Questions: Precipitation Types and Formation Processes

Question 1

Snow falls from a cloud, passes through a warm layer (above 0°C) aloft where it melts, then descends through a deep cold layer (below 0°C) near the surface. What precipitation type reaches the ground?

• A. Sleet — the melted drops refreeze into ice pellets while still in the air above the surface
• B. Freezing rain — the drops arrive as liquid and refreeze on contact with cold surfaces
• C. Snow — the cold near-surface layer causes the drops to re-solidify back into snowflakes
• D. Rain — once snow melts into liquid water, it cannot refreeze before reaching the ground

Answer: (A)

This temperature profile — warm layer aloft, deep cold layer below — produces sleet. The snow melts in the warm layer, then has sufficient time falling through the thick cold layer to refreeze into ice pellets before landing. Freezing rain (option B) requires a thin near-surface cold layer — too shallow for in-air refreezing, so drops arrive liquid and freeze on contact. The depth of the near-surface cold layer is the key variable distinguishing sleet from freezing rain.

Question 2

In a mixed-phase cloud containing both ice crystals and supercooled liquid droplets at the same subfreezing temperature, why do ice crystals grow at the expense of the liquid droplets?

• A. The saturation vapor pressure over ice is lower than over liquid water, so the air is simultaneously supersaturated with respect to ice and undersaturated with respect to liquid, causing ice to grow and droplets to evaporate
• B. Ice crystals fall faster than droplets and sweep them up by collision and coalescence
• C. Liquid droplets transfer latent heat to ice crystals on contact, freezing the droplets
• D. Ice crystals have greater surface area per unit mass than droplets, allowing faster vapor absorption

Answer: (A)

The Bergeron-Findeisen process exploits a physical asymmetry: at any given subfreezing temperature, equilibrium vapor pressure over liquid water is higher than over ice. When both phases coexist, air reaches a vapor pressure between the two equilibrium values — supersaturated with respect to ice (ice grows by deposition) but simultaneously undersaturated with respect to liquid water (droplets evaporate). Mass is transferred from liquid droplets to ice crystals through the vapor phase. Option B describes collision-coalescence, which is a separate mechanism operating in warm clouds.

Question 3

Sleet and freezing rain are essentially the same phenomenon — both occur when precipitation refreezes near the surface.

• T. True
• F. False

Answer: False

Sleet and freezing rain differ in where refreezing occurs. Sleet (ice pellets) forms when melted precipitation refreezes in the air before reaching the ground — the near-surface cold layer is deep enough to accomplish this. Freezing rain forms when that cold layer is too thin for in-air refreezing; drops arrive as liquid and freeze on contact with cold surfaces. This distinction matters practically: freezing rain creates invisible glaze ice on roads and power lines, making it far more dangerous than the visible pellets of sleet.

Question 4

Large raindrops falling from mature storm clouds are typically teardrop-shaped — pointed at the top and rounded at the bottom due to air resistance.

• T. True
• F. False

Answer: False

Despite being the intuitive image, large raindrops are not teardrop-shaped. Small drops are nearly spherical; larger drops are flattened into oblate spheroids (like a hamburger bun) due to air pressure on the falling drop's underside. True teardrop shapes do not occur — drops that grow too large break apart into smaller spherical drops rather than maintaining a teardrop form. The teardrop misconception is perpetuated by diagrams and logos, not by physics.

Question 5

Why is freezing rain generally more hazardous than sleet, even though both involve precipitation associated with subfreezing temperatures near the surface?

Model answer: Freezing rain arrives as liquid water and freezes on contact with objects, forming a continuous, transparent glaze of ice that coats roads, sidewalks, power lines, and tree branches. This glaze is difficult to see (especially 'black ice' on pavement) and extremely slippery. Sleet arrives as already-frozen ice pellets that bounce and pile up — still slippery, but more visible and less prone to uniformly coating surfaces. Freezing rain also accumulates weight on power lines and branches, causing widespread infrastructure damage from line breaks and falling trees.

The physical difference — liquid that freezes on contact versus solid pellets — produces entirely different hazard profiles. Freezing rain creates a uniform ice surface across all exposed infrastructure simultaneously, while sleet creates discrete accumulations. Major ice storms, like the 1998 North American Ice Storm that devastated the Canadian power grid, are caused by extended freezing rain, not sleet — illustrating how thin, invisible ice layers can cause catastrophic damage well beyond their apparent severity.