Convection, or hot air rising through cooler
air around it, is perhaps the most important process to
understand, in the weather around us. Convection can can
change our weather in a matter of minutes. It quickly changes
a clear blue sky into a cloudy one, and can turn harmless
puffy clouds into raging storms with lightning, hail and
unbelievably powerful winds. A great way to visualize
this is to show your students that convection happens in
water, too. When an area of water is warmer than the
water around it, it will try to rise through that water.
Just like air rising below a cloud, hot water will cool
as it rises through its cooler surroundings. With
some food colouring, water allows us to see convection as
it happens. Materials: one large jar and a much smaller
jar which easily fits inside it, a small piece of foil,
an elastic band, food colouring and warm and cold water.
Fill the larger jar with cold water. Fill the tiny one with warm
water and add a couple of drops of colouring. Now place the foil tightly
over the top and secure it with an elastic band. Gently lower this jar into
the large jar. Lastly, take a sharp pencil and gently poke two tiny holes in
You should see a flow of warm, coloured water, upward into the cold water in
the jar. This water moves quickly, just above the small jar, and slows down
as the warm, coloured water mixes with the colder air around it.
Why does it happen?
Warm water is lighter than cold water, just as warm air is lighter than cold air. The lighter, warm water in the small jar rises up through the colder,
heavier water around it. The colouring lets us see the same kind of upward motion that creates clouds and sometimes, storms.
See the Barometer Work
Barometers measure air pressure to give us valuable information about the changes we can expect in the weather. Italian scientist Evangelista
Torricelli invented the barometer nearly three hundred and fifty years
ago...and we've used them to predict weather ever since. Torricelli used
mercury in his instruments. Since mercury is extremely poisonous, we'll use a simple but effective design that you can safely make at home or direct the
class to build.
You can also carry out just part of this experiment, to make a simple, but sensitive barometer. If placed against a background of graph paper,
somewhere they won't be disturbed, your students' barometers will indicate
falls or rises in air pressure over time and allow them to make hands-on
a mid sized jar and a tiny one that fits inside, a 20 cm or larger balloon, two rubber bands, a toothpick or 5cm long
piece of drinking straw and a bit of rubber cement or stick glue.
Cut the balloon in two, about two thirds of the way to one end,
so that the large part has the neck and the smaller part has the round end
with no neck. Stretch the smaller part (the part without the neck) over the
little jar, and secure it tightly with an elastic band. Glue the large end of a flat toothpick or drinking straw to the balloon lid on the small jar,
so that it hangs over the end of the jar. Wait for the glue to dry.
Carefully place the small jar inside the larger one. Tie a knot in the end
of the neck of the other peice of balloon, then stretch it over the top of
the large jar, just like you did with the small jar.
Pull on the neck of the balloon to decrease the pressure in the big
jar...and carefully watch the little jar inside it. Does the toothpick move
up or down? Watch the little jar's rubber top as you pull out on the big
one's top; does air inside the little jar expand or get smaller? Why? If you
push on the rubber top, using a few fingers, what happens to the little jar?
Why does it happen?
When you pull on the balloon, you create more space for the molecules inside
the jar - but the same number of molecules remains. Making more space for
the molecules, or lowering the pressure, makes them spread apart, and take up that extra space. The lower pressure in the big jar pushes less on the
little jar, making its molecues spread apart, too. These molecules try to leave the little jar, pushing out on the rubber top.
Pushing on the balloon top does the opposite; leaving less space for the
molecules, it forces them closer together. This force also pushes on the lid of the small jar, making the molecules in the small jar come closer together
Heats Faster Than Water
The earth, heated by the sun, is responsible for a wide range of weather conditions. The way the sun warms some places more than
others is directly responsible for our local weather and all the different climates on earth. A
very important part of weather forecasting is predicting how the sun will warm the surface of the earth. Different surfaces heat up differently, when
the sun's rays hit them. Dark, dry surfaces heat up much more quickly than
bodies of water. In this experiment, you will prove that soil warms at a different speed than water.
A bright lamp or a window with direct sunshine for at least 30
minutes, two shallow cups or small pans, water, dry soil without stones, and
Fill one cup with the soil and the other with water. Carefully put a theromometer in each cup, wait five minutes and record the temperature
of the soil and the water. Place the cups in the sun or under the light.
Which heated faster - the soil or the water?
Predict the temperatures of the soil and water after another thirty minutes
in the sun. Record the temperatures again, after 30 minutes. Did the
temperature rise as much? Why or why not?
Why does it happen?
Sun, or strong lights, radiate heat toward all surfaces. However, dry and dark surfaces may heat up much more quickly. Dark
surfaces absorb much more heat. You have probably noticed that a driveway, a road, or a other dark
surface is a lot hotter on a sunny day than a field or a swimming pool. This is because the pavement, or soil, warms much more quickly than a green field
or a pool of water. Also, a pool of water shares its heat with the bottom of the pool better than soil shares its heat with the earth below; in other
words, for the pool, or your cup of water to warm up, it must warm the water below it, as well. This takes a lot more time, or a lot more light energy.
Clouds usually form high above the ground, in moist air which has cooled as
it rises. Clouds can also form on the ground, if it cools very quickly at night. Cooler air cannot hold as much moisture, so this moisture condenses
into tiny, but visible droplets or ice crystals - and these droplets are what we see as a cloud. You can demonstrate the formation of a cloud, with a
bottle full of warm water vapour and some ice.
A clear bottle that you can easily see into, very warm water and an ice cube.
Fill the bottle with warm water. Let it stand for a few minutes and pour out the water. Immediately place the ice cube on top of the bottle,
so that it covers the neck. Watch carefully under the ice cube.
A dense cloud should form, in the air at the top of the bottle, just below the ice cube.
Why does it happen?
Water exists in three forms: solid ice, liquid water and the gas form, water vapour. Warm air can contain a lot of water vapour...and your wet bottle has
a great deal of vapour inside. When you placed the ice cube on top, you cooled some of the air and vapour inside the bottle. This cool air cannot
hold as much water vapour, so some of it condenses into tiny droplets. You see these droplets as a cloud, visible below the ice cube.
Build an anemometer...your own wind speed
Meteorologists use anemometers to measure wind speed and with this simple hand-held one, your classes can, too. By counting the number of times the
unit spins, per minute, students can actually measure the wind's force.
The Beaufort wind scale uses visual cues, such as smoke rising, leaves rustling and branches breaking off trees, to estimate the speed of the wind.
These visual descriptions of wind speed can be compared with the results obtained from anemometers that the students build.
The actual wind speed, measured by Environment Canada, can be used to calibrate the students' anemometers. See below for addresses.
Four very small paper cups, long and sturdy plastic straws, tape,
pins, pencils with erasers.
Arrange two plastic drinking straws to form a cross and tape them securely together at the centre. Tape one drinking cup (the small paper
Dixie (tm) cups designed for bathroom dispensers work well) to the ends of each straw, so the closed ends of the cups all face the direction the
anemometer rotates. A straight pin can be pushed through the centre of the straws (where they overlap) into an eraser on the end of a pencil. This
provides the axle along which the straws and cups will rotate. Mark one
cup brightly, to act as a reference.
The unit spins, when wind blows past it; faster in stronger winds and slower in weaker winds. The speed of the wind can be measured by the number of
complete revolutions that the anemometer makes in one minute. The greater the speed at which it is rotating, the faster the wind is blowing. In high
winds, it's almost impossible to count; this is why professional anemometers use computerized, digital counters!
Why does it happen?
The tapered end of the cups moves more easily through the wind than the open, wider end. As wind pushes on the wide end, it creates a force greater
than the force pushing on the (opposing) small end, so the unit starts to spin.
a weather vane... your cue to the wind's direction
Wind direction is key to predicting weather;
a sudden shift in direction often signals a major change
in the weather.
Wind often follows patterns, associated with high and low
pressure areas; in Ontario, winds are often Northwest as
an area of high pressure (good weather) moves overhead,
and Southeast as the high pressure and good weather are
By measuring wind direction, students gain an important
look at one of the simplest, but most important variables
used to predict weather.
Modelling clay or play-dough, small triangles (5cm by
5cm by 5cm) of thin cardboard (cereal box material is ideal)
long and sturdy plastic straws, tape, pins, pencils with
Attach a few gram lump of clay to one end of the straw.
Slice the other end open about a centimetre, so a small
triangular piece of cardboard can be inserted and taped
(vertically and little end stuck into the straw) on to the
straw. A straight pin can be pushed through the middle of
the straw (where it balances) into an eraser on the end
of a pencil. This provides an axle along which the straw
The unit turns, so that the small end points into the wind.
This is the wind direction. Using a compass, or known
orientations in your schoolyard, the students can determine
the direction from which wind is blowing.
Why does it happen?
The heavy, smaller end of the straws provides less resistance
to the wind than the large, cardboard triangle end. As wind
pushes on the wide end, it creates a force greater than
the force pushing on the (opposing) small end, so that end
tries to blow with the wind as much as possible; and ends
up exactly downwind of the small end. This points the small
end into the wind.