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 (about 200 droplets per cubic centimeter). Cloud droplets range in size from microscopic to 0.002 inches (about 0.005 cm) in diameter. Although an individual cloud droplet is too small to see with the naked eye, the combined effects of many droplets within a small volume make clouds visible because they will affect the passage of visible light though them. Clouds are composed of tiny droplets of liquid water and/or ice crystals, which are really aerosols, and 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.

WORD document expanding on how water behaves and containg the numerical example given below

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 of the air surrounding the tiny droplets falls 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 temporarily "see" your breath on cold days.

In the atmosphere of Earth, clouds most often form as air rises and cools. As air cools, its relative humidity increases since the saturation mixing ratio decreases as temperature gets lower. 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, which was described in a previous reading page.

• 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. Since the relative humidity is less than 100%, 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.

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 started with. The excess water vapor (i.e., the water vapor in excess of the saturation mixing ratio at the lower temperature) condenses into liquid water, forming clouds.

As clouds form by condensation, latent heat is released. Under the right atmospheric conditions, this released heat can cause the air inside developing clouds to become warmer than the cloud-free air surrounding the clouds. Similar to a hot air balloon, warmer air surrounded by cooler air is buoyant because it is less dense (weighs less) than the surrounding cooler air, and thus this warmer air accelerates upward. This mechanism is the driving force for the formation of thunderstorms and hurricanes. We will expand on this concept later.