Understanding the Heat Budget
Tue Aug 25, 2009 12:39 pm
The heat that enters the earths atmosphere is called incoming solar radiation. Incoming heat is absorbed by the earth, on average this is balanced by outgoing heat escaping the earth. If this was unbalanced it would result, overall, in global heating or cooling.
Did you know that the atmosphere absorbs heat? This is true! Heat energy has trouble passing through the atmosphere, thus a lot of the heat is absorbed. This is why it is warm even when the sun goes down, as the atmosphere is retaining its warmth. CO2 (carbon dioxide) is very good at storing heat, so therefore as the amount of CO2 in the atmosphere increases, the overall temperature also rises.
The heat budget is like so, given that the earth receives 100% incoming solar radiation:
- 47% is absorbed by the earths surface and changed into heat energy
- 24% is reflected back into space by clouds (clouds reflect heat well)
- 14% is absorbed by particles in the atmosphere (such as carbon dioxide molecules)
- 6% is reflected back into space by particles in the atmosphere
- 6% is reflected back into space by the earths surface*
- 3% is absorbed by clouds
* this is called albedo. Albedo is the ability of a surface to reflect. The best reflectors are polar ice shields and clouds.
Three ways the earth heats the atmosphere
There are three such ways that the atmosphere is heated by the earth.
1. Thermal radiation
The earth absorbs "short wave solar radiation" (called insolation) and radiates outwards "long wave heat energy" (thermal radiation).
2. Conduction and convection
Convection currents carry heat up into the atmosphere. Air comes into contact with the warm earth, becomes heated and rises.
3. Evaporation and condensation
When water evaporates the water vapour contains heat. When this water condenses later, the heat is released.
Global temperature variations
The latitudes between 37 degrees North and 37 degrees South is called the heat surplus zone. This is the case because places in this region get the most amount of heat energy per unit area. Places in the heat deficit zones (such as the north and south poles) receive the same amount of energy however it has to be spread over a much larger area.
Height above sea level can also influence temperature. As the height above sea level increases, the temperature decreases. The technical term for this is the "environmental lapse rate", which refers to an average temperature drop of 6.5 degrees celcius per 1000m above sea level. Most greenhouse gases are close to the earths surface as this is where a lot of the heat is absorbed, while oceans tend to have a moderating effect on temperature (this is why cities right next to the coastline tend to stay a bit warmer at night, as the ocean allows the area to retain its warmth).
Urban areas with large areas of dark surfaces can sometimes be called urban heat islands, as they generally have higher temperatures than natural vegetation zones due to the higher rate of heat absorbtion.