On June 21, 2009, at 1:45am EDT, another summer* began. Astronomically speaking, of course.
We learn in middle-school science class that summer isn’t the hottest season because the Earth is nearest the Sun at that time. Actually, the Earth is about as far from the sun as it ever gets at the time of the summer solstice.
What makes summer hot and winter cold, as our junior high teachers informed us, is the angle at which the Sun’s rays strike the Earth. You can get a sense of this by extending your arm out the window of a moving car. If you face your palm in the direction of travel, so that the wind strikes perpendicular to the surface of your palm, you’ll experience a great force! Turn your hand, however, so that the wind hits you at an angle, and the force you’ll experience will decrease as the angle extends further from the perpendicular.
So, by that logic, why isn’t June 21, the day of the summer solstice, the hottest day of the year?
(Any sun-lovers who happen live in upstate New York have been wondering about the answer to this question, as our temperatures haven’t gotten much above 75 all year.)
Really — water. There’s a great deal of water on this planet. About 70% of the surface of the planet is covered in water. That’s millions of square kilometers.
And water has a very, very special property — it has a very high specific heat capacity. In other words, it takes a lot of heat energy to raise the temperature of water one degree. This is why boiling water for tea or pasta seems to take forever. Metals (like your tea kettle) have a very low heat capacity. They get hot quickly and cool down quickly.
As the summer goes on and June turns into July, heat is stored in the oceans, lakes, and ponds around us. The water continues to heat up as the Sun’s rays beat down. In the evenings, as the air temperature cools to below that of the water temperature, the heat from the warm water is slowly released into the atmosphere.
Because of this effect, the (average) warmest day of the year is sometime around July 22, a full month after the solstice. The reverse effect happens in the wintertime, and the coldest days of the year fall in late January, not December.
You can also see this regionally. The coasts of the United States tend to have more moderate temperatures than the Midwest, thanks to their proximity to the oceans. An extreme example, consider deserts — deserts get very cold at night due to the lack of water.
So if you’re wondering where the heat is, I only ask you to be patient. The solstice has passed, but the warm weather is yet to come.
*In the Earth’s Northern Hemisphere, which I assume throughout this post, since I and about 98.5% of the readership of this blog lives in the Northern Hemisphere. The same logic applies to the Southern Hemisphere, yet in December, not June.