Scientific explanations of weather phenomena primarily involve principles of heat transfer (convection, conduction, and radiation), energy transformations (in the productio of thunder and lightning), and changes or state within the water cycle--precipitation, evaporation, condensation.

As a result of these principles interacting, hot, cold, warm, and cool air masses develop. They are usually very large and cover several states in the United States. The develop as the air lingers in the same places for several days or weeks. The air masses become dry or humid matching the surface below. There are four air masses that affect the weather in the United States

1) continental polar air mass (over Northern Canada and Alaska)
2) maritime polar air mass (over North Pacific Ocean)
3) maritime tropical air mass (over the Gulf of Mexico and Caribbean)
4) continental air mass (over the southwestern states and Mexico)

Air is moving molecules of gas. These gas molecules are constantly moving and pushing on each other and anything in their way. This pushing power is called air pressure. Air pressure can change depending on how many molecules are in a given space and how fast they are moving. The more molecules in a given space and the faster the molecules move, the greater the air pressure. If air molecules are heated in a confined space, the pressure increases. If the space is decreased for the same amount of molecules, the pressure will rise.

The uneven heating of the earth results in uneven air pressure on the earth. ON a wather map, "H" marks areas that have high pressure and "L" marks the low pressure areas. High pressure areas are generally correlated to clear skies, while low pressure areas produce clouds and precipitation. High pressure prevents water vapor from rising to form clouds.

As air moves from high pressure to low pressure areas, wind is produced. The closer the high and low areas are to each other, the stronger the winds. Winds in a high pressure area move clockwise, while winds in a lower pressure area move counter-clockwise.

When air masses meet, a front develops. It is usally on a front where cold and warm air meet and the most violent storms and bad weather occur. Winds are bringing in air to equalize the pressure when a high pressure collides with a low pressure area. The kinds of fronts include:

1) warm = warm air replacing cold air; the lighter warm air slips over the heavier cold air forming a gradual slope; clouds increase and become lower; steady precipitation

2) cold = cold air displaces the warm air; the heavier cold air shoves under the lighter warm air pushing the warm air up very fast carrying moisture with it; forms clouds, steep slope between these two air masses --- violent storms!

3) stationary = neither the cold or warm air is advancing; clouds form on both sides of the front


Thunder is the sound that is made by air molecules when lightning strikes through the atmosphere--resulting from the rapid heating and expansion of air by lightning causing a "shock wave" in the air. Lightnining heats air molecules to 54,000 degrees F. This causes the surrounding molecules to explode outward and come back again Static electricity caused by the moving of water molecules in the clouds--lightning.

Humans can also use technology to predict the weather--thermometers (temperature readings), barometers (air pressure), hygrometer (relative humidity), wind vanes (direction of wind), anemometer (speed of wind), Beaufoart Wind Scale (spead of wind), rain gauge (amount of rainfall), weather satellites, weather planes and balloons, radar.

Use these data below to predict
what kind of front has just passed through Detroit: After you make your prediction, click here!


Data Set One

Data Set Two

relative humidity



barometric pressure

30.10 in.

30.14 in.


48 degrees F

75 degrees F

wind direction



Consider that the temperature increases in data set two, the relative humidity increases in data set two, and the winds shift direction. It is most likley that a
warm front has just passed through Detroit.