Facts of the Matter
Facts of the Matter
Weather is Earth’s way of
dissipating the sun’s energyThe vernal equinox is this Thursday, 3 p.m. Honolulu time. That's when astronomical spring will begin as the sun crosses the equator, heading back into the Northern Hemisphere after six months wintering in the south.
Because Earth's axis is tilted and points toward the same spot in space (near Polaris, the North Star), the sun moves a little northward or southward in the sky each day. This subtle north-and-south motion is superimposed on the sun's more rapid daily east-west motion, and is barely noticeable from day to day.
Between the December and June solstices (around Dec. 20 and June 20), the sun moves northward, crosses the equator at the vernal equinox and reaches its most northward point at the Tropic of Cancer (latitude 23.5 North) on the solstice around June 20. From June to December it moves southward, crosses the equator at the autumn equinox around Sept. 20, reaches its most southward point at the Tropic of Capricorn (latitude 23.5 degrees South) at the solstice around Dec. 20.
The region known as the tropics, which lies between the Tropic of Cancer and the Tropic of Capricorn, receives the most intense sunlight. Arctic regions receive ever smaller amounts the closer they are to the poles. This creates continent-size masses of air that have a relatively uniform temperature and humidity within them. There are warm, moist tropical air masses and cold, dry arctic air masses, with minor variations depending on whether they form over land (continental) or sea (maritime).
Most of Earth's large-scale weather patterns (hurricanes are an exception) develop at the interface of tropical and arctic air masses along a polar front that trends roughly west to east. Severe winter storms such as the blizzard that ravaged the Atlantic coast earlier this year, and the heavy rain that we experienced here last month, replete with lightning and thunder, are two notable examples.
If Earth's axis was not tilted, air masses would tend to remain where they form. As it is, arctic masses migrate toward the equator in the autumn, and tropical air masses migrate toward the pole in the spring, They interact and encroach on one another as they advance and retreat in a constant, raging battle between heat and cold.
To further complicate the picture, Earth's rotation is just fast enough that the air masses do not move directly north and south. They tend to spiral.
Equator-bound arctic air masses deflect to the west, while poleward-bound tropical air masses deflect to the east. This shear causes the polar front to twist into huge gyres that form and re-form as they move eastward around the earth like giant spinning tops.
These whirling cyclones and anticyclones aggravate and distort the front, mixing the cold, dry arctic air with the warm, moist tropical air while releasing condensed water vapor in the form of rain, sleet or snow. A further complicating factor is the expansion and contraction of the air, which form ridges and troughs in the slowly moving air masses. Add these effects together, and you have the unpredictable yet constrained mix that creates the climate system of this remarkable planet.
The key ingredient in the weather machine is the polar jet stream. This rapidly moving, turbulent, 2-mile-thick tube of air five to eight miles up in the atmosphere snakes around the globe, bending and twisting its way along eastward. In the middle latitudes it meanders along the polar front, but high above the surface. The position of the polar jet, the position of its bends and how far north or south it dips exert a strong influence in the location of the polar front and the storms that result from the interaction. The greater the temperature difference between the two air masses, the faster the jet stream flows, the less "typical" its behavior and the more extreme the resulting weather along the front.
These interactions, along with ocean currents, are Earth's way of transferring heat from the equator to the poles as it dissipates the sun's energy over the entire surface. To us it is weather. To Earth it is dissipating heat by whatever means available, while recycling water that is crucial to biological cycles of growth and decay.
If not for the combined motions of the daily rotation of the earth on its tilted axis, and its yearly revolution around the sun, our planet would be a much less interesting place to live. But then, we wouldn't be here anyway without those biological cycles to support and nourish us.
We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu