Met4: Conduction, Convection and Sphericity (v1.0)

 In the previous essay, I talked about the radiant energy received by the earth from the sun and the radiant energy emitted by the earth that is dissipated to space and the thermodynamic equilibrium achieved at some temperature where the two balance out. I outlined how greenhouse gases play a key role in this process. In this essay, I focus on three additional important concepts Conduction and Convection and Sphericity. Conduction and convection are two additional ways heat can be moved around besides radiation.

Three key concepts first.

1. The atmosphere is heated from below. The earth’s surface temperature and variation of temperature with height plays a very important role in weather. 
2. The oceans get their primary energy from radiation and are a huge reservoir of energy. About 90% of the water vapor in the air comes from evaporation from water bodies like oceans. The rate of evaporation for a given surface area depends on temperature, humidity and wind speed. The ocean’s high heat capacity and water vapor in the air plays a very important role in weather.
3. Earth is a sphere. Therefore, it is important to note that radiation is not equally distributed across the earth - tropics get much more than the poles. This creates a temperature difference between the equator and poles. Temperature differences cause pressure differences and pressure differences cause wind. The equator to pole temperature difference is the primary driver of atmospheric circulation.

There are a number of reasons the poles get less energy from radiation than the equator. The rays are at an acute angle at the pole. The rays go through more atmosphere at the poles resulting is more loss. The earth is tilted, and this obliquity causes seasons and causes a change in the temperature difference between the equator and poles over the seasons. In addition, the earth’s orbit is eccentric, and this affects it too.  We are closest to the sun in January. The average temperature at the poles is -30 degrees centigrade. At the tropics it is 25 degrees centigrade.

It is said that nature strives to eliminate imbalances. So, is nature poor at doing that? No. The earth’s equator to pole temperature difference is about 100-degree Fahrenheit. Let us look at the moon which has no atmosphere. There the temperature difference is 500-degree Fahrenheit!! So, Nature did a big improvement! But there are some things that hinder nature. The earth rotates for example.

Conduction is heat transfer by direct atomic contact. Conduction operates in one direction – warm to cold. Different objects conduct heat to different extents. Water is 26 times better at conducting heat than air is. But copper is 650 times better than water. The thermal conductivity of a material determines how efficiently it can transfer heat. Air is a terrible conductor of heat and is in-fact an insulator!  So, air does not carry heat away from a heated surface quickly.

Thermal inertia is another concept and is resistance to thermal change. Objects with thermal inertia can hold a lot of energy without the temperature rising by much. This is also sometimes called heat capacity. Sand has low thermal inertia. Water is slow to heat up and cool down. The ocean is a huge reservoir of heat energy. It moderates coastal climates and influences weather patterns. The thermal inertia of air is somewhere in between sand and sea.

Heat transfer in any direction needs some help since air conduction is poor. This is called convection. Convection involves the vertical movement of heat and moisture in the atmosphere. It occurs when warmer air rises from the surface (like the ground or a warm sea) and transports heat upwards. Sunlight heats the ground, which, in turn, warms the air directly above it through conduction. Uneven heating causes pockets of air to warm faster than others, making them less dense and causing them to rise. These rising columns of air are called "thermals". As air ascends, it carries heat and moisture vertically into the atmosphere.  

Wind is an example. When the wind picks up, there is more likelihood for vertical mixing. A temperature inversion is a layer in the atmosphere in which air temperature increases with height. 

Wind can affect the surface temperature.

1. When the wind blows, it can make the air seem colder than the actual temperature. This is called wind chill factor.
2. Wind accelerates the process of heat transfer from our bodies to the surrounding air.
3. Wind also enhances evaporation.
4. Wind can have localized effects on temperature.