Thursday, 12 April 2012

The Earth is hottest when it is furthest from the Sun

During the period when the Earth is furthest from the sun (aphelion), the average temperature of the entire planet is about 4°F (2.3°C) higher than when it is closest to the sun (perihelion). On average, the intensity of sunlight falling on Earth during aphelion is about 7% less than during perihelion. Despite this, the Earth ends up being warmer during the period in which it is furthest away from the sun.

So what’s going on here? During the winter months, for the Northern Hemisphere, the overall temperature of the Southern Hemisphere, where it is summer, doesn’t change as much as the other way around. This is because a much larger portion of the Southern Hemisphere, compared to the Northern Hemisphere, is made up of water and water has a significantly greater heat capacity than land. On a similar vein then, during the summer for the Southern Hemisphere, the overall average temperature of the Southern Hemisphere doesn’t increase as much as the Northern Hemisphere does during its summer, for this same reason.
Earth-lighting-summer-solstice ITEarth-lighting-summer-solstice IT (Photo credit: Wikipedia)

So the “tl;dr” version is: there is a lot more land in the Northern Hemisphere than the Southern Hemisphere; this land heats up much faster than water and water cools down much slower than land. So even though there is less intensity of sunlight during the summer in the Northern Hemisphere, the Earth’s average temperature is higher at this time when it’s furthest from the sun.

As you might have guessed then or already known, the seasons are not caused by the distance the Earth is from the sun, but rather are caused completely by the fact that the Earth is tilted on its axis 23.5°. This is why when it’s summer in the Northern Hemisphere, it’s winter in the Southern Hemisphere, and vice-verse. Without this tilt, there would be no seasons and the weather from day to day across the globe would be relatively uniform. In this case, there would only be a very slight variation in temperature as the Earth got closer or further away from the Sun, but for the most part, everything weather-wise would stay the same year round.

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