Sky Watchers Teachers’ Guide: chapter 1

Prior Knowledge

What do you know about the sun, how its energy reaches you and how ozone is involved?


Bright orange sun setting over the Atlantic Ocean. The sun provides heat and light energy to the Earth.

Section Summary

The sun provides heat and light energy to the earth. Some of the energy that reaches the earth passes through the atmosphere. This visible and invisible radiation reaches different parts of the earth at different times. It also builds protective ozone high in the atmosphere and polluting ozone down at ground level. The sun’s energy, and other factors like altitude and the air mass over your region, affect daily temperature and ultraviolet (UV) ratings.

There are direct and indirect ways to measure how much heat or light energy is in a certain location. Thermometers and UV meters can collect useful information for making everyday choices.

Everything on earth is affected by the energy from the sun. It influences our natural and built environments.

1.1 Read with Understanding: Energy

Energy From the Sun

The story of weather starts with the sun, which provides heat and light energy to the earth.

The sun’s energy travels 150 million kilometres to the outer edge of the earth’s atmosphere. The sun radiates energy in short waves to the earth but clouds, dust and water vapour in the atmosphere deflect about half of that energy back into space.

The energy that reaches the earth is either visible light or invisible radiation. One form of invisible solar radiation is ultraviolet or UV. It has a shorter wavelength than visible light but carries more energy. UV is classified into three types: UV-A, UV-B and UV-C. Much of the sun’s UV-A reaches the earth’s surface, whereas most of the UV-B, and all of the UV-C, are filtered out by the earth’s atmosphere, primarily by the ozone layer.

The ozone layer is produced naturally about 15 to 35 km from the ground, high up in the atmosphere. A cycle of forming, then breaking up ozone molecules, maintains a natural balance of high-altitude ozone, protecting us from harmful UV radiation. This protection is best when there is a lot of high-altitude ozone, and worst when manufactured products and greenhouse gases, such as chlorofluorocarbons (CFCs), react and destroy the ozone layer. The thicker the ozone layer, the more UV it can absorb.

ozone layer

A drawing depicting the Earth surrounded by the ozone layer with UVA, UVB and UVC rays coming from the sun and penetrating the ozone layer. The ozone layer absorbs some but not all types of ultraviolet radiation.

Ozone is also produced at ground level, but here it is harmful to health, rather than protective. Ground-level ozone is formed when pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) react using this energy from the sun. As a result, ground-level ozone is higher when there is more NOx from burning fossil fuels and more VOCs from the evaporation of liquid chemicals. Since sunlight is required to form ozone, the concentrations in the air are normally higher in the summer, when temperatures are warmer and the sun’s rays are stronger.

Once the sun’s visible and invisible energy passes through the atmosphere, it is absorbed by the land and water. Here, it is converted to heat to warm the earth and the air above it, as well as melt snow and evaporate water. The earth radiates this heat back as long-wave energy, which then warms the air above.

Because the earth moves, not all areas receive the same amount of energy at the same time. Also, even if the amount of energy received is the same, not all surfaces on earth absorb the same amount of energy.


Emissions filling the air around an industrial site. Ground-level ozone is higher when there is more NOx from burning fossil fuels and more VOCs from the evaporation of liquid chemicals.


Mist hanging over a lake at night. At night, the Earth continues to radiate heat, even though it receives no energy from the sun.

The Earth’s Rotation Causes Night and Day

The earth rotates on its axis every 24 hours, creating night and day. During the day, the earth receives more energy than it radiates back, so it warms up. At night, however, the earth continues to radiate heat, even though it receives no energy from the sun. Consequently, the earth cools down. This cooling process continues until after sunrise, which is one of the reasons why sunrise often has the lowest temperature for the day.

The Earth’s Orbit and Tilt Cause Winter and Summer

The earth revolves around the sun every year. During the earth’s orbit, some regions receive more of the sun’s energy than others.

The earth is also tilted on its axis at an angle of 23½ degrees. Without that tilt, the sun would shine directly over the equator all year and there would be no seasons. Instead, the sun’s energy hits different parts of the earth at different angles, affecting the amount of heat any one part of the earth receives. This unequal heating also sets the air in motion, creating global wind belts.

Canadians do not need to be reminded about the effect that the seasons have on temperature. Canada has its warmest weather when the sun is over the Northern Hemisphere. The sun passes over the equator around March 21st on the way north to the Tropic of Cancer at 23½° north latitude. This is the sun’s northern-most position, which it reaches around June 21st. From here, the sun starts the slow slide south again to the equator, reaching there about September 21st.

During the 6 months the sun is over the Northern Hemisphere, its rays shine down on Canada more directly than they do during the country’s winter months. The sun reaches the Tropic of Capricorn at 23½° south latitude around December 20th, when it starts moving north again to the equator.

North and South

The latitude of a region affects how much of the sun’s energy it receives. The countries around the equator receive more of the sun’s direct energy than those that lie farther north or south. That is because the sun’s rays are almost perpendicular to the earth’s surface at the equator. To reach areas closer to the poles, such as Canada, the sun’s energy must travel at an angle and pass through more of the atmosphere. Consequently, by the time the sun’s rays reach Canada, they are weaker, more spread out and diffuse than the rays that hit the earth around the equator.

Earth and atmosphere

A drawing that depicts the Earth and its atmosphere, and the angle that the sun’s energy must travel to get to the North Pole and to the Equator. Countries around the equator receive more of the sun’s direct energy than those that lie farther north or south.

Demonstration: What Energy Reaches the Earth’s Surface?Footnote 1


  • a globe or map of the earth
  • a small flashlight


  1. In a darkened classroom, shine the light directly down on the equator.
  2. Keeping the flashlight at the same height and over the equator, tilt the flashlight so that it shines on the Tropic of Cancer, north of the equator.


Where do you think the light will be brightest? Can you see its outline clearly? What does the shape of the light look like? How does the area of light change when it is tilted? Does the light cover a larger area? Are some parts of the area lit by the flashlight brighter than others?


The shape and the intensity of light changes when it shines at an angle. The earth’s tilt affects the strength of the sun’s rays at different points on the earth’s surface.

Geography Affects Heating and Cooling

Snow melting

Snow melting along the edge of dark pavement. Ice and snow reflect energy whereas pavement absorbs it.

The geography of an area plays a role in its heating and cooling because factors such as material and color affect how a surface absorbs the sun’s energy.

One reason for this is that water heats and cools more slowly than air. For example, coastal communities often have lower summer temperatures and warmer winter temperatures than communities that are farther inland.

A dark-coloured surface absorbs more of the sun’s energy than a light-coloured surface. Ice and snow reflect energy, whereas pavement absorbs it. You may have noticed that cities can experience different heat effects.

Record-Setting Temperatures Across Canada
Province or Territory Highest Temperature (°C) Lowest Temperature (°C)
Alberta 43.3 Bassano Dam -61.1 Fort Vermilion
British Columbia 44.4 Lytton, Lillooet -58.9 Smith River
Manitoba 44.4 St. Albans -52.8 Norway House
New Brunswick 39.4 Nepisiguit Falls -47.2 Sisson Dam
Newfoundland and Labrador 41.7 Northwest River -51.1 Esker 2
Northwest Territories 39.4 Fort Smith -57.2 Fort Smith
Nova Scotia 38.3 Collegeville -41.1 Upper Stewiacke
Nunavut 33.6 Baker Lake -57.8 Sheppard’s Bay
Ontario 42.2 Atikokan & Fort Frances -58.3 Iroquois Falls
Prince Edward Island 36.7 Charlottetown -37.2 Kilmahumaig
Quebec 40.0 Ville Marie -54.4 Doucet
Saskatchewan 45.0 Midale and Yellow GrassTable note a -56.7 Prince Albert

Heat Waves

Despite the country’s reputation as the land of snow and ice, Canada has heat waves. Environment and Climate Change Canada (ECCC) defines a heat wave as 3 or more consecutive days with temperatures of 32°C or higher. Most heat waves in Canada last about 5 or 6 days.

The worst heat wave on record was in July 1936. The heat rolled into the Prairies from the American southwest in early July and spread into Ontario. Temperatures climbed to 44.4°C at St. Albans in Manitoba and to 42.2°C at Atikokan and Fort Frances, Ontario. Those records stand today. The heat wave lasted a week and was directly or indirectly responsible for the deaths of 780 people in Canada.

1.2 Observe

Measuring Energy from the Sun

There are direct and indirect ways to measure how much heat or light energy is in a certain location. Two measures are of particular interest because they affect everyday choices: temperature and UV radiation. Temperature is a measure of the average heat energy in the air. UV radiation is only one part of the sun’s invisible energy that reaches the earth’s surface, but an important one.


The substance inside a thermometer reacts to temperature changes. This may be a liquid or a metal coil. Thermometers using a liquid are faster to respond to temperature changes than those with a metal coil. This is due, in part, to the sheer mass of the metal coil, and to the fact that the air flow over the coil is reduced.  

In Canada, we use metric units called degrees Celsius (˚C). Some thermometers can be set to track the current, maximum and minimum temperatures. Others only indicate the current temperature.

Hang a thermometer at eye level and away from direct sunlight. A shady, secure, grassy area on the north side of a school may be the best spot. Try to allow for air to flow across the unit. This will improve the reaction time of the thermometer to changes in air temperature.

If you do not have a secure area for permanent instrument exposure, store the thermometer in a classroom and hang it outside for 30 minutes before you take a reading.

Building a Weather Instrument: Thermometer


  • small, narrow glass jar with cork or stopper
  • cooking oil
  • a sealant (e.g., petroleum jelly, candle wax or modelling clay)
  • several drops of food colouring
  • clear narrow drinking straw at least 15 cm long
  • eye dropper
  • water
  • an index or recipe card about 8 cm by 13 cm
  • thermometer for reference

Note: The width of the straw and the amount of liquid in the jar will affect how quickly and accurately a thermometer will respond. With a narrow straw, a smaller volume of water is required to noticeably raise the level in the straw.


  1. Fill the glass jar with water and add a few drops of food colouring to make the water visible.
  2. Cut a hole in the stopper or cork, just large enough to fit the straw.
  3. Place the stopper in the jar and insert the straw through the hole.
  4. Add more water, but this time through the straw and until the water is about one quarter of the way up the straw.
  5. Seal the straw into the stopper and the stopper onto the jar using either the petroleum jelly, modeling clay or candle wax.
  6. Finally, put a drop of the cooking oil into the straw so that the oil sits on top of the water. The oil prevents the water from evaporating.
  7. Attach the index card to the straw. Allow the thermometer to settle for two or three hours.
  8. Use the reference thermometer to calibrate your home-made thermometer. To do this, note the level of water in the straw and mark a line on the card. Beside the line, record the temperature shown on your reference thermometer. Repeat this process over the next several days.

Why it Works

This thermometer is based on the principle that water expands when heated and contracts when cooled. It takes a long time to respond because the entire jar of water must adjust before it will register the new temperature.

UV Meter

A UV meter will measure the burning effect of UV radiation on human skin and express it using the UV Index. Many meters turn on automatically when exposed to sunlight. Consult the instruction manual to determine if the model should be held flat or on an angle. In either case, hold the meter about 30 cm in front of your body and out of the shade.

It is easiest to compare UV levels from 11 a.m. to 4 p.m. on sunny days in May or June. For best results, move away from buildings that might reflect additional light. Make sure your fingers or other objects do not shade the meter, particularly the sensor window on the front edge of the meter. Try not to touch the sensor window as this may scratch or streak the window and affect the readings.

1.3 Predict

The easiest way to get information about UV and temperature is using ECCC’s weather website at You can check the website daily to gather data for graphing or to do community comparisons.

Weather Reports: Temperature

Although it is easy to measure temperature with a thermometer, it is tricky to forecast it. ECCC feeds current weather conditions into complex computer models to predict temperature. Mathematical equations then predict how the weather may change over time because of wind direction, sky conditions, precipitation, how close the forecast area is to large bodies of waters, or the arrival of a new air mass into the region.

How did they predict temperatures before computer models? A simple start was to forecast the same temperature tomorrow as was observed today. Then, meteorologists had to ask a lot of questions about air masses, cold or warm air movement, expected sunshine or cloudiness, wind direction and strength, and whether the wind was blowing off the land or the water.

Nowadays, meteorologists have to be computer savvy to ensure that mathematical equations and weather models consider all of these things.

Weather Reports: UV Index

The UV index was developed by ECCC to inform Canadians about the strength of the sun’s UV rays. The higher the number, the stronger the UV rays, and the greater the need to protect yourself.

Canada was the first country in the world to issue daily UV index forecasts for major cities. Latitude, cloud cover and precipitation are factored into the local UV index. The UV Index is reported when the index is 3 or above.

ECCC’s computers produce a daily UV Index forecast for many Canadian communities based on the angle of the sun at midday, predictions of the ozone-layer thickness, observations at Canadian ozone-monitoring stations and the forecast cloud cover. The UV Index forecast is a prediction of the maximum UV Index value for that day (morning forecast) or the next day (evening forecast). Under clear skies, this will occur at midday when the sun is at its highest point in the sky. In the summer, this generally occurs from 1 to 2 p.m.

For more information about the UV Index, go to The UV Index and Ozone.

Weather Reports: Air Quality Health Index (AQHI)

Another index, the Air Quality Health Index (AQHI), gives information to the public about air pollution. One pollutant included in the AQHI is the amount of ground level ozone produced using energy from the sun. Since ground-level ozone is invisible, it is useful to have an index that measures air quality and suggests how to adjust activity levels for the conditions.

For more information about AQHI, go to Air Quality Health Index.

Classification: Compare and contrast the UV Index and the AQHI.

1.4 Reflect

What Does Temperature Mean to Me?

Temperature and Clothing

On cold days, you have probably been told to wear several layers of clothing to keep warm. That is because air trapped between layers acts as an insulator and slows down the loss of heat from your body.

Did you know that the colour of your clothes might affect how warm you feel? Light colours reflect more of the sun’s energy than dark colours, so are cooler to wear on a sunny day. By the same logic, black keeps you warmer in the sun.

Temperature and the Built Environment

Home builders consider the climate of a region when designing the layout of a building. In many parts of Canada, building a house with large windows facing south, for instance, can reduce heating costs. The sun is lower on the horizon in winter so the sunshine pouring in through the windows will partially heat the house. Interestingly, in summer, the sun is more directly overhead so it does not have the same effect.

Classification: What features of a home, building or landscape make good (or bad) sense for heating or cooling? For example, coniferous trees provide shade from the sun and shelter from cold winds.

Afternoon sun

Afternoon sun pouring in through a large, south-facing window. The sun is lower on the horizon in winter, so the sunshine pouring in through the windows will partially heat the house.

What Does UV Mean to Me?

Human Health Effects of UV Radiation

Although energy from the sun is essential because it sustains all life on earth, some forms of the sun’s energy can be harmful.

Skin damage caused by UV radiation is cumulative, whether it comes from the sun, or artificial sources such as tanning beds. Daily exposure to natural or artificial sunlight adds up, damaging the DNA of our skin cells. A tan, like a sunburn, is a sign that the skin has been damaged by UV radiation. If cells cannot repair the DNA, skin cancer can result. The occurrence of skin cancer has increased in Canada at a fairly constant rate over the past 30 years. In 2010, there were roughly 75,500 new cases of basal and squamous cell carcinomas reported in Canada, and about 5,300 new cases of malignant melanomas.

In addition to skin cancers and sunburns, overexposure to UV rays can lead to a weakening of the immune system, eye damage, eyelid cancers, and premature aging of the skin.

Silhouette of plant life

Silhouette of plant life on the edge of a lake. Increased UV exposure in lakes and oceans can damage tiny single-celled plants called phytoplankton that provide food for fish and other animals.

UV Effects on Plants and Wildlife

Ultraviolet radiation not only affects humans, but wildlife as well. Excessive UV-B inhibits the growth processes of almost all green plants. There is concern that ozone depletion may lead to a loss of plant species and reduce global food supply. Some agricultural crops, such as canola, oats, and even cucumbers, show reduced yields at higher levels of UV.

Ultraviolet radiation has an effect on natural communities. Increased UV exposure in lakes and oceans can damage tiny single-celled plants called phytoplankton that provide food for fish and other animals. Sudden, brief UV increases during early spring can damage young vegetation or the eggs of fish and frogs.

UV and the Built Environment

Community planners design for shade around buildings and recreational areas. UV also reduces the lifetime of the construction materials used in buildings.

Staying Protected

It is important to adopt good sun protection strategies at an early age, because skin damage caused by the sun builds up over time. Walking, biking, recreation and sports are great ways to be active outdoors. Simply check and plan for UV as you would check for the weather.

The following is a list of protective measures. Enjoy the sun safely. Protect your skin, protect your eyes.

  • Seek shade or bring your own (e.g., an umbrella).
  • Wear clothing that covers as much skin as possible and a wide-brimmed hat.
  • Wear sunglasses or eyeglasses with UV protective lenses
  • Wear approved UV-filtering sunglasses.
  • Use sunscreen labelled "broad spectrum" and "water resistant" with a sun protection factor (SPF) of at least 30 on skin not covered by clothing. Apply sunscreen generously and reapply often.
  • Avoid getting a sunburn and avoid intentional tanning.

The table below outlines the sun protection actions recommended at different levels of the UV Index.

UV Index Description Sun Protection Actions
0 to 2 Low Minimal sun protection required. If outside for more than one hour, wear sunglasses and sunscreen. Reflections can nearly double UV strength.
3 to 5 Moderate Take precautions. Cover up, wear a hat, sunglasses and sunscreen if outside for 30 minutes or more. Look for shade near midday.
6 to 7 High Protection required. UV damages skin and can cause sunburn. Reduce time in the sun between 11 a.m. and 3 p.m. Seek shade, cover up, wear a hat, sunglasses and sunscreen.
8 to 10 Very High Extra protection required. Unprotected skin can be damaged and burn quickly. Avoid the sun between 11 a.m. and 3 p.m. Seek shade, cover up, wear a hat, sunglasses and sunscreen.
11+ Extreme Maximum protection required. Unprotected skin will be damaged and burn in minutes. Avoid the sun between 11 a.m. and 3 p.m. Remain in the shade, cover up, wear a hat, sunglasses and sunscreen.

For more information on sun safety, consult Health Canada’s UV Index Sun Awareness Program or the Sun Safety website.

Investigation: Determining the Best Protection From UV


  • UV meter(s)
  • pencil and pad
  • three (3) different types of fabric (e.g., cotton, synthetic, silk) in the same colour
  • three (3) different colours of fabric of the same type and weight (e.g., black, white, and
  • red cotton)
  • three (3) pairs of sunglasses
  • several leaves and/or types of shade (e.g., buildings, tree(s), umbrella)
  • calculator


  1. Choose a day when the UV Index is forecast to be 6 or greater.
  2. Identify variables and assign to groups, each with their own UV meter.
  3. Make predictions about which materials or shade give the best protection against UV.
  4. Put the samples in a random order.
  5. Take a UV reading with the UV meter in direct sunlight and record the value.
  6. Place each sample, one at a time, on the UV meter over the sensor. Ensure that the sensor is completely covered. Do not touch the sensor with your fingers. To sample types of shade, move the UV meter from direct sunlight to the designated shaded area before taking the reading.
  7. Wait a minute for the UV meter to adjust between samples.
  8. Calculate the percentage of UV transmitted through each type of fabric sunglasses or shade compared to the full sun reading.

UV transmitted (%) = (reading under sample ÷ full sun reading) x 100%
UV reduction (%) = 100% - UV transmitted


  • Which fabrics give the best protection against UV?
  • Which sunglasses give the best protection against UV?
  • What type of leaf or shade gives the best protection against UV?
  • Why is it important to change only one variable at a time?
  • Why should the samples be in a random order?
  • How would variable cloudiness affect the results?


The more of the sky that is blocked by trees and other objects, the lower the amount of UV. In general, denim and polyester protect better than cotton; tight weaves, better than loose; dark colours, better than white; and heavy fabrics better than light. Sunglasses show approximately how well a pair of sunglasses protect your eyes from UV-B. Accurate UV testing for sunglasses can only be conducted by an optometrist or ophthalmologist. UV-A is also harmful to the eye. When buying sunglasses, it’s wise to check the label for the degree of protection from both types of UV.

What Does Ground-level Ozone Mean to Me?

Human Health Effects of Ground-level Ozone

Exposure to ozone can irritate the nose and throat and cause chest tightness, coughing and wheezing. Increases in ground-level ozone in Canada have been linked to increased mortality, emergency hospital visits and admissions for respiratory problems. In sensitive people, the stress of ozone exposure can be particularly damaging. There is also evidence that ozone heightens the sensitivity of asthmatics to allergens. Other studies on animals have indicated that ozone exposure decreases the ability of the lungs to ward off disease. The effects also include decreased lung capacity, which can impair performance in athletes

Demonstration: How it Feels to Have Breathing Problems When the AQHI is High

Health note: This activity must be voluntary. It is not for high-risk groups! Restricted airflow may cause dizziness or hyperventilation.


  • juice box straw or other very thin straws


  1. Take 10 normal breaths.
  2. Put the straw in your mouth and plug your nose.
  3. Take 10 breaths again.


  • What is the difference between breathing normally and breathing through a straw?
  • How would it feel to breathe like this all the time?
  • What do you think would happen if you tried to play sports or run, breathing only through the straw?
  • How could you simulate ground-level ozone in addition to a pre-existing health condition?


The straw simulates how air passages constrict, limiting air flow to the lungs of a healthy individual when the AQHI is high or to an individual with breathing difficulties when the AQHI is lower.

Effects of Ozone on Plants

Ozone interferes with the ability of plants to produce and store food. This threatens growth and reproduction and makes plants more susceptible to pests and disease.

Some estimate that the provinces of British Columbia and Ontario each lose millions of dollars per year because of lower crop productivity due to high levels of ground-level ozone. Ozone damage can be seen on the foliage of some potato varieties in Atlantic Canada. Beans, tomatoes, potatoes, soya beans and wheat are all crops that are sensitive to ozone. Trees, which live longer than the plants above, are exposed to ozone year after year. If the effects of exposure add up over many years, entire forests may be affected. This means that plants and animals that depend on the trees to provide shelter are also affected by prolonged exposure to ground-level ozone.

Classification... Compare and contrast ground-level ozone in rural and urban areas. (Rural pollution can be just as bad as urban pollution, depending on a combination of local weather conditions, topography, or the amount of pollution due to long-range transport.)

Peeling paint

Paint peeling off wood siding. Certain types of paints and other coatings are damaged or weakened by ozone exposure.

Ozone and the Built Environment

Materials you use in everyday life can be weakened by exposure to high levels of ozone. Rubber, textile dyes and fabrics, and certain types of paints and other coatings are either damaged or weakened by ozone exposure. Synthetic elastic materials can become brittle and crack, while the textiles and dyes fade faster than usual.

For more information on the effects of ground level ozone, go to Air Quality Health Index.

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