Wednesday April 18, 2018 (Discussion – Stationary and Occluded Fronts, Dew Points, Dry Lines, Wall Clouds)

The air masses separated by a front usually differ in which traits? The Launch Pad Wednesday, 4/18/18 1/1 The air masses separated by a front usually differ in which traits? humidity temperature

Remediation for Quiz 15 runs through Friday at 5:00! Announcements Remediation for Quiz 15 runs through Friday at 5:00!

QUIZ 15 F5 3/29 N/A F6 4/6 F7 4/9 F8 4/12 S3 4/13 F1 4/17 Assignment Summative or Formative # Date Issued Gone Missing Day Last Day Accepted XC Current Events Crossword F5 3/29 N/A Friday eResearch Paper – Atmospheric Heating F6 4/6 Worksheet – Atmospheric Conditions F7 4/9 eResearch Paper – Atmospheric Conditions, Circulation and Wind F8 4/12 QUIZ 15 S3 4/13 WS – Air Masses, Cold and Warm Fronts F1 4/17 Hold for eResearch Paper

Stationary Fronts A stationary front is a non-moving (or stalled) boundary between two air masses, neither of which is strong enough to replace the other. They tend to remain essentially in the same area for extended periods of time, usually moving in waves. There is normally a broad temperature gradient behind the boundary with more widely spaced isotherm packing.

Stationary Fronts A wide variety of weather can be found along a stationary front, but usually clouds and prolonged precipitation are found there. Stationary fronts either dissipate after several days or devolve into shear lines, but they can transform into a cold or warm front if conditions aloft change.

Stationary Fronts Stationary fronts are marked on weather maps with alternating red half-circles and blue spikes pointing in opposite directions, indicating no significant movement.

Stationary Front

Stationary Front

An occluded front is formed when a cold front overtakes a warm front. Occluded Fronts An occluded front is formed when a cold front overtakes a warm front. The cold and warm fronts curve naturally poleward into the point of occlusion, which is also known as the triple point. It lies within a sharp trough, but the air mass behind the boundary can be either warm or cold.

Occluded Fronts In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front and plows under both air masses. In a warm occlusion, the air mass overtaking the warm front is warmer than the cold air ahead of the warm front and rides over the colder air mass while lifting the warm air.

Occluded Fronts A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass.

Occluded fronts usually form around mature low- pressure areas. Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel. Occluded fronts usually form around mature low- pressure areas.

Occluded Front

Occluded Front

National Weather Map from Yesterday

DFW 10 Day Weather Forecast

Dew Point The dew point is the temperature at which dew forms and is a measure of atmospheric moisture. It is the temperature to which air must be cooled at constant pressure and water content to reach saturation. A higher dew point indicates more moisture in the air; a dew point greater than 68 °F is considered uncomfortable and greater than 72 °F is considered to be extremely humid. Frost point is the dew point when temperatures are below freezing.

Dew Point

Dry Line "Dryline" is the meteorological term applied to a surface boundary that separates moist air to the East from dry air to the West. Under certain conditions, the dryline serves as a focal point for thunderstorm development in the Plains during the spring and summer months. A dryline is represented on a weather map by a yellowish-brown line with scallops on the right side of the line.

Dry Line

Wall Cloud A wall cloud is a large, localized, persistent, and often abrupt lowering of cloud that develops beneath the surrounding base of a cumulonimbus cloud and from which sometimes forms tornadoes. It is typically beneath the rain-free base (RFB) portion of a thunderstorm, and indicates the area of the strongest updraft within a storm. Rotating wall clouds are an indication of a mesocyclone in a thunderstorm; most strong tornadoes form from these. Many wall clouds do not rotate, however.

Wall Cloud