Clouds

Visible aggregates of minute droplets of water or tiny ice crystals. Formed when water condenses aloft due to adiabatic cooling.

Condensation

Produces clouds, fog or dew.
Two Necessary Conditions:
1)
2)
Cloud Condensation Nuclei - dust, smoke, salt, solid particle, volcanic ash, pollen
Hygroscopic nuclei:
Hydrophobic nuclei:

Cloud Classification

Based on Form and Height
Form: Three basic types
Cirrus - high, thin clouds with a veil-like or wispy appearance.
Cumulus - globular, cauliflower- like masses, often with flat bases and as rising domes or towers.
Stratus - sheet-like or blanket-like clouds covering large areas.

Cloud Height

High Clouds

Form above 6,000 m
Do not tend to produce precipitation
May indicate impending stormy weather
Cirrus: delicate, icy clouds with bent, curled or hooked filaments.
Cirrostratus: transparent veils of fibrous smooth cover over large areas of sky. Will create halos around the Sun and Moon.
Cirrocumulus: patchy small cells or ripples, commonly showing a regular pattern known as "mackerel sky".

Middle Clouds

Form between 2000 to 6000 m
Altocumulus: large patches of round/roll-like masses. Easily confused with cirrocumulus (which are smaller, less dense masses) and stratocumulus (larger masses)
Altostratus: formless layer of gray cloud covering large areas of sky. Sun is visible through the layer but is indistinct and with no halo. Light precipitation (snow or drizzle) is uncommon. Associated with approaching warm fronts.

Low Clouds

Form below 2000 m
Stratus: uniform layer covering large areas. May produce light precipitation.
Stratocumulus: transitional form between stratus and cumulus.
Nimbostratus: "rain cloud" - major producer of precipitation. Dark gray layers with ragged bottoms. Light to moderate, but generally long duration and widespread precipitation. Form in stable air conditions when air is forced to rise along a front or converging air masses.

Vertical Development

Clouds with bases at low heights, and tops which may extend to middle or high heights. Typically associated with unstable air
Cumulus: Individual globular, cauliflower-like masses, often with flat bases and as rising domes or towers. Typically form on clear days due to uneven surface heating. Tend to increase in numbers as the day progresses into afternoon.
Small clouds: cumulus humilis - rarely produce precipitation. "Fair weather clouds"
Increased height on cloud top produces cumulus congestus.
Cumulonimbus: "Thunderstorms". Tall, tower-like, dark, dense, billowy clouds with tops at high heights (up to 12 - 20 km) In later stages of development - top spreads out into an anvil head. Produce heavy precipitation, lightning, thunder and occasionally hail and/or tornadoes.

Fog

Defined as a cloud with its base at or very near the ground
Physically no difference between fog and clouds.
Clouds form from rising and cooling air.
Fog forms from cooling only, or due to the addition of water to the air.

Fog Formed by Cooling

When the temperature of the air at ground level falls below the dew point, condensation produces fog
Radiation Fog:
Advection Fog:
Upslope Fog:

Evaporation Fogs

Fogs formed by the addition of water vapor to air causing saturation.
Steam Fog:
Frontal Fog:

Dew and Frost

Dew is condensation on objects that have been cooled below the dew point temperature of the surrounding air.
Hoar Frost (white frost) forms when the dew point is below freezing. Water vapor in the air changes directly to solid ice (deposition) upon contact with the cold surface.

Precipitation

Cloud droplets start at ~0.02 mm in size.
Typical raindrop is ~2 mm in diameter.
1 million time larger than a cloud droplet.
Condensation alone is insufficient to create large enough drops to precipitate.

Bergeron Process

Two important Properties of Water
1)
2)
Both ice and liquid are present.
Air around the ice crystal is super- saturated.
Water will deposit on the ice faster than it will sublimate.
Water evaporates from droplets providing water for ice growth.
Ice will grow large enough to fall.
During the fall, ice grows larger as it intercepts droplets
If ice breaks up during the fall, each small crystal becomes and additional freezing nuclei to continue this process.
Ice will fall as rain, if the ice melts before reaching the surface.

Collision-Coalescence

Warm clouds - clouds located below the freezing level
Large droplets form (>0.02 mm)
These large droplets fall colliding with smaller droplets
Then they fall faster as they grow larger, and increase their chance for more collisions.
Clouds with thick vertical development or strong updrafts have a better chance of producing precipitation in this manner.
Most efficient over tropical oceans (tends to produce large droplets due to fewer condensation nuclei).

Forms of Precipitation

Mist: 0.005 - 0.05 mm, liquid
Drizzle: less than 0.05 mm, liquid
Rain: 0.5 to 5 mm, liquid
Virga: rain which evaporates before reaching the ground
Sleet: 0.5 to 5 mm, solid: forms as rain falls through a cold layer of air and partially or completely freezes
Glaze (Ice Storms); layers of ice 1 mm - 2 cm thick, solid: super-cooled raindrops come into contact with surfaces that are below 0°C and freeze on that surface.
Rime: deposit of ice formed by fog or cloud droplets freezing on cold surfaces.
Snow: 1 mm - 2 cm, solid: Crystals of water are minerals. Different snow crystals form at different temperatures.
Graupel: 2 - 5 mm; rime encrusted snow
Hail: 5 mm - 10 cm, solid: hard, rounded pellets or irregular lumps of ice. Concentric shells which form as the stones travel on strong updrafts and downdrafts in cumulonimbus clouds

Precipitation Measurement

Rain gauge measurement: Units of depth of fall per unit time.
One cm of rainfall would cover the ground with one cm deep water if it did not run off, evaporate or soak into the ground.
Tipping-bucket Gauge
Weighing Gauge

Snow Fall

Measurement of Snow fall?
Water Equivalent - snow is melted to give a rain fall equivalent.

Measurement Errors

Rain and snow fall measurement tend to under-record the actual fall amount.
Turbulence, splash, surface tension and evaporation can all lead to lower recorded amounts in a rain gauge.
Under-recording in U.S. ranges from 7 to 20%; high latitude stations as high as 80%.

Radar Measurements

Weather Radar measures the intensity, rate, and movement of precipitation.

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