Climatology: Atmosphere, Weather and Climate

What is the atmosphere made of, and how is it layered?

• Air is a mixture of gases: nitrogen makes up about 78%, oxygen about 21%, with carbon dioxide, argon, helium, ozone and hydrogen making up the remaining roughly 1%, plus tiny dust particles.
• Plants cannot take nitrogen directly from the air. Soil bacteria convert it for them. Oxygen from green plants balances the oxygen used up in breathing.
• The atmosphere has five layers. The troposphere matters most, since almost all weather like rain, fog and hail happens there, up to about 13 km.
• Above it sits the stratosphere (to ~50 km, holds the ozone layer), the mesosphere (to ~80 km, where meteors burn up), the thermosphere (its ionosphere part reflects radio waves), and finally the exosphere, fading into space.
• Nearly 99% of the atmosphere’s total mass sits within just 32 km of the surface. Molecular oxygen (O2) thins out above about 120 km, and atomic oxygen (O) takes over higher up. Water vapour stays mostly in the lowest few kilometres.

How does the greenhouse effect actually warm the planet?

• Carbon dioxide acts as a greenhouse gas by trapping heat radiated from the earth. Without it, the earth would be far too cold to support life.
• The atmosphere is heated more by outgoing terrestrial (long-wave) radiation than by incoming solar (short-wave) radiation. Most sunlight passes straight through and heats the ground instead, and it is the ground’s own re-emitted long-wave radiation that greenhouse gases absorb well.
• Extra CO2 from factory smoke and car fumes raises the temperature further, called global warming, which melts polar snow, raises sea level, and may flood coasts.
• Dust particles come from sea salt, fine soil, smoke-soot, ash, pollen, and even burnt-up meteors. Water vapour condenses around them, which is exactly how clouds get their start.

What’s the real difference between weather and climate?

• Weather is the short-term atmospheric condition. Climate is the long-term pattern averaged over many years. Precipitation, humidity, and thunderstorms all result from temperature, moisture, and pressure interacting.
• Latitudinal changes drive much of the variation in weather systems. The angle and intensity of incoming sunlight varies systematically from the equator to the poles, and insolation falls the same way.
• Water vapour’s amount decreases with altitude, since colder air higher up holds less moisture. About 90% of atmospheric moisture sits within 6 km of the surface.
• Water vapour’s percentage is highest near the equator, not at the poles, since warmer tropical air can hold far more moisture than cold, dry polar air.

Why is the troposphere thicker at the equator than at the poles?

• The troposphere is far thicker at the equator, about 18 km, than at the poles, about 8 km. Intense equatorial solar heating drives strong convection currents that carry heat to great heights, pushing the tropopause boundary much higher there.
• The tropopause itself sits near -80°C over the equator, but only -45°C over the poles, since the equatorial troposphere is so much taller.
• Atmospheric dust is more concentrated over subtropical and temperate areas than over equatorial or polar regions. Frequent dry winds there pick up loose surface material and keep it suspended, unlike the wetter equatorial belt or snow-covered poles.
• Three types of rainfall follow from these same mechanisms: convectional (warm moist air rising and cooling on its own), orographic (moist wind forced up a mountain), and cyclonic (rising air within a cyclone’s low-pressure system).

What forces create pressure belts and wind?

• Air always moves from a high-pressure area to a low-pressure area, and this moving air is what we call wind. An anemometer measures wind speed.
• Earth has four pressure belts: the equatorial low, the subtropical highs near 30°N/S, the sub-polar lows near 60°N/S, and the polar highs. These belts shift with the sun’s seasonal path.
• Wind direction and speed come from three forces together: the pressure gradient force (stronger where isobars sit close together), friction (strongest near the surface), and the Coriolis force, which deflects moving air due to Earth’s rotation.
• The Coriolis force grows stronger as wind velocity increases, and is at its maximum at the poles while effectively absent at the equator. High above the friction layer, where isobars run straight, the pressure gradient force and Coriolis force balance out, producing the geostrophic wind, parallel to the isobars.

How do the three circulation cells and local winds work?

• The Hadley cell: air rises at the ITCZ near the equator, flows poleward aloft, sinks back around 30°N/S as the subtropical high, then returns to the equator at the surface as trade winds.
• The Ferrel cell: in the middle latitudes, surface winds called westerlies blow from the subtropical high toward the poles. The Polar cell: cold, dense air sinks near the poles and flows back as the polar easterlies.
• By day, land heats faster than the sea, so wind blows from sea to land as a sea breeze; by night, land cools faster, reversing the flow into a land breeze. Mountain slopes work the same way: a daytime valley breeze, a nighttime mountain wind.
• A katabatic wind is cold, dense air draining downhill from high plateaus or ice fields, a stronger cousin of the ordinary mountain wind. A foehn-type wind drops its rain crossing a mountain, then warms further through compression on the way down, and can melt snow quickly.

Why do land and ocean areas have such different climates?

• Land and ocean heat and cool at different rates, since water has a much higher specific heat capacity. In January, Northern Hemisphere land cools faster than the ocean, so isotherms bend equatorward over land and poleward over the relatively warmer oceans.
• The Gulf Stream and North Atlantic Drift are warm ocean currents, not cold ones. They keep the North Atlantic warmer than its latitude would suggest, which is exactly why January isotherms bend so far north over that ocean.
• The Marine West Coast climate has small annual and daily temperature ranges, thanks to moderating ocean influence, plus year-round precipitation of roughly 50 to 250 cm, carried inland by nearby currents and westerly winds.
• Around a low-pressure centre in the Northern Hemisphere, wind circulates anticlockwise; around a high, clockwise. Both patterns reverse in the Southern Hemisphere.