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# Jet Stream, Pressure, Humidity, and Fronts Help

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By McGraw-Hill Professional
Updated on Sep 4, 2011

## Jet Stream

When watching the evening weather report, chances are good that you will hear something about the jet stream . This speedy current is commonly thousands of kilometers long, a few hundred kilometers wide, and only a few kilometers thick. Jet streams are usually found somewhere between 10–18km above the earth’s surface in the troposphere. They blow across a continent at speeds of 240km/hr/hr, usually from west to east, but can dip northward and southward depending on atmospheric conditions.

The jet stream is a long, narrow current of fast moving air found in the upper levels of the atmosphere.

Air temperature differences cause jet streams. The bigger the temperature differences, the stronger the pressure differences between warm and cold air. Stronger pressure differences create stronger winds. This is why jet streams fluctuate so much in speed.

During the winter months, polar and equatorial air masses form a sharp surface temperature contrast causing an intense jet stream. Stronger jet streams push farther south in the winter. However, during summer months, when the surface temperature difference is less severe, the winds of the jet stream are weaker. The jet stream blows farther north.

## Pressure

Although air is invisible, it still has weight and takes up space. Since air molecules float freely in the vastness of the atmosphere, they get 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 kg/cm 2 . The force on 1000 square centimeters is nearly a ton!

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

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 cm of mercury is equivalent to 1013.25 millibars.

Air pressure can tell us a lot about the atmosphere. 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 storms.

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 the Earth’s surface on a topographical map.

## Wind

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 slight, the day is calm. However, if the 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. Wind speeds are described according to their effects on nature and surface structures. Table 14-1 lists the different wind effects by increasing Beaufort numbers.

Table 14-1 The Beaufort Wind Scale gives visual clues as to a wind’s speed.

 Beaufort Scale # Wind speed (km/hr) Wind Sign 0 <1 calm smoke rises vertically 1 1–3 light air smoke drifts 2 6–11 light breeze leaves rustle 3 12–19 gentle breeze small twigs rustle 4 20–29 moderate breeze small branches move 5 30–38 fresh breeze small trees move 6 39–50 strong breeze large branches move 7 51–61 moderate gale whole trees move 8 62–74 fresh gale twigs break off trees 9 75–86 strong gale branches break 10 87–101 whole gale some trees uprooted 11 102–119 storm widespread damage 12 >120 hurricane severe damage

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 are comfortable. But, if a sudden wind gust blows by, we get chilled. The warm protective air layer is lost and has to be reheated by the body. See Table 14-2 to get an idea of the wind chill equivalent temperatures at different wind speeds.

Table 14-2 Wind chill can bring down the temperature of the body quickly.

 Wind speed (km/hr) Temperature (°Celsius) −15°C −10°C −5°C 0°C 5°C 10°C 15°C 20°C 25°C 30°C 35°C 40°C 0 −15 −10 −5 0 5 10 15 20 25 30 35 40 5 −18 −13 −7 −2 3 9 14 19 25 31 36 41 10 −20 −14 −8 −3 2 8 13 19 25 31 37 42 30 −24 −18 −12 −6 1 7 12 18 25 32 38 43 50 −29 −21 −14 −7 0 6 12 18 25 32 38 44 70 −35 −24 −15 −8 −1 6 12 18 25 32 38 44 90 −41 −30 −19 −9 −2 5 12 18 25 32 38 45

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