Weather Maps

NGS

How Does Air Pressure Affect Weather?

• How much the earth’s atmosphere is pressing down on us

• Measured with a BAROMETER

• If it CHANGES, then new weather is on the way:– Falling Air Pressure = stormy weather

coming– Rising Air Pressure = fair weather coming– Steady Air Pressure = no change is

coming

Isobars

• Isobars is a word that describe constant pressure. Iso is a prefix that means the same and bar is the prefix that means pressure, so ISOBARS are lines of CONSTANT PRESSURE!!!

WEATHER MAPPING ACTIVITYSurface Pressure

• We will be drawing ISOBARS to find areas of High Pressure and areas of Low Pressure.

• Begin with the 1024 pressure over Salt Lake City, UT (the one that is boxed). Draw a line to the next 1024 value located to the northeast (upper right). Without lifting your pencil draw a line to the next 1024 value located to the south and then to the one located southwest, finally returning to the Salt Lake City value. Remember, isobars are smooth lines with few, if any, kinks.

• Repeat this process for the next lowest pressure reading, which is 1020.• Continue this process until you have completed the entire map of the

United States. • Normally, each ISOBAR is named after the last two numbers of the

pressure, so the 1024 isobar will be named 24.

WEATHER MAPPING ACTIVITYSurface Pressure

• We can use these maps to identify areas of high pressure and areas of low pressure. Using your colored pencils, label the center of the high pressure with an H. Label the center of the low pressure with an L.

• High pressure is normally associated with dry weather because the heat causes the water vapor to evaporate. Shade in yellow the states you’d expect to see clear skies.

• Low pressure is normally associated with precipitation because the pressure causes temperatures to condensate and be cloudy. Shade in green the states you’d expect to see storms.

Winds = created from differences in air pressure

• Moves from areas of HIGH to LOW pressure

• Greater the difference in pressure, the FASTER the wind blows

• Measured with wind vanes and anemometers or you can estimate with the Beaufort Wind Scale

• Winds moves counterclockwise around a low pressure system and clockwise around a high pressure system.

Winds

• On your surface pressure map, draw arrows that show the direction of the winds around the areas of high pressure and low pressure.

• Now your surface pressure map is done. Hold on to it, and let’s work on the air temperature analysis.

Temperature Analysis

• We can look at a weather map to analyze temperature trends. We will be doing a similar activity to find ISOTHERMS – or areas of warm or cool air masses. ISOTHERMS are lines connecting similar temperatures.

• Draw ISOTHERMS throughout your map. • Shade in blue, the areas of lowest

temperatures and shade in red the areas of highest temperatures.

Dew Point

=Temperature at which the air is saturated (100% relative humidity)

Several events can occur when the dew point temp. is reached:1. If dew point temp. is above freezing:

a. water vapor condenses as liquidb. dew will form on surfaces

Dew Point

c. cloud droplets will form in air

2. If dew point temp. is below freezing:a. water vapor condenses as a solidb. frost on surfacesc. snow (or hail) in the air

Dew Point Weather Mapping • Using a green colored pencil, draw lines connecting the same dew points (as we

have done in the previous two exercises).

• These lines that connect the same dew point temperatures are called isodrosotherms.

• Isodrosotherms are used to identify surface moisture. The closer the temperature and dewpoint are together, the greater the moisture in the atmosphere. As the moisture increases so does the chance of rain. Also, since moist air is lighter than dry air, the greater the moisture, the easier for the moist air to lift into the atmosphere resulting in a better chance for thunderstorms. Typically, dewpoint 70°F or greater have the potential energy needed to produce severe weather.

• Shade in the region where the dew point temperatures are above 70˚F.

Pressure Changes

• Using a blue colored pencil, beginning at any +2 value, lightly draw lines connecting equal values of the +2 millibars pressure change.

• Draw the remaining "positive" pressure change value(s) at two millibars intervals.

• Using red colored pencils lightly draw a line connecting equal pressure change values of less than zero (0). Finally, using black, draw a line connecting the zero (0) line.

Pressure Changes

• Cold fronts are often located in areas where the pressure change is the greatest. The front represents the boundary of different air masses. Cold air is more dense than warm air so when a cold front pass your location, the pressure increases. We analyze for pressure change to look for these boundaries. We can also tell where high pressure and low pressure systems are moving by looking where the greatest change is occurring.

• Shade, in red, the region where the surface pressure change is -2 millibars or less.

• Shade, in blue, the region where the surface pressure change is +2 millibars or more.