Direct FactorsFor any location on the earth's surface, the temperature is controlled by three overall factors:
- Heat transport (often called "heat flux")
- Heat storage (often called "heat capacity")
Water phase changes
Details of Direct FactorsHeat transport refers to the following processes:
- From above:
- Net Radiation
- Downwelling short wave (Solar) Radiation
- Upward (Reflected) short wave Radiation
- Downwelling Longwave (IR) Radiation
- Upward (Emitted) Longwave Radiation
- 1.-2. = Net short wave Radiation
- 2./1. = Surface Albedo
- 3.-4. = Net Longwave Radiation
- 1.-2.+3.-4. = Net Surface Radiation
- Turbulent Atmospheric Heat Transport
- Sensible Heat Flux
- Latent Heat Flux
- From below:
- Conductive Heat Transport (through ground, ice or snow)
- Turbulent Heat Flux (through ocean or other liquid water masses)
- From the sides:
- Horizontal Advection of Temperature and Humidity in the Atmosphere
- Horizontal Advection of Temperature in the Ocean
Heat storage refers to the heat which is stored in the lower atmosphere and in the material immediately below the surface. This can be in the form of latent heat, in which case temperature is not affected unless water phase changes occur. Materials such as light snow or dry soil have low heat capacities, which means that changes in surface temperature due to the heat transport mechanisms listed above will be greater than for surfaces with a high heat capacity, such as the ocean.
Water Phase Changes refer to the processes of evaporation, condensation, sublimation, freezing and melting. These affect the surface temperature by changing the form of heat from latent to sensible forms or vice-versa. Latent heat represents a form of heat storage that is not "realized" until there is water phase change.
The processes described above are called the "direct" controls on surface temperature because they are the processes that are directly responsible for controlling the surface temperature for any given time and location. Any numerical model attempting to simulate or predict surface temperature would need some type of mathematical representation of the above processes to provide a quantitative measure of their effects on surface temperature.
- The temperature of a ground, water or ice surface (surface temperature) and the temperature of the air just above the surface (near-surface air temperature) are not always the same, although they are usually closely related. In this section the term "surface temperature" will refer to either parameter, they will be distinguished when needed.
- For simplicity, we use the term 'ocean' for all liquid water masses. This includes lakes, rivers and all other surface liquid water masses.
The focus of this module will be on a qualitative understanding of the factors controlling the annual cycle of surface temperature for a given location. For this purpose, it is more useful and practical to examine indirect factors controlling surface temperature, indicated below in green. They are called 'indirect' because, in themselves, they do not control the temperature. They do, however, control the direct factors (red) which, in turn, control surface temperature. Here is a list of the most important indirect factors which control surface temperature, along with an explanation of how each one affects the direct factors and the annual cycle of surface temperature.
- Time of Day
- Solar radiation is a function of time of day.
- The surface is always emitting longwave radiation, usually at a rate greater than it is absorbing longwave radiation from above. Therefore, this tends to cool the surface. During midday hours, the net short wave radiation is typically greater than the net longwave radiation, the result is surface warming. At night, the short wave radiation is zero, therefore the net surface radiation cools the surface. These processes create the temperature cycle we call the diurnal temperature cycle.
Time of Year (season)
- Solar radiation is also a function of time of year, therefore the surface temperature typically follows a yearly cycle.
- Solar radiation is also a function of latitude. On the average, the poles receive much less than the tropics, but not during all months of the year. Examine Figure 1 to see how downwelling solar (shortwave) radiation varies with latitude and time of year. Note that on their respective summer solstices, the North and South Poles actually receive more solar radiation than anywhere else in the world, although in today's climate most of the solar radiation reaching the surface in polar regions is reflected back due to the high albedos of snow and ice.
Four important aspects of the surface are albedo, heat storage, moisture content, and vegetation type:
"Albedo" is the reflected solar radiation divided by the incoming solar radiation. Snow and ice have relatively high albedos. Therefore they tend to remain cooler than other surfaces exposed to short wave radiation.
- Heat storage
Dry snow and dry soil have low heat storage, causing larger diurnal and yearly temperature variations. The ocean has very large heat storage, due to both the the high specific heat capacity of water and the ability of turbulent mixing to transfer heat energy much deeper than solid surface types.
- Moisture content
Moisture at the surface will often decrease the albedo, but usually a more important effect is to convert sensible heat to latent heat by evaporation when warmed by short wave radiation. Moisture also tends to increase the surface heat storage. The net result of these effects usually causes moist surfaces to have smaller diurnal and yearly temperature variations than dry surfaces.
- Vegetation type
Higher and thicker vegetation lessens short wave radiation, while at the same time increasing downward longwave radiation at the surface. Transpiration of plants cool the surrounding air in the daytime. The net effect is that thicker and higher vegetation decreases the magnitude of both the diurnal and the yearly temperature cycles.
- In general, higher elevations surfaces will be colder due to adiabatic cooling of air parcels and decreased downward longwave radiation. Small scale elevation changes may have different effects and are discussed below in the local topography category.
- The ocean moderates surface temperature swings for reasons discussed above. Horizontal advection causes these effects to extend over adjacent land regions as well. Regions over the oceans and adjacent land areas are said to have a marine climate that is characterized by a relatively small magnitude yearly temperature cycle. Land regions far away from marine influences, particularly areas isolated by mountains, have very large differences in temperature between summer and winter. These areas experience a continental climate. Therefore, continentality is a measure of the marine influence (actually the lack of) and is quantified by the magnitude of the yearly temperature cycle. Synoptic activity and clouds (described below) are also related to marine influence and are therefore closely related to continentality.
- Cyclones are efficient at horizontally advecting heat between latitudes and between land and ocean areas. Therefore, polar regions that experience frequent cyclones are usually warmer than other polar regions. Storms are associated with high winds and cloudy conditions. The high winds tend to disrupt nocturnal surface inversions and cause vertical advection (mixing) of warmer air from above. The effects of the clouds, vertical and horizontal advection from strong synoptic activity causes areas in storm tracks generally to have more moderate diffferences in average temperature between winter and summer. (There will be larger 1-10 day temperature swings.)
- Clouds decrease downward short wave radiation and increase downward longwave radiation. The result is cooler days and summers, and warmer nights and winters.
- In the absence of strong mixing winds, shallow valleys will collect cold air that has flowed downhill from surrounding surfaces that are cooled by longwave radiation at night. Therefore, these areas will have colder nighttime temperatures. Conversely, the surrounding slopes will be warmer than would otherwise occur without the cold air drainage. This may seem to contradict the elevation effects described above. But the topographic effects described here result from a location's immediate surroundings, while the elevation effect described above is more general. These "cold drainage" valleys and adjacent warm slopes are not so different in elevation from each other that adiabatic heating of the downward flowing air parcels is able to counteract the longwave radiational cooling. Therefore, due to the combination of elevation and local topographic effects, high valleys tend to have the coldest temperatures in a region, with the possible exception of mountain peaks, where elevation effects (cold) may dominate local topographic effects. Deep valleys and areas in the lee of mountains may be warmer than nearby areas (at the same elevation) if air parcels are forced to move downhill before reaching the valley bottoms.
Ocean Currents and Convection
- Horizontal temperature advection by ocean currents and turbulent vertical heat flux during ocean convection can transport large amounts of heat energy and strongly affect surface temperature.
Last update: 6/22/2000
Please send all comments and suggestions to the author/instructor, Peter Guest.