to cause 1013 mb of air pressure.

Since the air (a gas) is a fluid, the pressure force acts in all directions, not just downward. The pressure force pushing downward due to the weight of the air is the same as the pressure force acting sideways and even upward. If you are having trouble understanding this, make an analogy with another fluid liquid water. Consider a deep swimming pool full of water. The water pressure anywhere in the pool depends on the weight of the water above (that is the deeper you dive downward in the pool, the stronger the water pressure.) The pressure force is not just downward though, it pushes in on your body from all directions. The average air pressure at sea level (1013 mb or sometimes called one atmosphere of pressure) is caused by the weight of all the air above sea level. In the same way water pressure is caused by the weight of water above you. At a depth of 32 feet (9.75 meters) below a water surface, the water pressure is about one atmosphere. Thus, the entire column of air from sea level to outer space weighs as much as a 32 foot column of water. Of course diving deeper than 32 feet downward into water means you will encouter an increasing water pressure (enough to crush you if you go too deep).

Typical change in air pressure with altitude. Note how rapidly air pressure falls with increasing altitude.

In the atmosphere, the air pressure at any point is caused by the weight per area of the air above that point. As we climb in elevation, fewer air molecules are above us (less weight of air above us); hence, atmospheric pressure always decreases as you move upward in the atmosphere (See Figure B). Another way to look at it is that the air pressure at any point in the atmosphere is exactly enough to support the weight of the column of air above it. A balance exists between the gravitational force pushing air downward and the upward directed pressure force. This balance is called hydrostatic balance (see figure).

Earlier we made an analogy between diving down in water and moving downward in the atmosphere. In both cases, the fluid presure increases as you move down because there is more and more weight of fluid above you. A big difference between water and air, though, is that air is compressible and water is not. This affects the rate of pressure changes as one moves up or down in the fluid as shown in Figure C, which compares the rate of vertical pressure changes between water and air.

Because air is compressed by its own weight, much of the mass of the atmosphere is squeezed into the troposphere where the air is most dense (higher number density), while only a small portion of the mass of the atmosphere remains above the stratosphere where air is less dense (lower number density). Since air pressure is directly related to the weight of air above a given point, a ratio of air pressure is equivalent to a ratio of weight.

		Measured air pressure at your location
Fraction of the atmosphere above you by weight	=
		Sea level air pressure

If you do not know the sea level air pressure, you can 1000 mb as a good approximation. Thus, at a location where the air pressure is 500 mb, roughly half the weight of the atmosphere is above you and the other half is below you. A typical 500 mb height is about 5500 meters or 5.5 km above sea level. Thus, half the weight of the atmosphere is compressed into the vertical column from sea level up to about 5.5 km above the surface while the other half is spread out from 5.5 km upward to the top of the atmosphere, somewhere around 500 km above sea level. This happens because the number density is greatest just above the ground surface and decreases as you move upward. Note in the figure above how rapidly air pressure drops as you move up above the surface where number density is largest, but that the rate of pressure drop slows down as you move to higher altitudes where the number density is much smaller. This is characteristic of an exponential decrease in air pressure with increasing height. On average in Earth's atmosphere, the air pressure approximately drops in half for every 5.5 km increase in altitude. The air pressure is roughly 1000 mb at sea level, 500 mb at 5.5 km above sea level, 250 mb at 11 km above sea level, 125 mb at 16.5 km above sea level, and so forth. The figure above also shows that on average the top of the troposphere is located at a pressure of about 250 mb. Using the equation above, this means that 25% of the mass of the atmosphere is above the troposphere and 75% of the mass of the atmosphere is contained in the troposphere as stated in the temperature section above.

In-class demonstration for air pressure