Geography Reference
In-Depth Information
this carbon is not released again by decay, then it remains stored on
land. In fact peatlands have preserved many interesting archaeologi-
cal features including almost perfectly preserved prehistoric human
remains with their leather shoes still intact. The world's peatlands
are a large store of carbon that was once in the atmosphere, and
they have actually reduced the amount of the greenhouse gas,
carbon dioxide. Peatlands have helped to cool the climate by a few
degrees. However, this all means that these peatlands could also be
a large potential source of carbon dioxide for the atmosphere if
they are rapidly degraded by human action such as through drain-
age or extraction for horticulture or fuel.
LARGE SCALE ATMOSPHERIC CIRCULATION
The atmosphere is in constant motion. Air moves vertically, prima-
rily due to heating from the Earth's surface below. This motion of
fluids (air is a fluid) is called
convection
and is the dominant
process for transferring heat upwards from the Earth's surface.
Because air pressure falls with height, then rising air expands and
therefore cools. The rate of temperature change with altitude is
known as the
lapse rate
. The normal rate of temperature change
with altitude is known as the
environmental lapse rate
and is
around 6.4°C per kilometre, but this is variable. However, a rapid
temperature change associated with a rising and expanding parcel
of air is described as 'adiabatic', meaning that there is no inter-
change of heat between the rising air parcel and its surroundings;
the temperature change is internal to the air parcel. The rate of
temperature decrease with altitude in the rising air parcel under
these conditions is known as the
dry adiabatic lapse rate
and is
9.8°C per kilometre. Cooling of a rising air parcel may result in the
air becoming saturated with water vapour, condensation of water
droplets and the formation of clouds and
precipitation
. When
water vapour condenses into liquid water it releases heat which
then warms the air slightly and therefore the lapse rate within this
air parcel is less than the dry adiabatic lapse rate and is known as
the
saturated adiabatic lapse rate
. The exact value of this lapse
rate varies depending on the amount of moisture in the air and the
temperature. When some air rises, other air descends to replace it
and so the troposphere is continually mixing.
