called an inversion. Generally in the range of 5–11 km of altitude, the decrease in
temperature averages 0.7
C/l00 m altitude (3
F/1000 ft). In the free atmosphere,
where there are no other sources of heat or means of removal, the rising air cools at
the dry adiabatic lapse rate. In the middle latitudes this decrease in temperature is
l
C/100 m (5.5
F/1000 ft). This decrease in temperature occurs until the dewpoint is
reached. It must be pointed out that the dewpoint also decreases at a rate of 0.2
C/
100 m (1.1
F/1000 ft). After the dewpoint has been reached, the wet or, as it is
sometimes called, retarded adiabatic lapse rate occurs. This results in a decrease in
0.6
C/100 m (3.2
F/1000 ft). The difference between the wet and dry adiabatic
lapse rate is caused by the latent heat of condensation, which releases energy as
condensation occurs.
Under adiabatic conditions, three scenarios can occur, as shown in Fig. 6.3. The
first scenario represents stable air in which the existing lapse rate is less than both the
wet and dry adiabatic lapse rate. Therefore any rising air would be cooled more
rapidly than the surrounding air and, since it would be cooler, it would not rise.
Under this condition there would be no convective rising. This is depicted in the first
diagram in Fig. 6.3. The second scenario is one of unstable air. If the existing lapse
rate is greater than both the wet and dry adiabatic lapse rates, any rising air will be
cooled less rapidly than the surrounding air and thus would be relatively warmer
than the surrounding air and would continue to rise. This creates the unstable
conditions and is shown in the middle diagram in Fig. 6.3. If the existing lapse
rate is less than the dry adiabatic lapse rate, but greater than the wet adiabatic lapse
rate, the scenario is one of conditional stability. The condition would be stable only
when no moisture is condensing. However, when moisture is condensing, unstable
conditions would develop and the water vapor would continue to rise. This condition
is useful in artificial rainmaking, which relies upon initiating the precipitation using
dry ice, silver iodide, or some other nucleating agent.
Upward movements of air that cause precipitation generally fall under three
categories. The first is convective movement, which is typified by tropical rains
and thunderstorms. The surface air is heated rapidly by a warm sun during the day.
The water vapor rises rapidly, expanding to the point at which it then cools,
condenses, and precipitates. In tropical areas this is responsible for the routine
daily precipitation that falls sometime in early afternoon. Thunderstorms may
occur any time of the day or night, but are most common late in the afternoon as a
result of the rapid daytime warming by the sun.
The second upward movement of air is called orographic precipitation. This
occurs when horizontal currents of warm air pass over large bodies of water, such
as the ocean, where they pick up moisture. When they reach land, the air is forced
upward by the coastal mountains where it is cooled and forms precipitation. Since
the moisture is removed in the upward motion that produces this cooling, the air
mass becomes warmer and hence drier as it proceeds down the other side of the
mountain. Thus on the windward side of the mountain, areas of heavy rainfall occur,
whereas on the leeward side deserts occur. As a typical example, in the Pacific
Northwest, 190–300 cm/yr of precipitation occurs (75–125 in/yr) producing rain
forests on the windward sides of the mountain ranges. On the leeward sides, only
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
239
temperature averages 0.7
C/l00 m altitude (3
F/1000 ft). In the free atmosphere,
where there are no other sources of heat or means of removal, the rising air cools at
the dry adiabatic lapse rate. In the middle latitudes this decrease in temperature is
l
C/100 m (5.5
F/1000 ft). This decrease in temperature occurs until the dewpoint is
reached. It must be pointed out that the dewpoint also decreases at a rate of 0.2
C/
100 m (1.1
F/1000 ft). After the dewpoint has been reached, the wet or, as it is
sometimes called, retarded adiabatic lapse rate occurs. This results in a decrease in
0.6
C/100 m (3.2
F/1000 ft). The difference between the wet and dry adiabatic
lapse rate is caused by the latent heat of condensation, which releases energy as
condensation occurs.
Under adiabatic conditions, three scenarios can occur, as shown in Fig. 6.3. The
first scenario represents stable air in which the existing lapse rate is less than both the
wet and dry adiabatic lapse rate. Therefore any rising air would be cooled more
rapidly than the surrounding air and, since it would be cooler, it would not rise.
Under this condition there would be no convective rising. This is depicted in the first
diagram in Fig. 6.3. The second scenario is one of unstable air. If the existing lapse
rate is greater than both the wet and dry adiabatic lapse rates, any rising air will be
cooled less rapidly than the surrounding air and thus would be relatively warmer
than the surrounding air and would continue to rise. This creates the unstable
conditions and is shown in the middle diagram in Fig. 6.3. If the existing lapse
rate is less than the dry adiabatic lapse rate, but greater than the wet adiabatic lapse
rate, the scenario is one of conditional stability. The condition would be stable only
when no moisture is condensing. However, when moisture is condensing, unstable
conditions would develop and the water vapor would continue to rise. This condition
is useful in artificial rainmaking, which relies upon initiating the precipitation using
dry ice, silver iodide, or some other nucleating agent.
Upward movements of air that cause precipitation generally fall under three
categories. The first is convective movement, which is typified by tropical rains
and thunderstorms. The surface air is heated rapidly by a warm sun during the day.
The water vapor rises rapidly, expanding to the point at which it then cools,
condenses, and precipitates. In tropical areas this is responsible for the routine
daily precipitation that falls sometime in early afternoon. Thunderstorms may
occur any time of the day or night, but are most common late in the afternoon as a
result of the rapid daytime warming by the sun.
The second upward movement of air is called orographic precipitation. This
occurs when horizontal currents of warm air pass over large bodies of water, such
as the ocean, where they pick up moisture. When they reach land, the air is forced
upward by the coastal mountains where it is cooled and forms precipitation. Since
the moisture is removed in the upward motion that produces this cooling, the air
mass becomes warmer and hence drier as it proceeds down the other side of the
mountain. Thus on the windward side of the mountain, areas of heavy rainfall occur,
whereas on the leeward side deserts occur. As a typical example, in the Pacific
Northwest, 190–300 cm/yr of precipitation occurs (75–125 in/yr) producing rain
forests on the windward sides of the mountain ranges. On the leeward sides, only
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
239