What is the relationship between air temperature and relative humidity? This graph assumes the amount of water in the air does NOT change as the temperature changes.

Think of air as a container, and we need to fit a fixed amount of water into the container. When the temperature drops, and water vapor amount stays the same, the air fills up with water, increasing the relative humidity. WARM AIR “container” FITS MORE WATER = 40% humidity COLD AIR “container” FITS LESS WATER= 90% humidity

Aim: Why does the weather change? Air masses and fronts Aim: Why does the weather change?

Vocabulary: Air mass Source regions Maritime Continental Polar Tropical Fronts Cold Front Warm Front Stationary Front Occluded Front

Air masses are… A volume of air characterized by similar pressure, moisture and temperature. Air masses acquire their properties from the source region.

Source regions are where the air mass comes from Maritime – from ocean Continental – from land Tropical – warm (L) Polar – cold (H)

Air Mass Type Map Symbol Formed over…. Characteristics Continental Tropical cT Continental Polar cP Maritime Polar mP Maritime Tropical mT

Naming air masses Air mass names are characterized by their source regions. Use a 2 letter symbol. Maritime tropical (mT) – Warm, humid air mass from over tropical oceans. Bring summer showers and thunderstorms, hurricanes Maritime polar (mP) – Cool, humid air from over cold oceans. Bring fog, rain and cool temperatures, noreasters

Continental Tropical (cT) – Hot, dry air from over the Southwest and Northern Mexico Continental Polar (cP) – Cool or cold, dry air from over Canada and Alaska

Warm moist air rises from the equator region, while cool, dry air sinks from the north. These air masses collide over the midwest in early spring causing tornados. TORNADO ALLEY

Fronts – where air masses collide The warmer air rises above the cooler air. It cools, condenses into clouds and causes precipitation around the front. Fronts signal changing weather, as a new air mass moves into the area.

There is always precipitation at a front! Why?

4 kinds of fronts Symbols (like flags) point in the direction the front and air masses are moving After the front passes, a new air mass brings new weather conditions

Cold fronts Cooler air mass runs into a warmer air mass Warm air is pushed up rapidly  cumulous clouds and thunderstorms, tornados Passes quickly, shorter period of rain, heavy downpours After  clear skies and cooler temps (H -cooler, clearer, drier) Cold fronts

Warm fronts A warmer air mass runs into cooler air mass Warm air rises over the cold Passes slowly, clouds over wider area Slower moving, drizzly or steady rain After  warm and humid weather (warmer, wetter, cloudier) Warm fronts

Stationary Fronts Cold and warm air masses meet but neither move “standoff” Water vapor condenses in the warm air  days of clouds and precipitation until one air mass becomes stronger than the other and begins to move

Occluded Front Warm air mass is caught between 2 colder air masses Cold masses combine and push the warm up Makes heavy clouds, rain or snow

SUMMARY Why does warm air always rise above cool air? What always happens at a front? What type of pressure characterizes a air behind a cold front? Warm front?