Bakers that live in the mountains have to consider the pressure of air when creating light cakes and soufflés. The decrease in pressure at high altitudes (over 6,000 meters) changes the baking process from that of sea-level baking. That is why some cake mixes give different directions for high-altitude baking, to make up for the difference of pressure on the rising cake.

Air pressure is the force applied on you by the weight of air molecules.

Although air is invisible, it still has weight and takes up space. Since air molecules float freely in the vastness of the atmosphere, they become pressurized when crowded into a small volume. The downward force of gravity gives the atmosphere a pressure or a force per unit area. The Earth's atmosphere presses down on every surface with a force of 1 kilogram per square centimeter.

Weather scientists measure air pressure with a barometer. Barometers are used to measure air pressure at a particular site in centimeters of mercury or millibars. A measurement of 76 centimeters of mercury is equivalent to 1013.25 millibars.

Air pressure can tell us a lot about the weather. If a high-pressure system is coming, there will be cooler temperatures and sunny skies. If a low-pressure system is moving in, then look for warmer temperatures and thunder showers.

Atmospheric pressure falls with increasing altitude. A pillar of air in cross section, measured from sea level to the top of the atmosphere, would weigh approximately 14.7 pounds per square inch (psi). The standard value for atmospheric pressure at sea level is equal to:

1 atm = 760 mm Hg (millimeters of mercury) = 1013 millibars = 14.7 psi (pounds force per square inch) = 1013.25 hPa (hectopascals)

On weather maps, changes in atmospheric pressure are shown by lines called isobars. An isobar is a line connecting areas of the same atmospheric pressure. It's very similar to the lines connecting equal elevations on a topographical map of the earth's surface.


Winds are a product of atmospheric pressure. Pressure differences cause air to move. Like fluids, air flows from areas of high pressure to areas of low pressure. Meteorologists predict winds by looking at the location and strength of regional high- and low-pressure air masses. If the changes are small, the day is calm. However, if pressure differences are high and close together, then strong winds whip up.

In 1806, Admiral Sir Francis Beaufort of the British Navy came up with a way of describing wind effects on the amount of canvas carried by a fully rigged frigate. This scale, named the Beaufort wind scale, has been updated for modern use. Wind speeds are described according to their effects on nature and surface structures. Table 3-2 lists the different wind effects using Beaufort numbers.

The wind chill factor measures the rate of heat loss from exposed skin to that of surrounding air temperatures.

Wind chill happens when winter winds cool objects down to the temperature of the surrounding area—the stronger the wind, the faster the rate of cooling. For example, the human body is usually around 36°C in temperature, a lot higher than a cool Montana day in November. Our body's heat loss is controlled by a thin insulating layer of warm air held in place above the skin's surface by friction. If there is no wind, the layer is undisturbed and we feel comfortable. However, if a sudden wind gust sweeps by, we feel chilled. Our protective warm

Table 3-2 Beaufort scale.

Beaufort scale no.

Wind speed (kilometers per hour)






Smoke rises vertically



Light air

Smoke drifts



Light breeze

Leaves rustle



Gentle breeze

Small twigs rustle



Moderate breeze

Small branches move



Fresh breeze

Small trees move



Strong breeze

Large branches move



Moderate gale

Whole trees move



Fresh gale

Twigs break off trees



Strong gale

Branches break



Whole gale

Some trees uprooted




Widespread damage




Severe damage

air cushion is blown away and has to be reheated by the body. See Table 3-3 to get an idea of the wind chill equivalent temperatures at different wind speeds.

The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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