Weather Facts

• All weather is caused by non-uniform heating of the Earth's surface by the Sun. The Sun's rays shine straight down on the tropics, but come in at a glancing angle at the poles.
• As heat migrates from the tropics to the poles, convection happens in the atmosphere causing the air to be constantly moving.
• As moving air carries heat from place to place, it carries water along with it. The transfer of heat from tropical ocean to atmosphere causes tremendous amounts of water to evaporate; this water vapor condenses and forms clouds and storms worldwide.
• Weather as we know it exists in the first six miles of altitude in the atmosphere. Over 90 % of atmospheric gas is concentrated this close to the Earth's surface, even though the atmosphere extends beyond an altitude of 200 miles.
• Weather as we know it exists because there happens to be a readily-available substance around (water) that changes phase (solid, liquid, vapor) easily over the range of temperatures on our planet. There are strong winds on Mars and Venus, but no "weather" because water is unavailable or always in one phase. Triton, one of the moons of Neptune, has an atmosphere with weather in it: Nitrogen plays the same role there that water plays here.
• Clouds are made up of microscopic drops of liquid water or crystals of ice, which typically "grow" around even tinier grains of dust or salt. Rain or snow (precipitation) results when these tiny particles come together in various ways and become too heavy to be supported by updrafts.
• Water vapor does not form clouds and storms unless it is somehow lifted and cooled: this can occur as air moves laterally over mountains (orographic lifting), or as cold dense dry air moving along the surface undercuts and lifts warm moist air (cold fronts).
• Lifted air in northern hemisphere storms always acquires a counter-clockwise rotation (cyclonic flow) caused by the rotation of the planet.
• Water vapor releases heat when it condenses into cloud droplets and rain. The most violent storms result from conditions where quantity of vapor, degree of lifting, and temperature change combine to cause very rapid heat release in the atmosphere during condensation. Rapidly heated air expands, drops in pressure, and causes the violent winds associated with hurricanes, thunderstorms and tornadoes.
• Since most of our storms are associated with cold fronts that move from west to east, short-term weather can be predicted if you know the atmospheric characteristics associated with an approaching front: clouds lower and thicken from west to east, wind blows from the south with increasing speed, surface atmospheric pressure decreases as lifting begins.
• Climate is the aggregate of weather over a long period in one place. Climate is strongly influenced by latitude, proximity to the ocean, altitude, and periodic variations in the motion of heat within the oceans (for example El Niño).
• The seasons are caused by the tilt of the Earth's rotation axis. In mid-latitudes like ours, seasonal climate varies the most as the Sun's rays change their angle with the ground significantly during the year.

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