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Summary Atmosphere and Weather
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Atmosphere and weather2.1) Diurnal energy budget
The energy budget is the amount of energy entering the system, the amount of energy
leaving the system and the transfer of energy within the system
Energy budgets are usually considered at local scale (micro-scale) and at a global scale
(macro-scale)
Micro-scale climates energy budgets usually describe regional climates i.e. those
associated with large urban areas, coastal areas and mountainous areas
Daytime and night-time energy budgets
Six components to the daytime energy budget
Incoming (shortwave) solar radiation
Insolation is the main energy input and is affected by latitude, season, and cloud
cover. The amount of insolation will vary with the angle of the sun and with
cloud type
e.g. strato-cumulus clouds when the sun is low only allow in ±23% of radiation
The less cloud cover or the higher the cloud, the more radiation reaches the
Earth’s surface
Reflected solar radiation
The albedo is the proportion of energy that is reflected back to the atmosphere
Light materials have a higher albedo than dark materials
e.g. glass has an albedo of 20-30% reflects back 20-30% of the radiation it
receives
Surface absorption
Energy that reaches the earth’s surface has the potential to heat it depends
greatly on the nature of the surface
e.g. if the surface can conduct heat to lower layers, the surface will remain cold,
if not, the heat will become concentrated at the surface
limestone is a poor conductor with surface temperatures reaching ±45°C
in deserts during the daytime
granite is a darker rock with a lower albedo and therefore absorbs heat
well
The conducting of heat in soils also depends on moisture content
The air in pores of dry sand is a poor conductor so heat remains concentrated
on the surface
Water in soil pores increases heat flow e.g. wet compared to dry beach sand
on a sunny day
Sensible heat transfer
The movement of particles of air into and out of the area being studied
Convective transfer is the air that is warmed by the surface, rises by convection
currents and is replaced by the cold air that sinks below the warm air (denser)
Common in warm areas in the early afternoons
Warming causes the air molecules to expand, becoming lighter and causing
them to rise through the air that is colder and denser. Cooler air then moves
down to replace the hot air that has moved up
, Longwave radiation
Shortwave radiation from the sun is absorbed by the Earth and is re-radiated as
long wave radiation from the Earth, some may make its way back to the
atmosphere, but most is reabsorbed by “greenhouse gases” in the atmosphere
(CO2 and water vapour) often causing a net loss of radiation
Latent heat transfer
When liquid is turned into water vapour (evaporation using heat), heat energy is
used up from the air
Latent heat is stored in the water vapour until such a time when condensation
can occur, changing the water vapour to water droplets or ice crystals when the
energy is released into the air/returned to the earth, warming the earth
Dew is condensation on a surface
Air is saturated (temperature on the surface has dropped to below dewpoint,
causing condensation to occur)
Condensation occurs as more moisture is introduced (e.g. by a sea breeze)
while the temperature remains constant
Absorbed energy returned to Earth
Insolation received by the Earth will re reradiated as long-wave radiation but
most of this will be absorbed by water vapour and CO2 causing an increase in
temperature
Influence of clouds
High, thin clouds (cirrus) allow incoming solar radiation to pass through but
absorb some long wave radiation which warms the earth’s surface
Deep convective clouds (cumulonimbus) do not heat or cool overall
An overcast sky with complete cloud cover of low, thick clouds (stratus and
stratocumulus) can reflect 80% of solar radiation and cool the Earth’s surface
Clouds usually have higher albedos than the surface below them so more short-
wave radiation is reflected back to space then would be the case if there were no
clouds clouds have a net cooling effect
*assumes horizontal,
grass-covered surface
energy available onthe surface=incoming solar radiation−¿
Four components to the night-time energy budget
Long-wave (Earth) radiation
, Amount of long-wave radiation escaping to space from the Earth’s surface
depends on cloud cover
Clear skies produce a very cold night as there is no cloud cover to stop the
longwave radiation from escaping to space, so the temperature falls quickly
Longwave radiation loss is reduced under cloudy conditions as clouds return
longwave radiation to the surface
Absence of cloud cover can to lead to large temperature differences between
day and night e.g. desert temperatures can reach ±45°C during the day and be
below 0°C at night
Latent heat transfer
On cloudless, calm nights, cooling is very intense as heat is lost by longwave
radiation and because no warm air is moving into the area
Dew is formed when water vapour comes into contact with an object who
temperature is below dewpoint (condenses on object) latent heat absorbed
during evaporation is released during condensation, adding warmth to the air
near the ground
Conduction of heat to surface (sub-surface supply)
Heat transferred to the soil/bedrock during the day may be released to the
surface at night and can partly offset the night-time cooling effect
In cities, man-made materials/surfaces (glass/concrete/tar) retain heat which is
transferred to the air at night causing a higher nigh-time temperature urban
heat island effect
Sensible heat transfer
Cold air moving into an area may reduce temperature whereas warm air moving
to a given point will contribute energy and keep the temperature higher
Convection is the vertical movement of air ¯ ¯
Advection is the horizonal movement of air «
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