Chapter 8 - Notes Flashcards
What does Supercooled **mean?* ***
Water in the liquid state below 0°C (freezing)
Pure water suspended in air does not freeze until it reaches a temperature of nearly -40°C
**Supercooled **water will rapidly freeze if it impacts an object
*Freezing rain, *or *glaze *is supercooled water.
What are Freezing Nuclei?
Supercooled droplets will freeze on contact with solid particles that have a crystal form closely resembling that of ice. These materials are called **Freezing Particles. **
- When a cloud is at temperatures between 0 and -10°C, clouds consist of?
- -10°C and -20°C?
- Below -20°C?
- Supercooled water droplets
- Liquid droplets coexist with ice crystals
- Entirely of ice crystals
Is the **Saturation Vapor Pressure **above ice crystals lower or higher than above supercooled liquid droplets?
Lower
This occurs because ice cyrstals are solid, which means that the individual water molecules are held together more tightly than those forming a liquid droplet. As a result, it is easier for water molecules to escape from the supercooled liquid droplets. Consequently, when air is saturated with respect to liquid droplets, it is Supersaturated** **with respect to ice crystals.
At -20°C when the relative humidity is 100% with respect to water, what is the relative humidity with respect to ice?
At -10°C when the relative humidity is 100% with respect to water, what is the relative humidity with respect to ice?
At 0°C when the relative humidity is 100% with respect to water, what is the relative humidity with respect to ice?
121%
110%
What is the Bergeron Process?
The Bergeron process can produce precipitation throughout the year in the middle latitude, providede at least the upper portions of clouds are cold enough to generate ice crystals.
The type of precipitation that reaches the ground depends on the temperature profile in the lower few km of the atmosphere.
When the surface temperature is above 4°C, snowflakes usually melt before they reach the ground and continue their descent as rain.
What is the Collision-Coalescence Process?
When there is a large cloud droplet, formed when “giant” condensation nucei are present, or when hygroscopic particles exist. The large cloud droplet falls at a greater rate than the other cloud droplets, as it falls it collides with more droplets, and grows. When it grows to a size of 5 millimeters it breaks into smaller drops.
After collecting a million or so cloud droplets, they are large enough to fall to the surface without evaporating.
Updrafts aid this process because they allow the droplets to traverse the cloud repeatedly, colliding with more droplets. Clouds that have great vertical thickness and contain large cloud droplets have the best chance of producing precipitation.
Collision does not guarantee coalescence. Experimentation has indicated that the presence of atmospheric electricity may be the key to what holds these droplets together once they collide. If a droplet with a negative charge should collide with a positivly charged droplet, their electrical attraction may bind them together.
Working with the Bergeron process, the Collision-Coalescence process may contribute to the precipitation from a large cumulonimbus cloud – particularly during the hot, humid summer months. High in these towers the Bergeron process generates snow that melts as it passes below the freezing level. Melting generates relatively large drops that starts the CC process.
Name the different Types of Precipitation
- **Mist, **Droplets large enough to be felt on the face when air is moving 1 meter/second. Associated with stratus Appr. Size: 0.005 to 0.05 mm
- **Drizzle. **Small uniform drops that fall from stratys clouds, generally for several hours. Appr. Size: Less than 0.5 mm
- **Rain. **Generally produced by nimbostratus or cumulonimbus clouds. When heavy, size can be highly variablee from one place to another. Appr. Size: 0.5 to 5mm
- **Sleet. **Small, spherical to lumpy ice particles that form when raindrops freeze while falling through a layer of subfreezing air. Because the ice particles are small, any damage is generally minor. Sleet can make travel hazardous. Appr. Size: 0.5 fo 5mm
- **Glaze. **Produced when supercooled raindrops freeze on contact with solid objects. Glaze can form a thick coating of ice having sufficient weight to seriously damage trees and power lines. Appr. Size: Layers 1mm to 2cm thick
- **Rime. **Deposits usually consisting of ice feathers that point into the wind. These delicate frostlike accumulations form as supercooled cloud or fog droplets encounter objects and freeze on contact. Appr. Size: Variable accumulations
- **Snow. **The crystalline nature of snow allows it to assume many shapes, including six sided crystals, plates, and needle. Appr. Size: 1mm to 2cm
- **Hail. **Precipitation in the form of hard, rounded pellets or irregular lumps of ice. Produced in large convective, cumulonimbus clouds, where frozen ice particles and supercooled water coexist. Appr. Size: 5mm to 10cm or larger
- **Graupel. **Sometimes called “soft hail,” graupel forms as rime collects on snow crystals to produce irregular masses of “soft” ice. Because these particles are softer than hailstones, they normally flatten our upon impact.
How can you determine how many up/down trips a piece of hail made?
The central graupel or hail nucleus in the middle of the first clear layer occurs at the beginning of the first down trip. The middle of the first clear layer to the middle of the first rime layer is the first up trip, etc.
