Penn State NASA

Process of Heat Transfer

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Process of Heat Transfer

The atmosphere and oceans are constantly flowing, and this motion is critical to the climate system. What makes them flow? In general, the movement is due to pressure differences — things flow from regions of high pressure to low pressure and the resulting surface winds distribute heat at the Earth's surface.

diagram showing two boxes. The taller one labeled High P, greater h and it moves to Low P and gets smaller
Movement of air masses from high to low-pressure regions.
Credit: © D. Bice. Used with permission.

The pressure changes are themselves due to density and height differences — higher density in the air or the oceans leads to higher pressures. The density differences are due to changes in composition and temperature; this works slightly differently for air and water. In air, the important compositional variable is water vapor content — more water means lower density air. When we say more water, we mean that for a given number of molecules in a volume of air, a greater percentage of them are water, and water, as shown below, is lighter than a nitrogen molecule, which is the most abundant molecule in our atmosphere.

Density of Air

Inverse relationship with Temperature:

Higher temp = lower density → rising air

Weight of H2O= 18

Lower temp = higher density → sinking air

Weight of N2 = 28

Inverse relationship with water content:

More water = lower density → rising air

Less water = higher density → sinking air

see accessible text description below
Explanation of how changes in temperature and water content control the density of air
Click here for a text description of the image above

Image Reads:

Density of Air

Inverse relationship with temperature

Higher temp = lower density ⇒ rising air

Lower temp = higher density ⇒ sinking Air

Weight of H20 18g/mol

Weight of N2 = 28g/mol

Inverse relationship with water content

More water = lower density ⇒ rising air

Less water = higher density ⇒ sinking air

Credit: © D. Bice. Used with permission.

As indicated above, density differences can cause either rising or sinking of air masses. Because Earth’s gravity decreases as you move away from the surface, there is a kind of equilibrium profile of density with height above the surface as shown by the green curve below:

Graph: density on x & height on y. Shows downward sloping equilibrium profile of density. When density decreases lifting occurs & vice versa
Lowering density of an air mass causes it to elevate
Credit: © D. Bice. Used with permission.

If we lower the density of air at the surface from A to B, then the air rises from B to C. Then, if we increase the density of air at point C, moving it to D, it will sink back down to point A near the surface.

We start with the movement of the atmosphere, which we will try to make as simple as possible by first concentrating on the flow as seen in a vertical slice from pole to pole. The story begins at the equator, where air is warmed and lots of evaporation adds water to the air, giving it a low density:

Hadley cell: Described below. Low pressure @ equator, air rises, moves toward 30*, descends at High pressure, goes to poles or to equator
Air masses spread out laterally when they reach the tropopause.
Credit: © D. Bice. Used with permission.

This air rises until it gets to the top of the tropopause, which is a bit like a lid on the lower atmosphere. It then diverges, with some of the air flowing north and some flowing south. As it rises and moves away from the equator, the air gets colder, water vapor condenses and rains out and the air grows drier — the cooling and drying both make the air grow denser and by the time it reaches about 30°N and 30°S latitude, it begins to sink down to the surface.

Hadley cell: Described below. Low pressure @ equator, air rises, moves toward 30*, descends at High pressure, goes to poles or to equator
Sinking of air masses at sub-tropical latitudes.
Credit: © D. Bice. Used with permission.

The sinking air is dense and dry, creating zones of high pressure in each hemisphere that are associated with very few clouds and rainfall — these are the desert latitudes. The sinking air hits the ground and then diverges. Some flows south and some flows north; the parts of this divergent flow that return towards the equator complete a loop or a convection cell, a Hadley Cell, named after Hadley, a famous meteorologist. Now, let’s turn our attention to the air that flows away from the equator. Moving along the surface, it warms and picks up water vapor, and so, its density decreases, and it eventually rises up when it gets to somewhere between 45° and 60° latitude in each hemisphere.

Diagram: Hadley cell – equator to 30* (low then high pressure) connects Ferrel cells – 30* to 45-60* (starts high then low pressure)
Rising of air masses between 45 and 60 degrees.
Credit: © D. Bice. Used with permission.

Once again, the rising air runs into the tropopause (which is lower at these higher latitudes) and diverges, with some of it returning toward the equator, thus completing another convection cell called the Ferrel Cell. The air that flows pole-ward sinks down at the poles, creating yet another convection cell known as the Polar Hadley Cell. These convection cells create bands of low and high pressure that roughly follow lines of latitude that exert a big influence on the climate at different latitudes. The air flowing within these convection cells does not simply move north and south as depicted above — the Coriolis effect alters the flow directions, giving us a surface pattern that is dominated by winds flowing east and west.

Globe pressure map. Low at equator, high at 30*, low at 60*. Ferrel, Hadley and polar Hadley (start @ 60*)cells & polar fronts at the poles
Simplified global atmospheric circulation.
Credit: © D. Bice. Used with permission.

Note that the boundary between the Polar Hadley Cell and the Ferrel Cell (often called the Polar Front, and associated with the mid-latitude jet stream) is highly variable, with big loops in it. These loops, or waves, change over time to a much greater extent than the boundaries between the other convection cells.