Lapse Rates

Atmospheric resistance to vertical motion (stability), depends upon the vertical distribution of the air’s weight at a particular time. The weight varies with air temperature and moisture content. As shown in figure 2-7, in comparing two parcels of air, hotter air is lighter than colder air; and moist air is lighter than dry air. If air is relatively warmer or more moist than its surroundings, it is forced to rise and is unstable. If the air is colder or dryer than its surroundings, it sinks until it reaches its equilibrium level and is stable. The atmosphere can only be at equilibrium when light air is above heavier air—just as oil poured into water rises to the top to obtain equilibrium. The stability of air depends a great deal on temperature distribution and to a lesser extent on moisture distribution. Since the temperature of air is an indication of its density, a comparison of temperatures from one level to another can indicate how stable or unstable a layer of air might be—that is, how much it tends to resist vertical motion. Lapse Rates In chapter 1, it was shown that temperature usually decreases with altitude and that the rate at which it decreases is called the lapse rate. The lapse rate, commonly expressed in degrees Fahrenheit per 1,000 feet, gives a direct measurement of the atmospheres s resistance to vertical motion. The degree of stability of the atmosphere may vary from layer to layer as 2-14 STANDARD AIR MOIST AIR IS LIGHTER THAN STANDARD AIR. HOT AIR IS LIGHTER THAN STANDARD AIR. - ON LOOSING ITS MOISTURE IT THEN BECOMES... DRY AIR IS HEAVIER THAN STANDARD AIR. COLD AIR IS HEAVIER THAN STANDARD AIR. - IF COOLED, IT THEN BECOMES... STANDARD AIR RELATIVE OR T AIR WILL RISE. (UNSTABLE) HOT MOIS RELATIVE OR AIR WILL SINK. (STABLE) COLD DRY B A AG5f0207 Figure 2-7.—Moisture content and temperature determines weight of air.