Clouds Flashcards
What is a cloud?
A cloud is formed from lots of small cloud droplets or ice crystals.
What kind of air are clouds formed in?
saturated air
What does CCN stand for?
cloud Condensation nucleus
Why do clouds require CCN
to grow they require CCN, growth occurs by deposition of water molecules from vapour
How big are cloud droplets typically
10 - 30 nano meters in diameter, growth and evaporation can occur within a few 10s of secconds
What temp to ice clouds form at?
ice clouds typically don’t form until well below 0 degrees c
what do ice crystals require to form
ice nuclei (IN)
are ice nuclei as abundant as CCNs
No they are much less abundant
If IN are not there what temp does ice spontaneously form
-40 degrees C
Explain the process of Ice clouds
Often there are just a few ice crystals, which then grow very fast, “stealing” all the available water, and falling out of the cloud as they grow large.
How large are raindrops ?
typically .5-5mm in diameter, growth from the vapour would take several hours - longer than lifetime of typical convective clouds
explain how ice crystals provide a more effective process of forming rain
Saturation vapour pressure over ice is less than that over water ice crystals grow at expense of water droplets
If ice crystal touches a cold droplet, the droplet freezes
Once large enough, ice crystals – or clumps of crystals – fall past droplets and collect them. Rapid growth of soft hail pellets (graupel) by riming.
Graupel falls from cloud, melting before reaching the surface as rain
Cloud Classification - cirrus
fibrous or hair-like (ice clouds)
Cloud Classification - cumulus
a heap or pile
Cloud Classification - stratus
a horizontal sheet or layer
