Question by Ofer: It is well known that August is the hottest month of the year in the northern hemisphere. But why is that? Intuitively I would think that June should be the hottest month since this is when the Sun is at its highest point in the sky and the level of solar radiation per unit area is maximal. In August the days are shorter than in June and the Sun is lower in the sky. However, as mentioned, August is the hottest month. How can this be explained?

First, is August indeed the hottest month in the northern hemisphere? Well, it seems that July is probably the winner of this contest. After I've checked almost all capital cities in the northern hemisphere, I found July temperatures to be as hot as or hotter than August temperatures. This is the situation in all western and eastern European capitals and most of the Middle East and northern Africa. The big cities in North America are also mostly characterized by hotter Julies. In fact, the only capital city in which August is significantly hotter than July is... Jerusalem (I intentionally ignored Asian cities – we'll get back to it).

So why is that? Well, on June 21st every year the Sun is located in the Tropic of Cancer - the circle of latitude on the Earth that marks the most northerly position at which the Sun appears directly overhead. On this day, the northern hemisphere absorbs the most sunlight, in contrast to the southern hemisphere which absorbs the least. As a result, June 21st is the longest day of the year in the northern hemisphere. But if on June 21st the most sunlight is absorbed, why isn't it the hottest day of the year?

This is because the temperature on a given day depends on several factors of which direct sunlight is not the most important one. The most influential factor is the Earth's heat capacity: the Sun warms up the Earth, which later on radiates off heat in the form of electromagnetic radiation at longer wavelengths than that absorbed. This process is time-consuming, and the difference usually sums up to 4-6 weeks.

This phenomenon is not unique to summertime; the day on which the northern hemisphere receives the least sunlight is December 22nd, the shortest day of the year. Despite this, the coldest months are usually January & February, once again due to the time it takes for the Earth to radiate off its absorbed heat.

Schematic representation of Earth on June 21st. Sunlight is coming at a right angle to the Tropic of Cancer and the northern hemisphere receives most of it. On December 22nd, in direct contrast, the solar rays are perpendicular to the Tropic of Capricorn and the southern hemisphere receives most of the solar radiation.

In fact, this explanation is also relevant to other climate phenomena, and not necessarily in the timescale of months. Anyone who's ever been up outdoors in the wee hours of the night, either night swimming or simply waiting for the Sun to rise, knows that the coldest hours are not at the peak of the night but rather when sunlight begins to appear. The temperature of the ocean water also drops to a minimum just before sunrise and not in the middle of the night.

Another reason has to do with precipitation, in any of it forms. Along with the wind that usually accompanies it, rain cools down the air and brings down the overall temperature. June is usually slightly wetter than July & August and hence cooler. In some parts of Asia, summer is the wet season (monsoon season). In this part of the world the hottest month is the last dry month of the year – usually April or May.

Shlomi Dagan
Department of Biological Chemistry
The Weizmann Institute of Science


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