OVERVIEW:

We will discuss cloud and precipitation formation processes, general cloud classification, and synoptic-scale cloud patterns.

OUTLINE:

1. What are these cloud things and how do they form? Where does precipitation come from?

1.1. Clouds form when an airmass is forced towards saturation. Precipitation becomes likely when the force pushing the airmass towards saturation continues after the airmass has become saturated.

1.1.1 Clouds are made of liquid water droplets that condense out of the water vapor in the saturated airmass (relative humidity 100%).

1.1.2. The droplets condense onto cloud condensation nuclei (CCN) that are made of tiny specks of dust, soot, and sea salt. CCN are about 1 tenth of a micrometer (100 nanometers) in diameter.

1.1.3. Clouds droplet diameters range from about 10 micrometers (stratus) to about 100 micrometers (cumulus). They are held aloft by air currents. Cloud droplets get progressively larger by absorbing more and more water vapor, and by coalescing with other cloud droplets.

1.1.4. Cloud droplets are often supercooled, i.e. in a liquid state even though their temperature is below zero degrees C.

1.2. A small rain drop is about 200 micrometers in diameter, and a typical rain drop is about 1 millimeter (1000 micrometers) in diameter.

1.2.1. Like cloud droplets, rain drops are often supercooled when they form, and may warm above the freezing point if they fall through warmer regions of the atmosphere.

1.2.2. Rain drops begin as large cloud droplets, and get progressively larger by absorbing more and more water vapor, and by coalescing with more cloud droplets and other raindrops. Eventually, they get too heavy to be held aloft by air currents, and fall to Earth.

1.2.3. Snow flakes begin as tiny crystals of frozen water molecules that grow as more and more water molecules are added to the structure. Like rain drops, they eventually get too heavy to be held aloft by air currents, and fall to Earth.

2.1. Radiational cooling -- Occurs at night in a relatively clear atmosphere. The Earth's surface cools by emitting infra-red (black body) radiation. A radiation inversion forms. Creates clouds with bases on or very near the Earth's surface (fog and stratus).

2.1.1. Continuous (steady) light rain or drizzle (very small rain drops) may occur as radiational cooling forces the temperature lower and lower.

2.1.2. More cooling -> saturation mixing ratio goes down -> more condensation -> more and larger fog or cloud droplets -> more precipitation.

2.2. Addition of moisture via evaporation -- Occurs under conditions favorable to strong evaporation, such as the presence of warm air above a field of snow, or dry air above a body of water. Dew point increases towards saturation near the Earth's surface. Creates clouds with bases on or very near the Earth's surface (fog and stratus).

2.2.1. Continuous light rain or drizzle may occur as continuing evaporation forces more and more condensation, growing the cloud droplets beyond the size where air currents can keep them aloft.

2.2.2. More evaporation -> more condensation (esp. after airmass reaches saturation) -> more and larger fog or cloud droplets -> more precipitation.

2.3. Cooling due to convective lift -- Occurs when an airmass is forced aloft and cools adiabatically.

2.3.1. Convective lift occurs when the air near the Earth's surface becomes significantly warmer than the air aloft, forcing the airmass to become unstable. Short-wave solar radiation (sunlight) is the forcing mechanism responsible for heating the Earth's surface.

2.3.2. The airmass cools dry-adiabatically until reaching saturation at the convective condensation level (CCL), then cools moist-adiabatically if lifting continues or if the parcel is warmer than its environment. Cloud bases are at the CCL.

2.3.3. Showery (varying intensity) rain or snow may occur as convective lift continues and rain drops (or snow flakes) grow beyond the point where air currents can keep them aloft.

2.3.4. More surface heating -> more convective lift -> more adiabatic cooling -> more condensation -> more and larger cloud droplets and more and larger rain drops/snow flakes -> heavier precipitation.

2.4. Cooling due to mechanical lift -- Also occurs when an airmass is forced aloft and cools adiabatically.

2.4.1. Mechanical lift occurs when the airmass is forced aloft by some mechanical means, such as being lifted by the wedge of an advancing front, or if the airmass is forced over a tall range of mountains.

2.4.2. The airmass cools dry-adiabatically until reaching saturation at the lifted condensation level (LCL), then cools moist-adiabatically if lifting continues or if the parcel is warmer than its environment. Cloud bases are at the LCL.

2.4.3. Showery rain or snow may occur if mechanical lift continues and rain drops (or snow flakes) grow beyond the point where air currents can keep them aloft.

2.4.4. More mechanical lift -> more adiabatic cooling -> more condensation -> more and larger cloud droplets and more and larger rain drops/snow flakes -> heavier precipitation.

3.1. Cumuliform -- "Billowy, individual cloud masses that often have flat bases" (Lutgens and Tarbuck).

3.1.1. Range from puffy, fair weather clouds (cumulus) with a few hundred feet of vertical development, to large thunderstorms (cumulonimbus) with tens of thousands of feet of vertical development and a cirriform anvil.

3.1.2. Often associated with the generally fair weather in the interior of high pressure systems (cumulus), and with fast-moving and summer-time cold fronts and occlusions (cumulonimbus). "Airmass" (non-frontal) thunderstorms are also possible.

3.1.3. Created by either convective or mechanical lift. Clouds are centered on regions of upward vertical motion (adiabatic cooling and condensation); clear areas mark regions of downward vertical motion (adiabatic warming and drying).

3.1.4. Showery precipitation (RASH/SNSH).

3.1.5. Downdrafts (as in a thunderstorm) are caused when falling precipitation drags colder air from the upper regions of the cloud down to the surface.

3.1.6. Lightning is caused when falling precipitation strip electrons off nearby atoms, which in turn concentrates negative electrical charges near the base of the cloud and positive electrical charges near the top of the cloud. A positive charge buildup is induced in the Earth's surface. Eventually, electricity arcs from the negative centers to the positive centers.

3.1.6.1. Lightning is visible because the arcing electricity superheats the surrounding air, causing it to glow.

3.1.6.2. Thunder is caused by the sudden (explosive) expansion of the superheated air.

3.2. Stratiform -- "Sheets or layers [of clouds] that cover much or all of the sky" (Lutgens and Tarbuck).

3.2.1. Range from tiny tufts of fog that float away from the main fog mass (stratus fractus), to giant clouds that reach from near the Earth's surface to heights of several kilometers and cover entire states or provinces (nimbostratus).

3.1.2. Associated with stable early morning conditions (radiation inversions) and the generally fair weather in the interior of high pressure systems (stratus fractus); with slow-moving and winter-time cold fronts (nimbostratus); and, with all warm fronts and most occluded fronts (nimbostratus).

3.1.3. Created by non-lifting saturation processes with or without the addition of weak mechanical lift.

3.1.4. Continuous precipitation (RA/SN/DRZL).

3.3. Cirriform -- "Thin, delicate ice-crystal clouds often appearing as veil-like patches or thin, wispy fibers" (Lutgens and Tarbuck).

3.2.1. Range from "mare's tails" (cirrus), to very high, very small tufts similar to low cumulus clouds (cirrocumulus), to wide-spread, thin, milky clouds that invade the sky like a sheet (cirrostratus).

3.1.2. Associated with all kinds of pressure systems. Cirrus generally indicates high pressure and fair weather; cirrocumulus generally indicates instability in the upper troposphere; and cirrostratus often indicates the approach of low pressure and unsettled weather (over-running).

3.1.3. No precipitation.

4.1. Look for regions where the relative humidity is high enough to support cloud formation. In general, clouds will form in regions where the dew-point depression (difference between the temperature and the dew point) is 5 degrees C or less and some kind of lifting is available. Regions where the DPD is less than 2 are probably already cloudy.

4.2. On surface and 850 millibar charts: -- Fronts, troughs, other regions of low-level convergence, and fair-weather areas that may become unstable under surface heating.

4.2.1. Cold fronts, troughs, convergence lines: Cumuliform clouds -- showery precipitation.

4.2.2. Warm fronts and occluded fronts: Stratiform clouds for both, esp. the warm front (nimbostratus) -- continuous precipitation. The occlusion will also have cumuliform because of the additional mechanical lifting -- therefore continuous precipitation with occasional showery precipitation.

4.2.3. High pressure systems: Morning stratus in areas where a moisture source or a surface-based cooling mechanism is available -- light, continuous precipitation (if any). Afternoon cumulus in areas that may become convectively unstable, esp. the southwest and western parts of high pressure areas -- showery precipitation (if any).

4.3. On 700, 500, and 300 millibar charts: -- Troughs, regions of cold-air advection, and regions of upper-level divergence (speed or directional).

4.3.1. Troughs: "Chimney effect" leads to mechanical lift, cold air associated with trough may advect over warmer air below, resulting in absolute instability -- cumuliform clouds.

4.3.2. Regions of divergence, esp. on 300 millibars chart: Upper-level divergence leads to mechanical lift; strong cold air advection may also result in absolute instability. Look for dense mid-level cumuliform (700, 500 mb) and cirriform (300 mb) clouds.

4.3.3. Saturated or near-saturated regions without a lifting mechanism: Stratiform clouds -- light continuous (if any) precipitation. 700/500 mb -- Mid-level stratiform clouds. 300 mb -- cirriform (cirrostratus) clouds.

LAB:

Divide up into groups of three. Each group should jointly:

1. Analyze an 850 millibar chart for fronts, troughs, and moisture (DPD .le. 5 dC).

2. Analyze a 700 millibar chart for moisture (DPD .le. 5 dC).

3. Analyze a 500 millibar chart for troughs, ridges, and moisture (DPD .le. 5 dC).

4. Compare areas of cloudiness on the displayed satellite photo to what you found on your charts. Do they coincide? Are there cloud areas on the satellite picture that don't show up on the charts?

5. Speculate on the causes and types of cloudiness in various areas of North America. Where would you expect precipitation?

HOMEWORK:

1. Read Lutgen and Tarbuck chapters 5 and 10.

2. Review "27 States of Sky" on-line.

3. Skim Lyons et al,, 2000.

4. Review notes and labs from meetings 1 through 7.