Types of Precipitation: Sixth Grade Science
Precipitation is any liquid or frozen water that condenses in the atmosphere and falls to Earth under gravity, categorized into rain, snow, sleet, freezing rain, and hail.
The Physics of Falling Moisture
Precipitation occurs when cloud water droplets or ice crystals become too heavy to remain suspended by atmospheric updrafts. Inside a cloud, a typical cloud droplet has a microscopic diameter of only 0.02 millimeters (roughly 20 micrometers). A cloud droplet is so light that downward gravitational pull is balanced by rising air currents. For precipitation to fall, millions of cloud droplets must collide and coalesce (merge) into a raindrop with a diameter of roughly 2 millimeters—containing approximately one million times more water volume than a single cloud droplet!
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| THE FIVE MAJOR FORMS OF PRECIPITATION |
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| Precipitation Type | Physical State | Atmospheric Temperature Profile |
| | at Ground Impact | During Descent |
+---------------------+-------------------+-----------------------------------+
| Rain | Liquid droplets | Deep warm layer above freezing |
| | (0.5 to 5 mm) | (> 0 °C) all the way to ground |
+---------------------+-------------------+-----------------------------------+
| Snow | Crystalline solid | Cold freezing layer (< 0 °C) |
| | ice flakes | continuous from cloud to ground |
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| Sleet (Ice Pellets) | Translucent hard | Snow melts in warm middle layer, |
| | frozen ice balls | then REFROZES into ice pellets in |
| | (< 5 mm) | deep freezing layer near ground |
+---------------------+-------------------+-----------------------------------+
| Freezing Rain | Supercooled liquid| Snow melts in warm layer; shallow |
| (Glaze Ice) | that freezes ON | freezing layer at ground; freezes |
| | contact with cold | instantly into lethal glaze on ice|
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| Hail | Layered ice balls | Convective cumulonimbus updrafts |
| | (5 mm to >15 cm!) | recycle ice through freezing zones|
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Atmospheric Temperature Profiles in Winter Precipitation
The crucial scientific factor that determines whether winter precipitation reaches the ground as snow, sleet, freezing rain, or rain is the vertical temperature profile of the lower atmosphere between the cloud base and the ground surface.
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| WINTER PRECIPITATION VERTICAL PROFILES |
| |
| RAIN SNOW SLEET FREEZING RAIN |
| |
| [ Cloud: Snow ] [ Cloud: Snow ] [ Cloud: Snow ] [ Cloud: Snow]|
| | | | | |
| =======+======= (0 °C) | =======+======= =======+======|
| | Warm Layer | | | Warm Layer | | Warm Layer ||
| | (Melts into | | | (Melts into | | (Melts into||
| | Liquid) | | | Liquid) | | Liquid) ||
| | | | =======+======= =======+======|
| | | | | Deep Sub- | | Thin Sub- ||
| | | | | Freezing | | Freezing ||
| | | | | (Refreezes | | (Freezes ON||
| | | | | into ICE) | | CONTACT) ||
| v (Liquid) v (Solid Flake) v (Ice Pellet) v (Glaze Ice) |
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Sleet Versus Freezing Rain: A Critical Distinction
People frequently confuse sleet and freezing rain, yet they represent completely different physical events:
- Sleet (Ice Pellets): Snow falls from a cloud, enters a warm atmospheric layer aloft, and melts into liquid raindrops. The drops then fall into a deep, sub-freezing layer of cold air near the ground. The cold layer is thick enough that the liquid drops have sufficient travel time to freeze solid into hard, bouncing translucent ice pellets before striking the ground. Sleet bounces off car hoods and windows with an audible clicking sound.
- Freezing Rain (Glaze): Snow falls and melts in a warm layer aloft, becoming liquid rain. However, the sub-freezing cold layer near the ground is extraordinarily shallow (often only a few hundred meters thick). The liquid raindrops do not have enough time to freeze in the air; instead, they become supercooled liquid. When these supercooled drops strike freezing cold roads, telephone wires, and tree branches, they freeze instantly on contact, coating everything in a heavy, glassy sheet of solid glaze ice. Freezing rain is the most dangerous winter weather hazard, snapping power lines and causing catastrophic highway black-ice pileups.
Hail: The Violent Product of Severe Thunderstorms
Unlike winter precipitation, hail is an exclusively warm-season phenomenon produced by severe cumulonimbus thunderstorms:
- Updraft Power: Towering supercell thunderstorms contain violent vertical updrafts exceeding 100 to 160 kilometers per hour.
- Formation Cycle: An ice pellet (graupel) is carried upward by powerful updrafts into the sub-freezing upper reaches of the cloud (-40 degrees Celsius), where supercooled water droplets freeze onto its surface, adding a layer of solid ice.
- Recirculation: The hailstone falls under gravity, is caught again by another violent updraft, and is hurled back into the freezing zone, adding another concentric layer of ice like the rings of an onion.
- Ground Impact: This cycle repeats until the hailstone grows too heavy for the storm's updrafts to support. A baseball-sized hailstone weighs roughly 150 grams and plummets to the ground at speeds exceeding 140 km/h, denting automobiles and destroying crops.
Exercises and Solutions
Exercise 1: Diagnosing Winter Precipitation Profiles. A meteorologist releases a weather balloon that transmits vertical temperature data during a December storm: Cloud altitude (3,000 m): -8 °C (snow forming); Middle altitude (1,500 m to 800 m): +4 °C; Surface layer (800 m down to ground): -5 °C. (a) Trace the physical phase changes of the precipitation as it descends through these three layers; (b) What type of precipitation will strike the ground?
Solution 1: (a) In the sub-freezing upper cloud (-8 °C), water vapor condenses into crystalline solid snowflakes. As the snowflakes descend into the warm middle layer (+4 °C), they absorb thermal energy and melt completely into liquid raindrops. As the liquid raindrops enter the deep, sub-freezing lower layer (-5 °C), they lose thermal energy. Because this freezing layer is deep (800 meters thick), the water droplets have enough time to refreeze into solid ice pellets before reaching the surface. (b) The precipitation that strikes the ground will be sleet (ice pellets).
Exercise 2: Analyzing Concentric Rings in Severe Hailstones. Following a severe summer thunderstorm, a homeowner slices a golf-ball-sized hailstone in half and examines it with a magnifying glass. The interior of the hailstone reveals five alternating concentric rings of clear ice and cloudy, milky-white ice. Explain what these concentric rings reveal about the hailstone's journey through the thunderstorm cloud.
Solution 2: The concentric rings reveal that the hailstone made multiple vertical trips through the thunderstorm cloud, carried by alternating cycles of powerful updrafts and gravitational falls. The milky, cloudy ice rings formed when the hailstone was in the super-cold upper cloud: supercooled water droplets froze so rapidly on contact that microscopic air bubbles were trapped inside the ice, giving it an opaque white appearance. The clear ice rings formed when the hailstone fell into warmer cloud zones where liquid water accumulated slowly across the surface, allowing air bubbles to escape before freezing into solid, transparent ice. The five distinct rings prove the hailstone was recycled through the cloud's updrafts and downdrafts five separate times before growing too heavy for the storm to suspend.