CH 14 Precipitation

14 Precipitation

14.1 Introduction

  • Definition of Precipitation:

    • Precipitation is described as any of the forms of water particles, whether they are liquid or solid, that fall from the atmosphere and reach the ground.

  • Chapter Overview:

    • This chapter focuses on the ingredients necessary for the formation of precipitation, the processes involved in its growth, and the various types of precipitation that can occur.

  • Types of Precipitation:

    • Examples of precipitation include:

    • Drizzle

    • Rain

    • Freezing rain

    • Freezing drizzle

    • Snow

    • Snow grains

    • Ice crystals

    • Ice pellets

    • Hail

    • Small hail and/or snow pellets

14.2 Necessary Ingredients for Formation

  • Three Essential Ingredients for Precipitation Formation:

    1. Water vapor

    2. Sufficient lift to condense the water vapor into clouds

    3. A growth process that allows cloud droplets to increase in size and weight until they can fall to the ground as precipitation

  • Cloud Thickness for Significant Precipitation:

    • For significant precipitation, clouds must typically be at least 4,000 feet thick.

    • Heavier precipitation usually correlates with thicker clouds.

  • Expectations Related to Aviation:

    • When arriving or departing from an airport reporting light or greater intensity precipitation, it's likely that clouds are more than 4,000 feet thick.

14.3 Growth Process

  • Cloud Composition:

    • All clouds consist of water, but only certain clouds produce precipitation.

    • Many cloud droplets and/or ice crystals are too small and light to contribute to precipitation.

  • Rate of Cloud Droplet Descent:

    • The descent rate of an average cloud droplet from a cloud base at 3,300 feet (1,000 meters) is very slow, taking approximately 48 hours to reach the ground.

    • During this time, the droplet would likely evaporate before reaching the surface.

  • Growth Processes:

    • Two primary processes contribute to the growth of cloud droplets or ice crystals large enough for precipitation:

    1. Collision-Coalescence Process:

      • Described as the warm rain process.

      • Occurs when collisions between cloud droplets of various sizes and fall speeds lead to coalescence, forming larger droplets.

      • Once droplets become too large to remain suspended, they fall as rain.

      • This process primarily occurs in warm, tropical air masses where the freezing level is notably high.

      • Larger cloud droplets help gather smaller droplets during their fall, facilitating growth.

    2. Ice Crystal Process:

      • Takes place in colder clouds containing both ice crystals and water droplets.

      • Water vapor tends to deposit onto ice crystals, enabling their growth at the expense of surrounding water droplets.

      • Eventually, ice crystals accumulate enough mass to fall; when the conditions near the surface are cold, snowfall occurs, but if warmer, the snowflakes may melt into rain.

      • This process is common in mid-latitude and high-latitude regions.

14.4 Precipitation Types

  • 14.4.1 Snow:

    • Condition for Snow:

    • Snow occurs when atmospheric temperatures remain below freezing at all levels throughout the depth of the atmosphere.

  • 14.4.2 Ice Pellets:

    • Formation of Ice Pellets (Sleet):

    • Occurs when there is a shallow layer of above-freezing temperatures above a deep layer of below-freezing air at the surface.

    • Snow falls into the warm layer, partially melting, and as it reenters the cold air, it refreezes into pellets.

  • 14.4.3 Freezing Rain:

    • Definition and Conditions for Freezing Rain:

    • This type of precipitation occurs when warm air is present above a layer of cold air near the surface.

    • Typically requires a temperature inversion with warmer air over a colder mass, often found along warm fronts.

    • Initial precipitation may start as snow or rain but becomes rainfall in warmer layers; it subsequently freezes upon contact with cold surfaces, forming a layer of ice.

  • 14.4.4 Rain:

    • Conditions for Rain Formation:

    • Rain results from a deep layer of above-freezing air at the surface.

  • 14.4.5 Hail:

    • Definition of Hail:

    • Hail consists of balls or irregular lumps of ice that develop in thunderstorms characterized by significant updrafts, high updraft content, large supercooled liquid water content (SLWC), and great vertical cloud heights.

  • Hail Formation Process:

    • Hail forms when supercooled water droplets freeze once they exceed the freezing level; added droplets can fuse to form larger hailstones.

    • Severe thunderstorms can produce large hailstones, which can reach sizes greater than a softball (4.5 inches in diameter).

    • A notable example includes a hailstone collected in Vivian, SD, which measured 8 inches in diameter, 18.62 inches in circumference, and weighed 1.93 lb.

  • Hail and Aviation Hazards:

    • Hail poses a significant threat to aircraft and can cause major damage, especially when stones exceed 0.75 inches in diameter.

    • Hail often occurs several miles away from thunderstorm activity, affecting flight safety and control.

  • Geographic Distribution of Hail:

    • Hail occurs mainly in mid-latitudes, commonly found in areas like the Great Plains region of the United States.

    • Hail tends to form at higher elevations where freezing levels are more prevalent.

  • Hail Fall Patterns:

    • From the air, hail falls in regions known as hail swaths, which can vary significantly in size from a few acres to 10 miles wide and 100 miles long.

  • Snow Plowing of Hail Swaths:

    • Hail accumulation can be significant enough that snowplows are needed for removal in some circumstances.