AOS100 Exam #1

Explain why the temperature decreases and the relative humidity increases in an air parcel that is lifted. (10 points)

When an air parcel is lifted, its pressure will increase relative to its environment, since atmospheric pressure always decreases with altitude. The parcel will cool adiabatically about 10º per kilometer, assuming it's a dry parcel. This cooling happens because the parcel's kinetic energy, which it had more of at the lower level, is converted to potential energy as it rises and expands (since potential energy is energy of position, a parcel at a higher elevation will have more potential/positional energy). Since temperature is a measure of the average kinetic energy of an object/atmosphere, as kinetic energy goes down due to conversion into potential energy, so will temperature. Relative humidity is a measure of how close the air is to saturation, which occurs when the temperature and dew point are the same. If dew point is the temperature of saturation (and is always lower or equal to the temperature), as temperature decreases, relative humidity will increase. Therefore, the relative humidity of the air parcel will increase as kinetic energy is converted to potential energy as it rises, causing temperature to go down and approach the dew point temperature (assuming the dew point temperature remains constant).

Describe how liquid water cloud drops form in a cloud and how they grow to reach the size of raindrops. (10 points)

To form cloud droplets, you need to start out with moist air and nuclei. These nuclei can either be water droplets or aerosols, but it's much more common to have them be aerosols (tiny particles in the air). Water condenses onto the particles as the air rises, usually via wind and/or evaporation. The reason why water will condense is because as the parcel rises and cools (see answer to #1), the relative humidity will increase, meaning there's more water vapor inside the parcel of air that's turning into a cloud. As the cloud rises, these new little droplets can grow by colliding with one another and coalescing (growing bigger). The deeper the cloud and the stronger the updraft (air carrying the cloud/moist air upwards), the greater the chances for collision/coalescence. It also helps to have droplets of different sizes, since bigger droplets are more likely to be hit by other, smaller ones. Another helpful factor in aiding collision/coalescence is to have droplets of different electrical charges, since opposite charges will attract one another. Rain falls when droplets grow big enough to fall out of the cloud.