14 



METEOROLOGY AND CLIMATOLOGY: 



production when water vapor condenses. Moreover, 

 this heat is released into the surrounding air. Natur- 

 ally, this warms the air, thereby causing further air 

 expansion, rising, cooling, and condensation which if 

 carried far enough results in precipitation. Therefore, 

 differences in rates of air heating and cooling cause 

 the water vapor, temperature, and pressure of adja- 

 cent areas of air to differ. These differences cause 

 local variations in winds and general climatic condi- 

 tions. 



MOISTURE 



The amount of water vapor necessary to saturate 

 air is determined by temperature. The higher the 

 temperature, the greater the amount of moisture 

 necessary to saturate the air. A standard measure- 

 ment of this relationship is relative humidity (the 

 amount of moisture in air in relation to the amount 

 that could be held at the same temperature). Rela- 

 tive humidity is always expressed as a percentage of 

 the total amount that could be held. Therefore, the 

 maximum amount of moisture air can hold is 100 per 

 cent relative humidity and the saturation, or dew, 

 point is immediately beyond that. 



In addition to relative humidity, water content is 

 measured by absolute humidity. This is a measurement 

 of water content per unit volume of air. However, 

 absolute humidity is generally ignored in natural 

 history studies, because environments are affected 

 most directly by available water and relative humidity 

 indicates this availability better than does absolute 

 humidity. 



Water availability is best explained in terms of the 

 biogeochemical cycle involving water, the hydrologic 

 cycle. In this cycle moisture is lost from the air in the 

 form of precipitation and is regained by the evapora- 

 tion of water from the land, living organisms, and 

 bodies of water (see Figure 17.4, p. 308). The condi- 

 tions already considered indicate why and how pre- 

 cipitation is formed. Moreover, the process whereby 

 increased temperature can change liquid water to 

 vapor (evaporation) can be readily appreciated. How- 

 ever, three aspects of moisture replenishment deserve 

 further consideration. First, the main source of water 

 vapor is standing bodies of water, especially the 

 oceans. Second, conditions that result in a continu- 

 ous supply of dry air will produce maximum evapora- 

 tion from the earth's surface into the air. Therefore, 

 lower relative humidity, higher temperature, and 



greater wind velocity each contribute to greater 

 evaporation of surface moisture. Third, and perhaps 

 obviously, the moisture-holding capacity of the air is 

 increased with any rise in temperature. Actually, for 

 every 18°F. increase in temperature, the moisture- 

 holding capacity of air is doubled. 



Air moisture is transported by winds, along the 

 surface of the land or upward into the higher levels of 

 air. The movement of water vapor, causing it to mix 

 with air, is the direct consequence of air mass move- 

 ments, our next topic. 



WIND 



Consideration of air mass movements, or winds, 

 must be on both a local and a worldwide scale. 

 Locally, four major factors characterize winds. First, 

 winds move from higher pressure areas into lower 

 pressure areas. This is the effect of a tendency 

 toward equalization of air pressure density, or 

 toward homogeneity of the number of air molecules 

 per unit volume in different places. Therefore, any 

 lower pressure area has fewer air molecules per unit 

 volume than does any higher pressure area. 



Second (actually another way of considering the 

 first characteristic), cold air is usually of higher pres- 

 sure and weight than is warm air. This is the case be- 

 cause cold air (in contrast to warm) has less energy 

 (heat) for molecular activity, and less molecular 

 movement enables individual air particles to be closer 

 to one another. As a result, cold air has greater 

 particle density, weight, and pressure. Therefore, 

 cold air, when it has the above features, penetrates 

 through and forces its way under warm air. Follow- 

 ing this penetration down and under warm air, the 

 cold air often continues as a wind over the land. Be- 

 fore considering the next local wind feature, it must 

 be emphasized that these temperature and pressure 

 relationships are general tendencies. There are ex- 

 ceptions in which warm air has greater pressure than 

 does cool air. For example, under local conditions 

 warm air may be compressed until it has greater 

 pressure than cooler air. 



Third, differential surface temperatures contribute 

 to local wind patterns. Any condition causing nearby 

 areas to have different temperatures usually causes 

 differences in air pressures. The role of such air 

 pressure variation in producing winds already was 

 mentioned. 



Finally, surface features alter wind direction or 



