How does condensation – the change from vapor to liquid – usually happen in the atmosphere? Consider this example. On a sunny spring afternoon the air temperature is 15.5 degrees Celsius and the specific humidity is 8 grams. The air’s capacity at this temperature is 11 grams, so it is not saturated. That night the air cools rapidly. When its temperature reaches 10 degrees Celsius, its capacity is only 8 grams. Since the specific humidity of the air is already 8 grams, the air is saturated.
What happens if the temperature drops below 10 degrees Celsius? All the water vapor above its capacity condenses. If the temperature drops to 4.5 degrees Celsius, the air’s capacity is 6 grams. Each kilogram of air releases 2 grams of water vapor, which then condenses. If the water vapor condenses on surfaces such as grass, in the form of a liquid, it is called dew. The water vapor could also condense into droplets, forming a cloud or fog.
The temperature at which saturation occurs is called the dew point. In the example just given, the dew point before condensation was 10 degrees Celsius. Dew point may be higher or lower, depending on the amount of water vapor in the air. The more water vapor the air starts with, the higher its dew point. When air cools to slightly below the dew point, water vapor begins to condense. In fact, when water vapor condenses in cooling air, air temperature and dew point are so close that they are considered equal. In the example, the temperature and dew point fall together until both equal 4.5 degrees Celsius.
Evaporating water molecules absorb heat energy from their surroundings. Condensing water molecules release the same amount of heat energy to their surroundings. Because of this, the process of condensation slows down the rate at which air cools. Thus, the temperature of cooling air drops more slowly after the dew point has been reached.
For water vapor to condense, air must be cooled below its dew point. This cooling can happen in four different ways. Air may lose heat by:
The last process is most important in producing clouds, rain, or snow. These processes will be discussed further in the next lesson. Even when air is cooled below its dew point, condensation may now occur. The air is then said to be supersaturated. Water vapor needs to condense on something. The tiny particles on which water vapor condenses are called condensation nuclei. If there are no condensation nuclei, condensation cannot occur.
Condensation nuclei are usually substances such as salt, sulfate particles, or nitrate particles. Salt enters the air when fine sea spray evaporates. The sulfates and nitrates come from natural sources and from the burning of fuels. Condensation nuclei are so tiny that a puff of smoke contains millions of them. Similarly, water vapor requires ice nuclei to form ice crystals. Some types of bacteria and clay particles contaminated with organic material are good ice nuclei.
Condensation usually happens when air is cooled below its dew point. If cooling occurs by contact with a colder surface, water vapor condenses directly on that surface. If the temperature is above 0 degrees Celsius, dew forms. The drops of water that form on the outside of a glass or ice water are an often-seen example of dew. Dew may form on the ground, on leaves and grass, and on other surfaces. At night these surfaces become cooler than the air because they lose heat more rapidly. The air reaches its dew point where it touches the cooler objects. Clear nights show greater cooling and heavier dew.
If the temperature at the surface is below 0 degrees Celsius, the water vapor condenses on surfaces as a solid, called frost. When the temperatures near the ground drop below –2 degrees Celsius, liquid in the cells of some plants may freeze. This freezing bursts the cell walls and kills the plants. Killing frost are caused not by atmospheric moisture but by the temperature of the plants themselves.
Often a surface layer of air a few hundred meters thick is cooled below the dew point. As water vapor condenses throughout the entire layer, tiny droplets fill the air and form fog. (If the temperature is well below 0 degrees Celsius the fog droplets will be ice.) Each droplet is centered about a condensation nucleus. The droplets are so tiny that they fall slowly. The lightest air movement keeps them suspended in the air.
Radiation fogs, or ground fogs, form under conditions similar to those that form dew. The nighttime sky is clear and the ground loses heat rapidly by radiation. Light winds mix the cold bottom air with the air a short distance from the surface. A whole layer of air is cooled below the dew point, and fog forms.
Radiation fogs are common in humid valleys near rivers or lakes. They are most frequent in the fall of the year. These fogs are thickest in the early morning and are “burned away” by the morning rays of the sun. The surface air in this fog is colder than the air above it. This arrangement of warmer air above cool fog is called a temperature inversion.
Advection fogs result when warm, moist air blows over cool surfaces. In the northern United States or southern Canada, advection fogs form when warm, moist southerly winds blow over snow-covered ground. The famous fogs of Newfoundland form when warm, moist air over the Gulf Stream blows over the cold Labrador Current. Summer fogs in coastal California form when warm ocean air strikes cold coastal waters. Winter fogs form along parts of the Gulf Coast when cold Mississippi River waters chill the warm gulf air at the river’s mouth.