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5.21 Weather Circulation

Where there is a significant planetary atmosphere, there is also weather. From the overheated atmosphere of Venus, to the icy clouds of Neptune, the gas surrounding planets circulates and at times may chemically separate into distinct clouds and bands. These atmospheric elements are energetically fueled by the flow of energy and the effects of planetary rotation, and they are shaped by the seasons and land formations they experience. At the most fundamental level, hot gases rise, shed their energy, and sink once cooled. This convective motion can take place across large spans or in a series of bands, or in chaotic patterns. 


Ultraviolet image of Venus' clouds as seen by the Pioneer Venus Orbiter (February 26, 1979). The immense C- or Y-shaped features which are visible only in these wavelengths are individually short lived, but reform often enough to be considered a permanent feature of Venus' clouds. The mechanism by which Venus' clouds absorb ultraviolet is not well understood. Click here for original source URL


It isn't possible for a planet to have a uniform temperature at all points. While one side faces its host star, the other side experiences night. This simple difference creates a thermal gradient between the day and night sides of the planet. If a planet has a tilt, such as the Earth has between its axis of rotation and the ecliptic, the planet will experience seasons. During one part of its year, one half of the planet will experience more sunlight than the other, setting up an additional temperature gradient.

On rocky worlds like Earth, the seasonal differences are able to heat the ground, creating an energy reservoir that can drive weather. This thermal energy build up is why the hottest part of summer (typically August in the Northern Hemisphere) and the coldest part of winter (typically February in the Northern Hemisphere) lag behind the longest and shortest days of the year. In the winter, the land is still radiating away its summer heat store when the winter solstice comes in December, and the land is still absorbing heat readily when the June solstice marks summer. Oceans change temperature much more slowly than land masses, but variations in sea temperatures, such as are seen with el Nino and la Nina both alter weather patterns as well. This build up and release of energy is seen in hurricane and monsoon seasons.

On Earth, the atmosphere is broken into 3 northern and 3 southern weather bands: the Hadley, Ferrel, and Polar cells. As the planet rotates, the equator moves faster than the polar areas, and this drives pressure differences. This rotating system also causes the atmosphere to experience a Coriolis forces that deflects southern winds in a counter clockwise motion and northern winds in a clockwise direction. As lower winds rush across land, they also experience friction, which can steal energy from the winds.

Rapidly rotating planets like Jupiter, with its approximately 10 hour rotation period, have many atmospheric bands while tidally locked planets, which rotate once per year, may only have a single day side and a night side convective cell.

While scientists understand the forces and thermodynamics that drive weather, it still isn't possible to accurately predict what will happen. This is in part a lack of fully accurate models. Also, the oceans are a large driver of weather and climate and data from the oceans is sparser than data taken over the land. Everything, from airplane contrails to the degree to which fall trees have lost their leaves, are the effects of energy transfer and weather. This is also a chaotic problem, where the same action can have varied outcomes when repeated. At the 24 hour time scale, reasonable brackets can be put on what should happen — estimates of highs and lows for temperature and projected precipitation — but it is generally agreed that a fully accurate weather prediction model isn't possible, and long term and accurate prediction beyond a week isn't possible even with powerful super computers.