Why is it colder at the top of a mountain? Heat rises, so it should be warm. [Bq0VEyVU4Wy]
Why is it colder at the top of a mountain? Heat rises, so the summit should be the warm end. It is the coldest place on it. The heat did rise. Rising is what cooled it. Climb, and there is less air above you pressing down, so a parcel of rising air expands into that lower pressure. Expanding costs energy, the parcel spends it out of its own warmth, and it arrives at the top cold. That is adiabatic cooling: about 9.8 degrees Celsius per kilometre for unsaturated air. There is a second half, and it is the one almost nobody is told. Air is nearly transparent to sunlight. Roughly 23 percent of incoming solar energy is absorbed by the atmosphere itself, while about 48 percent goes straight through and is absorbed by the ground. The surface then heats the air touching it, by conduction and convection. The atmosphere's fire is at the BOTTOM, so the higher you stand, the further you are from it. Climbing a mountain, a thermometer falls about 6.5 degrees Celsius every kilometre, which is about 3.6 degrees Fahrenheit every 1,000 feet. You can feel the mechanism in ten seconds. Pump a bicycle tire hard and the pump gets hot, because you are compressing air. Now let the air back out and the valve goes cold. Same physics, both directions, in your hands. And you have been reading it your whole life without knowing. Every cumulus cloud has a FLAT bottom, and that flat line is the exact altitude where rising air finally cooled to its dew point. The sky has been drawing the lapse rate above your head since you were a child. HONEST NOTES: this is the troposphere's rule, not the atmosphere's. Above the tropopause, around 11 km, it REVERSES and the stratosphere gets warmer with height, because ozone absorbs ultraviolet. The 6.5 degrees per kilometre figure is a global average and varies a lot with moisture, season and weather; saturated air cools more slowly, around 5 degrees per kilometre, because condensing water releases latent heat. Temperature inversions can flip it locally, so on a calm clear night a valley floor can be colder than the slope above it. Real summit temperatures vary; the minus 14 Celsius shown at 4,500 m is the standard-atmosphere value, not a forecast. Sources: UCAR Center for Science Education, "How the Atmosphere Changes with Altitude" (6.5 C per 1,000 m); NOAA National Weather Service Glossary, "lapse rate" (dry adiabatic 9.8 C per km); NOAA JetStream, "Layers of the Atmosphere" (the troposphere is heated from below); NASA Earth Observatory, "Climate and Earth's Energy Budget" (23 percent absorbed by the atmosphere, 48 percent by the surface). Look up at the next flat-bottomed cloud and you will know exactly what that line measures. Then tell someone why. #physics #weather #mountains #science #atmosphere