Environmental Engineering Reference
In-Depth Information
simple. The incoming radiation heats the ground through absorption. A small proportion
of this energy is transmitted downwards into the soil by conduction but the majority is
returned as either sensible heat or long-wave radiation to heat the atmosphere. Heating is
greatest close to the ground surface and declines with height.
The rate at which the temperature falls with increasing altitude is called the
environmental lapse rate
(ELR). It is not constant, however, for it is affected by
atmospheric and surface conditions. When moist air is turbulent, or is being mixed by
strong winds, the environmental lapse rate, at least in the lower layers of the atmosphere,
is low, perhaps 5° C/1000 m; with strong surface heating, it is steep, meaning that air
temperature cools rapidly with height and may reach 10° C/1000 m or locally even more.
Under still, calm anticyclonic conditions temperatures may even rise with height for short
distances. Whatever its value, this rate of temperature change greatly influences air
movement. On average, the environmental lapse rate is about 6·4° C/1000 m.
STABILITY AND INSTABILITY
We can start to understand the importance of the environmental lapse rate by considering
a simple example. Imagine local heating of the air above an island in the sea. The island,
because it converts sunlight to heat more effectively than the surrounding water, will act
as a thermal source. Air in contact with this source will be warmed, its density will
decrease, its surface pressure will fall and the air will tend to rise. Typically, after the
bubble of air has risen a distance equal to about once or twice its own diameter it sinks
back. New and larger bubbles form in its wake, however, and each rises a little higher.
What controls this movement? The answer, simply, is density or relative temperature.
If the bubble of air is warmer than its surroundings it will continue to rise; if it is cooler,
it will sink. We know already that the general temperature of the air declines upwards -
that is the environmental lapse rate. We might imagine, therefore, that once the bubble
starts to rise it will continue to do so indefinitely, for the air around it is becoming
progressively cooler with height. That does not happen, however, and the reason is that as
the air bubble rises it will also cool. The critical factors that determine the height to
which the bubble rises are the relative rates of cooling of the bubble and of the
surrounding air.
The next question, then, is why does the bubble get cooler? As we move away from
the surface, air pressure will decrease. As the air bubble rises, it encounters surrounding
air of lower density. The pressure confining the bubble is reduced and it expands. As it
does so heat is extracted from the bubble and it becomes cooler. This is in accord with the
gas laws, which state that:
In other words, the pressure (
P
), volume (
V
) and temperature (
T
) of a gas are
interdependent. A change in any one of these properties tends to cause changes in the
others.
The rate at which the air cools with height as a result of this expansion is constant, at
9·8° C for each 1000 m of ascent. It is known as the
dry adiabatic lapse rate
(DALR).
Adiabatic
means that there is no heat exchange between the bubble and its surroundings,
and so long as the bubble of air rises rapidly, this condition applies. It is called dry, not
