Chapter 18.2 Cloud Formation

Condensation can form dew, fog, or clouds Condensation can form dew, fog, or clouds. Condensation is when water vapor changes to a liquid. Condensation must have saturation to happen.

Air Compression and Expansion When air is compressed, the motion of the gas particles increases and the air temperature increases. When air pressure is released the air will expand and cool.

Adiabatic Temperature Changes Any temperature change that happens when no heat is added or taken away is called adiabatic temperature changes. This is the kind of temperature change that happens when air is compressed or expands. When air expands, it cools. When it is compressed, it warms.

Expansion and Cooling Remember as you move higher in the atmosphere the pressure decreases. As air moves upward through the atmosphere it expands and cools. The opposite is true for air moving back toward Earth. It will heat and compress. If air rises it will eventually cool to its dew point. This is when condensation takes place.

Processes that Lift Air Air usually resists vertical movement. There are 4 ways that cause air to rise: orographic lifting, frontal wedging, convergence, and localized convective lifting.

Orographic Lifting Orographic lifting – when elevated terrains (like mountains) act as barriers to air flow. As the air moves up the mountain slope clouds are made. As the clouds move over the mountain they lose moisture and become less likely to give off precipitation. This is very important for where we live. On the ocean side of the mountains they will get more rain (San Francisco area) than where we are and even less past the Sierra’s.

Frontal Wedging A front is when masses of warm air and cold air collide. When this happens the cooler, denser air will act as a barrier forcing the warmer, less dense air to rise. This is frontal wedging.

Convergence Convergence – whenever air in the low atmosphere flows together and lifting occurs.

Localized Convective Lifting On warm, summer day there may be unequal heating of the Earth’s surface and this causes pockets of air to be warmed more than the surrounding air. This warmer air will move upward These warmer air parts are called thermals. These are used for birds to fly very high or people to hang glide.

Stability Stable air will stay in its original position, while unstable air will rise. Air stability is determined by measuring the temperature of the atmosphere at different heights. Lifting of stable air will cause hardly any precipitation. Lifting of unstable air creates thunderstorms and occasionally tornadoes.

Condensation For condensation to take place, the air must be saturated. This happens most often when air is cooled to its dew point. For condensation to take place there usually needs to be a surface for water vapor to condense on. Dew forms near the ground. In the air there is dust, smoke, and salt particles (these are called condensation nuclei) and they allow water vapor to condense on them.