1. Basis of Cloud Formation
Water vapor (humidity) plays many important
roles in weather but none more
important then the formation of clouds

2. Cloud Formation
When air is saturated (100% relative humidity), the excess water vapor condenses to form clouds or fog.
Air becomes saturated by:
Addition of water vapor
Cooling the air below its dew point – usually through lifting

3. Cooling the Air
A common way for air to become saturated is for it to be adiabatically cooled.
Adiabatic temperature changes result when air is compressed or allowed to expand.
expanding air cools
compressed air warms

4. Cooling the Air Adiabatic cooling can be done by one of four ways:
Convective lifting
Orographic lifting
Frontal wedging
Low level convergence (upper level divergence)

5. Dew Point

6. Orographic lifting occurs when elevated terrain act as a barrier to flowing air. As the air is pushed up over the mountain, cooling generates clouds & precipitation

7. Frontal wedging occurs when cool air acts as the barrier over which warmer, less dense air rises. As the air is pushed up over the cool air, cooling generates clouds & precipitation

8. Convergence occurs whenever air masses flow together
Convergence occurs whenever air masses flow together. Such flow results in general uplift resulting in cooling of the air mass.

9. Convective lifting
Rising warm air cools at the rate of 1o/100 meters until the air reaches the dew point temperature and condensation occurs.

10. Cloud Formation
Once the air has been cooled below its dew point – condensation occurs
Condensation is the change of state from a gas to a liquid
It may result in the formation of dew, fog, or clouds
There must be something for the liquid to condense on (condensation nuclei)

11. Condensation Factors
Condensation nuclei – Tiny dust, dirt, smoke, and salt particles act as the surface upon which condensation occurs
Hydroscopic nuclei – Not all particles in the atmosphere make good condensation nuclei. Only those that can absorb water (called hydroscopic) are effective as nuclei.

12. Condensation Factors
Supersaturation – If the air contains few condensation nuclei, than the relative humidity must be greater than 100% (supersaturated) for condensation to occur.
Dew – Condensation can also occur on large surfaces, such as grass, cars, & windows. This is known as dew.

13. Cloud Composition
Clouds are made of a large number of very small droplets of liquid water and/or ice crystals.
Clouds are NOT water vapor.
The droplets are so small that they do not fall, but remain suspended in the air.
Clouds Typical cloud droplet concentration: 300 hundred/cm3.
500,000 droplets would fit in a 2L soft-drink bottle!

14. Cloud droplets start small and grow through coalescence.

15. Next time: Cloud Types

16. Credits
Web1.meso.com/wind-personal/glenn/171/lab05/clouds