Weather Science:What causes weather

Understanding weather charts

13th July 2017

CANNOCK CHASE U3A

SCIENCE & TECHNOLOGY GROUP

THE DIFFERENCE BETWEEN WEATHER

AND CLIMATE IS…

�The climate is what you expect;

�The weather is what you get.

� Frequently attributed to Mark Twain.

� So, why do we get what we get?

� There are loads of factors at play…

TEMPERATURE

� The Tropics are absorbing more energy than they are

losing, whilst the North and South Poles are losing more

energy than they are absorbing.

� This would mean that the Tropics would be always

warming, and the Polar regions always cooling.

� However this isn’t the case, something else must be

transporting heat from the Tropics to the poles – Winds.

PRESSURE & WINDPressure is how much air

there is above your head

Tropopause acts as a lid

Zero mb

1000 mb

� Temperature makes hot air rise,

with cooler air replacing it, causing a

flow of air or wind.

� If more air diverges at the top than

converges (friction), we end up with

less air in the column and the air

pressure falls (less weight).

� If there is more pressure in one

place, and less elsewhere, then the

air moves from high to low pressure.

� The bigger the difference between

high and low pressure, the faster

winds that blow.

� The units of pressure are mb or hPa

or inches of Hg, where 1mb = 1 hPa.Air weighs 1.225 kg/m3

Water is 1000kg/m3

Pressure is shown on a barometer and can indicate the weather approaching.

WHAT ARE ALL THE

WEATHER MAP

MARKINGS?

� Isobar contours are lines of equal pressure, usually ovals at 4mb

intervals. Pressures decrease towards centre of (L)ows & viceversa

� (H)igh and (L)ow pressure centres marked with a cross.

� Close together isobars means pressure rising rapidly, so wind

speed is higher and blows almost parallel to isobar lines.

� Spread out isobars mean little wind with slower pressure changes.

� Low pressure usually means cloud rain & wind (cyclone).

� High pressure usually means fine weather (anticyclone).

� Temperature, Pressure

contours, Wind & Fronts all

appear on various charts.

� Pressure is usually in the

range of 950-1050mb, UK,

marked by lines (isobars).

AIRMASSES AND FRONTS

ON CHARTS

� 6 sources of air mass as in picture.

� Weather “Fronts” are the boundaries

of two Air Masses.

� Northerly winds can bring cold air.

� Southerly winds can bring warm air.

� Warm fronts on charts are red

semicircles and mean rising air,

higher pressure (higher column of

air).

� Cold fronts are blue triangles and

mean falling air, falling pressure.

� Air masses/Fronts meeting will clash

at occluded front in purple, causing

bad storms.

[-------Cold showery weather-------]

THE CORIOLIS EFFECT &

WIND

� Normally the wind

tries to blow from high

to low pressure, but the

Earth’s spin acts on it,

diverting it.

� Wind then follows the

pressure contours and

blows around low

pressure in an

anticlockwise direction.

� And around high

pressure in a clockwise

direction.

� (True only for the

Northern Hemisphere).

Remember that we name winds by the direction they are blowing from

Video

DEPRESSION, FRONTS,

CLOUDS & RAIN

� 4 stages of a depression

Cold fronts are marked with triangles

(reminiscent of icicles) and warm fronts

with semi-circles (reminiscent of the

warm sun).

The lines are the boundaries of the wave

of warm air.

The occluded front (purple) occurs when

the wave breaks, and the cold front

catches up with the warm front.

The grey shading shows the

region of continuous cloud

cover.

WHAT HAPPENS ATWARM & COLD

FRONTS

APPROACHING FRONT:

DEPRESSION PASSING

OVERHEAD

1. The first thing that reaches you is the

warm front, far above your head. The

clouds are on the front, so you will see

high clouds, typically wispy, ice clouds

called cirrus, then Cumulonimbus

rising.

2. As the front moves towards you, it gets lower

and similarly the clouds get lower and heavier.

You’re going to need your umbrella.

3. On the warm front, you’ll typically see

featureless stratus or ‘sheets’ of cloud, becoming

nimbostratus or ‘rainy sheets’ with a prolonged

spell of rain when the front is near the ground.

Nimbostratus

Cirrus

4. As the front crosses you on

the ground, the

temperature rises as there

is a shift from cold to warm

air, typically by a few °C.

5. The air pressure falls

steadily ahead of and

during the passage of the

warm front, but then rises

slowly after its passage.

THE PHYSICS

� PV / T = constant

� So, if the pressure falls, the gas must

cool. As the rising air cools, the rate of

evaporation becomes less than the rate

of condensation until eventually cloud

droplets form.

Cumulus clouds are indicative of

convection. They can billow upwards as the

air rises within them.

� As water vapour condenses into liquid water, latent heat

is released providing the energy source for the developing

storm.

� The latent heat generated is significant. For example,

there is enough heat released in a small cumulus cloud to

power an average home for 17 years.

� As the storm continues to develop, some of the cloud

droplets will become large enough to fall as rain.

THE JET STREAMYet another force at play.

The polar front jet stream is a

belt of westerly winds in the

upper troposphere, with wind

speeds up to 250mph. It sits at a

around 10km, the height at

which aeroplanes fly. Trans-

Atlantic pilots make use of it.

The jet stream can split up warm

& cold fronts.

Summer 2012 UK Jet Stream

Normal Summer UK

Jet Stream

Powerful example of wind flow driven by

pressure & equator/pole temperature

differences, but shaped by earth’s spin.

FRONTS & FEATURES ONWEATHERMAPS

� To sum up so far:

� Let’s look at some features of a

weather map

� ANYONE FOR TEA?

�Next after tea…

�Will it snow?

�How high is a cloud?

�Making a cloud

WILL IT SNOW?

� To have snow, we need a

combination of two factors:

� A weather front, convection

clouds (cumulous) or mountains

need to be causing precipitation.

� The atmosphere needs to be cold

enough for precipitation to fall as

snow.

� Are there any cold fronts or air masses on Met Office

surface pressure charts bringing rain?

(www.metoffice.gov.uk)

� Arctic maritime or polar continental air masses can

bring snow in winter to the UK. Look for “528”

contours.

24 Dec 2010

1. ARE THERE COLD FRONTS? LIKELY TO RAIN?

LOOK ATMETOFFICE CHARTS. SEE 528 LINE (DOTTED)

SURFACE PRESSURE AT WWW.METOFFICE.GOV.UK

2. WILL IT SNOW?

� 500-1000mb is the bottom half mass of atmosphere.

� Height to 500mb pressure – Look for 528 lines = 5280m.

� 5280m = height below 500mb, snow is possible.

�Weather forecast charts from netweather.tv (charts/gfs)

� Select ‘HGT 500-1000’

© Netweather.tv

� Look at thickness of bottom

half of atmosphere.

� Hot air rises.

� Colder more dense air sinks to

bottom half.

� So smaller height = colder air.

2. Is the atmosphere cold

enough for precipitation to

fall as snow?

netweather.tv

500mb

1000mb

3. WILL IT SNOW?

� Check whether the atmosphere really is cold

enough throughout its entire depth:

� Go to wetter3.de …Select ‘Vertikalschnitte’

(Vertical section) and select latitude & longitude.

� wetter3.de

Saturday

Feb 11th

2017

Above

London

(00)

It

snowed!

EXTRA TIME…

�Measure the height of a cloud

�Making a cloud

MEASURE THE HEIGHT OF CLOUDS

� Temperature change from the ground:

� Approx -6 degrees C per km (-1.8C per 1000ft)

� Use an Infra Red non-contact temperature sensor.

� Point IR sensor to clouds for temp

measurement (eg -16C).

� Point to ground for ground temp

(eg +5C).

� Temp difference, say 5C – (-16C) =

21C

� Height = 21/6 km = 3.5km

�Making a cloud video

� [Copyright Reading University]

� THAT’S IT FOLKS

� See you next time

� References thanks to Reading University

RAIN, WHEN?

What causes precipitation (rain and snow)?

Precipitation forms when cloud droplets (or ice particles) in clouds grow and combine to become so large that their fall speed exceeds the updraft speed in the cloud, and they then fall out of the cloud. If these large water drops or ice particles do not re-evaporate as they fall farther below the cloud, they

reach the ground as precipitation.

CLOUDS