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Forecasting Assignment

Forecasting Assignment. Now available (links from class iLearn and backup Web pages) : Scores listed by student ID # (last four digits) Scores ranked by average of best 20 forecasts Item #13: Factors Affecting Precipitation moisture supply (relative humidity at 850 mb )

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Forecasting Assignment

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  1. Forecasting Assignment • Now available (links from class iLearn and backup Web pages): • Scores listed by student ID # (last four digits) • Scores ranked by average of best 20 forecasts • Item #13: Factors Affecting Precipitation • moisture supply (relative humidity at 850 mb) • frontal position (fronts within 140 miles) • atmospheric stability

  2. Atmospheric Stability • Most clouds (except most fog) form when air moves upward sufficiently far • Air cools as it rises and expands under lower pressure • Need sufficient cooling for water vapor to condense (and form cloud droplets) • Type of cloud depends on how easy it is for air to move upward • Upward motion resisted (must be forced): Smooth, extensive layers (stratiformclouds) • Upward motion easy (happens spontaneously): Puffy, individual clouds (cumuliform clouds) • Precipitation comes from some types of clouds • Atmospheric stability determines how easy it is for air to move vertically • Atmosphere stable: air resists vertical motion (must be forced) • Atmosphere unstable: air moves vertically spontaneously

  3. Atmospheric Stability (cont’d) • Stability of a layer of air depends partly on: • Temperature lapse rate in the layer • Amount by which atmosphere’s temperature decreases per unit of vertical distance • Measure of how rapidly temperature decreases with increasing altitude • Global average value: 6.5°C/kilometer (3.5°F/1,000 ft.) • Varies from place to place, time to time • Temperature inversions have negative lapse rate (very stable)

  4. Atmospheric Stability (cont’d) • Atmospheric stability often also depends on: • Whether or not air is saturated with water vapor • often, a layer is stable when air is not saturated but unstable when air becomes saturated • can saturate air by cooling it • can cool it by forcing it upward • expands under lower pressure, cools as a result

  5. Atmospheric Stability (cont’d) • Very rough measure of stability of lower troposphere (excluding next to surface): • Temperature difference between 850 and 500 mb • When less than ~25°C, air is more stable • When ~25°C or more, air is susceptible to becomin unstable • especially if forced up enough to cool enough to become saturated with water vapor • Consult forecast soundings or 500 and 850 mb forecast temperature maps

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