Weather fronts - cold fronts, warm fronts and air masses
A cold weather front is defined as the changeover region where a cold air mass is Warm fronts usually move from southwest to northeast and the air behind a. When large masses of warm air and cold air meet, they do not mix. Instead, they form a front, usually hundreds of miles long. When a front passes, the weather. Fronts are classified depending on what kind of air (warm, cold, The weather map symbol for a warm front is a red curved line with red semi-circles. If a cold air mass spills onto and overtakes a neighboring warm air mass.
If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.
Occluded front Occluded front depiction for the Northern Hemisphere An occluded front is formed when a cold front overtakes a warm front,  and usually forms around mature low-pressure areas. 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.
Within the occlusion of the front, a circulation of air brings warm air upward and sends drafts of cold air downward, or vice versa depending on the occlusion the front is experiencing.
Weather front - Wikipedia
Precipitations and clouds are associated with the trowalthe projection on the Earth's surface of the tongue of warm air aloft formed during the occlusion process of the depression. Stationary front 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. 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 are marked on weather maps with alternating red half-circles and blue spikes pointing in opposite directions, indicating no significant movement. When stationary fronts become smaller in scale, degenerating to a narrow zone where wind direction changes significantly over a relatively short distance, they become known as shearlines.
Dry line A similar phenomenon to a weather front is the dry linewhich is the boundary between air masses with significant moisture differences. When the westerlies increase on the north side of surface highs, areas of lowered pressure will form downwind of north—south oriented mountain chains, leading to the formation of a lee trough.
Near the surface during daylight hours, warm moist air is denser than dry air of greater temperature, and thus the warm moist air wedges under the drier air like a cold front. At higher altitudes, the warm moist air is less dense than the dry air and the boundary slope reverses. In the vicinity of the reversal aloft, severe weather is possible, especially when a triple point is formed with a cold front.
When moisture pools along the boundary during the warm season, it can be the focus of diurnal thunderstorms. The southern plains west of the Mississippi River in the United States are a particularly favored location.
- 1.2.3 Occluded fronts
- Weather front
The dry line normally moves eastward during the day and westward at night. A dry line is depicted on National Weather Service NWS surface analyses as an orange line with scallops facing into the moist sector.
Dry lines are one of the few surface fronts where the pips indicated do not necessarily reflect the direction of motion. Squall line A shelf cloud such as this one can be a sign that a squall is imminent Organized areas of thunderstorm activity not only reinforce pre-existing frontal zones, but can outrun cold fronts in a pattern where the upper level jet splits apart into two streams, with the resultant Mesoscale Convective System MCS forming at the point of the upper level split in the wind pattern running southeast into the warm sector parallel to low-level thickness lines.
When the convection is strong and linear or curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise. If outflow boundaries or squall lines form over arid regions, a haboob may result.
Precipitation Convective precipitation Fronts are the principal cause of significant weather. Convective precipitation showers, thundershowers, and related unstable weather is caused by air being lifted and condensing into clouds by the movement of the cold front or cold occlusion under a mass of warmer, moist air. If the temperature differences of the two air masses involved are large and the turbulence is extreme because of wind shear and the presence of a strong jet stream" roll clouds " and tornadoes may occur.
Air Masses and Fronts
Orographic precipitation is precipitation created through the lifting action of air moving over terrain such as mountains and hills, which is most common behind cold fronts that move into mountainous areas. Differential heating of the surface of the earth. Heating of the lower atmosphere by energy transfer from the surface. Air Mass Air Mass is an extremely large body of air whose properties of temperature and moisture content humidityat any given altitude, are fairly similar in any horizontal direction.
Require long, clear nights, which means strong radiational cooling of air near the surface. A stable air mass.
Little moisture added so air is dry mP -- Winter cP air moves over a region such as the NE Pacific, picking up some warmth and moisture from the warmer ocean. In the case of the Pacific NW mountains force the air to rise orographic lifting causing rain.
Occasionally, slow moving weather systems in SW flow aloft can draw up moisture at mid and low levels producing precipitation.
CLIMATE & WEATHER
It is usually fairly stable and dry, and if it becomes stagnant over the midwest, results in a drought. Deaths associated with the heat wave in the midwest were the result of cT and mT air which stagnated over the central and eastern part of the US this last summer.
Air masses can control the weather for a relatively long time period: Most weather occurs along the periphery of these air masses at boundaries called fronts.