processes increase precipitation developement Flashcards
increase raindrop diameter ________ number of raindrops per m3 of air of the same diameter
decreases
increase raindrop diameter ________ rain rate
increases
define terminal velocity
gravitational force = air resistance (friction)
what is the shape of a raindrop at terminal velocity
parachute like shape. the base is pushed up.
define precipitation
any form of water (liquid of solid) falling from the sky
other than condensation, droplets can also grow by collection (in warm clouds). define the terms collision and coalescence involved in collection
- collision: when larger droplets with higher velocity catch up with smaller droplets with lower velocity. they collide and become a larger droplet
- coalescence: droplets combine to become one due to opposite charges
what is low collection efficiency?
Not all droplets will collide and stick
what causes low collision efficiency
smaller droplets may be swept past the larger droplet
what causes low coalescence efficiency
droplets can bounce or re-separate after collision (usually due to identical charges)
the curve of growth by collection increases ___________
exponentially
how come only condensation is involved initially with the growth of droplets?
- droplets have to grow to a certain size that it can have velocity before collection can happen.
- time is needed for droplets to fall and collide with other droplets, so it can’t start immediately
define continuous collection
Droplets are uniformly distributed in space so can be collected uniformly at the same rate by all collector drops of a given size
define Stochastic collection
droplets are not uniformly distributed in space, therefore collisions are individual events, distributed statistically in time and space
what is the Cascade process of raindrops
continuous process where small droplet grows to be large droplets. large droplets then break and become small droplets again. this cycle continues
why raindrops reaching the surface rarely exceed 5 mm in diameter?
- Collisions between large raindrops break them into smaller drops.
- surface tension is too weak to hold the larger drops together
If RH below the cloud is high, then the droplets will arrive at the ground as ________
drizzle
what would happen to the raindrop if the cloud base is high?
drops will evaporate before reaching the surface
why do thicker clouds allow raindrops to grow to a greater size
Thicker clouds, formed by convective motion, can have stronger updrafts (due to convection motion )and can keep larger cloud droplets aloft, permitting them to join (coalesce) with more droplets and grow to greater sizes
describe homogenous nucleation (pure water) of both deposition and freezing in cold clouds
- deposition (vapour to ice): very unlikely to happen, temp has to be lower than -60C so the molecules have very low energy and stick together to form ice crystals
- freezing (liquid to ice): water molecules within the droplet must gather rigidly to form an ice embryo, large enough to survive and grow
- both very improbable
true or false:
water have a larger change to freeze at 0C if it is in a large volumn
true.
large volume means more water molecules –> higher chance a few of the molecules will line up properly to become crystal structure
true or false heterogenous freezing (ice nuclei involved) happens at a higher temperature compared to homogeneously
true. as ice nuclei aid the freezing process. Most effective ones have a crystal structure similar to ice hence ice can form around and on them easily
define effective ice nuclei and give some examples.
- effective ice nuclei has a molecular geometry similar to the molecular structure of ice. This geometry helps water molecules in the surrounding air to align in the molecular structure for forming ice when they deposit on the surface of the nucleus
- Clay minerals
- some bacteria from decaying plants
how come ice crystals and supercooled water can coexist at the same time when there is a relative sparseness of ice nuclei
- low ice nuclei population means not all supercooled water can form on the ice nuclei and be ice crystals. therefore they can coexist
- this allows Bergeron process to happen
4 ways ice particles can be formed and describe them briefly
- Heterogeneous Deposition - vapour transformed to ice on a nucleus
- Condensation followed by Freezing - droplet forms on a nucleus which freezes
- Contact - nuclei makes contact with a droplet which then freezes (airplane wing)
- Immersion - nuclei becomes immersed in a droplet which then freezes
true or false:
the saturation vapour pressure is lower in ice compared to supercooled water at same temperature ( for temperature below 0C)
true. this is because ice has larger volume and larger surface tension than water therefore ice has a lower pressure–> lower water vapour and saturation vapour pressure
true or false:
vapour pressure difference between ice and supercooled water is greatest at -12C
true. it is where freezing and deposition happens most frequent
what is the Bergeron process
- Bergeron process happens since the svp of supercooled water is higher than the svp of ice, which means that diffusion can happen due to the pressure gradient
- this diffusion allows the ice crystals to grow as the water molecules from the water moves to the ice
- this happens when ice and supercooled water both coexist together.
Bergeron process explains how precipitation consisting single ice crystals can be formed. however, for larger precipitation to be formed, ice crystal have to grow by accretion or aggregation. define these two terms
- Accretion: ice particles collide with supercooled water droplets, which freeze upon contact. (forms snow pellets/ graupels)
- Aggregation: collisions and aggregations of ice particles falling at different speeds (forms snowflakes, may melt to become rain)
Shape of snow crystals depends on _________ and ________
- temperature
- humidity
true or false:
clouds can be a water/ice mixture
true
what are the 4 types of precipitation and describe their formation from snow.
- snow: air temp is below 0C the whole time –> no ice melts
- sleet (ice pellet): the snow meets a shallow warm layer high above the surface –> ice partially melts, but the melted supercooled water refreezes afterwards.
- freezing rain: the snow meets a deep warm layer –> all ice melts completely. however near the surface, the supercooled water freezes again via contact
- rain: air temp is above 0C the whole time –> all ice melts.
