AVIATION 120

1. AVIATION 120 Meteorology

2. Today�s Agenda Fronts Study and Reference Material Introduction to fronts General characteristics of fronts Types of fronts and associated weather: Cold Warm Stationary Occluded Upper Frontal Waves

3. Fronts Chapter 5 and 7 in Meteorology Today Submit your questions for review no later than Friday December 5, 2008 Chapter 7 and 8 in the Air Command Weather Manual Submit your Air Command Supplement chapter 7 and 8 no later than Friday December 5, 2008 WW2010 � University of Illinois Fronts

4. Introduction to Fronts Since air masses of different air densities don�t readily mix, there are often distinct boundaries (on the order of 50-100 miles across) between air masses of different densities, and therefore, temperatures These boundaries are mixing zones and are known as fronts: Cold front when a cold, dense air mass is invading an area Warm front when a warm, less dense air mass is invading an area Stationary front (or quasi-stationary) when neither air mass is invading an area Occluded front (AKA: TROWAL) when a cold front catches up to and overruns a warm front Upper front when the frontal surface does not reach the surface of the earth

5. Introduction to Fronts Fronts may also be known by the name of the colder air mass at the boundary: Continental Arctic Front separates cA and a warmer air mass such as mA or mP Maritime Arctic Front separates mA and a warmer air mass such as mP or mT Continental Polar Front Separates cP and a warmer air mass such as mP or mT Maritime Polar Front Separates mP and a warmer air mass such as mT or cT Since air masses have significant vertical extent, a front may not only exist at the surface, but will usually extend upward from the surface of the earth along the �frontal surface� Frontal Surfaces will typically be more steep for a cold front than a warm front

6. General Characteristics of Fronts All fronts are unique� just as every person looks a little bit different, no two fronts will be exactly the same However, all fronts have specific characteristics that are the same and often predictable for that type of front Weather associated with a cold front will differ from weather associated with a warm front, and not every cold (or warm) front will have the same weather associated Frontal slope is a significant factor that will determine the intensity of the weather at the front and the movement of the fronts Frontal slope is the steepness of the frontal surface and can vary from a steep slope of around 1:50 (50 miles horizontally to each 1 mile vertically) to a shallow slope of 1:300 Steep frontal slopes tend to be associated with cold fronts and gradual slopes tend to be associated with warm fronts Clouds and weather primarily form in the warm air mass since this is the air mass that is forced to rise as the more dense cold air undercuts it Clouds and weather can form in the cold air mass as well due to phenomena such as evaporative cooling, turbulent mixing, etc Fast moving fronts generally are associated with more severe weather than slow moving fronts, but slow moving fronts will affect local weather for longer periods of time Frontal speed is a factor of the wind component behind the front

7. Cold Fronts A cold front represents an area where a cold air mass is replacing a warm air mass Frontal slope of a fast moving cold front is typically on the order of 1:50, less steep if moving slower As a cold front moves, surface friction tends to slow the lowest part of the front while the upper part of the front continues to move quickly, causing the frontal slope to steepen over time and distance A very fast moving cold front can develop a �bull nose� due to surface friction Cold fronts tend to be the fastest movers of all the fronts and will often overtake slower moving warm fronts Weather associated with a cold front generally occurs slightly ahead of and in a relatively narrow band behind the frontal position at the surface

8. Cold Fronts � Map View

9. Cold Fronts � Cross Section

10. Cold Fronts � Clouds and Weather Clouds and weather at any front are primarily due to frontal lift as the cold air mass undercuts the warm air mass The stability and moisture characteristics of the warm air mass are significant factors controlling the type of cloud and weather experienced at a cold front An unstable, moist warm air mass will often cause severe convective activity at the cold front � a squall line A stable warm air mass may cause layer-type cloud in a narrow band around a cold front A very dry warm air mass may not even produce any clouds or weather in some cases Clouds may form in the cold air mass due to the evaporative cooling of precipitation Arrival of a cold front can be preceded by altocumulus clouds, then towering type clouds and finally mature cumulonimbus clouds

11. Cold Fronts � Wind, Pressure and Temperature Changes When a cold front passes, wind will generally shift from a warm southerly or southwesterly to a cold westerly to northwesterly (the wind veers) Gusty, strong winds can be associated with a cold front Pressure will fall as the cold front approaches then will rise steadily after frontal passage (FROPA on a METAR) The temperature will drop with passage of a cold front

12. Warm Fronts A warm front represents an area where a warm air mass is replacing a cold air mass The warm air mass will overrun the cold air mass and be forced aloft Frontal slope of warm fronts is typically on the order of 1:150 but can be as gentle as 1:300, less steep if moving slower As a warm front moves, surface friction tends to slow the lowest part of the front while the upper part of the front continues to move quickly, causing the frontal slope to get less steep over time and distance Warm fronts tend to be the slow movers of the fronts and will often be overtaken by fast moving cold fronts Clouds and weather associated with a warm front generally occur well ahead of and in a wide band ahead of the frontal position at the surface

13. Warm Fronts � Map View

14. Warm Fronts � Cross Section

15. Warm Fronts � Clouds and Weather Clouds and weather at any front are primarily due to frontal lift as the warm air mass overruns the cold air mass Warm air overrunning colder air also causes the air mass to become more stable This overrunning tends to cause cloud to occur well in advance of the surface front � often as much as 1000 miles or more ahead of the front Since the air mass associated with warm fronts is generally stable, clouds are usually stratiform (layered) and precipitation is non-showery An approaching warm front will often be preceded by Cirrus cloud, then cirrostratus, then altostratus, then extensive nimbostratus and finally low lying stratus cloud and fog near the frontal boundary at the surface Precipitation associated with a warm front is generally continuous rain or snow over widespread areas Depending on the temperature of the air below the frontal surface, precipitation can exist as liquid, super cooled liquid or solid Extensive freezing rain can be associated with warm fronts in North America Generally, the deeper the layer of freezing air below the warm front, the more likely snow will occur Shallow layers of freezing air can contain snow, sleet, rain or freezing rain

16. Warm Fronts � Wind, Pressure and Temperature Changes When a warm front passes, wind will generally shift from a cold southerly or southeasterly to a warm southerly or southwesterly (the wind veers) The transition from one air mass to another through a warm front is generally much less abrupt than that of a cold front Pressure will fall as the warm front approaches then will stop falling after frontal passage The temperature will rise with passage of a warm front

17. Stationary Fronts A stationary front represents an area where neither air mass is moving into an area Since overrunning occurs with a stationary front as well, there often is light to moderate precipitation associated Since stationary fronts can persist for several days, the constant precipitation can cause flooding, heavy snowfall etc.

18. Occluded Fronts Occluded fronts occur when a cold front overtakes a slower moving warm front Occluded fronts are also known as a TROWAL � TROugh of Warm air Aloft Occluded fronts are associated with unstable frontal waves Occlusions can be either a warm or cold occlusion A warm occlusion occurs when the cold front overruns the warm front A cold occlusion occurs when the cold front undercuts the warm front

19. Occluded Fronts - Developement

20. Occluded Front � Associated Weather

21. Unstable Frontal Wave - Map View

22. Frontal Waves

23. Frontal Waves

24. Frontolysis and Frontogenesis Frontolysis � A dying front The discontinuity between the air masses is disappearing, the frontal mixing zone is widening The front weakens and eventually disappears Frontogenesis � The birth of a front The discontinuity between the air masses is getting more pronounced, the frontal mixing zone is getting narrower The front strengthens

25. Upper Fronts An upper front is a cold or warm front that does not reach the surface of the earth Upper fronts are depicted with hollow cusps on a weather map There are many reasons that a front may not reach the surface of the earth: Very cold air trapped near the surface due to terrain � the front may not be able to penetrate through the dense air mass Steepening of a frontal surface associated with a warm front Low level air mass modification due to daytime heating may cause the discontinuity between air masses to disappear The tropopause may fold under a jet stream forming an upper air front Associated clouds and weather are similar to surface fronts with the exception that there may be less tendency for low level cloud formed by frontal lift Hazardous freezing precipitation can occur with upper fronts that are overrunning a very cold air mass