How the Earth's Rotation Affects the Weather

The weather on Earth, as on any other planet with an atmosphere, is a

dynamic system

that is powered by the sun, the interior heat and its rotation. Heat generated by the sun on the lit side provides the temperature differential to the chill of the dark side away from the sun. Also the Polar Regions are cold compared to the equator. As the atmosphere seeks to balance out the extremes, movement of air is generated and that movement is mediated by the spin of the planet. The ocean surfaces also act as a heat sink and work in concert with the atmosphere.

Generally, heat causes air to expand and become less dense. Cold has the opposite effect. The result is that

heated air

rises and cold air sinks. Weather on the Earth is divided into two large conveyor groups; one for the northern hemisphere and the other for the southern hemisphere. The heated atmosphere near the equator rises, causing cold air to be pulled in from the

north and south

poles. The heated air moves up and then toward the poles. The cool air hugs the surface and flows toward the equator from both poles and the dark side. These influences have their effects over the entire planet.

Now the spin rate for the planet is not equal for all latitudes, with the poles having the slowest spin rate and the equator the greatest one. The outflow from the poles tends to get skewed due in part to the unequal atmospheric movement at different latitudes. The movement due to the spin, generates sustained wind directions, especially over the oceans, that tends to blow from the north east toward the south west in the northern hemisphere. In the

southern hemisphere

, the sustained winds tend from the southeast toward the north west. These are referred to as the trade winds and were of profound significance in the days of sail from the days of Viking travel to the famous clipper ships of the 19th century. Wind flow pattern is profoundly different in the southern hemisphere at the latitude called the roaring 40's that have a sustained west to east wind flow most of the year with severe storms being the norm. This is due to the lack of blocking land masses that alter wind patterns. Otherwise, intervening land masses and islands create eddies in the outflow from the poles. Land surfaces heat faster than expanses of ocean, so there is an unequal and combined effect on the Earth's weather. This causes local patterns that create turbulence in the over all flow.

Recall that latitudes close to the poles tend to spin more slowly than the equatorial regions due to a shorter distance around the globe at those latitudes. The Earth as a whole has a more or less constant rate of rotation. The slower spin closer to the poles, combined with the faster spin near the equator causes the winds not only to skew, but to break into rotating cells that are anti-clockwise in the north and clockwise in the south. This is the famed Coriolis Effect that is responsible for the direction of hurricane, tornado and hot/cold front rotations in both hemispheres. Without the Earth's spin, hurricanes and tornadoes would not be able to organize themselves in their characteristic patterns.

Oceans tend to be heated at the top, as opposed to atmospheric

heating

at the bottom. As a result, ocean currents move on the surface away from the equator and toward the poles. The atmosphere obtains heat and moisture from the heated ocean surface close to the equator. As the ocean currents, directed by land masses, move to the poles, they cool and sink to the bottom, then move back to the equator along the bottom in great conveyor belts. The most famous of these oceanic currents is the Gulf Stream. Out of the combined effects of Earth's spin, ocean and atmosphere as well as land masses, great sea storms like hurricanes are born with their characteristic cyclonic spin. On land, tornadoes are born from north moving warm moist air and south moving cold dry air. The spin of the Earth causing the Coriolis Effect creates the conditions for anti-clockwise rotation and storm intensity in the northern hemisphere.

Warm moisture picked up near the equator tends to give warm air weight, holding it close to the surface and intensifying hurricanes. As long as hurricanes remain over the ocean, they will intensify. Hurricanes weaken only by moving over land, breaking the cycle of feeding from warm moisture and "eating" smaller storms at the fringes.

Suppose for a moment, the Earth spun very slowly like Venus, or not at all due to tidal locking to the sun. Weather patterns would differ in the extreme, with the sun facing side baking and the dark side well below zero. The wind patterns would differ radically. The rising, heated outflow would occur from the sun side to the dark side, with cold air rushing at the surface from the dark side to the sunward side. A single gigantic storm would dominate on the sunward side, with the eye of the storm under the sun. There would be little or no storm rotation as we see it with our storms.