If you purchased the Goodes World Atlas, turn to the January and July normal temperature maps. Compare the two and what do you notice immediately? First, in January the northern hemisphere is very cold, and the southern hemisphere is very warm. Next, the overall temperature in July is much warmer than that in January. Both the northern and southern hemisphere are colored yellow, indicating a temperature range of 10 to 21 degrees Celsius. Why is this? You have explored this relationship extensively in Lab 2, so you have probably figured out by now that land and water react differently to the incoming solar radiation (insolation) of the sun.
Most of the earth’s surface ranges between –34 degrees C and 32 degrees C. Temperature is controlled by four factors: latitude, altitude, cloud cover and land-water heating differences.
Let's consider the January Normal Temperature. What are the coldest areas, what are the warmest areas? What about the January Pressure? Where is the pressure the highest? There is a band of very high pressure over eastern Asia. This is known as the Siberian Highs and is the location of some of the consistently highest pressure readings on earth. Next, there is a band of low pressure around the equator: Does it correspond to a warm temperature pattern? The correlation in July is a little less dramatic than January. This is because the earth is overall much warmer in July.
Recall from lecture 1 the unequal distribution of isolation over the earth. Areas that are located at higher latitudes (very far north or south) receive much less isolation than areas near to the equator. In fact, it is worth pointing out that the regions that we call the ‘tropics’ (between 23.5N and 23.5S latitude), or regions that are generally considered to be the warmest, are the regions that receive direct sunlight for part of the year. These regions are much warmer than the rest of the earth simply because they receive much more energy than other parts of the globe.
The effect of altitude and latitude on temperature is nicely illustrated in figures. A common way of illustrating temperature ranges includes the x-axis (the months of the year) and the y-axis (the temperature). The horizontal lines represent the average temperature for each location, and the vertical bars represent the possible ranges of temperature. For example, in La Paz in June, the temperature can range between –1 and 18 degrees, but the average temperature is 5 degrees.
Recall that the lapse rate for the atmosphere is 6.4 degrees C/per 1000 meters. In addition, as the density of the atmosphere decreases with elevation, its ability to absorb radiant heat is reduced. The result is that at high elevations the temperature difference between day and night is much greater, as heat absorbed during the day is re-radiated at night (and the thin atmosphere does not absorb it well).
Low clouds reflect or absorb up to one-quarter of the isolation, and thereby have the effect of cooling our planet. Weather satellites show that at any one time, about 50% of earth is covered by clouds. Think about how daytime temperatures are much lower on a foggy day than on a sunny day. Clouds can also keep air temperatures higher at night by trapping the long wave radiation released by the cooling earth (this is just a magnified version of what the atmosphere already does). Therefore, a low cloud ‘blanket’ regulates earth temperature, keeping it from becoming too hot during the day, or too cold at night. On the other hand, high clouds can have the effect of heating up the earth, and causing the ‘greenhouse effect’.
K. Allison Lenkeit-Meezan (Foothill College)