(C) Daily Kos This story was originally published by Daily Kos and is unaltered. . . . . . . . . . . Climate Science 101 Hot air rises [1] ['This Content Is Not Subject To Review Daily Kos Staff Prior To Publication.'] Date: 2023-05-31 I haven’t seen any diaries that cover basic climate science even though it is itself very interesting. And it was only covered in a few blogs like “real climate,” which spent very little time on deep background. While climate is complicated enough that models are need to understand it, much can be understood without quantitative analysis. Most of the energy absorbed by the earths system is visible light absorbed at the earth’s surface. About 71 percent of solar energy is absorbed at the surface vs. 29 percent by the air. So the earth’s biosphere is heated mostly from the ground, and hot air rises. In the lower troposphere, the bottom layer of the atmosphere, where people live, rising and falling air transfers most of the heat, also mixing the air. Convection and latent heat dominate heat transfer in the bottom of the atmosphere. Directly above the surface a net 25W/M2 are transferred by radiation vs 104W/M by thermals and latent heat of water. In meteorology there is a condition called a temperature inversion, where hot air is over cold air. Isn’t hot air supposed to be on top? In fact, even though hot air near the surface is always rising, the air cools as we go up in altitude. Having hot air above cold air stops this circulation, with very smoggy results. The rate of cooling as air ascends is called the lapse rate, and weather forecasters use the local lapse rates for forecasts. Three common lapse rates that are calculated are the dry adiabatic lapse rates, the moist adiabatic lapse rates and the environmental lapse rate. The dry adiabatic lapse rate average is 9.8 degrees C per Kilometer of altitude. The moist adiabatic lapse rate varies with pressure and temperature but is always lower than the dry lapse rate, and the environmental rate is the actually measured rate of cooling. The word adiabatic is a give away. Adiabatic is a term of art expressing that heat conduction into and out of a parcel of air can be ignored. When a parcel of air rises the pressure on it decreases. The parcel expands, in doing this it does work on its surroundings and cools. ( Because of the rule “energy cannot be created or destroyed”, the parcel when it expands exchanges heat energy for mechanical energy thus cooling. ) If the parcel falls then the surroundings do work on it and it warms. The video in the link shows somebody lighting cotton on fire by compressing the air around it adiabatically, kind of like what diesel does. Weather people keep track of the lapse rate. What they call high instability is a high environmental lapse rate compared than the dry lapse rate. Under these conditions air from the ground will rise. It will cool but it will stay warmer than its surroundings which are cooling faster. It will keep rising. Moist air will rise also, but when the temperature reduction due to adiabatic expansion reaches the dew point some of the water vapor condenses. That water vapor releases the latent heat that was needed to evaporate it ( 540 cal/gram. Enough energy to heat liquid water to 540C), back to the gas. Most of this energy heats the surrounding air which then gains more altitude and cools, but it cools more slowly than the surrounding air. But it’s still cooling so more condenses. This causes clouds and rain, and rain is always associated with rising columns of air. With enough moisture deep convection occurs in which the air reaches the stratosphere. The general flow pattern associated with storms is low pressure at ground level sucks in moist air. Note that warm air at the same pressure is less dense than cold air surrounding it, which is why it rises. But since the gas is less dense as it rises the pressure falls off more slowly than the pressure of the colder gasses around it which are denser. At high altitude the air expands side wise because it is now at a high pressure compared to the surroundings. So moist air goes from a low at the surface to a high pressure at high altitudes. The result is low pressure at ground level under rain, which is the classic predictor of storms. Radiation Heat Transfer Air temperature falls as altitude increases till the tropopause which is the boundary with the stratosphere. Most of the air in the atmosphere is in below the tropopause. The stratosphere warms as the altitude increases, because convection is less important there. Once our theoretical rising parcel of air reaches the tropopause marking the boarder between the troposphere and the stratosphere, infrared radiation from greenhouse gasses in the atmosphere can reach space. So radiation heat transfer becomes important. This is the original theory of global warming. When you add greenhouse gasses to the atmosphere, the height of the radiation emission into space rises too. Because greenhouse gasses only emit at the frequencies they absorb at, adding greenhouse gasses “thickens” the atmosphere, meaning that they can only emit when there aren’t many molecules of the same gas above them, and yes the tropo-pause height is increasing. The increase in height means that the radiation is emitted from a colder place so there is less of it, since colder objects radiate less. Since the atmosphere was cooling due to adiabatic cooling not heat transfer, it will still be colder at that high place. The earth will store the heat not emitted until it has warmed enough so the outgoing radiation will increase to balance the absorbed incoming radiation. Because most of the energy not emitted goes into the ocean which acts as a heat reservoir, it was expected to take 20 years for the ocean to warm enough to warm the surface and upper atmosphere enough, to increase the outgoing long wave radiation enough to balance the greenhouse effect. The theory has been added to over the years. Global warming appears to have caused a moistening in the troposphere. This caused a water vapor feedback from an increase in absorption of solar radiation by water vapor of the visible and near infrared. Returning to an equilibrium condition then requires an actual increase in outgoing long wave radiation from the upper atmosphere compared to prewarming conditions. Some call this a negative feedback, but it is simply a fact that the only way energy leaves the earth system is by emission of long wave (infrared) radiation. Graph of the intensity of out going long wave radiation vs the wavelength. The temperature of a black body (ideal emitter) vs temperature is plotted also. The atmospheric window is the hump between 6-14 microns. It emits near the surface temperature of the earth 286K (13C,57F). The emission from the very long wave wave rotational bands is at 260k (-13C or 8F). The CO2 bands are near 220k (-53C or -63F) the temperature of the tropopause. Radiation energy transfer is where quantum mechanics started. Although we talk about waves, the energy of the light can only be absorbed in discrete amounts called photons. The energy of the photon increases as the wavelength gets smaller. So ultraviolet less than 124nm can ionize atoms. Those wavelengths are absorbed by almost all gasses. Visible light photons from 400 to 700nm from the sun are powerful enough to change the electronic states of atoms, but they don’t tend to knock atoms electrons away from atoms. Infrared photons from 700nm to 5000nm are usually in the range of molecular vibrations, and are very much like heat. Above 6000 nanometers till about 14000nm there is a gap. It is usually called the atmospheric window. There are vibrational energy levels in this window, but it is mostly transparent to IR. Thermal imaging cameras work in this region. Above this wavelength the atmosphere is opaque again due to rotational energy levels. The figures below plotted in wavelength. When the energy is plotted by wavelength the more energetic reactions are to the left. Wikipedia shown above graphs the outgoing radiation vs the emission vs wavelength. The radiation from an ideal emitter that temperature is shown in the background. The earths average surface temperature is 286K (13C,57F) the emission from the very long wave wave rotational bands is at 260k (-13C or 8F). The atmospheric window from 6 to 14 microns is near the earth’s surface temperature. The CO2 bands are near 220k (-53C or -63F) the temperature of the tropo-pause. Changes those emission temperatures are the basis of radiative forcing. This figure was prepared by Robert A. Rohde for the Global Warming Art project. It indicates the upgoing long wave radiation probably near the surface. It also shows the absorption bands of greenhouse gasses. The near IR bands of water are shown as are is the absorption or emission spectra of the various greenhouse gasses. Image from Global Warming Art. This image is an original work created for Global Warming Art. Please refer to the image description page for more information. Element removed Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled GNU Free Documentation License. The figures above show most of the aspects of radiative heat transfer in the earths atmosphere. The figure contains the short wave solar radiation reaching earth on the left. The up going radiation shows on the right, shows the spectrum Long wave emission by the earth surface. The atmospheric window is shown. All of the other wavelengths shown are absorbed by the atmosphere. That energy is transported upward mainly by convection, and is released to space at these same long wave lengths, because that is the only way it can leave earth. Greenhouse gasses only emit at the same wavelengths that they absorb, and oxygen and nitrogen have no infrared spectrum, so don’t emit long wave radiation. So the radiation leaving the earth has a spectrum like the absorption spectrum. Summary We live in the troposphere. Due to the fact that most of the heating is at the surface, and that heat is transferred to the top of the troposphere by convection, the air in the troposphere is constantly mixing. The lapse rate is the rate of cooling due to increases in altitude in the troposphere and is used to predict the weather. Rising moist air causes rain, falling air is dry. Global warming mostly works by optically thickening the atmosphere and causing the emission to happen from a higher colder place. By warming the upper troposphere to increase emission, and warming the surface to increase emissions through the atmospheric window the earth rebalances incoming and outgoing heat energy. 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