Forecasting Precipitation
scaffolding activity teacher's guide

1) Moisture is required for precipitation to develop. As a parcel of air rises, water vapor will cool and condense, forming tiny water droplets. If there is a lack of moisture, there will not be enough water droplets to form a cloud, let alone precipitation.

2) They are lifting mechanisms, responsible for generating upward motion necessary for precipitation to develop.

3) An air parcel cools as it rises because it expands due to decreasing pressure with height. This expansion causes the air to cool and consequently the water vapor condenses into tiny water droplets. With continued condensation, clouds develop and eventually precipitation.

Weather Scenario Precipitation?
Example Scenario:
Boulder, CO, a city on the east side of the Rockies. Downslope winds are expected.
Unlikely Downslope winds (or wind blowing down the mountain) tend to be very dry, warming as it descends, creating an unfavorable environment for the development of precipitation (since rising air in the presence of downslope winds is unlikely).
Scenario 1:
A cold front is approaching from the west, but the air both ahead of and behind the front is very dry.
Unlikely The cold front is an important lifting mechanism, but there isn't enough moisture for precipitation to develop.
Scenario 2:
A warm front is approaching and the air behind and ahead of the front is very moist.
Likely With moisture present and lifting forced by "overrunning" of the warm front, these are favorable conditions for precipitation to develop.
Scenario 3:
Upslope winds are expected in Boulder, CO and the air has been very moist for the past couple of days.
Likely With moisture available, winds blowing up the mountain will provide the lifting of air (since air can't go through the mountain, it must go up and over), and this combination creates a favorable environment for the development of precipitation
Scenario 4: The trend for the latest batch of precipitation is a steady eastward movement of 30 miles/hour. The latest position is roughly 700 west of here. Will precipitation arrive within 24 hours? Likely Using mathematics, at 30 miles/hour the precipitation should move 720 miles over a 24 hour period, which means that it will have reached us by then (though just barely).

forecasting temps
Terms for using data resources. CD-ROM available.
Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.

Case Studies