Introduction to Condensation

By: PoonamShah | Views: 2612 | Date: 20-Feb-2012

Condensation is the phase change of water vapor into a liquid. During the condensation process, water molecules lose the 600 cal/gm of latent heat that was added during the evaporation process. When latent heat is released it is converted into sensible heat which warms the surrounding air. Warming the air increases its buoyancy and fuels the development of storms. Condensation takes place in the presence of condensation nuclei and when the air is nearly saturated.

Condensation is the phase change of water vapor into a liquid. During the condensation process, water molecules lose the 600 cal/gm of latent heat that was added during the evaporation process. When latent heat is released it is converted into sensible heat which warms the surrounding air. Warming the air increases its buoyancy and fuels the development of storms. Condensation takes place in the presence of condensation nuclei and when the air is nearly saturated.

Water vapor is darting around so fast in the air that the molecules tend to bounce off one another without bonding. Even if a few pure water molecules were to collide and bind together, the surface tension created by such a tiny sphere is so great that it is extremely difficult for additional water molecules to become incorporated into the mass. Hence condensation nuclei act as a platform for condensation to take place, increasing the size of a droplet and decreasing surface tension. Water absorbent clay minerals and sea salt are good condensation nuclei. Sulfates and nitrates are water absorbent and are responsible for creating acid rain.

The air must be at or near its saturation point for condensation to take place. Air can become saturated in two ways, 1) add water to the air by evaporation thus bringing it to saturation given its present temperature, or 2) cool the air to its dew point temperature. Cooling the air is the most common way for condensation to occur and create clouds. Air can be cooled through contact with a cold surface or by uplift. Contact cooling occurs when air comes in contact with a cooler surface and conduction transfers heat out of the air.

Uplift Mechanisms

Adiabatic cooling occurs when air is uplifted from the surface causing the air to lose heat through the work of expansion. A parcel of air is uplifted when it initially gains heat from the surface causing convective uplift. When the air is warmed by the surface it will expand and become less dense relative to air that surrounds it. Being less dense than the air that surrounds it, the air becomes buoyant and begins to rise. Because atmospheric pressure decreases with height, the parcel of air expands and cools. If the air cools to its dew point temperature saturation occurs and condensation begins. The elevation above the surface where condensation begins is called the condensation level.

Figure 7.8 Convergent uplift

Convergent uplift occurs when air enters a center of low pressure. As air converges into the center of a cyclone it is forced to rise off the surface. As the air rises it expands, cools, and water vapor condenses. Convergent and convective uplift are the two most important uplift mechanisms for condensation in the tropics. Under the intense sun, surface heating causes the moist tropical air to rise. Convergence of the trade winds in the Intertropical convergence zone creates copious rainfall in the wet tropics as well.

Orographic uplift is the forced ascent of air when it collides with a mountain. As air strikes the windward side, it is uplifted and cooled. Windward slopes of mountains tend to be the rainy sides while the leeward side is dry. Dry climates like steppes and desert are often found in the "rain shadow" of tall mountain systems that are oriented perpendicular to the flow of air. A rainshadow in the lee of the trade winds as they cross the mountainous northwest portion of the Big Island of Hawaii (Figure 7.10). Cloud formation and green vegetation identifies the windward, while the reddish brown indicates the dry leeward side.

Figure 7.10 Rainshadow on the Big Island of Hawaii. (Source NASA EOS)

Frontal uplift occurs when greatly contrasting air masses meet along a weather front. For instance, when warm air collides with cool air along a warm front, the warm air is forced to rise up and over the cool air. As the air gently rises over the cool air, horizontally developed stratus-type clouds form. If cold air collides with warm air along a cold front, the more dense cold air can force the warmer air ahead to rise rapidly creating vertically developed cumulus-type clouds.

Assess your basic understanding of the preceding material by "Looking Back at Phase Changes of Water" or skip and continue reading.
Adiabatic Temperature Change and Stability

In "The Atmosphere" we discovered that air temperature usually decreases with an increase in elevation through the troposphere. The decrease in temperature with elevation is called the environmental lapse rate of temperature or normal lapse rate of temperature. Recall that  the normal lapse rate of temperature is the average lapse rate of temperature of .65o C / 100 meters. The environmental lapse rate of temperature is the actual vertical change in temperature on any given day and can be greater or less than .65o C / 100 meters.  Also recall that the decrease in temperature with height is caused by increasing distance from the source of energy that heats the air, the Earth's surface. Air is warmer near the surface because it's closer to its source of heat. The further away from the surface, the cooler the air will be. It's like standing next to a fire, the closer you are the warmer you'll feel. Temperature change caused by an exchange of heat between two bodies is called diabatic temperature change. There is another very important way to change the temperature of air called adiabatic temperature change.

Adiabatic temperature change of air occurs without the addition or removal of energy. That is, there is no exchange of heat with the surrounding environment to cause the cooling or heating of the air. The temperature change is due to work done on a parcel of air by the external environment, or work done by a parcel of air on the air that surrounds it. What kind of work can be done? The work that is done is the expansion or compression of air.

Imagine an isolated parcel of air that is moving vertically through the troposphere. We know that air pressure decreases with increasing elevation. As the parcel of air moves upward the pressure exerted on the parcel decreases and the parcel expands in volume as a result. In order to expand (i.e.. do work), the parcel must use its internal energy to do so. As the air expands, the molecules spread out and ultimately collide less with one another. The work of expansion causes the air's temperature to decrease. You might have had personal experience with this kind of cooling if you've let the air out of an automobile or bicycle tire. Air inside the tire is under a great deal of pressure, and as it rushes outside it moves into a lower pressure environment. In so doing, the parcel quickly expands against the outside environment air. By placing your hand in front of the valve stem, you can feel the air cool as it expands. This is called adiabatic cooling.
As air descends through the troposphere it experiences increasing atmospheric pressure. This causes the parcel volume to decrease in size, squeezing the air molecules closer together. In this case, work is being done on the parcel. As the volume shrinks, air molecules bounce off one another more often ricocheting with greater speed. The increase in molecular movement causes an increase in the temperature of the parcel. This process is referred to as adiabatic warming.

The rate at which air cools or warms depends on the moisture status of the air. If the air is dry, the rate of temperature change is 1oC/100 meters and is called the dry adiabatic rate (DAR). If the air is saturated, the rate of temperature change is .6oC/100 meters and is called the saturated adiabatic rate (SAR). The DAR is a constant value, that is, it's always 1oC/100 meters. The SAR varies somewhat with how much moisture is in the air, but we'll assume it to be a constant value here. The reason for the difference in the two rates is due to the liberation of latent heat as a result of condensation. As saturated air rises and cools, condensation takes place. Recall that as water vapor condenses, latent heat is released. This heat is transferred into the other molecules of air inside the parcel causing a reduction in the rate of cooling.

Stability of Air

Adiabatic temperature change is an important factor in determining the stability of the air. We can think of air stability as the tendency for air to rise or fall through the atmosphere under its own "power". Stable air has a tendency to resist movement. On the other hand, unstable air will easily rise. What gives air "power" to rise? The tendency for air to rise or fall depends on the adiabatic and environmental lapse rates.

Stable atmospheric conditions prevail when the environmental lapse rate is less than the saturated adiabatic rate. At the surface (0 meters) both the parcel of air (red line) and the air of the surrounding environment (blue line) have the same temperature. The surrounding air is changing its temperature at a rate of .65oC/100 meters. The parcel on this day is "dry" and will rise and cool at a temperature of 1oC/100 meters. After giving the parcel a slight upward push, it rises to a level of 1000 meters where it cools to a temperature of 20oC. A measurement of the air surrounding the parcel shows a temperature of 23.5oC. In other words, the parcel is colder (and more dense) than the surrounding air at 1000 meters. If the uplift mechanism ceased, the parcel of air would return to the surface.

Unstable air

Air is unstable when the environmental lapse rate is greater than the dry adiabatic rate. Under these conditions, a rising parcel of air is warmer and less dense than the air surrounding it at any given elevation. Figure 7.13 depicts unstable conditions. Follow up the graph for the rising parcel of air. Note that at any elevation above the surface the parcel temperature is higher than the air that surrounds it. Even as it reaches the dew point temperature at 2000 meters, the air remains warmer than the surrounding air. As a result it continues to rise and cool at the saturated adiabatic rate. Vertically developed clouds are likely to develop under unstable conditions such as this.


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