Lecture 5 Precipitation (2) Clouds Weather Systems Global Precipitation Patterns Regional Precipitation Patterns

Clouds Form Cirrus: fibrous Stratus: sheets Cumulus: heaped Height High clouds (>6 km) Mid-level clouds (2-6 km) Low-level clouds (<2 km) Aherns, C.D., 1991, Meteorology Today, An Introduction to Weather, Climate and the Environment. West Publishing Company. Chapter 7.

Clouds Aherns, C.D., 1991, Meteorology Today, An Introduction to Weather, Climate and the Environment. West Publishing Company. Chapter 7. 1.High clouds ( ,000 m, mid-latitudes): Cirrus (Ci) Cirrostratus (Cs) Cirrocumulus (Cc) 2.Middle clouds ( m, mid-latitudes): Altostratus (As) Altocumulus (Ac) 3.Low Clouds ( m) Stratus (St) Stratocumulus (Sc) Nimbostratus (Ns) 4.Clouds with vertical development Cumulus (Cu) Cumulonimbus (Cb)

Cirrus Clouds thin and wispy The most common form of high- level clouds are thin and often wispy cirrus clouds. Typically found at heights greater than 20,000 feet (6,000 meters), cirrus clouds are composed of ice crystals that originate from the freezing of supercooled water droplets. Cirrus generally occur in fair weather and point in the direction of air movement at their elevation.

Cirrostratus Clouds sheet-like and nearly transparent Cirrostratus are sheet-like, high-level clouds composed of ice crystals. Though cirrostratus can cover the entire sky and be up to several thousand feet thick, they are relatively transparent, as the sun or the moon can easily be seen through them. These high-level clouds typically form when a broad layer of air is lifted by large-scale convergence

Altocumulus Clouds parallel bands or rounded masses Altocumulus may appear as parallel bands (top photograph) or rounded masses (bottom photograph). Typically a portion of an altocumulus cloud is shaded, a characteristic which makes them distinguishable from the high- level cirrocumulus. Altocumulus clouds usually form by convection in an unstable layer aloft, which may result from the gradual lifting of air in advance of a cold front. The presence of altocumulus clouds on a warm and humid summer morning is commonly followed by thunderstorms later in the day.

Nimbostratus Clouds dark, low-level clouds with precipitation Nimbostratus are dark, low-level clouds accompanied by light to moderately falling precipitation. Low clouds are primarily composed of water droplets since their bases generally lie below 6,500 feet (2,000 meters). However, when temperatures are cold enough, these clouds may also contain ice particles and snow.

Stratocumulus Clouds low, lumpy layer of clouds Stratocumulus clouds generally appear as a low, lumpy layer of clouds that is sometimes accompanied by weak intensity precipitation. Stratocumulus vary in color from dark gray to light gray and may appear as rounded masses, rolls, etc., with breaks of clear sky in between.

Fair Weather Cumulus Clouds puffy cotton balls floating in the sky Fair weather cumulus have the appearance of floating cotton and have a lifetime of 5-40 minutes. Known for their flat bases and distinct outlines, fair weather cumulus exhibit only slight vertical growth, with the cloud tops designating the limit of the rising air. Given suitable conditions, however, harmless fair weather cumulus can later develop into towering cumulonimbus clouds associated with powerful thunderstorms.

Cumulonimbus Clouds reaching high into the atmosphere Cumulonimbus clouds (Cb) are much larger and more vertically developed than fair weather cumulus. They can exist as individual towers or form a line of towers called a squall line. Fueled by vigorous convective updrafts (sometimes in excess 50 knots), the tops of cumulonimbus clouds can easily reach 39,000 feet (12,000 meters) or higher.

Global Circulation Pattern Chrisopherson (2000), Figure 6-13, or Jones (2000), Figure 2.4