Grade 4 Science
Wk – 13 11/8
How can weather be predicted?
Air masses
Have you ever wondered why the weather can be sunny one day and
rainy the next?
Movements of air masses cause weather changes.
An air mass is a large body of air. All of the air in an air mass has a
similar temperature and moisture level.
Moisture level means the amount of water that is in air.
Air masses that form over land are dry. Air masses that form over
water have a lot of moisture in them.
The temperature and moisture level of an air mass affect the kind
of weather the air mass brings. Cold, wet air masses can bring snow
to an area. But cold, dry air masses can bring cool weather with no
rain.
As air masses move, they stay separate from each other. That’s
because warm air is lighter than cold air. When they come in near
each other, warm air masses are pushed upward and cold air masses
sink.
Fronts
When air masses move, they come into contact with other air
masses. The border between one air mass and another is called a
front. Most storms happen in fronts.
There are two main types of fronts: cold fronts and warm fronts.
A cold front forms where a cold air mass moves under a warm air
mass. Thunderstorms will often develop.
A warm front forms where warm air moves over cold air. Warm
fronts move slowly and bring steady rain.
Weather maps
A weather map helps you know what the weather is like in an area.
Weather maps use symbols to show the weather.
A sun symbol means it is sunny in the area.
A symbol of a cloud with rain means it is raining in the area.
Fronts are also shown on weather maps.
The symbol for a warm front is a red line with half circles along it.
A blue line with triangles shows a cold front.
Many weather maps show temperature.
Temperature is shown in degrees Celsius. When the temperature is
not written, it may be shown using colours.
When an area is warm, it will be coloured red (very hot), orange
(warm), or yellow (mild).
When an area is cold, it will be green (cool) or blue (very cold).
What is the weather like in
PTA (Pretoria)?
____________________
What is the weather like in
CTN (Cape Town)?
____________________
Where does all the information on a weather map come from? It is
collected at thousands of weather stations across each country.
Measuring weather
When you say that it is hot or cold outside, you are describing one
part of weather – the temperature.
The most accurate way to describe weather is to use data from
weather instruments.
Barometers measure air pressure.
Anemometers measure wind speed.
Rain gauges measure rainfall.
Thermometers measure temperature.
Make an anemometer or rain gauge by re-using materials.
Draw a picture or stick a photo of what you made.
My Dictionary
air mass: ______________________________________
moisture level: __________________________________
cold front: _____________________________________
warm front: ____________________________________
symbol: _______________________________________
barometer: ____________________________________
anemometer: ___________________________________
rain gauge: _____________________________________
thermometer: ___________________________________