The Subtle Workings of Mother Nature

You know (or should know) that it is the tilt of Earth's rotation axis relative to its orbital plane (by 23.44 degrees) about the Sun that accounts for the changes in the seasons. Greater amounts of light energy are deposited within the northern hemisphere during its summer because (1) the Sun's rays are then more direct and so more intense, and (2) the number of daylight hours are longer (and so the duration of heating
in our hemisphere is thus longer). Just the opposite holds in winter.

But here is a surprising tidbit regarding Earth's seasons...

The Earth's global average temperature is actually higher in July, when it is the furthest from the Sun.

Averaged over the globe, sunlight falling on Earth in July (at aphelion - Earth's most distant point from the Sun) is about 6.8% less intense than it is in January (at perihelion - Earth's closest point to the Sun). Still, the average temperature of Earth's entire surface at aphelion is about 4 degrees F (2.3 degrees C) higher than it is at perihelion!

Why is the Earth's average surface warmer when we're farther from the Sun? All else being equal, the flux of light radiation arriving at Earth from the Sun is proportional to the Sun's luminosity divided by the square of the Sun's distance from Earth (and a geometric constant).  The change in Earth's distance from the Sun accounts for the 6.8% deficit of sunlight arriving at Earth in July as compared to January. But the rise in temperature of a substance due to an increase in the amount of energy deposited depends upon a quantity called specific heat. It's a measure of the amount of energy required to raise the temperature of a unit mass of material by 1 degree Kelvin. It's because there's more landmass in the northern hemisphere and more water in the south that accounts for the surprising result. During July the land-crowded northern half of our planet is tilted toward the Sun during the long day time hours. Land warms more rapidly than does liquid water1 due to the deposition of solar energy. Earth's average global surface temperature is slightly higher in July because the Sun is shining down on all that land, which "heats up" more rapidly.2  

This effect is similarly responsible for the lag in time between the summer and winter solstices (June 21 and December 22) and when we in the northern hemisphere experience the hottest and coldest temperatures of the season, respectively. It simply takes time to deposit sufficient energy in the northern hemisphere to bring us our consistently hottest days of summer, usually occuring in mid-late July. Likewise, it takes time for the northern hemisphere to reach it's minimum average temperature due to the deficiency of solar energy, this usually occuring between mid January - early February.



1Although, this effect is ameliorated somewhat by the fact that a large body of liquid water, e.g., an ocean, absorbs more solar energy than does typical land of equal area, due to water's generally lower albedo (reflectivity). Spatial and temporal differences in land and ice albedos, as well as in hemispheric cloud cover also affect how much light energy of the Sun is deposited into Earth.
2
One can also consider a further effect that the northern hemisphere winter is approximately 3 days shorter than its summers because Earth is traveling more quickly in its orbit about the Sun near perihelion than near aphelion.