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Forecasting Factors Affecting Precipitation with Atmospheric Stability Details

Learn about the factors influencing precipitation, such as moisture supply, frontal position, and atmospheric stability. Understand how cloud formation and precipitation are related to air movement and stability in the atmosphere.

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Forecasting Factors Affecting Precipitation with Atmospheric Stability Details

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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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