Precipitation%20I - PowerPoint PPT Presentation

About This Presentation
Title:

Precipitation%20I

Description:

... atmosphere: determines the type of clouds ... Thick clouds are associated with strong ... 'Cloud seeding and precipitation' and 'Precipitation in clouds' ... – PowerPoint PPT presentation

Number of Views:57
Avg rating:3.0/5.0
Slides: 17
Provided by: konstanti
Learn more at: http://www.phys.ufl.edu
Category:

less

Transcript and Presenter's Notes

Title: Precipitation%20I


1
Precipitation I
2
RECAP
  • Moisture in the air (different types of
    humidity).
  • Condensation and evaporation in the air (dew
    point).
  • Stability of the atmosphere determines the type
    of clouds
  • Cloud formation moist air rises up in the
    atmosphere where it expands adiabatically, cools
    down, saturates and forms clouds.
  • Chapter 7 precipitation. Any form of water
    (liquid or solid) that falls from a cloud and
    reaches the ground.

3
From droplets to raindrops
  • Typical sizes (diameter)
  • Condensation nuclei 0.2 mm
  • Cloud droplet 20 mm
  • Raindrop 2000 mm
  • The cloud droplets need to grow
  • in order to become raindrops!
  • Growth is determined by the balance of
    condensation (C) and evaporation (E)
  • CgtE the droplet grows
  • CltE the droplet gets smaller
  • CE the droplet stays the same
  • (in equilibrium), hence
  • Saturation (equilibrium) vapor pressure

4
The growth of cloud droplets
  • Curvature effect The saturation water vapor
    pressure depends on the curvature of the water
    surface.
  • The larger the curvature the easier it is for the
    water molecules to leave the surface of the
    liquid water.
  • The saturation vapor pressure for small droplets
    is higher therefore they require more vapor to
    keep their size
  • Bottom line the smaller the droplet, the more
    difficult it is to grow. Small droplets dont
    make it as raindrops.

5
The growth of cloud droplets
  • If small droplets were to survive and grow, they
    require supersaturation of the air and the help
    of condensation nuclei
  • Condensation nuclei jump start the formation of
    the cloud droplets.
  • Hygroscopic nuclei condensation begins at
    RHlt100.
  • Decrease the starting curvature of the initial
    droplet.
  • Solute effect salt particles partially dissolve
    in the water and decrease the saturation vapor
    pressure.

6
From droplets to raindrops
  • Condensation by itself is a very slow process,
    cannot produce raindrops, only cloud droplets
    (20 mm).
  • The cloud droplets may stay suspended in the air
    by air currents for a long time
  • Those which happen to descend below the cloud,
    evaporate and dont make it to the ground
  • There must be other ways to grow raindrops
  • Collision-coalescence process first must discuss
    how droplets fall.
  • Ice-crystal (Bergeron) process

7
Terminal Velocity
  • Gravity makes things fall, constant force -gt
    acceleration
  • Air-drag force increases with the speed of the
    falling object
  • Eventually gravity and air-drag become equal -gt
    no net force -gt the body falls at constant speed.
  • This velocity is called terminal velocity.
  • The terminal velocity depends on
  • Shape of the body
  • Size of the body
  • Mass of the body
  • Air properties (density).

8
Terminal Velocity-Examples
Falling object Mass Area Terminal velocity
Skydiver 75 kg 0.7 m2 60 m/s 134
mi/hr Baseball (3.66 cm) 145 gm 42 cm2 33
m/s 74 mi/hr Golf ball (2 cm) 46 gm 14
cm2 32 m/s 72 mi/hr Hail stone (0.5 cm
radius) .48 gm .79 cm2 14 m/s 31 mi/hr
Raindrop (0.2 cm radius) .034 gm .13 cm2
9 m/s 20 mi/hr
m mass, g gravity acceleration, C-shape
coefficient of proportionality, r-air density, A
- area of the cross section of the body, v-
velocity
9
(No Transcript)
10
Collision and Coalescence Processes
  • Larger drops fall faster, overtake and absorb
    smaller drops
  • coalescence
  • Larger drops grow faster than smaller drops.
  • Larger drops are the first to hit the ground,
    the first raindrops are very large and heavy.
  • Smaller drops evaporate before they reach the
    ground.
  • Smaller drops are absorbed on the way down

11
Droplet growth in warm clouds
  • Warm clouds above freezing temperature
  • Thick clouds are associated with strong updraft
    winds.
  • This maximizes the time the droplet spends in the
    cloud -gt more time for coalescence to take place
  • Thick clouds produce large rain drops.
  • Thin clouds produce at most a drizzle.

12
Cold clouds
  • The temperature of a cold cloud drops below the
    water freezing point.
  • Below 0 deg C the cloud water droplets are
    supercooled.
  • The smaller the droplet, the lower the
    temperature at which it will freeze.
  • Below -40 deg C almost all droplets freeze and
    form ice crystals.
  • Small particles in the air serve as ice nuclei
  • deposition nuclei
  • freezing nuclei
  • contact nuclei

13
Saturation Vapor Pressure
saturated
  • The saturation vapor pressure Ps, depends on the
    temperature. It increases with temperature.
  • Ps over water is larger than it is over an ice
    surface at the same temperature.

Super-saturated
unsaturated
14
Ice Crystal Particles
  • There are many more condensation nuclei than ice
    nuclei -gt there are many more water droplets than
    ice crystals in the cloud
  • How does snow form?
  • The saturation vapor pressure above a water
    surface is larger than the saturation vapor
    pressure above an ice surface.
  • Water molecules evaporate more easily than ice
    molecules

15
Ice-crystal (Bergeron) process
  • Water vapor molecules migrate towards the ice
    crystals.
  • Cloud ice crystals grow at the expense of the
    water droplets.

16
Ice crystals in the clouds
Accretion
Fracture
Aggregation
Skip the sections Cloud seeding and
precipitation and Precipitation in clouds
Write a Comment
User Comments (0)
About PowerShow.com