Weather Part I

Companion to Ck-12 Earth Science Chapter 16: WeatherAuthor: Robert Smith

What is Weather

• Weather is what is going on in the atmosphere at a particular place at a particular time. Weather can change rapidly

Weather depends on• air temperature• air pressure• fog• humidity• cloud cover• precipitation• wind speed and direction• All of these are directly related to the amount of energy

that is in the system and where that energy is. • The ultimate source of this energy is the sun.

Climate

• Climate is the average of a region’s weather over time

• climate for a particular place is steady, and changes only very slowly

Humidity

• Humidity is the amount of water vapor in the air in a particular spot

• Relative humidity: the percentage of water vapor a certain volume of air is holding relative to the maximum amount it can contain

• What will happen if the humidity increases to more than 100%?

• The excess water condenses and• forms precipitation

Dew Point

• warm air can hold more water vapor than cool air

• temperature at which air becomes saturated with water is called the air’s dew point

• water condenses from the air as dew

Clouds Influence Weather

• by preventing solar radiation from reaching the ground.

• by absorbing warmth that is re-emitted from the ground.

• as the source of precipitation

Clouds form

• Clouds form when air reaches its dew point• Form two ways: • (1) Air temperature stays the same but

humidity increases. This is common in locations that are warm and humid.

• (2) Humidity can remain the same, but temperature decreases.

Examples of cloud types

High clouds

• form from ice crystals where the air is extremely cold and can hold little water vapor.

• Cirrus, cirrostratus, and cirrocumulus are all names of high clouds

Middle Clouds

• made of water droplets, ice crystals or both, depending on the air temperatures

• altocumulus and altostratus clouds• usually mean a large storm, bearing a lot of

precipitation, is coming

Low Clouds

• Composed of nearly all water droplets• Stratus, stratocumulus and nimbostratus

clouds are common• Nimbostratus clouds are thick and dark. They

bring steady rain or snow

Vertical Clouds

• Clouds with the prefix ’cumulo-’• grow vertically instead of horizontally• Clouds grow vertically when strong air

currents are rising upward

Fog

• a cloud located at or near the ground• Radiation fog forms at night when skies are clear

and the relative humidity is high. As the ground cools, the bottom layer of air cools below its dew point.

• Advection fog: Warm, moist Pacific Ocean air blows over the cold.

• Steam fog appears in autumn when cool air moves over a warm lake

Precipitation

• most common precipitation comes from clouds

• Rain or snow droplets grow as they ride air currents in a cloud and collect other droplets

• One million cloud droplets will combine to make only one rain drop

Precipitation

(a) Sleet is rain that becomes ice as it hits a layer of freezing air near the ground. (b) If a frigid raindrop freezeson the frigid ground, it forms glaze. (c) Hail forms in cumulonimbus clouds with strong updrafts. An ice particletravels until it finally becomes too heavy and it drops. This large hail stone is about 6 cm (2.5 inches) in diameter

Use your Notes

• Five in Five Quiz is about to begin.