understanding fronts

Understanding FrontsUnderstanding Fronts

A 3-D grasp on fronts and A 3-D grasp on fronts and frontal movements and frontal movements and

cyclonescyclones

Air MassesAir Masses

►An air mass is a defined as a large An air mass is a defined as a large body of air with very similar body of air with very similar characteristics. characteristics.

►Generally speaking, air masses are Generally speaking, air masses are generally defined by temperature and generally defined by temperature and dewpoints (moisture parameters.)dewpoints (moisture parameters.)

►Air masses are named based on the Air masses are named based on the source region of the air mass itselfsource region of the air mass itself

Types of Air MassesTypes of Air Masses► Continental Polar, “cold and dry”Continental Polar, “cold and dry”

Originates closer to the Poles over Originates closer to the Poles over land-locked regions.land-locked regions.

► Continental Tropical, “warm and Continental Tropical, “warm and dry”dry” Originates closer to the Tropics Originates closer to the Tropics

over land-locked regions. over land-locked regions. ► Maritime Polar, “cold and damp”Maritime Polar, “cold and damp”

Originates closer to the Poles over Originates closer to the Poles over water. water.

► Maritime Tropical, “warm and Maritime Tropical, “warm and humid”humid” Originates closer to the Tropics Originates closer to the Tropics

over water. over water. ► Arctic, “very cold”Arctic, “very cold”

Originates in the very cold land-Originates in the very cold land-locked areas locked areas

Analyzing Air MassesAnalyzing Air Masses

►An air mass is most easily identified by An air mass is most easily identified by comparing it to other air masses. comparing it to other air masses.

►Air masses can be modified with time, Air masses can be modified with time, most notably by days of sunshine or most notably by days of sunshine or lack thereof. lack thereof.

►Fronts are the dividing line between air Fronts are the dividing line between air masses so understanding air masses, masses so understanding air masses, means understanding where fronts are means understanding where fronts are located. located.

What is a Front?What is a Front?

►Definition: A narrow transition zone, or Definition: A narrow transition zone, or boundary, between disparate synoptic scale boundary, between disparate synoptic scale air masses whose primary discontinuity is air masses whose primary discontinuity is density. It is synoptic scale along the length of density. It is synoptic scale along the length of the front but mesoscale across the front itself. the front but mesoscale across the front itself.

► Commonly associated with ..Commonly associated with .. Moisture gradientMoisture gradient Temperature gradientTemperature gradient Wind shiftWind shift Pressure TroughPressure Trough Convergent boundaryConvergent boundary

Rules for finding frontsRules for finding fronts► Look for a strong temperature gradient. The front is Look for a strong temperature gradient. The front is

located on the warm side of the sharpest gradient.located on the warm side of the sharpest gradient.► Likewise, look for a strong dewpoint gradient. The Likewise, look for a strong dewpoint gradient. The

front is located on the moist side of the sharpest front is located on the moist side of the sharpest gradient. gradient.

► Generally found in a pressure trough – look for three Generally found in a pressure trough – look for three hour pressure changes. Fronts will show a decrease hour pressure changes. Fronts will show a decrease in pressure followed by a rapid increase in pressure in pressure followed by a rapid increase in pressure after the frontal passage. after the frontal passage.

► Look for a sharp change in wind direction. A cyclonic Look for a sharp change in wind direction. A cyclonic shear in the wind direction usually indicates a frontal shear in the wind direction usually indicates a frontal passagepassage

► Check weather and cloud patterns that are usually Check weather and cloud patterns that are usually associated with different kinds of fronts (more later associated with different kinds of fronts (more later on this.)on this.)

Types of FrontsTypes of Fronts► Cold Cold

Noted by cold air advancing Noted by cold air advancing and displacing warmer air and displacing warmer air that exists. that exists.

► Warm Warm Noted by cold air retreating Noted by cold air retreating

from an area. from an area. ► StationaryStationary

While differing air masses While differing air masses exist along a boundary, exist along a boundary, little movement is analyzed little movement is analyzed of the air masses. of the air masses.

► OccludedOccluded A complicated process A complicated process

where the surface low where the surface low becomes completely becomes completely surrounded by cooler/cold surrounded by cooler/cold air. Occlusion processes can air. Occlusion processes can be a “cool type” or “cold be a “cool type” or “cold type” (more later.)type” (more later.)

Cold FrontCold Front

►Marked on a map with a blue line and Marked on a map with a blue line and blue triangles pointing towards the blue triangles pointing towards the warm air. warm air.

►Slopes of 1/50 to 1/150Slopes of 1/50 to 1/150►Associated with cumulus & Associated with cumulus &

cumulonimbus clouds ahead of the cumulonimbus clouds ahead of the front in the warm air, producing front in the warm air, producing showers and thunderstorms. showers and thunderstorms.


►Simple 3-D idea: Simple 3-D idea:

http://www.physicalgeography.net/fundamentals/7r.html


http://www.free-online-private-pilot-ground-school.com/images/cold-front.gif


►An animation:An animation:

Warm FrontWarm Front

►Marked on a map by a red line with red Marked on a map by a red line with red semi-circles pointed towards the cool air semi-circles pointed towards the cool air (in the direction the warm air is (in the direction the warm air is retreating to.)retreating to.)

►Slope ranges from 1/100 to 1/300.Slope ranges from 1/100 to 1/300.►Generally associated with stratus type Generally associated with stratus type

clouds, overcast skies, fog, and general clouds, overcast skies, fog, and general rain or snow. rain or snow.


►Simple 3-D idea:Simple 3-D idea:



http://www.free-online-private-pilot-ground-school.com/images/warm-front.gif


►An animation:An animation:

Stationary FrontStationary Front

►Marked by alternating blue lines & blue Marked by alternating blue lines & blue triangles (pointed in the direction of the triangles (pointed in the direction of the warmer air) and red lines & red semi-warmer air) and red lines & red semi-circles (pointed in the direction of the circles (pointed in the direction of the cooler air)cooler air)

►Usually noted as *quasi*-stationary as it Usually noted as *quasi*-stationary as it is rarely ever completely stationary. It is rarely ever completely stationary. It tends to meander a bit. tends to meander a bit.

Occluded FrontOccluded Front

►Marked by a purple line with Marked by a purple line with alternating purple triangles and purple alternating purple triangles and purple semi-circles, all pointing in the semi-circles, all pointing in the direction of the frontal movement. direction of the frontal movement.

►There are two general types of There are two general types of occlusions, cool-type and cold-type. occlusions, cool-type and cold-type. Examples to follow. Examples to follow.


►Simple 3-D idea:Simple 3-D idea:



http://www.free-online-private-pilot-ground-school.com/images/occluded-front.gif

Cyclones: Putting it all Cyclones: Putting it all togethertogether

►There are two types of cyclones, There are two types of cyclones, tropical/warm core and extratropical/cold tropical/warm core and extratropical/cold core. core.

►Extratropical cyclones are characterized Extratropical cyclones are characterized by having differing air masses frontal by having differing air masses frontal movements where tropical cyclones do movements where tropical cyclones do not. not.

►We will concern ourselves with We will concern ourselves with extratropical cyclones in this presentation. extratropical cyclones in this presentation.

Cyclones: Putting it all Cyclones: Putting it all togethertogether

► Typically, cyclones are represented by using Typically, cyclones are represented by using the Norwegian Cyclone model. It is the Norwegian Cyclone model. It is simplistic, but a good way to start when simplistic, but a good way to start when understanding surface lows and cyclones. understanding surface lows and cyclones.

► Students must begin to understand the four Students must begin to understand the four dimensional view of a cyclonedimensional view of a cyclone North-SouthNorth-South East-WestEast-West Up-DownUp-Down TimeTime

The cycloneThe cyclone

Cyclones: Norwegian ModelCyclones: Norwegian Model

►Step 1: A subtle boundary existsStep 1: A subtle boundary exists

Surface View 3-D View

Images source: http://www.srh.noaa.gov/jetstream//synoptic/cyclone.htm


►Step 2: A wave forms on the boundaryStep 2: A wave forms on the boundary




►Step 3: Cyclone becomes matureStep 3: Cyclone becomes mature




►Step 4: The occlusion processStep 4: The occlusion process




►Step 5: Cold air dominates, near the Step 5: Cold air dominates, near the end.end.



Cyclone: On satelliteCyclone: On satellite

CycloneCyclone

CycloneCyclone

►Radar & Satellite view of a cycloneRadar & Satellite view of a cyclone


Warm FrontWarm Front Warm FrontWarm Front


OcclusionOcclusionOcclusionOcclusion

A cyclone centered just south of Wichita, KS. Fronts well developed

The cyclone six hours later, near Kansas City. A mature cyclone at this point with well developed fronts. Do you see them?

Another six hours, the cyclone (low) is beginning to occlude. The surface low is being surrounded by the colder air.

Another six hours, the cyclone (low) is now fully occluded. The surface low is completely within the colder air mass.

Another six hours, the cold front has surged well east while the center of the cyclone has remained over the Midwest.

Animation of a cycloneAnimation of a cyclone

Review: Finding the frontsReview: Finding the fronts

► Rule number one for Rule number one for finding fronts was to finding fronts was to look for a strong look for a strong temperature temperature gradient or change gradient or change in temperature. in temperature.

► Do you see one Do you see one here?here?

► Would you agree Would you agree that the air mass in that the air mass in the Dakotas is not the Dakotas is not the same as the one the same as the one that exists near that exists near Kentucky?Kentucky?


► Rule number two for Rule number two for finding fronts was to finding fronts was to look for a strong look for a strong dewpoint gradient, dewpoint gradient, or change. or change.

► Do you see one Do you see one here? here?

► If so, is it roughly in If so, is it roughly in the same location the same location as the temperature as the temperature gradient in the gradient in the previous slide?previous slide?


► Rule number three Rule number three for finding fronts for finding fronts was to look for a was to look for a strong pressure strong pressure gradient, or change. gradient, or change. Strong falls followed Strong falls followed by strong rises. by strong rises.

► Rule number four is Rule number four is to look for a strong to look for a strong wind shift (the wind shift (the yellow vectors.)yellow vectors.)

► Do you see these Do you see these here? here? Note: in this image, the dashed blue lines

represent pressure rises while the solid blue lines represent pressure falls. The numbers represent millibars per three hours of fall or rise. The yellow vectors point in the direction

the wind is blowing towards.


►The last rule is to check cloud and The last rule is to check cloud and weather patterns. weather patterns.

Review: Finding the frontsReview: Finding the fronts► Every cyclone presents Every cyclone presents

unique characteristics. unique characteristics. ► While the satellite and While the satellite and

radar didn’t show the radar didn’t show the front well, the front well, the temperature, wind, and temperature, wind, and moisture analyses moisture analyses provided a clear cut provided a clear cut picture of where this picture of where this front should be analyzed. front should be analyzed.

► The rules for finding The rules for finding fronts should not imply fronts should not imply that all the rules need to that all the rules need to be met. It means that be met. It means that you need to use all those you need to use all those tools to find a front. tools to find a front. Often, fronts are very Often, fronts are very subtle and only one rule subtle and only one rule might be useful. might be useful.

► Did you get it right? Did you get it right?

Advanced tools: SoundingsAdvanced tools: Soundings► From the same date as From the same date as

the previous images, the previous images, this is the sounding from this is the sounding from Omaha at 6pm. Omaha at 6pm.

► The sounding shows The sounding shows well the slope of the well the slope of the cold front. Notice the cold front. Notice the cold air has settled in cold air has settled in very well from the very well from the surface to around surface to around 880mb, and notice the 880mb, and notice the wind shift to go along wind shift to go along with that. with that.

Advanced tools: SoundingsAdvanced tools: Soundings► From the same date as From the same date as

the previous images, the previous images, this is the sounding from this is the sounding from Lincoln, IL at 6pm. Lincoln, IL at 6pm.

► The sounding is taken The sounding is taken before the front has before the front has passed, within the warm passed, within the warm sector (where a sector (where a maritime tropical air maritime tropical air mass exists.)mass exists.)

► Notice the difference Notice the difference between the low levels between the low levels of the Omaha soundings of the Omaha soundings vs. this sounding, vs. this sounding, including the low level including the low level winds. winds.

Advanced: Cross SectionAdvanced: Cross Section

Riverton North Platte Omaha Davenport Gaylord Rapid City Aberdeen Chanhassen Lincoln White Lake

Advanced: Using ProfilersAdvanced: Using Profilers

Advanced: Using forecast LIAdvanced: Using forecast LI

Advanced: Forecast Precip & Advanced: Forecast Precip & ThicksThicks

The slope of the frontThe slope of the front

Temperatures at the surface with frontal features overlaid.


Temperatures at 850 millibars (about 1400 meters above the surface) with frontal features overlaid. Notice the cold front is a little further west and the warm front is a little further north. The low is a little further northwest, as well.


Temperatures at 700 millibars (about 2900 meters above the surface) with frontal features overlaid. Notice, again, the cold front is a little further west and the warm front is a little further north than at 850. The low is even further northwest.

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