It is my (barely informed) understanding that summer is caused primarily by the larger amount of sunlight hitting that hemisphere during the season. So why is it that (in the northern hemisphere) July and August are so often hotter than June (I know it’s not always the case but usually).
Also why wouldn’t May, getting just as much direct sunlight per day as July, be as hot on average as July? Why is August even hot when the end of the month is getting close to the equinox?
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It takes time to heat up the air, oceans, and land.
In May, the entire world is cooler because it hasn’t had as much time to heat up from winter as in August. The Earth, especially the ocean, keeps the weather cooler because the water is still slowly warming up from December.
By the time August comes around, May, June, and July have been working to overcome this massive heat sink. The Earth in general, and especially the ocean, has had time to become warm. This makes it easier for the same amount of sunlight to heat up the air.
During summer, over months of sun exposure, the dirt and water literally get hotter. Air temperature is a balance of absorption and emission of heat of the surface. Then during winter the balance leans toward cooler. There’s more emission than absorption because there’s less daily sun, so by the end of winter temperatures are at their coldest.
Let’s say you’re trying to fill a leaky bucket at a water tap.
As you turn the tap on to start filling the bucket, you slowly turn it more and more open, until it reaches its maximum flow rate, and then you slowly turn it back down until it’s off.
In the meantime, the bucket is slowly draining out water at a constant rate. Less than the tap can fill it at partial blast.
Now ask yourself: when is the water level in the bucket at its *highest*? Is it when you had the tap all the way open? Not necessarily. Even when you start turning the tap down, water is still flowing into the bucket faster than it leaves the bucket, so the water level continues to increase, even though the tap is getting weaker.
Only once the flow rate of the tap becomes equal to the flow rate of the leak does the bucket’s water level actually stop rising. And once the flow rate of the tap becomes less, the water level of the bucket starts to decrease again.
Heating the Earth is kind of similar; just instead of water filling a bucket from a tap, it’s heat energy warming up the Earth from the Sun. The maximum “fill level” of heat doesn’t arrive when the sunlight is at its maximum, it’s always increasing until the sunlight input starts to equal the output of the heat radiating away from Earth. That doesn’t start to happen until later into the summer.
Think of the each hemisphere as a large sink, with the temperature represented by water in the sink. The sink is constantly filling from the tap, as the Sun keeps throwing energy at us, summer or winter, but it fills up faster – aka the tap is putting water in more quickly – in the the summer months.
But the sink is constantly draining – this represents the heat lost by things like heat loss to space, or the other hemisphere, or any other sources. This loss is not constant, but it doesn’t vary THAT much compared to how much the tap varies.
If water drains out of the sink faster than it fills, it gets colder. If water fills the sink faster than it drains, it gets warmer. And this is why June isn’t the warmest month. The tap might be the most open in June, but it’s still filling faster than its draining until August or so.
So, you know how if you put a bowl of water (or other microwaveable soup/similar), in the microwave, just putting it in for 5 seconds isn’t enough to get it boiling hot, despite being microwave amounts of energy being zapped into it? It doesn’t just instantly heat up to boiling because you added a bit of energy to it, a bowl of soup likely takes around 30-60 seconds to get ‘warm’, and another 30-60 seconds to reach boiling, with a microwave.
The reason for that is that there’s a lot of mass (and energy) that goes into changing temperature. Water is one of those things that is, naturally speaking, really difficult to change it’s temperature; it absorbs heat quite a bit more than it radiates heat.
Apply the same kind of idea, just with the sun and the earth. The earth has a lot of mass, and all the time the sun spends heating things up, is also counter-balanced by the angle of rotation the earth has (why we have seasons at all), and how much sunlight, for how long, is hitting each hemisphere at a time. It’s why it isn’t really until mid/late November or December that the northern hemisphere really “feels like” winter temperatures – why it doesn’t feel like “spring has sprung” in the northern hemisphere until around mid/late March or April – why the “hottest of summer” doesn’t feel like it’s a thing until late July and August. There’s a lot of mass to heat up (and/or cooldown), and a lot of thermal energy to move around – it isn’t going to happen instantly, because all that mass is holding on to it – it takes time for that energy to dissipate.
Because things take a while to heat up.
If you put a kettle of water on a hot plate the water will not be the hottest immediately after you put it on the plate but some time later.
The ground and the water that gets more sunlight will take a while to heat up and water and ground don’t heat up at the same speed, which is why some places have their temperature peak later or sooner than others.
There is also an unrelated but somewhat mind-blowing idea that in addition the seasons being caused by the tilt of the earth’s axis we also have the minor effect that the earth’s distance to the sun varies by about 5 million kilometers (out 150 million) over the year.
The point where Earth is closest to the sun is (this year) January 4 and it is farthest from the sun on July 6. This means the planet as a whole gets lees sunlight during summer in the northern Hemisphere and more during winter in the northern hemisphere. Despite that overall the planet is hottest during summer in the northern hemisphere, because there is more land in the north than the south and land absorbs sunlight better than water.
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