Heat Transfer: The Basis of Weather and Climate

Heat is a form of energy -- "thermal energy" -- so according to the Law of Conservation of Energy, it cannot be created nor destroyed, only converted from other forms of energy and transferred from one mass to another.  First, here are some examples of conversion of energy from various other forms into thermal energy, and vice versa.  Combustion of (burning) wood, coal or gasoline converts chemical energy (the bonds holding together atoms of the chemicals being burned) into thermal energy, and in the interior of the Sun nuclear energy (much stronger bonds holding sub-atomic particles together as atoms) is converted into thermal energy (as well as light and electromagnetic radiation at many invisible wavelengths).  Friction causes mechanical energy to be converted to thermal energy, and by containing the combustion of gasoline in pressurized cylinders with a movable piston in cars' engines, thermal energy is converted to mechanical energy, on such a scale that this website had to be created.

Besides conversion of other forms of energy into heat, an object (or mass) can acquire heat from another warm object or mass by heat transfer, which has three modes.  The most familiar mode of heat transfer (with the least familiar name!) is conduction.  When you put ice on an injury, you do it to cool the injury, but the way that happens is by heat conduction, the transfer of thermal energy from the warmer object to the colder one, which is always how heat conduction works.  So you're "really" heating the ice, not cooling a sprained ankle.  Conduction also occurs in gases and liquids, not just solid objects.  When cold water and hot water mix in a tub or sink to make lukewarm water, the mixture reaches a uniform temperature by conduction.  (Anybody who takes milk in their coffee or tea, or has separate "hot" and "cold" taps, knows that stirring accelerates the process of reaching a uniform temperature, but it only does so by causing more hot and cold portions of the mixture to come into thermal contact with one another sooner.  Either way, all that happens thermally is heat transfer by conduction.)  Warm air from a heating duct causes the temperature in a cold room to increase, also by conduction.

An important point about conduction is that conduction always causes heat to be transferred from the warmer mass to the colder, so that given enough time, a perfectly insulated system with no way of gaining or losing heat would always reach a uniform temperature, between the initial temperatures of the masses within it.  So, for example, putting an ice cube in a glass of warm water will always cool the water and melt the ice.  An ice cube at 31°F or 32°F is hundreds of degrees above Absolute Zero, meaning that if it could lose all its heat, that would be enough thermal energy to boil several ounces of warm water (if that water was initially near but slightly below the boiling point). Nobody is ever surprised that an ice cube cannot cause water to boil, but it's worth mentioning this to extend our vague intuition to the specific statement that thermal conduction is the transfer of heat between two objects or masses in contact, always from the warmer to the colder.

Another mode of heat transfer is radiation, which is explained in Radiation: What Makes Some Gases Greenhouse Gases.  In short, radiation is the transfer of thermal energy by the release of a photon whose wavelength is in the infrared range of the electromagnetic spectrum.

That leaves only one more mode of heat transfer, convection, which is simply and accurately defined by the phrase "hot air rises."  The reason it does is that hot air has higher pressure, which means that it has more tendency to spread out ("expand" in physics terminology).  Spreading the same amount of mass over more space makes it less dense, so whenever cold air is above hot, the denser cold air falls down, forcing the hot air up.  True story!

Of course, in nature, it's a little more interesting than a block of cold air right on top of a block of hot air.  Instead, when masses of air with significant temperature difference come into contact, they begin to mix, and the warmer air from one front rises above the cold air from the other front along the boundary between the warm front and the cold front, forming a slanted, ramp-like or inclined boundary between the masses, called a thermocline.  The cold mass then slides along below the mass of warm air, causing the winds we experience, which are basically all horizontal, or close enough that they feel perfectly horizontal, especially at ground level.

Now you know how its inability to emit certain wavelengths of infrared radiation makes CO₂ trap thermal energy, and you know enough about how heat is transferred around physical systems to start to seriously examine both the observed and predicted results of adding more CO₂ to the atmosphere.  The Earth's tilt, rotation, and orbit around the Sun are responsible for weather variations known as days and nights and seasons, and other more subtle drivers are responsible for multi-year cycles like El Niño and the Pacific Decadal Oscillation, but the key to understanding them all is the transfer of thermal energy.  And of course, adding thermal energy, or to be more exact preventing thermal energy from leaving, will of course effect how heat is transferred around the rest of the Earth's climate system.  With these principles of physics in mind, a key to common acronyms at your fingertips, and a significant* effort, you will no longer need to depend on others to summarize climate science for you. You can read and understand the original research, and I think I can answer any questions you have. More to the point, I can show you how to find the answers for yourself.

* Honestly, it's going to take quite a lot of effort.  I think I can make it possible for just about anybody to understand the original scientific reports on climate science but I have no delusions of making it easy.  It's difficult for me and it's not easy for the pros on the front lines of original research.  Okay?