Why isn’t it warmer?
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.)
Water.
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.
June 22nd, 2009 at 10:14 pm
Please tell me you remember a cartoon that used to be on called “Science Court” like in the late 90’s. I used to watch it all the time, and they had an episode directly on point about how the Earth was moving away from the Sun in the summer months. No one else I know has ever heard of the show, but it was like my life in the second grade.
June 22nd, 2009 at 11:18 pm
@Pat R.: Never heard of it, but I was out of college in the late ’90s. Wow, that makes me feel old.
Anyway, I must learn more about this show… (goes to search Youtube)
June 22nd, 2009 at 11:26 pm
Yeah I only found like an episode and a half on YouTube, I’ve been scouring the depths of the Interweb for a DVD but to no avail, only a teaching course selling for way more money than it’s worth. It’s absolutely nowhere to be found.
June 23rd, 2009 at 3:54 pm
Also, do Equator-dwelling folks have seasons? Because it seems like during summer and winter, due to the 23.5° tilt of the axis, they’d always have an indirect exposure, whereas during our spring and/or fall months they might have more direct exposure, I think? I actually never learned about seasons at the Equator, apparently no one felt it very pressing to teach me as such.
June 23rd, 2009 at 9:55 pm
@Pat R.: No real seasons to speak of.
I took a look at the climates of two cities close to the equator: Quito, Ecuador and Nairobi, Kenya.
Both have climates (Quito, Nairobi) that are pretty much constant year-round. But even being off the equator a slight bit seems to make a difference. Nairobi is at about 1° south latitude. If you look closely at Nairobi’s climate, you can see about a 10°F temperature difference between winter highs (July) and summer highs (January).