EASC 11 Clouds and Precipitation Fill in your note outline as you follow along with clouds and precipitation…. Water exists in all three states of matter in the atmosphere – solid (hail, ice particles, snow), liquid (rain, cloud droplets), gas (water vapour), and changes between these states evaporation: when liquid becomes gas condensation: when gas becomes liquid freezing: when liquid becomes solid melting: when solid becomes liquid sublimation: when solid becomes gas without passing through liquid phase

Molecules of liquid water are always in motion EASC 11 Water vapour is spread throughout the troposphere by convection currents and winds – there is little water vapour above the troposphere because there are no rising air currents to carry water vapour Molecules of liquid water are always in motion Air’s capacity for holding water vapour roughly doubles for every 11° C increase in temperature Saturated air has a relative humidity of 100%

EASC 11 Specific humidity is the amount of water vapour actually in the air Relative humidity compares the actual amount of water vapour in the air (specific humidity) with the maximum amount of water vapour the air could hold at that temperature Relative humidity is calculated as follows: Relative humidity = specific humidity x 100% maximum capacity (@ that temp) E.g.: if specific humidity = 11 maximum capacity = 22 RH = 11/22 x 100% = 50%

EASC 11 Instruments used to measure relative humidity include a hygrometer and psychrometer Clouds are simply high mists, fogs, or hazes Clouds form when water vapour is cooled below its dewpoint – water condenses on “condensation nuclei”, which are tiny particles in the air such as dust

EASC 11 If there are no condensation nuclei, air can be cooled below the dewpoint with no condensation, and air is said to be supersaturated The shape of clouds show how air is moving in stratiform (layered) clouds, air movement is mainly horizontal; in cumuliform clouds, air movement is mainly vertical At temperatures above 0°C clouds consist of only liquid water; from 0° to -18°C clouds consist of liquid water and ice below -18°C clouds consist only of snow and ice

EASC 11 There are four cloud locations or “families” high clouds middle clouds low clouds Clouds with vertical development There are four cloud structures stratus – layers or sheets cumulus – clouds with vertical development cirrus – thin, feathery, tufted “nimbus” clouds are literally rain clouds

EASC 11 Fog is a cloud that cannot fly, and forms when a surface layer of air cools below the dewpoint and water vapour condenses at the surface of the Earth Radiation fog (ground fog) forms when the night time sky is clear after a warm day: the ground radiates heat, winds mix the cold air with surface air, and this layer cools below the dewpoint (occurs often in late summer/early fall) Advection fog forms when warm moist air blows off water and over much colder land – this is the fog common in the Maritimes (e.g., Newfoundland & Labrador) and in California (e.g., San Francisco)

EASC 11 Precipitation is the falling of any form of water to Earth’s surface; occurs when cloud droplets grow into drops heavy enough to fall In warm clouds condensation forms tiny droplets then grow by bumping into and combining with other droplets In ice processes, supercooled water droplets evaporate and are deposited on ice crystals, which will fall when they become heavy enough If the temperature in the lower part of the cloud is warmer, the ice crystals melt and become rain (may be sleet or freezing rain) Rainfall is measured by a rain guage in mm – measures the depth of water rain would leave if it didn’t evaporate or soak into the ground Snowfall is measured in cm or inches

EASC 11 Cloud Classification Cloud Family Average Height Types Features Weather High Above 6000m (20000 ft) Cirrus Cirrostratus Cirrocumulus Feathery, tufted. Thin, smooth Granular patches Good Rain, snow Middle 2000-6000m (6500-20000 ft) Altostratus Altocumulus High layers Layers of puffs “fish scales” Rain/snow Rain coming Low Below 2000m (6500 ft) Stratocumulus Stratus Nimbostratus Low layers Low sheets Dark, grey circles Weak rain Drizzle Steady rain Vertical Development 500-18000m (1600-60000 ft) Cumulus Cumulonimbus Thick vertical masses Huge “cauliflower” top Showers Thundershower

EASC 11 2. Types of Precipitation Type Size H20 Description Mist 0.005 to 0.05 mm Liquid From stratus clouds Drizzle <0.5 mm Rain 0.5 to 5 mm Nimbostratus or cumulonimbus Sleet 0.5 to 5mm Solid Frozen raindrops Glaze Layers 1mm to 2 cm thick Supercooled raindrops freeze on contact with objects Rime Variable accumulations Ice feathers form from supercooled cloud/fog Snow 1mm to 2cm Crystals of frozen water Hail 5mm to 10cm+ Hard round pellets Gaupel 2 to 5mm “soft hail”