Predicting Weather

A Skill For Outdoors People

Weather

• Weather simply refers to the condition of the air on earth at a given place and time - whether it is warm or cold, dry or wet, blowing or calm.  The condition of air and how it acts to create weather is influenced primarily by two things:– Heat (the sun) – Water

Sun’s Influence on the Weather

• Weather on earth starts with the sun.  The sun's heat warms our atmosphere and causes two things to happen:– water evaporates into the air – air rises

Precipitation

• As air rises, its temperature drops.  As the moisture in the air cools, it condenses into tiny suspended droplets, forming clouds.  The droplets inside the clouds become larger as more moisture is evaporated into the air.  Eventually, they are too heavy to remain suspended and fall to earth as precipitation - rain, snow, sleet or hail.

Air Pressure & Weather Maps Maps

• Weather maps show the positions of air masses. An air mass is a very large body of air that has similar properties. Air masses move, so they can be tracked over time. They are clues to the future temperature, moisture level and air pressure of the area into which they are moving.

• How is air pressure measured, and plotted on a Map?

Weather Maps Show Air Pressure

H

L

Winds move Anti-Clock Wise around a high

Winds move Clock Wise around a Low

The Closer together the Isobars the stronger the winds.

Colliding Air Masses

• When one air mass meets another, the air in different masses usually does not mix because the properties of the air are different. A boundary, or front, forms between the two air masses. Stormy weather often occurs along fronts.

Cold Fronts

• A cold front forms where a cold air mass moves into a warm air mass. Cold air contracts, becomes more dense, and thus heavier than warm air, so the cold air pushes underneath the warm air. Cumulus clouds and showers or thunderstorms may be scattered along cold fronts. As the front passes, the wind changes direction. Skies begin to clear, and the temperature usually drops.

Pressure Systems, Wind &

Clouds

High Cirrus Cloud ahead of Cold Front

Cold front Cloud with cool SW winds behind it.

Clouds spiralling clockwise around the low pressure

Clear stable conditions under the high pressure

Rain can occur under the influence of a high pressure when it pumps moist winds off the ocean like here in Qld.

Using Pressure Systems to Predict the Weather

• Certain weather conditions are associated with high and low pressure systems.

• Knowing the characteristics of these systems can help you predict the weather.

• Lets take a look at a high and low pressure system and determine what each would look like if we were in the field on a trip.

High Pressure• High pressure areas, or highs, are shown by "H" symbols.

In a high pressure system, air pressure is greater than the surrounding areas. This difference in air pressure results in wind, or moving air. In a high pressure area, air is more dense than in areas of lower pressure. The result is that air will move from the high pressure area to an area of less density, or lower pressure. Winds blow away from high pressure areas toward areas having lower air pressure.

High Pressure

Low Pressure• Low pressure areas, or lows, are shown by "L"

symbols. Winds tend to blow into low pressure areas because air, like other gases, moves from areas of higher pressure into areas of lower pressure. As winds blow into a low, the air moves up. This upward flow of air can cause clouds and precipitation to form.

Low Pressure

Measuring Air Pressure

• Air pressure is measured by barometers.  In general, weather will improve when pressure increases and worsen when pressure decreases.

High Pressure Characteristics

• Stable descending air mass

• Calm winds• Often see fog or frost• Clear skies• In Australia winds

from NE area. Clear day above, fog in the valleys, under high pressure influence.

Low Pressure Characteristics

• Unstable ascending air mass.

• Winds increasing• Increasing cloud cover• As the low approaches

in Australia, temperatures increase.

• Winds swing to NW ahead of change.

High cirrus cloud, with increasing NE then NW winds, with increasing temperatures are a sign of approaching low pressure.

What Would We See in Perth• Winds flowing anti-

Clock wise around the high.

• As the cold front approaches winds swinging to the NW & strengthening. Closer together isobars = stronger winds.

• High Cirrus then stratus cloud increasing

What the sky would look like in Perth

Knowing What Conditions Clouds Form Under Can Help You Predict Changing Weather

Early Signs In The Sky For Approaching Bad Weather

High Cirrus Cloud = Bad Weather on the way!

Hot Days with Unstable Air = Cumulus Nimbus Clouds & Thunder Storms

If you see these clouds developing, think storm. Have a plan for shelter.

Storm Cells taken from a SatelliteClassic Anvil of a Cumulus Cloud

Run for CoverBegin To Worry

Cumulus Storm Clouds & Precipitation

Lightning is our biggest threat when in the outdoors in storms!

Lightning: Our biggest threat to safety when in the outdoors during a storm.

Precautions against Lightning Strikes

The weather is dynamic. You must be prepared

for all conditions.

Always be prepared for the worst!

Monitoring the Weather in the Field

• To complete the class we will now attempt to identify the weather in different parts of the country.

• The map on the following slide identifies a town or city in Australia. You have to identify the wind direction, cloud type, and temperature estimate solely from looking at the weather map.

• Your teacher will click on the hyper link to get the answers! Good luck.

Test Your Weather SkillsWhat is the Weather at…? Click on the town for the answer!

PerthKalgoorlie

Melbourne

Class, discuss the forecast for each place; then Click on the City for the answer

Monitoring the Weather on Solo

• Later this term you will be required to monitor the weather for the three days of your solo experience.

• You will then have to make a forecast as to what you think the weather will do based on your observations.

• You will need to monitor: Wind, clouds, temperatures, precipitation.

• The following sheet is a copy of the one you will fill out on solo.

• Let us attempt to fill it out for today’s conditions.

Weather Observation Recording Sheet

Date Time Location Elevation Cloud & Cover

Precipitation Wind Direction & Strength

Forecast

3 march 2000 9.00am GR666543 100 mts Rain showers earlier

South Light Wet to continue

Cloud Cover Wind Strength

Clear O Light

Scattered Cloudy ½ covered O Moderate

Partly Cloudy ¾ covered O Fresh Obscured X Strong

The End

Perth

• Hot Conditions• Winds Strengthening out

of the North West• Increasing cloud cover,

most likely High Cirrus (1), descending to alto-stratus (2) as the front approaches.

1

2

Click to return to Map

Kalgoorlie• Clear Skies• Stable Conditions• Dew on grass first

thing in morning• Light but warm to hot

northerly winds

Click to return to Map

Melbourne

• Cold Front and Low Pressure over city

• Unstable conditions

• Cumulus Clouds

• Possible storms & rain

• Warm to hot early, temperatures dropping after front has passed

• Winds turning fresh to strong from the Southwest

Click to return to Map