3 upvotes, 3 direct replies (showing 3)
View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
Why is the bottom of the grand canyon hotter than the top? The sun heats the earth, which heats the air, right? All points along the canyon walls are adjacent to "earth", and heat rises. These two things seem to suggest the bottom should not be hotter.
Comment by deadmoby5 at 26/04/2023 at 22:00 UTC
2 upvotes, 1 direct replies
As the surface of land absorbs sunlight it gets hotter. The heat "rising" is simply the surrounding air which gets heated around the sunlit surface, which dissipates the further it moves from the heated surface. The air above the canyon is simply cooler as it is as removed from the hot surface as possible.
Comment by cosmicosmo4 at 27/04/2023 at 04:29 UTC
2 upvotes, 1 direct replies
Air at higher elevations is colder because it is at a lower pressure. When any mass of air drops in pressure, it gets cold. You can observe this by letting some air out of a car tire on a hot day and noting that the air comes out cold. Likewise when air is compressed, it heats up.
So places with higher elevation are colder simply because the air that moves there from lower elevations cools down in the process as it expands, and vice versa.
In addition there are local effects, such as the canyon trapping heat due to its shape.
Comment by visualmath at 27/04/2023 at 21:18 UTC
1 upvotes, 0 direct replies
The temperature of a location is affected by 3 factors:
1. radiation - energy transferred by electro magnetic radiation incident on a surface
2. conduction - transfer of energy through direct contact between adjacent surfaces
3. mass flow - energy transfer due to movement of particles in a medium e.g. breeze, convection currents, etc
You and other posters took into account 2 and 3 but I believe the largest factor causing the difference in temperature here is #1 -- radiation
Canyon is surrounded by rock on 3 sides which stores a lot of heat from solar radiation and then radiates it back. While the plateau above is exposed to the sky which is much much cooler. The walls also receive more radiation from other earth surfaces than the plateau but less than the canyon floor and still be some temperature in between.
You can experience this if you are out under the open sky at night. It feels a lot colder when you are exposed to the sky than when you shelter next to a large object like a rock