The Atmosphere

It is best to read the weather forecast before praying for rain. - Mark Twain


A knowledge of the atmosphere's structure is required for an understanding of daily weather events. The lower portion, called the troposphere, is the region where vertical mixing of hot and cold currents takes place. In this zone temperature usually decreases with increasing altitude. The altitude of the troposphere upper limit, called the tropopause, varies with season and latitude. It is highest where the air is warmest. On average about 12 miles above the equator and 5 miles above the poles.
The layer directly above the tropopause is the stratosphere. Here the temperature remains nearly constant through the first 18 miles then increases sharply up to the stratopause about 30 miles.
Above the stratopause lies the zone called mesosphere, where temperatures again decrease with altitude to a level of about 50 miles. A thin boundary layer called the mesopause seperates he mesosphere from the next atmospheric level called the thermosphere, where temperatures again reverse increasing with height up to about 120 miles.
Still another zone called the ionosphere extends above upward to about 250 miles. No meterological activity takes place here, but it is responsible for the phenomena of the aurora borealis.
Most of the atmosphere's mass lies in its very lowest levels , about 99 percent within 18 miles of the earth's surface, and 99.999 percent in the lower 50 miles. The tropopause confines almost all significant weather to the troposphere.


In the atmosphere, most heat is transferred by the process of convection, or the transport of heat by the movement of parcels of air. Convection is usually described as taking place vertically. Air heated at the surface of the earth tends to rise as column through surrounding coller air. The rising column of air cools in its acent until it reaches its dew point. At this point also called the condensation level, water vapor in the rising air becomes liquid and is visible as a cloud.


Water vapor constitutes a very significant, although small portion, of the atmosphere, varying from very little in desert and polar regions to as much as 4 percent by volume in jungle regions. When water changes from one state to another a exchange of latent or "hidden" heat is involved. Latent heat is stored in the molecular structure. When water vapor is cooled and condenses into a cloud or rain droplet, latent heat is released into the air, where it becomes sensible or "measureable" heat, rasing the air temperature. In the opposiite process, when water droplets evaporate, sensible heat is removed from the air, lowering the air temperature.
It is now belived that a small impurity, called a condensation nucleus, is required to form raindrops or ice crystals. The nucleus come from many sources, such as blowing soil, volcanoes, and the spray of oceans.


Two processes have been identified in the formation of precipitation. The first, coalescence, takes place when tiny droplets of water vapor within a cloud merge to form larger raindrops. The second, process takes place at higher elevations, where below-freezing temperatures prevail and supercooled (below freezing) liquid water droplets and ice crystals are present. The supercooled water droplets are attracted to the ice crystals. The crystals grow into snowflakes, and when they become heavy enough they fall from the cloud. Depending on surrounding temperatures they may fall as snowflakes or raindrops.

Stability and Instability of Air

As air moves up through the troposphere it cools and expands. The swelling atop a cumulus cloud and the rising turrent of a cumulonimbus , or thunderhead, are evidence of instability within the cloud. On the other hand, stable air inhibits vertical motion and the formation of clouds.
Changes in the air's stability cause many weather process. For example, intense heating of the ground in summer can cause instability and air parcels to rise. This can lead to cumulus cloud formation and thunderstorm activity. At night, when the surface cools, the air returns to a stable condition.

Subsidence and Lifting

There are other ways in which air moves vertically. When high pressure prevails over an area, the air aloft tends to descend and spread out, near the ground. This process known as subsidence, the sinking air is heated by the compression and clouds tend to dissipate and skys clear.
The opposite of Subsidence is lifting, where temperatures of a n ascending column of air is lowered as it rises into coller air. The air column's tempreature eventually reaches its condensation level where water vapor turns into water droplets. these form clouds and may lead to precipitation.
Air may also be lifted through a process called orographiclifting, which occurs when a vertical barrier such as a mountain range or a front forces an advancing flow of air to ascend.

Air Masses

An air mass is a large body of air with relatively uniform properties of heat, moisture, and stability. Air masses are moved by the general circulation of winds around the globe. As masses move there temerature and moisture are modified by different land surfaces and the by mixing with other airstreams. Colliding air masses are know as fronts.


The general circulation of air around the globe is a response to differences in atmospheric pressure. The three main factors are, The unequal distribution of land and oceans around the globe, the influence of the rotation of the earth and solar heating related to latitude.
The speed of westerly air flow increase to a reach a maximum just below the tropopause. The high speed flow aloft is called the jet stream, a narrow ribben of very fast westerly wind over the middle latitudes about 7 miles above the earths suraface. The jet stream is a meeting place for both cold and warm airstream from the north and south. Storms tend to generate in the boundary zone below the jet stream, known as a front, and are pushed to the east by the jet stream.


Probably the easiest way to become a weather watcher is to observe the clouds. Clouds can serve as a weather station aloft, revealing what is going on at different levels of the atmosphere, and can give indications of what may happen in the hours and days to come.