3.6 - Controls on Temperature and Patterns of Temperature

Controls on Temperature

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We have already talked about a number of factors which influence temperature around the globe:

• latitude
• the angle of the sun (higher sun angles = higher receipt of solar radiation)
• the length of the day (more consistent length of day = higher receipt of solar energy)
• the distance solar energy must travel through the atmosphere (higher sun angles = more direct trip through the atmosphere and higher rates of transmission)

These factors give the Tropics the edge when it comes to the receipt of  solar energy. There is a surplus of solar energy in the Tropics and a deficit in high latitudes. This imbalance triggers what we call weather. It drives differences in atmospheric pressure which in turn drives the wind and precipitation. It drives the ocean currents. You and I live at 38°N where there is a balance between incoming and outgoing solar insolation. Our weather is, nevertheless, caused by this exchange of energy from low latitudes to high latitudes.

There are other factors that cause variations in temperature from place to place around the Earth.  These are the Controls on Temperature.

1. The Differential Heating of Land and Water:

 

Land heats up much more rapidly and to much higher temperatures than does water and it cools off much more rapidly and to much lower temperatures than does water. Water is highly mobile and convection effectively moves heat through the water column. Land is opaque and energy moves through soil very inefficiently through conduction. Temperatures change daily in water down to an average of 20 feet and down to 650 feet annually.  Land surfaces only see a daily temperature change down to only 4 inches and annually to 50 feet. We store roots in cellars and wine in caves precisely because the temperature changes in soil are confined to the upper few inches. Water has a very high specific heat and evaporation over water bodies is much higher than over land.   Summer temperatures are higher over continents than oceans and winter temperatures are lower over the continents than over the oceans.

2. Ocean Currents:

Warm ocean currents, like the poleward moving North Atlantic Drift keep winter temperatures moderate. For example both Fairbanks, AK and Reykjavik, Iceland are located at about 65°N latitude. Reykjavik is located on the ocean and in particular, on an ocean warmed by the North Atlantic Drift. Fairbanks is far inland.  In Reykjavik not one month averages below freezing while in Fairbanks five months during the year average below 32°F. The effects of warm currents are most apparent during the winter in middle and high latitudes. The effects of cold currents are most pronounced in the Tropics or in the summer in mid-latitudes. Off our coast we have the cold, California current.  In summer, warm air blows across the Pacific and hits the cold current. Its temperature plummets causing  condensation and the formation of fog.  San Francisco and Sacramento are both located at 38°N. Sacramento is inland from the ocean and has an average July high temperature of 94°F while San Francisco, right on our very cold ocean, has an average July high temperature of 67°F.

3. Altitude:

As you increase in elevation, there is less air above you.  Therefore, atmospheric pressure decreases as your elevation increases. As the pressure decreases, the molecules spread out or expand and the temperature drops. Conversely, as  you drop in elevation there is more of a column of air above you, the air is compressed and the pressure increases. As the pressure increases the temperature goes up. On average, for each 1000 feet you rise in elevation the temperature drops 3.5°F (the average lapse rate).  In the US we don't really have many high altitude cities. Denver is the highest major city in the U.S.  with an elevation of 5,280 feet.  But around the world major population centers are located at very high elevations. The capitol of Bolivia ( La Paz) is located at 11,942 ft with a population of over 11 million people. The capitol of Ecuador (Quito) is located at 9,350 with a population of 3 million.  More than 7 million people live in Bogota, Columbia at 8,612 ft and Mexico City is located at 7,350 with almost 9 million residents.  Why are these cities at these altitudes? Defense; the ability to grow different crops; the availability of water; and lower temperatures with less disease (less disease carrying mosquitos) make high elevations desirable.

4. Geographic Position:

As we've seen, where you are located with respect to the ocean can have a profound impact on temperature. Coastal locations have milder summers and winters than do inland locations. This effect is called continentality.  Being in a windward location (a location facing the prevailing wind) can also impact temperature. If that location is a coastal location it will experience the full moderating impacts of the ocean.

Global Temperature Patterns

 

The map on the right, is an isotherm map showing average summer temperatures in the Northern Hemisphere (and winter in the Southern).  The most obvious pattern is that temperature is lowest in the high latitudes. What may be more surprising is the that the highest temperatures are not at the Equator, and in some cases not in the Tropics, instead they are located about 30° N (or S) and are in the center of landmasses. North central Mexico, north central Africa and central Asia have the highest summer temperatures in the Northern Hemisphere. These locations are all far from the oceans and low enough in latitude to experience high temperatures. Also note the isotherm along the West Coast of the U.S.. It shows the impact of that cold current moderating the summer temperatures.

In Winter, we see that the lowest temperatures are in the high latitudes in the center of the landmasses. The center of Greenland and Siberia are among the coldest. Again, we see continentality at work. These locations are a long way from the moderating influences of the ocean. If you take a look at the isotherm on the west coast of Norway, you'll notice how much milder the winter temperatures are there than they are in the interior of Eurasia. Norway is close to the ocean and it is an ocean warmed by the North Atlantic Drift creating milder winter conditions than you might expect at that latitude.

It is pretty clear looking at the two maps above,  that the temperatures are highest and lowest over land. When we look at the annual temperature range (map to the left)  it is  greatest at high latitudes in the center of continents and the least over oceans. We also see very little temperature range at the Equator.  When we compare the Northern and Southern Hemispheres, we have to keep in mind that the Southern Hemisphere is 81% ocean while the Northern Hemisphere is 61% water. This explains why the Southern Hemisphere sees less extreme temperature ranges than does the North.  The most significant temperature ranges are seen in the largest landmasses in both hemispheres.