Air Masses, Fronts, and Middle-Latitude Cyclones-I

GEOL 1350: Introduction To Meteorology

1 2 3 4 5 6 7 8 9 Air Mass – extremely large body of air with similar temperature and moisture

Best source regions for air masses are large flat areas with light winds where air can be stagnant long enough to take on the characteristics of the surface below.

Air mass are classified according to their “origin” -Geographical characteristic: tropical, polar, arctic -Surface properties: maritime, continental

10 Air Mass Classification Source Region POLAR (P) TROPICAL (T) ------Land cP cT Continental Cold, dry, Hot, dry, stable aloft (c) stable unstable below

Water mP mT Maritime Cool, moist, Warm, moist (m) Unstable usually unstable 11 Air Masses of North America

12 Air Masses of North America

summer only

13 cP (Continental Polar)

Note: T rises towards south – air mass modification 14 15 16 cT (Continental Tropical)

Upper level winds

17 What type of air mass? mP (Maritime Polar)

18 What type is this? mP (Maritime Polar)

19 20 21 Fronts - The boundary between two air masses with very difference properties

cP

mT

22 Who formulated the ideas of fronts and air masses? During World War I, Norwegian meteorologists were largely cut off from weather information outside of their neutral country due to restrictions imposed by the warring nations of Europe. In response Norway established a dense network of weather stations within their own country. Led by the father and son team of Vilhem and Jacob Bjerknes, a dedicated group of scientists now known as the the Bergen School went to work analyzing the resulting data. From this work they developed the concept of air masses and the weather fronts. They studied instabilities on the polar front and from this developed the basic theory of mid-latitude storms. This theory has become a cornerstone of modern meteorology.

23 Fronts A Front - is the boundary between air masses; normally refers to where this interface intersects the ground (in all cases except stationary fronts, the symbols are placed pointing to the direction of movement of the interface (front))

Warm Front

Cold Front

Stationary Front

Occluded Front 24 How to identify fronts

– Change of Temperature

– Change of Moisture characteristic – Change of Wind Direction – Change in Pressure Tendency – Characteristic Precipitation Patterns

25 How do we decide what kind of front it is?

• From the vantage point of the ground, if warm air replaces colder air, the front is a warm front

• If cold air replaces warmer air, the front is a cold front

• If the front does not move, it is a stationary front

• Occluded fronts do not intersect the ground; the interface between the air masses is aloft 26 Cold Front

• Cold air replacing warm air • Typically move from northwest to southeast • Air behind the front is noticeably colder and drier than air ahead

27 Cold front – 3D view

• Frontal surface rises steeply at ~ 1km : 50 km • Warm air rises along the frontal surface • cirrus clouds before surface front arrives

• cumulus clouds at the front 28 Surface weather associated with a typical cold front

29 Cold Front Weather Condition

Weather Before passing While passing After Passing ------Winds S or SW gusty, shifting W or NW Temperature warm sudden drop steady drop Pressure falling steadily minimum, then sharp rise rising steadily Clouds Ci, Cs, Tcu, or Cb Tcu or Cb Cu, Sc Precipitation short period of showers heavy shower, Tstorms decreasing,clearing Visibility fair to poor Poor, improving good Dew Pont high, steady sharp drop lower

30 Warm Front • Warm air mass replacing cold air mass • Typically moving from southwest to northeast • Air behind the front is much warmer and more moist than air ahead of front

31 Warm Front - 3D view

• Warm air rides up cold air • Slope ( ~ 1km:300km) is not as steep as cold front • Clouds well in advance the surface front • Ci, Cs, and As far before front, changing to Ns and St as the surface front approaches 32 33 Warm Front Weather Condition

Weather Before passing While passing After Passing ------Winds S or SE Variable S or SW Temperature Cool to cold, slow warming steady rise Warmer, then steady Pressure Usually falling Leveling off Slight rise, followed by fall Clouds Ci, Cs, As, Ns, St Stratus type Clearing, Sc scatter Precipitation light to moderate drizzle or none Usually none, or light rain Visibility Poor Poor, improving Fair in haze Dew Pont Steady rise Steady Rise, then steady

34 Stationary Front

Front that does not move Notable temperature change or shift in wind direction

35 A developing low pressure system typically has a warm front preceding it and a fast moving cold front. Cold front may catches up to and overtakes the warm front. The boundary that separates the cool air already in place north of the warm front and the cold air behind the cold front is Occluded Front

36 Air west of the front is colder and drier than air east of front Strong wind direction shift across the front 37 Cold occluded front (behave similar to cold front)– air behind the cold front is much colder than air ahead of warm front.

Warm occluded front (behave similar to warm front)– air ahead of warm front is colder than air behind the cold front

38 Weather associated with occluded front

Weather Before passing While passing After Passing ------Winds E, SE, or S Variable W or NW Temperature Cold type cold or cool dropping colder Warm type cold rising milder Pressure Usually falling low point usually rising Clouds Ci, Cs, As, Ns Ns, (Tcu or Cb) Ns, As, or Cu Precipitation light, moderate, heavy continuous precip or shower preci, then clearing Visibility Poor in precip Poor, in precip improving Dew Pont Steady slight drop slight drop

39 40 41 SUMMARY 1. Air mass is a large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction. 2. Source regions for air masses tend to be generally flat, of uniform composition, and in an area of light winds dominated by surface high pressure. 3. Continental air masses form over land. Maritime air masses form over water. Polar air masses originate in cold, polar regions, and extremely cold air masses form over arctic regions. Tropical air masses originate in warm, tropical regions. 4. Along the leading edge of a cold front, where colder air replaces warmer air, showers are prevalent. 5. Along a warm front, warmer air rides up and over colder surface air, producing widespread cloudiness and light-to-moderate precipitation that can cover thousands of square kilometers. 6. Occluded fronts, which are often difficult to locate and define on a surface weather map, may have characteristics of both cold and 42 warm fronts.