Explain how precipitation occurs when temperature of moisture in air suddenly increases?
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Moisture in the Atmosphere
Reading Assignment
The textbook for this course is entitled: eWeather and Climate by Netoff, Gillespie, Fujimoto-Strait and Tiller. Upon purchase of the Lab Manual for the Course GEO 1401/Lab, you will be provided a link to a downloadable pdf which will serve as the textbook for this course GEOG 1401/Lecture. The Lab Manual/cd text is only available for purchase online at: bearkatsonline.com.
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01. When the word Storms is used in relation to climatic controls, we are not talking about tornadoes, hurricanes and the like. Rather this control deals with the presence of moisture in the atmosphere, the various means by which moisture is removed from the atmosphere and the forms the moisture takes. Meteorologists are very interested in the amount of moisture in the air not only because the presence of water vapor is an indicator of the potential for precipitation, but the energy released as water vapor changes states provides the energy for storms.
Moisture, or humidity, is a general term we will use to describe the amount of water vapor in the air. There are a number of ways to express the amount of humidity in the air -- the most common being absolute humidity, relative humidity and mixing ratio. We will take up each of these shortly.
02. Let's begin our look at moisture by considering the ideas of saturation and vapor pressure. In the graphic below, suppose we have a sealed container that is half full of water. The temperature of both the water and the totally dry air (the air contains no water vapor) above the water is 70 degrees F. Assume that if we were to insert a barometer into the air overlying the water we would get a pressure reading of 29.92 inches of mercury.
03. In the graphic below the process of evaporation begins with the movement of one molecule of water vapor from the liquid water into the dry air above. As this first molecule makes its way into the air, we should expect to see an increase in the air pressure being registered on the barometer -- after all we have now added one additional gas (water vapor) molecule to the air above the liquid water. Now granted, in order to measure the pressure increase of one molecule, we would have to be dealing with a very sensitive barometer -- but the point is one additional molecule into the air would result in some increase in the air pressure within the sealed container. This portion of the total air pressure that is accounted for by the presence of water vapor (in this case just one molecule) is called vapor pressure.
As the process of condensation continues and the presence of water molecules grows greater, we will see an increase in vapor pressure within the sealed container.
04. In our final graphic in this sequence, we have reached the point where the air can not hold any additional water molecules. At this point, the air is said to be saturated -- it is at capacity. Any increase in vapor pressure (the result of any additional water molecules being moved into the air over the water) will cause a corresponding number of water vapor molecules to move from the air to (because the air temperature is above 32 degrees F) water.
05. Air is said to be saturated when a balance is achieved between those water vapor molecules leaving the water and those being returned to the water. The air can hold no more water vapor -- it is filled to capacity.
You will note that the capacity (ability) of the air to hold water vapor increases sharply with increases in temperature. In the graphic below observe that air at 30 degrees F is saturated when 3.3 grams of water vapor are present in a kilogram of otherwise dry air. Were you to heat the air to 60 degrees F, you can see that the capacity of the air increases to 10.7 g/kg (an increase of 7.4 grams). But when we increase the air temperature another 30 degrees F (to 90 degrees F), the capacity of the air increases 19.3 grams.
As you increase air temperature, the ability of the air to hold water vapor increases at an increasing rate.