Chapter 7 Flashcards
What is the average diameter of condensation nuclei?
0.2 microns, smaller than visible light.
What is the average diameter of a cloud droplet?
20 microns, 100 times larger than condensation nuclei.
What is the average diameter of a raindrop?
2000 microns (2mm), 100 times larger than cloud droplets.
What is the curvature effect?
Smaller cloud droplets evaporate faster due to greater curvature, requiring a higher vapor pressure to remain stable.
What is the solute effect?
Hygroscopic particles like salt allow cloud droplets to form at RH < 100%, speeding up their growth.
What are the two main processes for precipitation formation?
Collision-Coalescence Process (warm clouds) and Bergeron Process (cold clouds).
What are the conditions for the Collision-Coalescence Process?
Cloud tops warmer than -15°C, occurs mainly in tropical regions, requires large drops falling faster than small drops.
What factors enhance Collision-Coalescence?
Electrically charged droplets, thick clouds (more collision time), strong updrafts (slows falling droplets).
What is the Bergeron Process?
Ice crystals grow by taking water vapor from supercooled droplets, most efficient at -15°C.
What is accretion?
Ice crystals collide with supercooled droplets, freezing on contact to form graupel/hail.
What is aggregation?
Ice crystals collide and stick together, forming snowflakes.
What is the required ice crystal ratio for precipitation?
1 ice crystal per 500,000 supercooled water droplets.
What are the different types of precipitation?
Rain, drizzle, snow, sleet (ice pellets), freezing rain, hail.
How does virga form?
Precipitation falls into low RH air and evaporates before reaching the ground.
What are the four main types of snowflakes?
Plate, Column, Dendrite (most common at -15°C), Needle.
What determines snowfall intensity?
Surface visibility, not snowfall rate.
What conditions allow snow to fall at above-freezing temperatures?
Wet-bulb temperature at freezing or below, or if snow falls faster than it melts.
What is sleet?
Rain that refreezes into ice pellets after passing through a shallow above-freezing layer.
What is freezing rain?
Rain that falls through a deep above-freezing layer and refreezes on contact with a subfreezing surface.
How does hail form?
Strong thunderstorm updrafts keep hailstones suspended, allowing them to accumulate layers of ice.
What are the two main types of rain gauges?
Standard rain gauge (funnel collector) and tipping bucket rain gauge (automated measurement).
What is the average snow-to-water ratio?
10:1 (10 inches of snow = 1 inch of liquid water), but can vary from 6:1 (wet snow) to 40:1 (dry snow).
What is cloud seeding?
Adding ice nuclei (e.g., silver iodide) to clouds to enhance precipitation using the Bergeron Process.
Does cloud seeding work?
Difficult to prove; some studies show 5–20% increased precipitation, but natural variations make results uncertain.
What was the National Hail Research Experiment’s finding on hail suppression?
Seeding may actually increase hail size due to excess supercooled water.
