Clouds and Precipitation Section 11 3 Cloud Formation

Clouds and Precipitation Section 11. 3

Cloud Formation Ø Condensation nucleus – a small particle in the atmosphere around which water droplets form; <. 001 mm in diameter Ø A cloud is visible when the number of these droplets gets large Ø Stable air resists rising Ø Air mass stability determines the types of clouds that form and weather patterns

Stable Air Ø Stability of air depends on how the temperature of the air mass changes compared to the atmosphere Ø Fair weather clouds form under stable conditions

Unstable Air Ø The atmosphere is considered unstable when air masses continue to rise because the air mass is less dense than the surrounding air Ø Unstable clouds produce thunderstorms

Atmospheric Lifting Ø Clouds form when moist air rises, expands, and cools Ø Convective lifting occurs when air rises when it is heated and becomes warmer than the surrounding air Ø Describe Orographic lifting and convergence.

Atmospheric Lifting Ø Orographic lifting occurs when an air mass is forced to rise over a topographic barrier; Sierra Nevadas and heavy rain Ø Convergence occurs when air flows into the same area from different directions; common at mid-latitudes and equator

Types of Clouds Ø Cloud shapes are due to differences in how clouds form Ø Cloud formation occurs at different altitudes Ø First classified in 1803 Ø Classified by altitudes formed and shape Ø Low (<2000 m), Middle (2000 m – 6000 m), and High (>6000 m)

Low Clouds Ø Cumulus – puffy, lumpy looking clouds Ø Stratus – a layered, sheetlike cloud that covers most of the sky in an area; fog that has lifted

Middle Clouds Ø Altocumulus and altostratus are made up of ice crystals and water droplets due to lower temps Ø Altocumulus – white or gray in color and form large round masses or wavy rows Ø Altostratus clouds are gray and form sheets of clouds Ø Sometimes produce mild precipitation

High Clouds Ø Made up of ice crystals Ø Cirrus – have a wispy indistinct appearance Ø Cirrostratus – a continuous layer that can cover the sky; vary in thickness

Vertical Development Clouds Ø Cumulonimbus clouds can reach into the tropopause; the top is composed entirely of ice; Ø These giant clouds can produce torrential rains, strong winds, and hail

Precipitation Ø All forms of water that fall from the clouds to the ground Ø Rain, snow, sleet, and hail Ø Coalescence occurs when cloud droplets collide and join together to form a larger droplet Ø Rain is precipitation that reaches Earth’s surface as liquid; between 0. 5 – 5 mm

Snow, sleet, and hail Ø Ice crystals form in clouds with a temp. below freezing causing snow to fall Ø Sleet forms when air currents move droplets to move up and down between freezing and non-freezing air Ø If this up and down motion occurs over a large area then hail forms (large ice pellets)

The Water Cycle Ø 97% of Earth’s water is in the oceans Ø The constant movement of water between the atmosphere and Earth’s surface Ø Radiation causes evaporation of water Ø Water rises and condenses into clouds Ø Water falls to Earth as precipitation Ø Cycle continues and nourishes living things