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Cancel the Cardinals Home Opener?!?

Lessons in Melting and Evaporation



Author:

Patrick S. Market
Department of Soil, Environment, and Atmospheric Sciences
University of Missouri–Columbia
marketp@missouri.edu

Abstract:

The St. Louis Cardinals are scheduled to play their home opener the next day and Megan Riley, a young meteorologist who works for a private weather consulting firm, is responsible for developing the weather forecast. It’s starting to look like she may need to change her current forecast for rain to snow. Students work in small groups to analyze information presented in each part of this multi-part dilemma case and, along the way, are asked to update their forecast: keep it as rain, or revise it to snow. The case has been used both in an introductory meteorology course and in a capstone course for seniors in atmospheric science.

Objectives:
  • Undertstand that temperature is a measure of energy.
  • Learn that energy transfer impact on water (vapor) has the opposite effect on the air.
  • Learn key weather forecasting concepts.
Keywords: Wet bulb temperature; dew point temperature; precipitation; rain; snow; sublimation; evaporation; condensation; freezing; melting; weather forecasting
Topical Area: N/A
Educational Level: High school, Undergraduate lower division, Undergraduate upper division
Formats: PDF
Type/Method: Dilemma/Decision, Interrupted
Language: English
Subject Headings: Climatology / Meteorology   Atmospheric Science   Earth Science  
Date Posted: 04/24/05
Date Modified: N/A
Copyright: Copyright held by the National Center for Case Study Teaching in Science, University at Buffalo, State University of New York. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.

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We received a comment from a teacher, who wrote that:

Some of my students were working on an assignment using the internet and saw a picture of phase changes on your site. It shows a cloud as being the result of sublimation and evaporation. They thought that it was incorrect since we had learned that clouds are the result of condensation or deposition, and are composed of either ice or liquid water droplets.

So, I checked out the image, and they were correct! The image shows clouds depicted as water vapor, though clouds are actually liquid or ice.

Sorry to seem nit-picky, bu I had to point that out!

We asked the author, Patrick Market, about this. His reply:

What is happening in this instance is not all that common but certainly does hold with the usual "textbook" method of cloud creation. The parent cloud is the result of the more typical process of air being lifted, being cooled, and having water vapor condense into liquid droplets (what we start with in Figure 4 in the case study).

What happens in Figures 5 and 6 is different. Snow is falling into a dry layer beneath the cloud base. Two things happen: (1) the snow crystals shrink (sublimate) as water vapor molecules break free of the crystal lattice in which they had been trapped, and (2) the process described in (1) takes energy from the air in order to invigorate the newly liberated water vapor molecule. And these two processes lead to a new environment beneath the original (Figure 4) cloud base: one where the air temperature becomes cooler (and closer to its dew point) and the humidity is increasing (because of the sublimated water vapor). The sublimation leads to cooling and increased humidity. Eventually, saturation is attained, and the cloud builds downward toward the surface.


Editor
National Center for Case Study Teaching in Science
University at Buffalo
Buffalo, New York

1/8/2008




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