The atmosphere and the Weather

Clouds

A Cloud is a visible assemblage of small water droplets or ice crystals suspended in the air. A cloud might contain about 3,000 droplets per cubic inch of air. Cloud droplets sizes range from microscopic to 0.002 inches. Although an individual cloud droplet is too small to see with the naked eye, the combined effects of many droplets make clouds visible. Clouds are NOT water vapor, which is an invisible gas.

Clouds are extremely important in regulating the climate of Earth. As they form, vast quantities of heat are released into the atmosphere. In addition, clouds regulate the earth's energy balance by reflecting solar radiation back to space and by absorbing the earth's infrared energy. We will come back to these topics later in the semester. Presently we will discuss how clouds form.

All clouds result from net condensation (or deposition), that is, water vapor in the air condenses (or deposits) into liquid water droplets (or ice crystals). Interestingly, the formation of water droplets requires a surface be available for condesation to begin. In the atmosphere, water vapor condenses onto microscopic aerosol particles, such as dust, smoke, and salt, which are called cloud condensation nuclei. There are always enough cloud condensation nuclei around for clouds to form. The actual physical processes that take place during the intial stages of cloud formation are quite complicated and will not be covered in this course.

Steam is basically a cloud. Above a boiling pot of water, lots of water vapor molecules are released into the air. As hot humid air moves away from the boiling pot of water, it cools sufficiently for net condensation to occur, and tiny droplets of liquid water form. As the steamy air spreads out, the concentration of water vapor decreases, the relative humidity drops below 100%, and the liquid droplets evaporate. Are you able to explain this situation in terms of the rates of evaporation and condensation? The same process explains why you are able to "see" your breath on cold days.

Clouds most often form as air rises and cools. As air cools, its relative humidity increases. Once the relative humidity reaches 100%, any further cooling results in net condensation and cloud formation. In fact, just enough water condenses to keep the relative humidity at 100%. A numerical example may help you to understand what happens. We will use Table 4-1 from an in-class handout.

• Suppose a parcel of air at a temperature of 25°C and RH of 70% is lifted upward in the atmosphere and cools to a temperature of 0°C. If the parcel contains 1 kg of dry air, how much water vapor and liquid water are contained in the parcel (a)before it is lifted upward; and (b)after it has been lifted and cooled to 0°C.
• ANSWER: (a)Before lifing the parcel, its mixing ratio is U=(RH)x(U_s) = (0.7)x(20g/kg)=14g/kg. There is no liquid water at first. Thus, there is 14 g of water vapor and no liquid water. From the table, the dew point for the parcel is 20°C.
• (b)If a parcel of air is cooled below its original dew point, just enough liquid water will condense so that the air in the parcel is saturated (relative humidity = 100%). From the table, this means that after the parcel has been cooled to 0°C, its mixing ratio will be 3.5g/kg, thus the lifted and cooled parcel now contains 3.5 g of water vapor. The remaining water vapor has condensed into liquid, therefore the parcel contains, (14g - 3.5g) = 10.5 g of liquid water in the form of tiny cloud droplets.

The next question we need to answer is 'Why does rising air cool'? In order to answer this question, we first need to describe how the atmospheric air pressure changes as we move up and down in the atmosphere.