Weather & Climate- Atmosphere

The atmospheric heat budget

The Earth and the atmosphere are heated by energy from the sun. The atmospheric heat budget of the Earth depends on the balance between insolation and out going radiation. This budget has remained constant over the last few thousand years. However there is evidence of global warming in recent decades.

The amount of energy received from the sun is determined by:

• The solar constant- varies slightly and affects longer term climate rather than short term weather variations.
• The distance from the sun- the eccentric orbit of the Earth can cause a variation of up to 6% in the solar constant.
• The altitude of the sun in the sky- the equator receives more energy as solar radiation strikes the Earth head on, whereas at 60 N or 60 S the angle creates twice the area to cover and increases the amount of atmosphere to go through.
• The length of day and night

The Earth receives energy from the sun as insolation. Some is lost as it passes through the atmosphere but overall the surface has a net gain of energy, the exception being the polar regions. Only about 24% of this insolation reaches the surface as it is either absorb, reflected or scattered.

The atmosphere in contrast has a net deficit of energy. Because of this difference, heat is transferred from the surface to the atmosphere by radiation, conduction and by the release of latent heat

Heat budget by latitude

There are variations in energy and heat between latitudes. 

• Low latitudes have a net surplus of energy, mainly because of their relative proximity to the sun. 
• The high latitudes (pole wards of 40 N and 40 S) have a net deficit. As the tropics are not heating up and the poles are not cooling down, a transfer of heat must occur.

This occurs by:

• Horizontal heat transfers: air movement (winds, 80%, including the jet streams, hurricanes and depressions) and water movement (ocean currents).
• Vertical heat transfers: energy is transferred from the warm surface vertically by radiation, conduction and convection. Latent heat also helps to transfer energy, e,g, when water is evaporated. This energy is released when condensation occurs in the upper atmosphere.

Factors affecting insolation and the heating of the atmosphere:

Longer term effects:

1) Altitude of the land- insolation heats the surface of the land which warms the air above by conduction & convection. As higher land is further away from heat source, the main mass of land heated by insolation is cooler. Density of air decreases with height adding to cooler effect.

2) Altitude of the sun- At higher latitudes the heat energy from sun has to pass through more atmospheres so more heat energy is lost to absorption or scattering.

3) Proportion of land & sea- land and sea react differently to insolation. Land heats up quicker than sea. Water has a greater specific heat capacity which means it requires twice as much energy to raise 1kg of water by 1 degree than it does for land. But oceans retain heat for longer.

4) Prevailing winds- As the temperature of an air mass is determined by the area which it originated from and the surfaces it passes over, wind from the sea is cooler in summer and warmer in winter, than wind that has travelled over land.

5) Ocean Currents- Surface ocean currents are caused by influence of prevailing winds blowing across the sea. Warmer ocean currents migrate pole wards, away from the equator & colder currents replace them by moving towards equator in a circulatory motion known as a gyre. Clockwise in N hemisphere and anticlockwise in S hemisphere. The rotation of the Earth causes water to move westwards. North eastwards movement of warmer waters is a key factor in raising air temp that deliver mild winters and cool summers to Britain. Labrador Current off the N E of N America can reduce summer temps. 

6) Ocean Conveyor belt- The transfer of cold water at depth of Polar Regions to the equator is like a conveyor belt. As water cools at the poles the formation of ice, leaves the remaining water- saltier and denser. Denser water sinks. It sweeps the Antarctic continent. These motions are reciprocated by movement of the less salty and less dense surface temp which moves north towards N Atlantic from India & pacific oceans. These warmer waters have a significant effect on temp of N Atlantic. North Atlantic is warmer than North Pacific. 

Short Term Factors:

1) Seasonal Changes- Insolation is distributed equally in each hemisphere at the spring and Autumn equinoxes, when the sun is at the equator. In the summer around 21st June and winter around 22nd December 22nd solstices when the sun is directly overhead at the tropics, maximum insolation is experienced in the N hemisphere and the S hemisphere respectively.

2) Diurnal Range- the length of day & night varies in all locations away from the equator. At the poles there is no insolation during winter months when the regions are tilted away from the sun and there are up to 24 hours of daylight in summer when they are tilted towards the sun.

3) Local

• Aspect- slopes alter the angel at which the sun strikes Earth. South facing slopes in the N hemisphere receive more of the available insolation than N facing. This has impact on agriculture.
• Cloud cover- clouds may reflect, absorb and scatter incoming radiation, but can also act as an insulating blanket, keeping the heat in the lower atmosphere. Therefore when there are clear days temperatures rise more rapidly as more insolation reaches the surface but when there are clear nights they fall quickly as terrestrial radiation reduces the surface temp. Conversely when it is cloudy temperatures do not rise as high or fall as low. 
• Urbanisation- Urban surfaces tend to absorb more heat than natural surfaces during the day and radiate more at night creating urban heat island effects.