3: Clouds, Precipitation, and Weather Radar Flashcards
What is cloudy air
- water vapour is invisible
cloudy air is condense liquid water droplets or sublimated ice crystals
contrails form when hot humid exhaust from jet engines mixes with cold, drier air
what is utility classification
- all clouds consist of water substance
- there is a continuum of different cloud bae heights
- classified by: high, middle, low
Types of clouds and their altitudes
High: 5000-13000
Cirrus
Cirrostratus
Cirrocumulus
Middle: 2000-7000
Altostratus
Altocumulus
Low: 2000-surface
Stratus
Nimbostratus
Stratocumulus
Clouds with vertical development: to 3000
Cumulus
Cumulonimbus
Cirrus clouds
- when the sky is very clear, wispy and looks like cotton balls
- consist of ice crystals
Cumuliform clouds
Cumulonimbus: indication of very violent weather
Cumulus: fair weather
Cumulus congestus: towering cumulus. Some precipitation
Mountain waves or gravity waves
- North-south mountain range deflects west wind into an up down wave like pattern called lee waves
- clouds form in crests (tip) where ascending air expands and cools
- Clouds dissipate in wave troughs where descending air is compressed and warms. When this happens energy breaks which results in clear air turbulence (CAT0
What are clouds
aggregate of tiny water droplets and/or ice crystals
What are aerosols and their importance
Aerosols are solid and liquid particles suspended in the atmosphere that are key for cloud formation
- have radius > 1 micrometer and act as a nuclei vapouring condensation or deposition of water vapour to form cloud droplet —> called cloud condensation nuclei (CCN)
What is CCN and IN
- CCN: Cloud condensation nuclei —> aerosols that favour condensation of water vapour to form cloud droplet. Natural sources are dust from volcanic eruptions, wind eroded soil particles, particles released from forest fires, ocean salt spray, vehicular emissions
- IN: Ice forming nuclei —> promote ice crystal formation under sub zero temperatures
What is terminal velocity
- how fast the particles fall from the sky: the heavier the faster
Ex (from small to large): cloud droplets, drizzle drops, rain drops, hail
drag and gravitation force
Drag force: pulls everything up
Gravitation force: pulls everything down
balance between these two determine where particles are in the atmosphere
- this means that cloud droplet has to grow into a bigger size so it can fall
terminal fall velocity of ice crystal
plate shaped ice crystal: uniform, about 1m/s
two processes of precipitation formation
- Collision-coalescence process: warm-cloud precipitation formation
- Bergeron-Findeisen process: cold-cloud precipitation formation
Collision-coalescence process
- warm cloud precipitation formation
- a large droplet falls through a cloud of smaller droplets and collides with them. They merge, increasing the size of the larger droplet
Bergeron-Findeisen process
- cold-cloud precipitation formation
- precipitation falling in middle and high latitudes form in cold clouds
- ice crystals grow at the expense of supercooled water droplets because the SVP of ice is lower than the water at the same temperature
- These snowflakes/ice crystals may melt on their way down, producing rain
- few water droplets will freeze and whatever available supercooled water droplet will freeze over the surface of the crystal and make it bigger
7 Precipitation types
Rain: water droplets
Drizzle: smaller water droplets
Snow: loose aggregates of ice crystals
Sleet: partly melted snowflakes
Grapple: soft hail (liquid inside)
Hail: balls of ice
Freezing Rain: super-cooled rain drops that freeze on contact with cold surfaces
snowflake forms and types
- all snowflake forms are hexagonal (6 sided)
- snow forms depends on temperature
4 types:
Needles: 0C
Dendrites: -10C
Plates: -20C
Columns: -30C
what affects the look of a hailstone
depends on the environment they grow in
largest hailstone, mass, dimension, shape
mass: 220g
max dimension: 9cm
oblate spheroid
rough surface prior to melting
olds hailstorm
- August 11, 2003
- multiple supercells
- $10 million in hail damage
what is weather radar
remote sensing tool that helps in determining location, movement, and intensity of precipitation
- radar emits microwave signals and receives reflected signals from the targets (precipitates)
- radar antenna sits on tower and rotates around, rotation at different angle allows measuring of 3D rain field
-modern weather radar can detect the movement of violent weather
Mod of weather radar
- reflectivity mode
- velocity mode
Reflectivity mode
radars emit pulses of microwave energy at wavelength between 10-11.1cm
- at this wavelength radar signals are reflected by rain, snow, or hail, and cannot properly detect cloud droplets or crystals
- reflected radar signals are processed as radar echo
- of above 40 db then that is heavy precipitation
Velocity mode
- doppler radar: works on the principle of doppler effect
- velocity detection capabilities
- stationary sound wave sources generates acoustics waves with uniform frequency
- a moving wave source generates wave pattern, with a frequency higher ahead of the source than behind of the source
how does doppler radar work
- captures the movement of precipitation particles moving towards and away from the radar
- the frequency oof radar signal shifts between emission and the return of the signal
- if the rain shower is stationary: frequency of returned echo = frequency of emitted signal
- if rain shower is approaching the radar: frequency of returned echo > frequency of emitted signal
- if rain shower is moving away: frequency of returned echo < frequency of emitted signal
