The Weather Map

Objectives

1) Explain the history of a frontal low
2) Read and interpret a weather map including all major symbols

In this lesson, you will learn how to read a weather map and look at frontal precipitation in greater detail.

Weather Maps are absolutely critical for forecasters. They contain information regarding fronts, highs and lows, isobars, and weather data for specific locations expressed through the use of symbols.
Before delving into the various components of a weather map, one should have a more in depth understanding of frontal precipitation.

Frontal Precipitation In Depth

Read pages 172 to 175 in Planet Earth: A Physical Geography.

Satellite Photo
December 23, 2003 (10pm PST)

Courtesy: University of Washington

Weather Data for Vancouver
December 23, 2003 (11 pm PST)
Courtesy: Environment Canada

Present State of Weather: Light Rain
Temp.: 7 degrees Celsius
Dew Point: 5 degrees Celsius
Relative Humidity: 88%
Air Pressure: 99.4 kPa and Falling
Wind: E16 km/h

Isobars

Air pressure is shown on a weather map using isobars - lines joining points of equal pressure at the earth's surface. You may think of isobars as being similar to contour lines on a topographic map but instead of elevation, air pressure is represented. Recall that strong winds occur where there is a strong pressure differential. This can be seen on a weather map where the isobars are close together. Again similar to a topographic map where contour lines being close together indicates a steep slope / gradient, isobars being close together indicates a significant pressure gradient.

Reading a Weather Map and Predicting the Weather

Read pages 186 to 190 in Planet Earth: A Physical Geography.

Simplified Station Model and Other Weather Map Symbols

Carefully note the symbols used on a Weather Map as outlined on page 189.

Some further explanation is needed with respect to interpreting the air pressure number for a station model.

With the example above, the air pressure number is 013.

To determine the actual air pressure, follow these steps:

Step 1: Add a 9 in front of the number (i.e. 9013) and move the decimal two places to the left (i.e. 90.13).

Step 2: Add a 10 in front of the number (i.e. 10013) and move the decimal two places to the left. (i.e. 100.13).

Step 3: Compare the two numbers. The air pressure at this station will either be 90.13 kPa or 100.13 kPa. So which one is correct?

The generally acceptable pressure range is 94.00 kPa to 106.00 kPa. Hence, 90.13 kPa is outside the acceptable range so 100.13 kPa is the answer.

Now, even if you did not know the generally acceptable range or if both answers fit into the range, you could still figure out the answer. How?

Look at the other conditions at the station. Above it shows partly cloudy conditions. Remember that low air pressure is associated with clouds and precipitation. Since 90.13 kPa is very low air pressure, it would definitely be associated with stormy conditions and would not make sense in relation to the other information given.

Converting kPa to millibars

Recall that converting kilopascals to millibars is as simple as multiplying by 10. Hence 100.13 kPa is 1001.3 mb.

Air Pressure Change

With the Weather Station model above, +08 represents the air pressure change over the past three hours. To interpret this number, simply move the decimal two places to the left and it is automatically in kPa. Therefore, at this station over the past three hours, the air pressure has risen (+) 0.08 kPa or 0.8 mb.

Wind Direction

With the Weather Station model above, the wind is coming from the southwest as the initial line coming off of the station circle is pointing in that direction. Always look at the direction that the initial line is pointing towards to get the wind direction. The bars attached to this line represent wind speed.

Assignment Work

Read the information presented above and then answer the following questions in your notebook. *E-mail your responses to Questions #A5, C2, and C3 to your instructor as a document attachment*. This assignment is worth 10 marks.

A) Frontal Precipitation in Depth

1) Carefully read and take notes from pages 172 to 175 in Planet Earth: A Physical Geography.

2) Sketch Figure 9.6a (page 175) "The Stages of a Mid-Latitude Cyclone" in your notebook.

3a) Which cloud is associated with the leading edge of a warm front?

b) Which cloud produces precipitation with a warm front?

c) Which cloud produces precipitation with a cold front?

4) Answer Questions #6 and #7 from p. 177 of Planet Earth: A Physical Geography.

5) Examine the satellite photo and data given above under "Frontal Precipitation In Depth". This photo shows a mid-latitude cyclone approaching the BC Coast during December. Using Figure 9.6 as a guide, summarize the changes in weather that the BC Coast would expect to experience as this low pressure system approaches, passes over, and then leaves the area. In your answer, refer to wind direction, air pressure, temperature, and precipitation changes.

6) Using an internet source, give a definition for an occluded front and a stationary front. Include the symbol for each front as well.

B) Isobars

1) Where on the weather map shown would the strongest winds be occurring (specify locations)?

2) The isobars are labeled in millibars. What is 1012 mb in kilopascals?

C) Reading the Symbols on a Weather Map and
Predicting the Weather

1) Carefully read and take notes from pages 186 to 190 in Planet Earth: A Physical Geography.

2) With the aid of Figure 9.17 (Map and Legend) on pages 188 and 189, answer the following:

a) What is the air pressure at San Francisco?
b) What is the dew point at Yellowknife?
c) What direction is the wind coming from at Fairbanks, Alaska?
d) What is the temperature at Winnipeg?
e) Which station has higher relative humidity, Denver, Colorado or Schefferville, Quebec? Explain.
f) What is the present state of weather at Victoria?
g) What is the change in air pressure at Prince George?
h) What does the shaded area over the southeast United States indicate?
i) Predict what will happen to the conditions at Minot, North Dakota over the next 24 hours. Explain.
j) Predict what will happen to the conditions at Aklavik, NWT over the next 24 hours. Explain.
k) Although it is not labeled, what air mass dominates over the southwest United States?(Hint: See a previous lesson.) Give data to support your answer.
l) Name the stage of development of the cyclone over Hudson Bay. See Figure 9.6a on page 175 for stages of development.

3) With reference to the weather map below, answer the following:

a) Describe the characteristics of a Continental Arctic air mass. Give data from the weather map to support your answer.
b) What is the relative humidity at the station circled in red? Explain.
c) What cloud types are at the station circled in green?
d) What is the direction of the wind at the station circled in purple?
e) What is the air pressure at the station circled in yellow?

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