1 Clouds Katy Ginger Meteorologist, NCAR

2 Relationship between changes in air pressure & temperature? As air pressure increases, temperature increases As air pressure decreases, temperature decreases

3 What must happen to the temperature of rising air? Atmospheric air pressure decreases with an increase in altitude. This is because air pressure is determined by the weight of the overlying air Consequently, air rising upward experiences lower pressure and expands Consequently, air rising upwards expands and (warms or cools)?

4 Why is there no cloud in our bottle? Clouds generally exist off the Earth’s surface, in air that is at a lower pressure What else is needed? What is a cloud?

5 Liquid water droplets or ice crystals suspended in the air

6 Why no cloud yet We added water and saturated the air so that a small change in pressure would be enough to cool the air and cause condensation Condensation is the changing of water vapor (gas) to liquid water when air is cooled

7 Recipe for a cloud Moisture Lift - changes in pressure brought about by: – Surface heating and convection – Topography (mountains) – Ascent of air from air mass convergence – Uplift along weather fronts Cloud condensation nuclei (water needs to condense on something) Cloud is when Temp = Dewpoint Temp

8 Atmospheric Facts Temperature generally decrease with height in the troposphere (0-10km) and then increases with height in the stratosphere (10-50km) Day to day weather occurs in the troposphere Pressure decreases with height because there is less mass (air) above Air contains of N 2 (78%) O 2 (21%) H 2 O (1-4%) plus other gas molecules

9 Atmospheric Structure

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13 Heating of the Atmosphere The atmosphere is heated from below because sunlight passing through clear air does not really heat the air.

14 Forms of Heat Transfer Conduction - air molecules in contact with the the ground are heated (important within a few centimeters of the ground) Convection - parcels of heated air become less dense, rise and redistribute heat Radiation - Earth’s surface emits infrared radiation (heat) that is absorbed Latent Heat - heat given off to the atmosphere when water changes phase