The atmosphere and the Weather

Clouds

A Cloud is a visible assemblage of small water droplets and/or ice crystals suspended in the air. A cloud might contain about 3,000 droplets per cubic inch of air. Cloud droplets range in size 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 by evaproation. 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. This is visible as steam. 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 the table of saturation mixing ratios 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(20.1g/kg)=14.1g/kg. There is no liquid water at first. Thus, there is 14.1 g of water vapor and no liquid water. From the table, the dew point for the parcel is slightly less than 20°C.

(b)If a parcel of air is cooled below its dew point temperture, 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 3.8g/kg, thus the lifted and cooled parcel now contains 3.8 g of water vapor. The remaining water vapor has condensed into liquid, therefore the parcel contains, (14.1g - 3.8g) = 10.3 g of liquid water in the form of tiny cloud droplets.

Why do rising parcels of air cool?

Remember that an air parcel is a bubble of air enclosed within an imaginary, elastic boundary keeping it separated from the rest of the air. No gas is allowed to leave or enter the parcel, but the shape of the parcel is allowed to change. As an air parcel moves around in the atmosphere, its shape changes in response to changes in air pressure surrounding the parcel. What happens is that the size of the air parcel will change so that the air pressure inside the air parcel becomes the same as the air pressure surrounding the air parcel. Why? If the air pressure surrounding the parcel is different than the air pressure inside the parcel, then the pressure forces on the parcel surface are not balanced, and the parcel is forced to change its shape (volume).

If the air pressure on the outside of the parcel (pushing inward) is greater than the air pressure inside the parcel (pushing outward), then the parcel will be squeezed together or compressed. As the parcel is compressed, the air pressure inside starts to increase. This continues until the air pressure inside the parcel equals the air pressure outside.
If the air pressure on the outside of the parcel (pushing inward) is less than the air pressure inside the parcel (pushing outward), then the parcel will expand outward. As the parcel expands the air pressure inside starts to decrease. This continues until the air pressure inside the parcel equals the air pressure outside.

So when an air parcel rises upward in the atmosphere, it must expand because the air pressure surrounding the parcel gets lower (See Figure H.) This means that the number density of air in the parcel decreases (same number of gas molecules but in a larger or expanded volume). The air in the parcel uses energy to expand (increase its size), so the temperature of the parcel also decreases. Mathematically, the pressure, number density, and temperature of a gas are related in an equation called the gas law:

pressure = temperature x number density x constant

Pressure decreases when a parcel is lifted. From the gas law, the (density x temperature) must also decrease. Both number density and temperature decrease when a parcel expands. By the way, you may remember the gas law equation from chemistry class, where it is typically expressed in the form PV=nRT. This expression above is the same equation except number density is used instead of volume and number of moles of gas.

Conversely, when an air parcel is lowered in the atmosphere, it is compressed because the air pressure outside the parcel increases. When parcels are lowered the air pressure, number density, and temperature of the gas in the parcel increase. In this case energy from the surrounding atmosphere is used to compress the parcel (decrease its size). This transfer of energy from surrounding environment to parcel acts to increase the parcel temperature.

A couple of everyday examples may help you to keep this straight. When air under high pressure, like that contained in an spray can or a bicycle tire, is released it feels cold. The spray nozzle or tire valve get cold because the air that is coming out is expanding and cooling. When air is compressed, like when operating a bicycle pump, it feels warm. The pump gets warm because air is getting compressed and warmed.

Key points:

When air parcels move in the atmosphere, their sizes (volumes) change in response to the changing air pressure surrounding them. The air pressure inside parcels will always equal the air pressure immediately surrounding the parcel. Importantly, this is NOT the case for temperature and number density. As we will see the air temperature and density inside parcels DO NOT have to equal the temperature and number density in the air immediately surrounding the parcel.
When an air parcel moves upward, the air temperature in the parcel decreases because it is expanding.
When an air parcel moves down, the air temperature in the parcel increases because it is being compressed.

Putting This All Together

Most clouds form well above the ground surface. Air parcels near the Earth's surface contain water vapor, which evaporated from liquid surfaces, e.g., oceans, lakes. When these parcels move upward, they take this water in the form of water vapor with them. As parcels move upward, they expand and cool, i.e., the temperature in the air parcel decreases. This increases the relative humidity of the air in the parcel because as air temperature falls, the capacity for water vapor in the air decreases. If the air is cooled enough (down to its dew point temperature), the relative humidity becomes 100%. Once this point is reached, when air parcels rise higher, their temperature becomes so low that they cannot hold all of the water vapor that they contain. The excess water vapor condenses into liquid water, forming clouds.