https://www.reddit.com/r/askscience/comments/1h19uqo/why_does_it_get_cold_at_night/
created by SussyNerd on 27/11/2024 at 17:33 UTC
7 upvotes, 7 top-level comments (showing 7)
I know it sounds like a question for 6 year olds but Where does the heat go ? What I mean is short term the ground that would only work for so long as it would eventually heat up as well. The IR radiation from everything would cool us down but it doesn't seem like it would be so high and iirc the atmosphere absorbs a lot of IR already so it's not that. The atoms escaping our planet might be contain a lot of energy but very low in mass so they likely don't cool us down much so How does the heat escape us ?
Comment by spartout at 27/11/2024 at 22:28 UTC
74 upvotes, 4 direct replies
IR waves which are mostly transparent to the atmosphere remove all of the thermal energy. They just leave into space.
There is also a neat thing clouds do as they are a big factor in retaining heat at night. Water happens to not be transparent at the wavelength most IR waves escape as and as clouds it forms large absorption and reflection regions. CO2 is also not very transparent but as it doesn't form clouds on earth it is spread evenly in the atmosphere so it only determines how slowly the IR energy escapes.
Comment by tacoman202 at 27/11/2024 at 22:24 UTC*
25 upvotes, 0 direct replies
It's largely emitted by the ground as infrared light, which carries the thermal energy away. You're right that Earth's atmosphere contains particular gases (Greenhouse gases, like carbon dioxide or methane) that absorb certain IR wavelengths, but it doesn't absorb such a large amount as to prevent cooling via blackbody radiation. The majority of the energy is simply radiated out into space. Earth's atmosphere retains some of the heat, hence why we don't have as dramatic of a shift in daytime/nighttime temperature as, say, Mercury, but the basic process by which the heat leaves Earth is identical.
Comment by certainlynotonreddit at 27/11/2024 at 23:15 UTC
18 upvotes, 1 direct replies
This is a fantastic question. You also already bring up all the important effects, and the only thing that's missing is putting numbers on them.
Maybe IR radiation seems like it wouldn't be so high, but in reality, without an atmosphere, the equilibrium temperature would be -18°C. It's, as you say, the greenhouse effect (water and CO2) that put us at a much more comfortable +15°C on average.
Evaporative cooling is I think negligible but I'd love to be corrected.
Comment by fsurfer4 at 27/11/2024 at 23:08 UTC
2 upvotes, 0 direct replies
When heat transfers from a hot object to a cold object, it essentially "goes to" entropy, meaning the energy becomes more dispersed and disordered as it spreads out to the colder object, causing an overall increase in the system's entropy; this is a fundamental principle of the Second Law of Thermodynamics
https://openstax.org/books/physics/pages/12-3-second-law-of-thermodynamics-entropy[1][2]
1: https://openstax.org/books/physics/pages/12-3-second-law-of-thermodynamics-entropy
2: https://openstax.org/books/physics/pages/12-3-second-law-of-thermodynamics-entropy
Comment by PD_31 at 28/11/2024 at 02:19 UTC
2 upvotes, 0 direct replies
Heat is constantly being radiated away from the planet. During the day, this is replaced by "new" heat absorbed from the Sun.
At night the Sun is no longer shining on this part of the planet so heat continues to be lost but not replenished by then Sun.
Clouds (water) help retain heat, which explains why on a cloudy night it doesn't cool down as much. On a clear night we get much colder temperatures as more energy is lost.
Comment by BondoDeWashington at 28/11/2024 at 00:30 UTC
1 upvotes, 0 direct replies
It's mostly radiated out into space, and in the summertime some of it is conducted down into the ground where it is cooler. If you live near water some of it will be exchanged with the cooler air from over the water, where the surface does not get so hot. Basically the same as being conducted down into the ground but it happens faster with water because it is flowing. This is why islands don't have temperature extremes but places far inland like interior Canada and Siberia have horrible extremes.
Comment by Notforyou1315 at 29/11/2024 at 08:41 UTC
1 upvotes, 0 direct replies
I live nearish to the ocean, but also in a valley. In my area, it is blazing hot in the summer and really cold in the winter. The water acts like a thermal blanket, moderating the temperature so that it doesn't get too cold in the winter. I am thankful for that. I am also thankful for the valley, as heat tends to get trapped down here, so it is never really that cold at night because there are usually clouds to trap the heat. When you can see the stars, that is when you know to put on that extra blanket because it is going to get super cold.