Occluded Fronts

hey I know that you're sad about this

but this is the last video in our

weather series and today we're going to

talk about occluded fronts occluded

fronts are probably one that you are a

little bit less familiar with they don't

necessarily talk about them directly on

the forecast that you're going to watch

on TV but they're pretty important when

it comes to understanding common weather

patterns that we get here in the United

States so we're going to dive in kind of

apply some of the things that we know

and get to better understand what

they're all about so let's consider this

situation continental polar airmass

forming in Canada meeting up with a

maritime tropical airmass from the Gulf

of Mexico so we know the characteristics

of each of these air masses and that

they definitely will have very different

densities a continental polar cold dry

air high density whereas maritime

tropical is going to be humid and warm

very low density both of these air

masses will have higher pressure at

their centers and are pushing towards a

pocket of lower pressure in the middle

of the United States as we're saying on

this map so as they push towards each

other if neither air mass actually is

advancing into the other let's say

they're happening to push with the same

amount of force what you're going to end

up with is what we call a stationary

front and this would be the initial

stage stage would stage one of our

process of a storm development and

sometimes experts refer to this process

as the mid-latitude cyclone it's

responsible for many of our storms that

we get so what we're going to see is a

symbol that looks like this one where if

you saw it in color you get the

alternating red and blue but as we

pointed out you got to recognize the

shape of the symbols so up when you find

semi circles and triangles that are


and on opposite sides of the line that's

when we know this is a stationary front

and it tells us which direction the warm

air is pushing which direction the cold

air is pushing so we can clearly see

that with this symbol so this is our

opening stage now what is the stationary

front well it's as we said to Hermes our

meeting neither is advancing as far as

what you get you know or how do you get

that it's often well you know that

there's necessarily that big of a

pressure difference or a temperature

difference now the case that I just

brought up we did say that we would

expect a big temperature or pressure

difference but sometimes when you see

stationary fronts it's because they

don't have a very big one in some cases

it's just that they're pushing with

about the same amount of force what

often happens with the situation that I

present it is the stationary front

doesn't last for all that long it's it's

an initial stage and after a little bit

it's going to change we're going to show

you exactly why far as what do you get

for whether you might get clouds I mean

it's very common that you would get

clouds because after all when the warm

air is bumping into that colder air the

warm air is less dense it's going to get

shoved up and if you shove up warm air

you often will get adiabatic cooling

condensation cloud formation so we're

seeing some evidence of clouds in this

picture it is possible that you could

get a stationary front with no clouds

but it's also possible that you could

get it with steady precipitation so it

really does range there's not a

universal when you have a stationary

front you're going to get this so back

to our map and our stationary front

what's going what we're going to see is

as the two air masses are pushing

towards each other we are going to see

that the winds are going to be deflected

by Coriolis effect and so here comes the

deflection as we're pushing that air

mass and remember the deflection is

right but because these two air masses

are pushing in opposite directions right

ends up seeming to go in different

directions so look at the net

effect of this the maritime tropical

airmass is pushing more to the eastern

side whereas the continental polar is

pushing more to the western side the end

result is that we're going to change

this from a stationary front into

because we developed the center of low

pressure because as the air is pushing

in it's kind of being swirled to the

side and you can see that we would end

up with counterclockwise flow around the

center of low pressure just like that

and we end up developing these two

different fronts we get a cold front off

to the west and a warm front to the east

and so this is stage two the frontal way

and so here we have these two different

fronts with a center of low pressure and

what we're going to see is that over

time this is going to continue to change

doubt we get to stage three you notice

that the shape looks different the warm

front doesn't really seem like it's

moved all that much yet the cold front

has moved significantly now recall the

reasons why this would happen cold air

being more dense is going to more

effectively push through the warm air

and therefore as a result the front

frontal boundary moves faster so look at

these pictures and this kind of shows us

what we would expect in each location

the the off to the west find the cold

front we would expect perhaps uh some

heavy rain perhaps a thunderstorm or so

along that boundary whereas we would get

the steady precipitation associated with

the warm front over in that region so we

have two different fronts that have

developed out of the initial stationary

front that we have but as you can

imagine this doesn't last forever either

and so as time goes on we're going to be

changing this and you can probably

predict what's going to happen that cold

front that is moving faster continues to

move faster so it keeps going and well

actually I forgot I'm going to show you

this profile stage first so if we were

going to look at it in profile

we would see there is a cold front with

a little gap in between where you see


Marit surface and then we have a warm

front out off to the east so we can tell

that the cold front and the one from are

both moving in the same direction

however we know that the cold front is

moving faster and so it is going to

catch up and when it does we call that

an occlusion so if a cold front moves

fast enough that it actually catches up

to a warm front as it does in this

situation because after all remember

it's being driven by the winds which are

pushing it and how high goes too low but

Coriolis deflecting it to the right so

we're getting counterclockwise flow

around that low-pressure Center and the

cold front now has caught up to the warm

front and it's caused the occlusion to

happen so let's take a look at what that

looks like in profile view so as you saw

with picture a here it's just like what

we were showing before with the cold

separate cold front and separate warm

front then we have the beginning of the

occlusion where it's just started to

catch up to the warm front and so at

that point we're kind of merging these

storms together and then finally we get

to the mature occlusion in picture see

and so when we see this occlusion that's

shown in picture see what we're seeing

is our a so it looks like a pretty

intense storm at this time however we

would say that this is a storm that's in

its early headed towards its dying stage

because once you get a full occlusion

like this notice how the ground has no

more warm air all of the warm air has

been lifted off the ground and if you've

already lifted all of the warm less

dense air off the ground then you're not

going to get rising air so that you

don't get condensation and you don't

feed your storm so the storm is on its

way out so here is the recap of

everything we just saw we have

stationary from that because of Coriolis

effect acting differently on the two

different air masses produces a frontal

wave where we have both a cold front and

a warm front with a center of low

pressure but because cold fronts move

faster begin an open wave stage where

the cold front is sweeping around

catching up to that warm front as it

actually does begin to catch up we call

that our initial occlusion stage and

then we get the advanced occlusion or

mature occlusion stage and then finally

it dissipates as they call it a cutoff

cyclone here it's often referred to as

the dissipated stage where it basically

is just a little pocket of low pressure

and a stationary front but basically at

this point the storm has died out and

it's just you know slight differences in

pressure that remain at this point so

what I want you to kind of reflect on is

ok so do I understand what a

mid-latitude cyclone is how does it

involve both stationary fronts and

occluded fronts and how does this end up

affecting our weather at different

points in time so as the next time

you're looking at a map look for the

development of frontal waves open waves

and occluded fronts because they're

there they're there common patterns in

our weather a one final note on occluded

fronts in terms of when you see those I

forgot to point out that the symbol is

going to be when you see triangle half

circle on the same side of the line

you're noting that you can see that in

the pictures that we're showing here

both in starting up in day three where

you have the initial occlusion day for

when you have a more mature occlusion

and then finally day five is still shown

as an occluded front but it were pretty

much hitting the dissipating stage by

WIPP the time we get to stay five here

so that's going to do it for our lesson

on occluded fronts and mid-latitude

cyclones we'll see you in class