Weather

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Meteorology - the study of the atmosphere (including weather).

The atmosphere is the layer of air that surrounds the Earth.  It is made up of about:
   78% Nitrogen
   21% Oxygen
   0.9% Argon
   0.03% Carbon Dioxide
   <1 to 3% Water Vapor (varies by region and season)

Atmospheric (Air) Pressure is the weight of the atmosphere pushing down on you.  This is about 1,000 kg (about 2,200 pounds) of air!  Air pressure is measured using a barometer.  Air pressure drops as you rise higher into the atmosphere.

The atmosphere is divided into several layers:
   1. The troposphere is closest to the ground.  It extends
       upwards for about 10 km.  The air is thickest in this level and
       most of our weather takes place here.  For each kilometer
       you climb into the troposphere, the temperature will drop 6.5
       degrees C.
   2. The stratosphere reaches from about 10 to 50 km above
       the ground.  The temperature in this layer rises to about 0
       degrees C.  This is due to the ozone layer, which absorbs
       ultraviolet light (UV) from the sun and turns it into heat.
   3. The mesosphere reaches from about 50 to 85 km above the
       ground.  This is the coldest layer, with temperatures dropping
       as low as -90 degrees C.
   4. The thermosphere reaches from about 85 to 600 km above
       the ground.  The temperature rises again in this layer, reaching
       as high as 2,000 degrees C.
   4a. The ionosphere is inside the thermosphere.  It reaches from
       about 80 to about 400 km above the ground.  This layer
       contains electrically-charged particles (ions), which are
       formed when this layer absorbs solar energy.  The ionosphere
       reflects radio waves.

Energy in the Atmosphere
Earth receives about 2/1,000,000,000 of the sun's total energy.  Of that, about half is either absorbed by the atmosphere or reflected back into space.

The type of energy that we get from the sun is called radiant energy.  Radiant energy is energy that travels through space in waves.  This includes gamma rays, x-rays, ultraviolet (UV) light, visible light, infrared light, microwaves, and radio waves.

The Law of Conservation of Energy states that energy cannot be created or destroyed, but it can change form.  This means that light that is absorbed can be converted into heat.

Some of the energy that the Earth receives from the sun is trapped in our atmosphere by gasses, such as carbon ioxide and water vapor.  This trapped energy is converted into heat and helps to warm the Earth.  This process is called the Greenhouse Effect.

Convection is the movement of fluids (gas or liquid) due to changes in temperature.  In other words, warm air rises and cool air sinks.  A complete cycle of air moving due to differences in temperature is called a convection cell.  Convection cells can cause land and sea breezes, mountain and valley breezes, and monsoons(These were discussed, with diagrams, in class.)

Convection drives the three major wind belts: the trade winds (closest to the equator), the westerlies (in the middle latitudes), and the polar easterlies (near the poles).

Humidity
Humidity is moisture in the air.  It can be described in one of two ways:
  Absolute Humidity is a measure of how much water is in the
     air.  This is expressed in terms of grams per cubic meter.
  Relative Humidity is a measure of how much water is in the air
     compared to how much water the air can hold.  This is
     expressed as a percent.  For example, air that has 100%
     humidity cannot take in any more water.  This means that the
     air is saturated.
Rule of Thumb: The warmer the air is, the more moisture it can hold.

When the temperature drops to the point where the air cannot hold the humidity that it had, this extra water comes out of the air as condensation (water turning from a gas into a liquid).  This water that appears out of the air forms dew - or frost, if the temperature is below freezing.  The dew point is the temperature at which the air is saturated, and dew can form.

Clouds
Clouds are classified by their shapes:
  - Cumulus clouds are puffy and shaped like cottonballs.
  - Stratus clouds form flat layers.
  - Cirrus clouds are wispy and feathery.
As a general rule, clouds get more wispy as you go higher into the atmosphere.  Below 2 km, you find cumulus and stratus clouds.  Sometimes you'll see clouds that seem both puffy and layered.  These are called stratocumulus.  Clouds between 2 and 7 km high add "alto-" to their name; thus, altocumulus and altostratus.  Cirrus clouds form above 7km.  Cumulus and stratus clouds at this height add "cirro-" to their name (cirrocumulus and cirrostratus).

Rainclouds can be either cumulonimbus (puffy) or nimbostratus (layered).  Cumulonimbus clouds are sometimes called thunderheads.  These tend to bring heavy rains for shorter periods of time.  Nimbostratus tend to bring lighter rains over a longer period of time.

Precipitation
Precipitation is basically water falling form the sky.  There are four basic types:
  1. Rain
      - Cloud droplets grow as moisture is added until they are too
         heavy to stay in the cloud.  They then fall to the ground as
         rain.
  2. Snow
      - If it's cold enough, the cloud droplets freeze - which forms
        ice crystals.  As moisture is added, the crystals grow until
        they are heavy enough to fall as snow.
  3. Sleet(a.k.a.: freezing rain)
      - Sleet begins as rain or snow.
      - As rain falls, it may pass through a layer of cold air and
        freeze into sleet.
      - As snow falls, it may pass through a layer of warm air and
        melt, then pass back into cold air and re-freeze into sleet.
  4. Hail
      - Hail begins as sleet.
      - As it falls, wind blows it back up into the clouds, where it
        picks up more moisture.  This causes the hail to get bigger.
      - This process repeats until the hail is too heavy for the wind to
        carry it back up into the clouds.
      - Depending on how strong the wind is, hail can grow as large
        as grapefruit or softballs.

Air Masses & Fronts
An air mass is a large body of air that has taken on the temperature and humidity of a part of the Earth's surface.  Air masses can be warm or cold, dry or moist.

Rule of thumb: Air masses don't mix - but they do push against each other.

The boundary between two air masses is called a front.  There are basically three types of fronts:
  1. Cold Front
      - Cold fronts occur where cold air moves in and replaces
         warm air.  Because the cold air is denser, it can push up
         the warm air the way that a snowplow pushes up snow in
         front of it.  The rising warm air cools off and forms
         cumulonimbus (thunderhead) clouds.  A squall line is a
         string of thunderstorms along a cold front.
  2. Warm Front
      - Warm fronts occur when warm air moves in and replaces
         cold air.  Because the cold air is denser, the warm air will
         tend to ride up over it.  As it rides higher, clouds form:
         nimbostratus at the low end, followed by stratus,
         altostratus, and finally cirrus clouds at the higher, leading end.
  3. Stationary Front
      - The air masses on either side of this front are not moving.

Isotherm - A line on a map that passes through locations that have the same temperature.  Isotherms are useful in locating air masses.

Climate
The climate is the average weather in an area over a long period of time (decades or centuries).  There are three main climate zones in the world: tropical (near the equator), polar (near the poles), and temperate (between tropical and polar).

The Jet Stream
The Jet Stream is a band of winds that form along the Polar Front (the boundary between cold polar air and warm tropical air).  The Jet Stream winds move at speeds of about 120-240 km per hour, and occur at an altitude of about 10-15 km above the ground.

Storms
A cyclone is a huge mass of spinning air.  In the Northern Hemisphere, cycles generally move in a couterclockwise direction.

 

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Supporting files for this material can be found here.

Subpages (1): Weather Files