WEATHER

The blanket of air around the earth is called the atmosphere.  All our weather happens in the bottom layer of the atmosphere called the troposphere, which is six to ten miles thick.  Meteorology is the study of the changes in temperature, air pressure, moisture, and wind direction in the troposphere.

All of our weather comes from basically two things:  the sun and the moisture in the air.  These two work together to form clouds, to make rain and thunderstorms, and to cause winds to blow.  Without the sun or moisture, our earth would have no weather.

An air mass is a large area of air that may be warm or cold or dry or moist.  When two air masses come together, the line between them is called a front.  Along a front there are usually clouds and rain and maybe thunderstorms.  The movement of air masses helps to cause our weather.

Air density
Air is matter.
Air has density.  Density is affected by gravity.
Gravity decreases as you go higher.  Density of the air decreases too. Temperature also affects density.
Cold air is more dense than warm air.

Air pressure - * warm air is less dense and exerts less pressure.
                        * cooler air is denser and exerts more pressure.
                        * water vapor in the air affects pressure. Water molecules are lighter than gas molecules. If
                            more water molecules are in the air, then the air pressure decreases.
 

Water vapor enters the air when liquid evaporates. The water vapor is invisible, but when the air contains a lot of water vapor, particles condense to form droplets when air is cooled.  the amount of water vapor in the air is called humidity.  Warm air can hold much more water than cool air.

Clouds form by surface heating and convection.  The sun heats an area of the ground, the ground heats the air above it, this air becomes less dense as molecules move farther apart, the warm air rises in a column, it begins to cool in the troposphere and condense to form a cloud.  When moisture falls from a cloud to the ground we have precipitaion. (rain, snow, sleet, freezing rain, hail)

Air Masses
When a large amount of air sits over one location an air mass with certain characteristics forms.
 
    Polar
 
    Tropical
        
    Equatorial

    Arctic

FRONTS - the boundary where two air masses come together
Collisions of the air masses at these boundaries cause WEATHER CHANGES.

Cold front - a cold air mass pushes away a warm, moist air mass.  These move quickly and produce wind and rain.  Thunderstorms are common. See more about cold fronts here: http://www.usatoday.com/weather/tg/wcfront/wcfront.htm

Warm front - a warm air mass creeps over a cold air mass.  Cloudy skies with rain or snow develop.  See more about warm fronts here: http://www.usatoday.com/weather/tg/wwfront/wwfront.htm

Air has pressure.  Colder air exerts more presssure than warmer air.  (remember molecules move apart in warmer air)

Different pressure in different types of air masses cause movement of the air mass.  Do you remember "Science in Your Bathroom" from sixth grade?  A shower curtain will blow in toward you because warm air in the shower is rising, so cooler air from outside rushes in.

So differences in air pressure can cause winds.
Temperature differences occur because the sun heats the earth's surface unevenly.
Temperature differences lead to pressure differences.

The wind blows because air has weight.  Cold air weighs more than warm air, so the pressure of cold air is greater.  When the sun warms the air, the air expands, gets lighter, and rises.  Cooler, heavier air blows to where the warmer and lighter air was, or in other words, wind usually blows from areas of high air pressure to areas of low pressure. If the high pressure area is very close to the low pressure area, or if the pressure difference (or temperature difference) is very great, the wind can blow very fast.
 

Think back to sixth grade again and remember the spinning cardboard.  You spun cardboard on a pencil point and tried to draw a straight line with a ruler.  You did not get a straight line, but  a curved line.  The earth's rotation causes wind to move in circular pattern (due to the Coriolis Force.)

Causes of Climate

    1) Latitude - As latitude increases (gets closer to the poles) the sun's rays strike at more of an angle.

    2) Altitude - As altitude increases less air presses down on earth's surface.  Particles of air spread farther apart, and the air is less dense.  Less dense air cannot hold as much heat so the temperature decreases.

    3) Distance from the ocean - water heats up and cools down more slowly than land.  So coastal areas have more "moderate" temperatures.  Inland, far from oceans the land heats up and cools down quickly so there are hot summers and cold winters inland.
    Ocean currents can also carry currents of warm or cool water - these cool or warm currents can warm(or cool) the air above them.

The state wants you to know that:
    Uneven heating of the earth's surface produces weather systems.
    The cycling of water in and out of the atmosphere plays an important role in determining weather and climate.
    Ocean currents affect local and global weather conditions.
    A weather map can be used to predict temperature and precipitation for several days in a given area.
 
SCIENCE PROVIDES INFORMATION.
TECHNOLOGY USES THE INFORMATION.
Student should be able to:  Describe how the tools of today (technology) are different from those of the past but may be modifications of ancient tools and techniques.
 

weather vane	balloons carry	radiosondes
radar	satellites	hurricane hunter airplane

Links
How the Weather Works http://www.usatoday.com/weather/wworks0.htm