In the northern hemisphere summer, why are places in the north cooler than those in the south?

Question

As the Earth's axis is inclined, during summer in the northern hemisphere places in the north are exposed to the Sun's rays for longer, so days are longer during summer (say in June - Aug). Yet, countries like the UK have a lower summer temperature than countries like India, particularly given that the UK is to the north of India and has longer days in summers). I was just wondering about this. Feel free to share your insights.

Answer 1

The image below (apparently from "physicalgeography.net" is somewhat instructive:

Notice that even in the middle of summer, India (at about 30 °N) still gets slightly more sun than the Britain (at 60 °N). That's because although the days are longer in Britain, the sun is lower in the sky and so its heat is spread out over a wider area.

But the North Pole gets even more sun than either Britain or India in June, and the equator gets substantially less. So there is something to explain. The reason is thermal inertia.

_{This graph shows insolation averaged over 24 hours at four different Latitudes. This shows the theoretical insolation. The model used to calculate these curves includes the effects of Earth's elliptical orbit, but do not take account of any atmospheric effects, such as the energy absorbed by the atmosphere, nor any weather effects such as clouds. It shows that a perfectly black disc with an area of 1 m² placed just above the atmosphere and oriented horizontal to the Earth surface would receive an average of about 515 Joules per second at the North Pole on June 21st and an average of about 380 Joules per second at the Equator. The main reason for the Equator value being so low is that it is night time for about 12 hours at the equator.}

Imagine pushing a heavy ball. It can take some time for it to begin to move quickly because it has inertia. Similarly, it takes time for the ground and water to heat up when the sun shines upon them. India gets much more sun in winter, and so the ground and water remain warm. When summer comes, it heats up further and ends up much hotter than Britain. In Britain there is very little sun in winter, and the ground and water cool down (and freeze). Then, even when the sun comes in summer, it takes time to warm up. By the time the ground and water have warmed up, the sun is already starting to weaken. And so the temperature never rises as much as it can in India.

There are lots of details and local effects (the gulf stream in Britain, and the monsoon in India are significant) but the basic reason that India is hotter in summer is that it gets more sun in winter!

Pidwirny, M. (2006). "Earth-Sun Relationships and Insolation". Fundamentals of Physical Geography, 2nd Edition.

Answer 2

Hours of sunlight are not the only factor that determines insolation (solar energy at the surface).

Thought experiment: spread a bed sheet on the floor, switch the lights off, and shine a flashlight directly down on that bed sheet from about one foot above it. You will get a small, fairly bright spot of light on the sheet.

Now tilt the flashlight, and shine it across it at a flat angle - say, 45 or 60 degrees - still keeping it about one foot away from the spot it's illuminating. As you flatten the angle, that spot elongates to cover a larger and larger area. But the amount of light coming out of the flashlight is still the same, spread over a larger area, which means lower intensity on the sheet. The same amount of light that was previously hitting a 100 cm² spot might now be spread across 500 cm².

This is part of what's happening at the high latitudes. During the summer months they are getting many hours of sunlight, but they're not getting a lot of energy per square metre compared to the tropics. In early April, the North Pole is getting sunlight 24 hours a day, but much less sunlight (in terms of energy/m² etc.) than the equator gets in 12.)