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What are weather fronts?

you may have heard weather forecasters

on television talking about a warm front

or a cold front approaching bringing

with it a change in the weather but what

are they going on about what is a

weather front and how does it affect our

weather a weather front is the boundary

between two bodies of air with different

temperature and humidity these

differences depend on where the air has

come from on the globe for example air

sitting over tropical oceans becomes

very hot and humid while in contrast air

over the Arctic becomes very cold and

dry

that's logical enough but what happens

somewhere like Britain where our climate

is neither tropically warm and humid nor

polar cold and dry over the Atlantic

these warm cold air masses often meet

just like oil and water they don't mix

easily and can become unstable as a

result giving us an active area of

weather in which fronts form when air

mass is first collide it's impossible to

know if we're looking at a warm or cold

front it's only when one air mass

dominates that we can distinguish

between the two if the warm air is

stronger than the cold air then this is

called a warm front you may have seen

this on TV as a line with red semi

circles along it the semi circles point

towards the cold air and the direction

in which the front is moving if the cold

air has the upper hand then it's a cold

front which is shown by a line with blue

triangles along it the blue triangles

point towards the warm air and again in

the direction of movement but what is

happening when two opposing air masses

meet and what kind of weather does each

front produce on a fine day if you see a

layer of very high cloud beginning to

spread across the sky usually from the

southwest in Britain it may mean that a

warm front is approaching warm air is

less dense than cold air so it tends to

ride over the top of the cool air it's

replacing this creates a vertical slope

to the frontal system with the front

first being seen high in the atmosphere

miles ahead of the front at ground level

as the warm moist air is forced to rise

it cools and water vapor condenses into

water droplets or ice crystals these

form clouds that you see approaching

first as high cirrus clouds and then as

thicker clouds

it's these interactions which form

clouds if you're measuring the air

pressure is the front approaching you'd

see that it's falling this is because

the column of dense cold air above you

is gradually being replaced with more

and more warm and less dense air as the

front moves closer layers of cloud also

deepen and rain begins to fall

a warm front usually produces quite a

long period of light and drizzly rain

with a shorter spell of heavier rain at

the beginning of the front the rain or

drizzle will eventually die out once the

front has moved through but it will

often stay pretty cloudy you might think

that a warm front would mean nice warm

weather but unfortunately that's not

always the case

all that rain and loss of sunshine can

mean that temperatures are actually

lower than they were before the front

arrived so why is it called a warm front

well the cold dry air mass has been

replaced by a warm humid airmass it just

doesn't feel much warmer but what about

cold fronts warm fronts are easy to spot

because you see the leading edge of high

cloud approaching on the horizon across

clear skies however warm fronts leave

clouds behind and the following cold

front sneaks in under the cover of the

clouds this makes them hard to spot just

using your eyes as it's difficult to see

any changes that might be on the way but

remember that the pressure Falls with a

warm front because the air is less dense

well the opposite is true with a cold

front a cold front is a bit like a wedge

the air is colder and more dense than

the warm air and it scoops it up causing

the pressure to rise and we can spot

that using a barometer so what kind of

weather does a cold front bring deep

cloud layers form as the warm moist air

is forced to rise and cool these cloud

layers produce lots of rain often in

bands and these bands tend to be quite

narrow with heavy rain in them sometimes

with even hail and thunder once the

front passes there's often a dramatic

clearance with blue skies bright

sunshine as the cold air descends as the

front passes temperatures fall at first

but they may rise again when the

sunshine appears however this sunny

weather doesn't always last for very

long because if the cold air is warmed

it will rise or convect as weather men

say and produce sherry rain clouds

just cumulus or Kuno Nimbus as we've

seen weather fronts can be complicated

things and bring with them some very

varied weather which is one of the

reasons we have such an interesting time

of it here in this country we've looked

at how warm and cold fronts are formed

and what sort of weather they bring but

if you want to learn anything more about

weather fronts or anything to do with

weather and climate visit the Met Office

website

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