Atmospheric Water & Changing Weather

Weather Weather can change rapidly. The weather at a location depends on: air temperature air pressure fog humidity cloud cover precipitation wind speed and direction

Climate Climate is the average of a region’s weather over time. The climate for a particular place is steady, and changes only very slowly. Determined by: angle of the Sun likelihood of cloud cover air pressure

Humidity Relative humidity is the percentage of water vapor a certain volume of air is holding relative to the maximum amount it can contain. Since warm air can hold more water vapor than cool air, raising or lowering temperature can change air's relative humidity.  The temperature at which air becomes saturated with water is called the air's dew point.

Clouds Clouds have a big influence on weather: by preventing solar radiation from reaching the ground. by absorbing warmth that is re-emitted from the ground. as the source of precipitation. Water vapor is not visible unless it condenses to become a cloud. Water vapor condenses around particles: Dust Smoke Salt crystal Billions of these water droplets together make a cloud.

Cloud Classification The most common classification used today divides clouds based on their altitude.

Cirro: Cirrus: Alto: Cumulus: Strato: Stratus: Nimbus: Height Prefixes Shape Prefixes Cirro: High clouds bases starting above 6000 m. Cirrus: Latin meaning “hair”; Wispy, stringy clouds. Alto: Middle clouds with bases between 2000-6000m. Cumulus: Latin meaning “pile or heap”; Puffy, lumpy looking clouds. Strato: Low clouds below 2000m Stratus: Latin meaning “layer”; Featureless sheets of clouds Nimbus: Latin meaning “cloud”; Low, gray rain clouds

High Clouds Cirro Form from ice crystals where the air is extremely cold and can hold little water vapor. Cirrus, cirrostratus, and cirrocumulus

Middle Clouds Alto Made of water droplets, ice crystals or both, depending on the air temperatures. altocumulus and altostratus clouds

Low Clouds Strato Nearly all water droplets. Stratus, stratocumulus, and nimbostratus

Vertical Clouds Clouds with the prefix 'cumulo-' grow vertically instead of horizontally. Have their bases at low altitude and their tops at high or middle altitude. Clouds grow vertically when strong air currents are rising upward.

Fog A cloud located at or near the ground . When humid air near the ground cools below its dew point, fog is formed. Science on the SPOT: The Science of Fog

Precipitation Can form in place. Dew forms when moist air cools below its dew point on a cold surface, such as a flower. Frost is dew that forms when the air temperature is below freezing.

Precipitation The most common precipitation comes from clouds. Rain & Snow Rain falls from clouds when the temperature is fairly warm. If temperatures are cold, the droplet will hit the ground as a snowflake.

Precipitation Less common types are sleet and hail. Sleet is rain that becomes ice as it hits a layer of freezing air near the ground.  Hail forms in cumulonimbus clouds with strong updrafts. An ice particle travels until it finally becomes too heavy and it drops. 

Air Masses Large body of air that takes on the characteristics of the area over which it forms. Where the air mass forms is its source region.

Surface of Source Regions: Land: Continental (c); Dry Water: Maritime (m); Humid/Wet Temperature of Source Regions: Warm: Tropical (T) Cold: Polar (P) VERY Cold: Arctic (A)

Arctic: Very cold, dry Maritime Polar: (mP) Cold, humid Continental Polar: (cP) Cold, dry Maritime Tropical: (mT) Warm, humid Continental Tropical: (cT) Warm, dry

Air Mass Movement Air masses are slowly pushed along by high-level winds. When an air mass moves over a new region, it shares its temperature and humidity with that region. Storms arise if the air mass and the region it moves over have different characteristics.  Cold air masses tend to flow toward the equator. Warm air masses tend to flow toward the poles. This is one of the many processes that act towards balancing out the planet’s temperatures.

Fronts Where two air masses meet. The air masses have different densities: One is lifted over the other, creating a low pressure zone.  The greater the temperature difference between the two air masses, the stronger the winds will be.

Cold Fronts Cold dense air displaces warm air, and pushes the warm air up along a steep front. This front moves rapidly. Cumulus and cirrus clouds can form. Strong cold fronts may lead to: Thunderstorms Squall lines Tornadoes High winds Snow showers

Seasonal Cold Fronts Spring and summer: Spring: Autumn: Winter: The air is unstable so thunderstorms or tornadoes may form. Spring: If the temperature gradient is high, strong winds blow. Autumn: Strong rains fall over a large area. Winter: The cold air mass is likely to have formed in the frigid arctic so there are frigid temperatures and heavy snows. After the front passes, temperatures usually drop and skies clear.

Warm Fronts Advancing warm air slides over cold air. Atmosphere is relatively stable: The less dense, warm air moves over the colder, denser air. Warm fronts are characterized by less severe weather but they last longer. Extensive cloudiness (cirrus & stratus) and precipitation (steady light rain or snow in front of them)

Warm Fronts Warm fronts move slower than cold fronts After the front passes, skies clear and usually temperatures increase.

Stationary Front When two air masses meet and neither is strong enough to replace the other. The boundary stays where it is, and this is called a stationary front. Can bring days of rain, drizzle, and fog. Winds usually blow parallel to the front, but in opposite directions. Can last several days, then will likely break apart.

Occluded Front Forms when a cold front is moving faster than a warm front; the cold front catches up to and takes over, the warm front. Precipitation is common on both sides of the front.