What Happens to a Parcel of Air as It Is Lifted Up Through the Atmosphere?

Intertropical Convergence Zone

• Energy from the sun causes almost all of the Earth's weather. Earth rotates around a central axis during the 24-hour day cycle and is tilted at about a 23.5-degree angle of latitude. This is why the planet experiences seasons as the Earth orbits the sun. If the Earth were not titled, there would be no seasons. Because of this tilt, more sunlight strikes the planet around its equator throughout the year, creating more air uplift and instability. This area of air movement and persistent atmospheric instability is called the intertropical convergence zone.

Hadley Cell Circulation

• As warmer air rises from equatorial regions, it cools at higher altitudes in the troposphere. Air parcels are less able to hold moisture at lower temperatures, creating clouds and rain in the form of condensation. Eventually, these air columns reach what is called the tropopause at the top layer of the troposphere and lose nearly all of their moisture. Here, they turn and are redirected back to the Earth's surface. As they fall, they heat up in the lower troposphere, creating many of Earth's dry desert climates at about 30 degrees latitude. This general pattern repeats itself at 60 and 90 degrees latitude, creating what are called Hadley Cells, the dominant features of air movement patterns on Earth.

Relative Humidity

• On a local scale, a parcel of air goes through specific changes as it rises, depending on the air, atmospheric pressure and humidity conditions at different layers of the troposphere. Relative humidity is measured as a percentage of the maximum moisture that a parcel of air can hold. A 50 percent humidity means that the air is saturated with half of all the moisture it could possibly hold at a specific temperature and pressure.

Dew Point and Rising Air

• Warm air can hold much more moisture than cold air. As a parcel of air at the Earth's surface is heated by the sun, it rises and cools as it interacts with the cooler air found at higher altitudes. When it cools to the point that it becomes completely saturated, it reaches its dew point and cannot hold any more moisture. If the air cools further, clouds and rain result as excess moisture falls out of the air parcel.

Stable and Unstable Air

• Stable air parcels and air masses usually occur in areas of high air pressure. Higher pressure causes air to resist vertical movement within the atmosphere, generally leading to sunny conditions and little to no wind. An unstable air mass occurs when there is a change in air pressures, often occurring if a cold air mass moves under a warmer one. The colder mass of air forces the warmer air to rise. As it rises, the warmer air reaches its dew point quickly, leading to clouds, rain and wind. If the lifting force is strong, air can become very unstable, creating severe thunderstorms and rapid air circulation patterns that can lead to tornadoes.