TOPICS:  Weather

LEVEL:  Middle school or high school

TIME:  Daily data collection for two or more weeks, one class period for discussion .

PREDICTION: In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students:

Heavy synoptic snowfall is most likely when the air pressure is falling, the winds are shifting counter-clockwise from the E or SE, and cirrostratus or altostratus clouds are thickening and lowering. Lake effect snow occurs when a mass of cold air moves over a large unfrozen lake. Pressure changes will depend on the location relative to the lake. Stratocumulus clouds are usually associated with lake effect snow .

PROCEDURE: You may wish to have different groups of students take responsibility for recording different weather parameters.  It is important to collect data at consistent times and places using the same type of instrument.  Be sure students know how to use weather instruments, are able to convert units of measurement, and identify cloud types.

QUESTIONS 3-12 require higher order thinking skills.  We recommend that you discuss these questions in class before having students write out their answers .

2. Temperature - thermometer or thermograph

Humidity - hygrometer or psychrometer

Air pressure - barometer or barograph

Wind speed - anemometer .

3. Probably not, except for very cold conditions (see question 11) .

4. The relative humidity (RH) near the ground may be high during snowfall; the RH in the cloud producing the snow is at least 100%.  Time of day and temperature are likely to have a greater influence on RH near the earth's surface than snowfall (see question 5) .

5. Yes.  The relative humidity goes up when there is a drop in temperature .

6. Snowfall is usually associated with approaching low pressure systems, except for lake effect snowstorms .

7. The relationship may vary.  Severe storms may have high winds and lots of snow.  In contrast, a stalled low pressure system or lake effect snow may result in heavy snows with low winds .

8. On snowy days the winds are usually from the NE to SE, occasionally from the SE to SW.

Wind direction with lake effect snow depends on your position relative to the lake .

9. Snow clouds are usually stratus, nimbostratus or stratocumulus.

QUESTIONS (Continued) .

10. Heavy synoptic snowfall is most likely when the air pressure is falling, the winds are shifting counter-clockwise from the E or SE, and cirrostratus or altostratus clouds are thickening and lowering.  Lake effect snow occurs when a mass of cold air moves over a large unfrozen lake.  Pressure changes will depend on the location relative to the lake.  Stratocumulus clouds are usually associated with lake effect snow .

11. Cold air holds less water vapor than warm air, so snowfall tends to be very light in extreme cold.  For example, saturated air can only hold 1/6 as much water at -20^oC as at 0^oC; only 1/16 as much at -30^oC and only 1/47 at -40^oC .

12. Storms can change direction or speed at the last minute, and sometimes conditions causing storm intensification are difficult to predict.

DATA SHEET:  Changes in pressure, wind direction and cloud type may be associated with snowfall.  Temperature will determine if precipitation is rain or snow.  If you are in a lake effect snow area, you may get two separate sets of conditions, one for synoptic snow and one for lake effect snow.

Data sheet courtesy of Daniel Barstow (Public Schools of Hartford, Connecticut). (1987). Weatherwise 40:146-148.