Average Annual Temperature
The temperature at any given location is a function of how much energy that area has received from the sun recently. Thus the poles, which only receive sunlight (and indirect sunlight at that) 6 months out of the year, are a lot colder than the regions of the Earth near the equator (which receive near direct sunlight year round).
Average annual temperature helps you get a general picture of the Earth's climate, however it only tells part of the story. The biggest problem with average annual temperature is that all of the climate's seasonality is removed. In other words, looking at average annual temperature, you'd never know that in New York City the temperature can reach over 25°C in the summer and can stay below 0°C in winter. Looking at annual temperatures is really only a starting point. Temperature must be examined at shorter timesteps, such as by seasons or even by months, to really understand how the Earth's climate functions.
Climate Research Unit, Univ. of East Anglia. Available at this address.
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