Eli5: How is it possible that deserts are super hot at day time and below freezing point at night time?
In: Earth Science
This is actually the default for most things in space. When you’re facing a star you’re blasted with radiation, and when you’re facing the void your heat quickly escapes into it.
On earth we have a humid atmosphere, groundwater and biomass that all hold onto heat very well. It’s like we have a sweater made of water and life on us that helps keep in the heat while we’re in the sun’s shadow at night. Deserts have little water, dry air and less life. With this thin sweater they get cold fast, and also have less protection from the direct heating of the sun during the day. They’re basically a little closer to a planet like Mars than most places on Earth.
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Things stay warm if:
1. There is a lot of the object to heat up. In the desert there isn’t much stuff.
2. The object needs to take in a lot of heat energy as it heats up – the more heat it takes in the longer it takes to cool down. Dry things, particularly sand, get hot quickly without needing a lot of heat energy, so cool down quickly
3. The object can pass it’s heat to others that aren’t heated directly. The better an object does this, the further heat gets into it (this includes the heat getting deeper into the ground), and the more heat it needs to give out to cool back down again. Dry things are bad at passing heat around.
This one has a complex effect, as passing heat quickly also helps things cool down to a certain extent, however the important part of this effect is allowing the object to take in more total heat energy.
4. When the heat that they give out is trapped near them. If the air near them warms up and stays near them, they don’t cool down as quickly – like wearing a sweater traps air near your skin. Exposed areas have the warmed air move away very quickly, and additionally provide no protection from windchill.
No water. Water is a very interesting substance for a variety of reasons, but one of those reasons is that they have a high heat capacity, which is literally the ability to hold a lot of heat energy.
What does that mean? Have you ever noticed how long it takes a simple pot of water to come to a boil, while everything else in your kitchen seems to heat up rather easily? That’s heat capacity. You can put a lot of energy into the water, and the temperature doesn’t go up a lot; and you can take energy out of water without the temperature going down a lot.
What this basically means is that water acts like a “heat battery”. During the day, the sun will heat the water and, during the night, the water will release that heat back into the surrounding landscape.
What this means is that large bodies of water act like nature’s HVAC for the surrounding area. It helps keep a climate moderate.
Now, deserts often occur somewhat far from natural bodies of water. This means that, at night, there are no large bodies of water releasing excess heat into the surrounding environment. On top of that, you don’t have any kind of clouds or weather up above to keep the heat from radiating away.
In other words, because deserts aren’t typically near large bodies of water, don’t receive a lot of rain, don’t typically have any kind of humidity, and don’t have any clouds, they don’t have nature’s heat battery to keep them warm during the night.
All the heat that gets put into them during the day just gets radiated away at night.
Afaik, sand doesn’t absorb heat well, so when temperature is high, sand temperature is also high, but as soon as temperature drops (so, night) sand quickly releases all it’s heat, cooling itself rapidly. Since there’s no longer anything warm, the desert is cold.
On the other hand, water DOES absorb heat really well and unlike sand can release the heat it absorbed during the day much much slower, making lakes/beaches less cold during the night.
Maybe “absorb” isn’t the best term, but I can’t think of another one
If you’re in space on the night side of the planet, you would see it “glow” in infrared (using an infraded camera). So you can compare the whole earth to a dim light bulb the size of a planet (assuming . The energy to keep that light bulb on (heat) is the energy you lose into space. in place with lots of stuff to accumulate heat during the day (from sunlight), you’ll lose a couple of degrees during the night, which in a desert, you lose the same amount of energy, but you have very few thing to store heat, so you lose more degrees for the same amount of energy sent to space (radiated).
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Thermal mass. (Actual ELI5 answer here, not perfectly accurate.)
If you put equal size chunks of metal and plastic in the freezer, wait a week, then take them out, the plastic will warm up to room temperature very fast while the metal will remain cold for hours. This is because solid metal has very high thermal mass. It stores a lot of cold while it’s in the freezer, and takes a long time to release all that stored cold when its out in the open.
Trees, lakes, and even rocks and dirt absorb and store the heat from the sun during the day, then release it at night. This keeps the environment around the same temperature at all hours. Sand does not absorb heat. It reflects the sunlight like a mirror during the day, heating up other things (like people). At night the sand has no stored energy to release so it gets very cold.
ELI5: What is “heat”? It’s the measurement of things jiggling around. When it’s daytime, the heat from the sun hits everything and makes it jiggle faster and makes it “hot”. When the sun goes away, everything jiggles slower and slower until the sun comes back again.
It turns out water is *really* good and jiggling long after the sun goes away, so it is able to store that heat energy for a long time. Since desert air lacks a lot of water, it can’t store that heat, and has massive fluctuations in temperature from day and night.
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