Temperatures on the Earth are largely dependent upon how much energy is received from the Sun, which varies both with latitude on the Earth and the time of year. At present, the global average surface temperature is about 15°C.
Regions nearer the equator are much warmer than regions nearer the poles, with average surface temperature 25°C or higher. In contrast, average temperature on Antarctica is way below freezing. This latitudinal temperature gradient however, generates a global transfer of heat from equator to poles via the general circulation of winds, such that the equator is kept cooler and the poles warmer than they would be if the Earth had no atmosphere.
The equator however, does not experience the highest temperatures on Earth. Here, rising air generates daily thunderstorms that consume considerable amounts of heat energy, suppressing the air temperature by several degrees Celsius. The greater cloud cover also helps to reduce the amount of sunlight. In fact, the hottest places on Earth lie in the subtropical climate zone of high pressure between latitudes 25 to 40°, where cloud cover is virtually non-existent and sunshine levels are very high throughout the year. Parts of the Sahara Desert, Saudi Arabia, California and Australia can experience daytime temperatures of over 50°C, although during the winter months, severe radiation cooling under the clear skies at night can drop the air temperature close to or even below freezing.
The most pleasant climates occur in the temperate mid-latitude zone. Here summer temperatures rarely go above 40°C, and winters are usually fairly mild, on account of the influence of the warm westerly winds that originate from the subtropics.
The changing of the seasons also influences temperature. During the Northern Hemisphere summer from April to September, the Northern Hemisphere of the Earth is tilted towards the Sun, and consequently receives more sunlight, raising the surface temperatures. From October to March, the Southern Hemisphere receives more energy from the Sun. While it is winter in the UK it is summer in Australia. Differences between wintertime and summertime temperatures tend to be greatest in higher latitudes, and particularly in the interior of large landmasses such as North America and Asia far from the moderating influence of the oceans. Winter temperatures in Siberia and Canada, for example, can fall below -40°C, whilst in summer, they may rise above 30°C. The UK, by contrast, has a more maritime climate, and the yearly temperature range is only 10 to 15°C. The smallest annual temperature range occurs in the equatorial tropical climate zone, where the changing angle of the Sun through the seasons has a proportionally smaller effect on the total amount of sunlight received.