GES 121 - Precipitation and Lifts

For Precipitation to Occur

there needs to be growth of droplets, sufficient size to fall

almost all precipitation in mid-latitudes begins to fall from cloud as snow

cirrus clouds can seeds lower clouds with ice crystals

if colder than 4°C at the surface, most likely snow because not enough time to melt; if warmer, probably rain.

as it falls, rain will evaporate

Types of Precipitation

Rain:

• liquid droplets that reach ground, >.5mm

• likely to be continuous, intermittent called a shower

• usually starts as snow but melts on the way down

• don’t usually get larger than 7mm in diameter

Drizzle:

• liquid droplets that reach ground, <.5mm

• likely to be continuous

• tends to fall more slowly because of smaller particle size, and therefore will restrict visibility more than rain

Snow:

• ice crystals that have not melted

Sleet:

• frozen rain - opaque ice pellets

• melts from warm air, then falls through thick layer of cold air at the surface, which freezes solid

  • snow → (melting) → rain → (freezing) → sleet

Hail

• ice pellet, >5mm in diameter

• forms because of multiple passes through a cloud, number of passes determines size (and often collides with other hail stones)

Freezing Precipitation

• (rain or drizzle) that freezes with surfaces at or near the ground

  • the pure precipitation is supercooled in a shallow layer of cold air at the surface, and freezes as it hits the dirty surface

  • rime - dangerous, pure ice

    • can also be danger to plane wings when flying through supercooled clouds

Graupel / Ice Pellets (“soft hail”)

• crystalline/snow-like internal structure with ice coating around it

  • fluffy inside, crunchy outside. yum!

• partial melting and coalescence of snow with refreezing

Intensity of Precipitation

may fall either:

• continuously

  • longer period, ~hour or more

• intermittently

  • shorter period, limited in both time and space

  • shower as rain, flurry as snow

may be described as:

• very light (slight)

• light

• moderate

• heavy 

Lifting Mechanisms

lifting humid air to greater heights (cooler temperatures) is essential for the formation of precipitation

Lift → Condensation → Cloud → Precipitation

4 Types of Lift Mechanisms

1. Convergent

2. Mechanical (Orographic)

3. Convective

4. Air Mass Frontal

Convergent Lift

air moving towards one another, towards particular point/line and then must go upwards

typically large scale, surface low pressure

• Intertropical Convergence Zone (ITC)

• often what drives tropical storms (hurricanes/typhoons/cyclones)

Mechanical (Orographic) Lift

“oro” means mountain

over top of landscape barriers, adds energy to clouds, SALR

• windward slope (faces wind) tends to be a wet slope - “rain shed”, warm/moist

• lots of the precipitation is removed as it travels up, but the energy/heat is still there

• on the leeward slope, it descends adiabatically into warming air, into the “rain shadow”/sheltered side, hot/dry

ex. Victoria Island gets 3 meters of rain a year (very wet), interior of BC is very dry (semi-arid)

Convective Lift

surface heating, density differences, rising thermals of air

very few cumulus clouds last more than 20 minutes (because cuts off insolation of the surface, so can’t get more latent heat energy from there)

• but if there’s lots of Lift, latent heat input with moisture within the cloud keeps it lifting, so last longer

Air Mass Frontal Lift

based on concept of air masses

air mass: region of relatively homogeneous air (so all one character), described by its temperature and moisture content (fairly uniform temp, humidity, stability)

• forms because air sits over a surface for a while (at least a few days) and takes on those characteristics

  • ex. Gulf of Mexico → warm/moist

  • ex. Northern Canada winter → cold/dry

• not static, moves around

at some point, air masses will meet, and forms an air mass front

• front: boundary between two dissimilar air masses