Weather Systems and Severe Weather

9.2 Weather Forecasting

How do TV weather reports make their maps?

Step 1: Go to the ArcGIS Online map, How’s the Weather?, and explore the map.

  • Which color indicates atmospheric high pressure?

Step 2: With the Details button underlined, click the button, Show Contents of Map (Content).

Step 3: In the upper-right corner, click the link, Modify Map.

Step 4: Click the button, Edit.

Step 5: Label the atmospheric high- and low-pressure areas on the map. Add at least three low-pressure “L” and three high-pressure “H” symbols. Click the text marker labeled H or L. Click on the map, and type in an H or L to indicate high- and low-pressure areas.

How do air masses generally move across the country?

Step 6: Hover over the layer name, Weather Stations.

Step 7: Click the button, Change Style. Set Step 1: Temp F. Set Step 2: Counts and Amounts (Color). Click the button, Options. Set Theme: Above and Below. Click Symbols. Click Fill. Select a Red-to-blue color ramp. If necessary, click Invert Colors to indicate red for warmer and blue for cooler temperatures. Press OK repeatedly to close the menus. Press Done.

  • Identify the states that are particularly warm.

What happens when air masses meet?

Radiated infrared energy warms air masses over darker ground or at lower latitudes. Warm, moist air expands and rises, and a low-pressure air mass results. The air over lighter-colored ground (such as snow) is generally cooler. Atmospheric molecules then pack more closely together, making dense, dry high-pressure air.

The differential heating of air masses and the atmospheric pressure difference between them causes the winds to blow. Where two air masses of different temperatures and humidity collide, a weather front is formed. Weather changes occur when two air masses meet along a weather front. Whether a cold or warm air mass is pushing them determines the type of clouds and weather systems. Fronts are symbolized for the atmosphere that is coming into an area.

Step 8: Click the Edit tool. Draw a blue line to indicate where cold air is advancing eastward, and draw a red line to indicate where warm air is advancing eastward. These lines indicate weather fronts where there are rapid changes in temperature.

How are precipitation and wind speed studied?

Step 9: Air always flows from high- to low-pressure. Hover over the layer name, Weather Stations.

Step 10: Click on the ellipse. Select Copy. This creates a new layer named, Weather Stations – Copy.

Step 11: On the new layer, change the style: Set Step 1: AVERASPEED. Set Step 2: Counts and Amounts (Size). Click Options. Click Symbols, and switch from Shapes to Arrows. Choose any colored downward arrow, and click OK in the symbol box. While still in the Options window, scroll down to check the box by Rotate Symbols. Choose ADIRECTION (which represents the wind direction), and ensure that Geographic is chosen.

  • In the northern hemisphere, winds curve outward from high pressure (clockwise), and inward towards low pressure (counterclockwise).

How will air masses affect the weather in front of them?

  • What will the weather be like tomorrow in Boston, Massachusetts, according to the map?
  • How should a person in Nebraska dress to go to work or school, according to the map?


Share This Book