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Sky Watchers Teachers’ Guide

Section 2 Water Vapour

Prior Knowledge

What do you know about humidity and clouds, and why we should care about particles in the air?

Puffy white clouds

Description

Puffy white clouds in a bright blue sky. When water droplets or ice crystals condense around particulate matter, it creates clouds.

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Section Summary

Water cycles around the earth as liquid, vapour, or solid ice and snow. Humidity represents the amount of water vapour in the air. As the temperature increases, the air can hold more water vapour. Water starts to condense, or come out of the air, at its dew point temperature. When these water droplets or ice crystals condense around particulate matter, many types of clouds are created at different altitudes. In certain cases when hot, humid air is forced to rise quickly, tall, turbulent thunderstorm clouds are formed.

Weather instruments such as a hygrometer or sling psychrometer can be used to measure humidity and relative humidity. Cloud cover indirectly represents the amount of condensation taking place in the air. It is one of the weather observations collected and plotted on weather maps using an internationally accepted code called a station model.

Canadians rely on weather reports for information that helps inform their day-to-day decisions. In summer this includes the humidex, which indicates how high the temperature feels when combined with humidity.

Clouds are visible using satellite imaging, and give forecasters clues to help track and predict local and regional weather. Forecasters at Environment and Climate Change Canada (ECCC) regularly use satellite imaging to monitor weather systems. They also issue severe weather bulletins to keep the Canadian public advised of weather events that could affect their safety or property.

If folks don’t want to leave it to the experts, they can look around for condensation and humidity clues to predict the weather themselves. However, they should pay attention when there are clues they can’t ignore, like thunder. Thunderstorms come hand in hand with lightning, hail and downbursts, which are serious hazards.

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2.1 Read with Understanding: Water Vapour

The Water Cycle

On Earth, water is constant in quantity and continuous in motion. Little has been added or lost over the years. The same water molecules have been transferred time and time again from the oceans and the land into the atmosphere by evaporation, dropped on the land as precipitation, and transferred back to the sea by rivers and groundwater. This endless circulation is known as the “hydrologic cycle”.

Drawing of the hydrologic cycle

Description

A drawing of the hydrologic cycle showing the processes of condensation, precipitation, transpiration, evaporation and surface runoff. On Earth, water is constant in quantity and continuous in motion.

In this cycle, the energy from the sun evaporates liquid water and sublimates solid water. The heat changes water directly from a liquid or solid to a gas. Plants transpire. They release water as a gas through their leaves.

Water Vapour

People often think of water as a liquid, but Canadians know that water is also solid ice and snow. Likewise, in the hot summer humidity, it is impossible not to feel the water that exists as a gas, or vapour, in the air.

There is always a small amount of water vapour in the air, but warmer air can hold more water vapour than cooler air.

When meteorologists talk about the amount of water in the air, the terms they use most frequently are relative humidity and the dew point temperature.

Relative Humidity

The relative humidity is the amount of water vapour that is actually in the air compared to the amount of water vapour that could be in the air at that temperature. Relative humidity is given as a percentage. For example, a relative humidity of 100% means the air is full of water vapour. It is saturated. Relative humidity of 25% means the air contains one quarter of the moisture that it could hold at that temperature.

Demonstration: From Liquid to Vapour and Back Again

Materials

  • water
  • saucer or shallow bowl
  • tape
  • a healthy houseplant
  • clear plastic bag (light has to shine through bag)

Method

  1. Pour some water onto the saucer or shallow bowl.
  2. Mark the level of the water with a piece of tape.
  3. Place the dish with the water in it on a windowsill for the day.
  4. Wrap the plant (pot and all) in the plastic bag and put it on the windowsill for a few hours.
  5. Predict what will happen to the water and the plant.

Questions

  • Why does the level of the water drop below the tape mark?
  • Where did the water go?
  • Why place the items on the windowsill?
  • What does a plant have to do with the water cycle and weather?

Explanation

Liquid water enters the atmosphere through evaporation. This can happen directly using heat energy from the sun. The evaporation of water through pores on the leaves of plants is called transpiration. Transpiration is essentially a way that water moves from deep within the soil up into the air. Underground water can be drawn up through the roots of plants, through the plants themselves, and released from the underside of plant leaves.

Dew Point Temperature

The dew point temperature is the temperature cool enough for the moisture to condense out of the air, forming dew. For example, if the dew point temperature is 10°C, then the air temperature has to fall to 10°C before it becomes saturated and its water vapour condenses to form water droplets or dew.

That is why dew forms on clear nights when the earth’s heat radiates back into space. The air cools down to the dew point temperature, water condenses out of the air, and dew forms on objects at the earth’s surface such as grass and flowers.

Outline of a spider's web

Description

Dew revealing the intricate outline of a spider's web. The dew point temperature is the temperature at which moisture will begin to condense out of the air, forming dew.

Emissions from a power-generating station

Description

Emissions from a power-generating station. The major sources of particulate pollution are factories, power plants, trash incinerators, motor vehicles, construction activities, wood stoves, fireplaces, forest fires and natural dust blown around by the wind.

Meteorologists use the term dew point temperature even on the coldest winter day, although frost point temperature may be a better name for it. On cold winter days, the water vapour changes from a gas directly to a solid without becoming a liquid first.

Particulate Matter

Clouds form when moisture laden air cools to its dew point and water droplets or ice crystals form around little particles such as dust, pollution and volcanic ash. These little particles are called particulate matter (PM).

Particulate matter is made up of very tiny solid or liquid particles that are small enough to remain suspended in the air. Even coarse particulate matter (PM10) is under 10 micrometres (µm) in size, or 1/8th the size of a human hair. The fine particles (PM2.5) are less than 2.5µm in size. These particles are smaller than a single particle of flour.

Particulate matter includes dust, dirt, soot, smoke and tiny particles of chemical pollutants. The major sources of particulate pollution are factories, power plants, trash incinerators, motor vehicles, construction activity, wood stoves, fireplaces, forest fires and natural dust blown around by the wind.

The amount of particulate matter in the air can be worse in winter from the burning of wood and other fuel to heat houses. In big cities, particulate matter can be worse than in rural areas where there are fewer cars.

Clouds

Particulate matter is so tiny and light that the water droplets or ice crystals that form around it stay up in the sky as clouds. More than two billion of them are needed to fill one teaspoon with water.

Clouds form at different levels in the atmosphere. The air stability and humidity affects their size, shape, and type.

clouds

Description

A drawing of clouds that shows a close-up view of particulate matter inside the clouds, surrounded by a droplet of water. Particulate matter is so tiny and light that the water droplets or ice crystals that form around it stay up in the sky as clouds.

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Naming Clouds

In the early years of the nineteenth century, an Englishman named Luke Howard classified the clouds according Naming Clouds to their appearance and behaviour. Mr. Howard was an apothecary or pharmacist with a keen interest in the atmosphere and all that it contained. He used Latin to name the types of clouds.

Stratus - Stratus means stretched out or layered

Cirrus - Cirrus means curl, lock of hair

Cumulus - Cumulus means heap

Nimbus - Nimbus means rain cloud, cloud burst, shower and clouds

From the Ground Up

Low Clouds

The bases of low clouds begin from 0 to 2 km above the earth’s surface. Depending on the season, these clouds may contain water droplets, ice crystals, or a mixture of both.

Stratus Clouds

Stratus clouds are the low, uniformly dull, gray clouds that hang heavily in the sky. Their bases may cover the tops of hills or high buildings. If it is not drizzling already, stratus clouds are a good sign that precipitation in the form of drizzle may be on the way.

Fog and Mist

These are thin layers of stratus cloud that form at ground level. Like any cloud, fog is composed of millions of tiny droplets of water, or in cold weather, tiny floating ice crystals. The thickness of fog depends on the concentration of water droplets. Weather observers report fog if the visibility is less than 1 km, and mist if the visibility is 1 to 10 km.

Smog

Smog is a brownish-yellow or greyish-white haze in the air, originally named as a combination of “smoke” and “fog”. This haze is a mixture of pollutants made mostly from particulate matter (PM) and ground-level ozone (O3). Other pollutants include sulphur dioxide (SO2), nitrogen oxides (NOx), carbon momonoxideide (CO) and hydrogen sulphide (H22S). Motor vehicle exhaust contains five of the components of smog: carbon momonoxideide, particulate matter, lead, nitrogen oxides, and volatile organic compounds.

Heavy clouds in Halifax

Description

Heavy clouds covering the top of office buildings in downtown Halifax. Stratus clouds are the low, uniformly dull gray clouds which hang heavily in the sky. Their bases may cover the tops of hills or high buildings.

 

Nimbostratus

As the name suggests these are low-lying, dense, gray clouds that may produce continuous rain, or if it is cold enough, snow. These clouds are thicker or deeper than stratus clouds.

Stratocumulus

These clouds have a well-defined rounded appearance and are often organized into rolls with flat bases that have gray or dark gray patches. Stratocumulus clouds are common in late autumn or winter.

Fog covering the base of a suspension bridge

Description

Fog covering the base of a suspension bridge. Fog and mist are thin layers of stratus cloud that form at ground-level.

 

Cumulus

These little, puffy, fair-weather clouds commonly form on a summer afternoon. They usually cover less than half the sky and produce no precipitation.

Towering Cumulus

These begin as cumulus clouds but grow vigorously into rising mounds or towers. Their tops are well-defined and often resemble cauliflowers. The bases are flat and dark. These clouds may produce showers or flurries.

Cumulonimbus

Some clouds extend high into the atmosphere, as high as 14 km and more than 25 km long. These very tall clouds are called cumulonimbus or thunderstorm clouds. Meteorologists call these clouds CBs. Temperatures at their tops are as low as -55˚C, even in the summer.

If you look at this cloud from a distance, it has a well-defined, whitish, anvil-shaped top and a ragged and low bottom. When you look at this cloud from below, it has a dark base with curtains made of heavy rain.

Stages of a Thunderstorm

Cloud types during the stages of a thunderstorm
Description

A drawing that shows the cloud types that form during each stage of a thunderstorm. The first cloud type is cumulus, followed by towering cumulus and then cumulonimbus. Stages of a thunderstorm.

Cumulonimbus clouds form when a parcel of humid air is so warm it rises very far up before it cools enough to reach its dew point. Here, there is a unique combination of strong updrafts (rising air) and downdrafts (sinking air).

As cumulonimbus clouds develop, water droplets bounce around so hard they collide with others, creating ever larger water droplets. Eventually, these water droplets become too heavy for even the strong updrafts to support, and rain falls.

The turbulence in cumulonimbus clouds creates positive and negative electrical charges. Scientists do not know why but, generally speaking, the positive charge develops in the cold upper reaches of a cloud and the negative charge develops in the lower portions of a cloud. This, in turn, induces positive charges in objects on the ground below.

Although air is a notoriously poor conductor of electricity, the electrical charge in the cloud above grows until it overcomes the air’s resistance. Interestingly, even though lightning looks like one bolt hurtling towards earth, it is not. Lightning usually occurs when the electrons holding a negative charge begin moving downward from the cloud to the earth in what is called a step leader. As they get closer to the earth, the negative force of the electrons attracts the positive charge from the earth. This flows upward in what is called a streamer. This streamer or return stroke flows upwards at about 96,000 km/s and at temperatures of 30,000°C, which is six times hotter than the sun.

Demonstration: Warm Air Rises and then Condenses as it Cools

Materials

  • kettle with boiled water
  • clear, heat-proof jar
  • aluminum tray or pie plate
  • ice

Method

  1. Pour 2.5 centimetres of hot water into a jar.
  2. Put ice cubes onto the tray or pie plate and place that over the opening of the jar.

Questions

What happens as the air inside the jar rises and is cooled by the ice?

Explanation

The water vapour in the air condenses onto the cold surface and collects to form droplets.

The same process occurs when lightning travels from one cloud to another. In fact, 90% of lightning strokes flash from cloud to cloud or within the same cloud.

Thunder is the sound produced by the sudden and rapid expansion of the narrow channel of air heated by the lightning stroke. You see the lightning, then hear the thunder because the speed of light is about a million times faster than the speed of sound.

Thunderstorms often change as they travel across the countryside. Lakes and the local terrain may affect the strength, movement and duration of storms. For example, if a thunderstorm passes over hills and ridges, it may grow stronger as it climbs up one side and weaker as it goes down the other. A thunderstorm may grow stronger if it moves over a long stretch of flat land that has been baking in the sun all afternoon, or weaker if it passes over a large body of cool water in the late spring.

Only a small percentage of the thunderstorms during Canadian summers unleash enough energy to produce severe weather: high winds, heavy downpours, damaging hail or tornadoes. Remember, to stay safe during a thunderstorm, when thunder roars, go indoors!

Middle Clouds

Clouds with the prefix alto are middle-range clouds. Their bases usually range from 2 to 6 km above the earth’s surface.

Altostratus

These are gray or blue-coloured sheets of clouds with little texture. They cover most of the sky. In some spots, altostratus clouds may be thin enough to reveal the sun.

Altocumulus

These are white or sometimes gray clouds with rounded bottoms. The clouds may look as if they are arranged in rolls, rounded masses or thin layers. The individual rolls of cloud appear smaller than those in stratocumulus clouds because altocumulus clouds are farther away. Sometimes you can see the sky or the sun between the rolls, but often these clouds cover the whole sky.

Altocumulus Lenticularis

These lens-shaped clouds form when a mountain range deflects strong winds upwards on the windward slopes and downward on the leeward slopes. This creates a giant wave or ripple several kilometres in length. Moist air enters the crests of the waves, cools as it rises and forms a cloud. When the air descends, it warms up and condensation stops. Groups of these clouds floating along in what appears to be a formation, may look like a fleet of flying saucers.

The clouds form at the top of the wave where the air cools, and disappear at the bottom of the wave where the temperatures are slightly warmer.

High Clouds

The bottoms of these clouds generally run from 5 to 12 km above the ground. These clouds are composed mostly of ice crystals.

White and orange clouds

Description

Wispy white and orange clouds, high in an evening sky over a silhouetted coastline. High clouds are composed mostly of ice crystals.

Cirrus

These thin clouds may appear as wispy streaks or streamers high in the sky. Extensive cirrus clouds may be the first sign of an approaching warm front.

Cirrocumulus

These are thin, white bands of clouds with tufted bottoms. These clouds often look like the ripples in the sand left by waves.

Cirrostratus

This whitish cloud covers the sky in a transparent veil or sheet. The cloud is usually thin enough for the sun to shine through, often producing a halo.

Cloud Movement

Clouds move in the direction that the wind at their altitude is blowing. This is why clouds may travel in one direction while the wind at the surface is blowing in another. That also explains why two types of clouds that form at different heights, such as cirrus and cumulus clouds, may blow across the sky from different directions.

For more information on clouds, visit the Sky Watchers Guide to Cloud Identification.

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2.2 Observe

Since clouds depend on water vapour condensing around particulate matter, the three are closely linked. However, there can be high humidity without much particulate matter, and high levels of particulate matter without much humidity. This can occur at ground level or high up in the atmosphere.

Measuring Humidity

Weather instruments such as a hygrometer or sling psychrometer can be used to measure humidity and relative humidity.

Hygrometer

Close-up of the needle on a hygrometer
Description

Close-up of the needle on a hygrometer. Not too many years ago, humidity was measured with a mechanical hygrometer.

Humidity is measured with an electronic hygrometer, but not too many years ago, it was measured with a mechanical hygrometer. This instrument had a long, naturally blond hair as one of the principal components. As the humidity increased, the hair absorbed moisture and stretched. This caused the indicator arm on the hygrometer to change readings. Blond hair was used because it absorbs moisture more readily than other naturally coloured hair.

Sling Psychrometer

Child holding up a sling psychrometer
Description

Child holding up a sling psychrometer. A sling psychrometer is used to determine dew point temperature and relative humidity.

A sling psychrometer is used to determine dew point temperature and relative humidity. It contains two thermometers. The bulb of one of the thermometers is covered with a wet cloth, so it is called a wet bulb thermometer. The other is called a dry bulb thermometer. The wet-bulb reading will always be equal to or less than the dry bulb because some of the heat energy is used for evaporation.

In dry air, the water evaporates quickly and causes a large drop in the wet-bulb temperature. This makes the difference in temperatures on the two thermometers greater than if the air was humid. This difference in temperatures indicates the amount of water vapour in the air.

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Observing Clouds

Cloud Cover

Cloud cover indirectly represents the amount of condensation taking place in the air. It describes the amount of sky covered by clouds from horizon to horizon.

  • Clear – No clouds in the sky.
  • A few clouds – Less than half the sky is covered with clouds.
  • Cloudy – More than half the sky is covered with clouds.
  • Overcast – All the sky is covered with clouds.

Weather Station Model

Cloud cover is one of the weather observations collected from hundreds of places and plotted on weather maps using an internationally accepted code called a station model.

The station model fits the following information into a space about the size of a dime:

  • Types of clouds present
  • Air pressure
  • Dew point
  • Cloud cover
  • Precipitation
  • Air temperature
  • Wind direction and speed

Using a Variety of Sources...How could you find out more about the different weather station model symbols for clouds and cloud cover? Do you think these symbols will continue to be used, or are they a thing of the past? Why?

Illustration showing weather at a given reporting station
Description

A symbolic illustration showing the weather occurring at a given reporting station. The information is arranged around a circle in the middle. The weather station model arranges information around a circle. Each placeholder in this diagram could be one of many possible symbols.

2.3 Predict

Canadians rely on weather reports for information that helps inform their day-to-day decisions. In summer this includes the humidex, which indicates how high the temperature feels when combined with humidity. It also includes the Air Quality Health Index (AQHI), a tool based in part on particulate matter. AQHI communicates the health risks posed by this and other air pollution.

Weather Reports: Clouds

Clouds are visible using satellite imaging, and give forecasters clues that help them track and predict local and regional weather.

Satellites

Each of ECCC’s weather centres has its own satellite receiver to pick up photos transmitted from space. Animated satellite images are available at ECCC’s weather website at, weather.gc.ca.

Weather satellites have become an indispensable tool for observing and forecasting weather because they give forecasters the ability to see the movement of entire weather systems.

ECCC uses images from two types of weather satellites: the geostationary satellite (GOES) and the polar orbit satellite (NOAA polar orbiting).

GOES

Each geostationary satellite orbits around the earth’s equator at an altitude of about 36,000 km. The geostationary satellite monitors the same portion of the earth continuously, producing a picture every 15 minutes. Because its position relative to the earth stays the same, forecasters are able to put together and animate consecutive pictures from the same satellite to show a movie of the weather. This is the view normally seen on the news.

NOAA Polar Orbiting

A polar orbit satellite travels at a much lower altitude (about 860 km above the earth) and provides more detailed images. The polar orbiter circles the earth about 14 times each day. However, as the earth rotates under it, each successive orbit covers a swath about two time zones further west.

Visible

Weather satellites use visible light cameras that take images of what you would see with your own eyes if you were on the satellite. This type of image is only transmitted during the daylight hours and cannot be used overnight.

Infrared

The second type of image is infrared (IR). The equipment senses temperatures and displays them in shades of grey--the colder the temperature of the ground or cloud top, the whiter it appears on the image. Conversely, the warmer a surface, the darker it appears. This type of picture also allows forecasters to monitor clouds overnight.

Environment and Climate Change Canada Severe Weather Bulletins

ECCC issues severe weather bulletins to keep the Canadian public advised of weather events that could affect their safety or property. These weather alerts can be special weather statements, advisories, weather watches, or weather warnings, depending on their severity.

Special weather statements and advisories are issued for events that are not severe enough to merit a warning, yet might cause general inconvenience or public concern. For example, an advisory might be issued to highlight widespread dense fog that could pose a transportation challenge, or a special weather statement might be issued to clarify a weather warning that may be in effect near our borders.

Weather watches provide a heads-up that conditions are favourable for severe weather to develop. A watch might be issued as much as 12 hours in advance, when the potential for dangerous weather has been identified, but the track and strength of the system are still uncertain. Watches may be issued for six different severe weather events to provide more advance notice of the threat.  These include a severe thunderstorm watch, a tornado watch, a winter storm watch, a snow squall watch, a tropical storm watch and a hurricane watch.

Water and ice flooding a street

Description

Water and ice flooding a street. ECCC issues severe weather bulletins to keep the Canadian public advised of weather events that could affect their safety or property.

Weather warnings are issued when severe weather is occurring or about to occur. The threshold for issuing various types of warnings will depend on the climate of an area as well as local needs. A severe thunderstorm warning is issued when a severe storm has developed, producing flooding rain, destructive winds with gusts of at least 90 km/h and/or hail at least 10 to 20 mm in diameter.

Environment Canada strives for a lead time of 6 to 18 hours, depending on the type of event. However, thunderstorms often develop rapidly, so lead times on occasion may be less than an hour. 

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Weather Reports: Humidex

The humidex is a comfort index that was invented in Canada during the 1960s. It is a measure of what high temperature and relative humidity feels like to the average person. When the relative humidity is high, perspiration does not evaporate well and it is difficult to cool down. For example, when the temperature is 32°C and the relative humidity is 75%, the air feels as if it is 46°C. 46 is the humidex reading.

Humidex And Your Comfort Level
HumidexComfort Level
20-29Comfortable
30-39Varying degrees of discomfort
40-45Almost everyone is uncomfortable
Above 45Many types of work and exercise should be restricted

ECCC displays humidex values of 25 or higher for a location with a dew point temperature above zero (0°C) and an air temperature of 20°C or more.

The highest humidex ever recorded in a Canadian city was in Windsor, Ontario, on June 20, 1953. That day, the humidex was 52 (ECCC’s climatologists worked this out using records for temperature and relative humidity).

Dew covering an autumn leaf

Description

Dew covering an autumn leaf. Dew in the morning means that there is a good chance of a bright day ahead.

 

Predict Weather Using Signs Found in Nature: Condensation and Humidity

If folks don’t want to leave it to the experts, they can look around for condensation and humidity clues to predict the weather themselves.

Dew

When dew or frost appear on the ground early in the morning, there is a good chance of a bright day ahead. That is because frost, dew or fog form more readily on clear, cool and calm nights when there are no clouds to interfere with the cooling of the ground. As calm, clear nights are typical of high pressure areas, the fair weather is likely to continue for at least another day.

Clouds

Generally speaking, the more types of clouds there are in the sky, the greater the chance of rain or snow.

Nimbostratus

These dull, gray clouds that cover the sky from horizon to horizon usually mean rain or drizzle all day.

Long white jet vapour trail crossing a clear blue sky

Description

Long white jet vapour trail crossing a clear blue sky. If you look up on a sunny day and see a jet leaving a long, white plume in a clear sky, it means the surrounding air is humid. Rain, or some other form of precipitation, may be on the way.

Cirrus

This bank of wispy clouds coming in high in the sky on a sunny day may mean a change in the weather. They are sometimes the first sign of an approaching warm front.

Towering Cumulus

When these clouds pop up rapidly on hot, humid days, it means that showers are likely. There is also a possibility that a thunderstorm will develop. If you see the sun shining behind a thundercloud, you know the cumulonimbus cloud is moving on, and the end of that particular thunderstorm is in sight.

Cirrostratus

Cirrostratus clouds can sometimes appear as halos around the sun or moon. The halo is caused by the refraction of the sun’s or moon’s rays through the clouds’ ice crystals. These clouds are an early sign that a warm front is approaching and that rain may be on the way within the next 20 to 24 hours.

Jet Trails

Airline jets sometimes leave white plumes called contrails behind them. It is the condensation trail of ice crystals left behind by the exhaust of a flying jet aircraft. These aircraft fly 8 to 12 km above the ground, pulling in very cold, dry air and spewing out hot, water-filled exhaust. The hot water vapour mixes with the colder surrounding air, and in the process, expands and then freezes in 1 or 2 seconds, forming a trail of ice crystals.

If you look up on a sunny day and see a jet leaving a long, white plume in a clear sky, it means the surrounding air is humid. Rain, or some other form of precipitation, may be on the way.

If a jet leaves no trail or only a short trail, or if the trail fades quickly, then the air at that level is relatively dry. This means the fair weather is likely to continue.

Humidity and Plants

Pine cones, tulips and daisies close when the relative humidity is high and rain may be on the way. One theory suggests that flowers do this to prevent pollen from washing away.

Exhaust filling the air over a fuel transport truck

Description

Exhaust filling the air over a fuel transport truck. You can’t always feel, see, or smell air pollution, even though it may start to affect your health.

Weather Reports: Particulate Matter

Although you can feel humidity in the air, you can’t always feel, see or smell air pollution, even though it may start to affect your health.

The Air Quality Health Index (AQHI) is a scale designed to help you understand what the quality of the air around you means to your health. It is a tool developed by health and environmental professionals to communicate the health risk posed by air pollution. The AQHI is calculated based on risks from fine and coarse particulate matter (PM2.5, PM10) as well as other common air pollutants that are known to harm human health, such as ground-level ozone and nitrogen dioxide. The index provides specific advice for people who are especially vulnerable to the effects of air pollution as well as the general public.

The AQHI communicates four primary things:

  • It measures the air quality in relation to your health on a scale from 1 to 10. The higher the number, the greater the health risk associated with the air quality. When the amount of air pollution is very high, the number will be reported as 10+. 
  • A category that describes the level of health risk associated with the index reading (e.g., Low, Moderate, High, or Very High Health Risk).
  • Health messages customized to each category for both the general population and the “at risk” population.
  • Current hourly AQHI readings and maximum forecasted values for today, tonight and tomorrow.

The AQHI is designed to give you this information along with some suggestions on how you might adjust your activity levels depending on your individual health risk from air pollution.

The AQHI is measured on a scale ranging from 1-10+. The AQHI index values are also grouped into health risk categories as shown below. These categories help you to easily and quickly identify your level of risk.

AQHI scale

Description

AQHI is measured on a scale ranging from 1--10+

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AQHI index
Health RiskAQHIHealth Messages
At Risk Population
Health Messages
General Population
Low1–3Enjoy your usual outdoor activities.Ideal air quality for outdoor activities.
Moderate4–6Consider reducing or rescheduling strenuous activities outdoor if you are experiencing symptoms.No need to modify your usual outdoor activities unless you experience symptoms such as coughing or throat irritation.
High7–10Reduce or reschedule strenuous activities outdoors. Children and the elderly should also take it easy.Consider reducing or rescheduling strenuous activities outdoors if you experience symptoms such as coughing and throat irritation.
Very HighAbove 10Avoid strenuous activities outdoors. Children and the elderly should also avoid outdoor physical exertion.Reduce or reschedule strenuous activities outdoors, especially if you experience symptoms such as coughing and throat irritation.

Illustration showing physical activities

Description

A drawing that depicts multiple physical activities that are appropriate when the Air Quality Health Index (AQHI) is low, moderate, high and very high. Always look at the AQHI health messages when choosing outdoor activities.

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2.4 Reflect

Take some time to think about how you react to a weather report. Do you look for information on weather when you make day-to-day choices? What will you do with a report about particulate matter, clouds or humidity?

What does Particulate Matter Mean to Me?

Many of the adverse health effects resulting from exposure to particulate matter are specific to the cardio-respiratory system. When we inhale particulate matter, the particles can penetrate deep into the lungs. The smaller the particle, the deeper into the lungs it can penetrate. Scientific studies have identified strong links between high levels of airborne particles and increased hospital admissions for heart and respiratory problems, as well as higher death rates from these ailments.

Depending on the length of time you are exposed, your health status, your genetic background and the concentration of pollutants, air pollution can have a negative effect on your heart and lungs. It can:

  • Make it harder to breathe;
  • Irritate your lungs and airways; and
  • Worsen chronic diseases such as heart disease, chronic bronchitis, emphysema and asthma.

Each individual reacts differently to air pollution. Children, the elderly and those with diabetes, heart or lung disease are most sensitive to the adverse health effects of air pollution.

Negative health effects increase as air pollution worsens. Small increases in air pollution over a short period of time can increase symptoms of pre-existing illness among those at risk.

For more information on the health effects of air quality, visit the Air Quality Health Index.

What Do Clouds and Humidity Mean to Me?

Clouds and humidity can send more obvious signals to you as you go about your day. Unlike particulate matter, which you may not notice without a weather report, it is hard not to detect a particularly high humidex or an overcast sky.

Pay special attention when there are clues about clouds and humidity that you can’t ignore, like thunder. Thunderstorms come hand in hand with lightning, hail and downbursts, which are serious hazards.

Thunderstorm Hazards

Lightning

Lightning kills an average of 7 people and injures 60 to 70 people each year in Canada. Lightning is also responsible for 42% of the country’s forest fires. The cost of forest fires caused by lightning has been estimated at $14 billion annually in recent years.

There is no truth to the saying that lightning never strikes the same place twice. Lightning strikes the CN Tower in Toronto about 70 times a year.

Lightning Safety Tips for Kids

You can work out how many kilometres away a thunderstorm is by counting the number of seconds between the time you see the lightning and hear the thunder and dividing the answer by 3. For example, if you count 15 seconds between the lightning flash and the crack of thunder, then the storm is about five (5) kilometres away.

Every thunderstorm produces lightning. Remember: When thunder roars, go indoors.

When thunder roars, go indoors
Description

A cartoon drawing of a cumulonimbus cloud with an angry face, and a bolt of lightning coming from the bottom of the cloud. When thunder roars, go indoors.

Indoors

  • Stay away from windows and doors.
  • Don’t use the telephone, take a shower or wash dishes. Avoid indoor swimming pools. Don’t even touch water faucets, electrical appliances or metal items that would conduct electricity.

Outdoors

  • Unsafe places include open fields, high places, tents, picnic shelters or pavilions, baseball dugouts, swimming pools and things that could conduct electricity, like metal fences.
  • If you cannot find safe shelter, make yourself as small a target as possible. Don’t lie flat. Instead, crouch down with only your toes touching the ground and lower your head.
  • Safety also means no bike riding, skateboarding or golfing until the storm has passed.
  • If you’re swimming or boating, return to shore immediately.
  • In wooded areas, go deep into a stand of trees and find a low-lying area, but never seek shelter under a solitary tree.

In a Vehicle:

  • You’re safe inside a hard-topped vehicle like a car or RV, because the outer metal body of the vehicle will divert the lightning. Keep your hands in your lap and don’t touch anything metal inside the vehicle.

Hail

Hail forms when updrafts carry water droplets into the colder reaches of a cumulonimbus cloud, where they freeze. More layers of ice are added when updrafts hurl other water droplets up and they collide with the now-frozen particles. This continues until the ice particles are too heavy for the updrafts to support and the ice particles fall to the ground as hail.

Hail Safety Tips for Kids

Indoors:

  • Follow your lightning safety plan.
  • Stay inside and away from windows that may be struck by hail.
  • Keep pets indoors.

Outdoors:

  • Find something to protect your body or at least your head.
  • Stay out of ditches or low areas that might suddenly fill with water.

In a Vehicle:

  • A car can give you reasonable protection, but be aware that extremely large hail could break windows.
  • Hail is one of the most destructive forms of severe weather in Canada. Hail stones destroy crops, kill farm animals and cause millions of dollars in damage. Fortunately, hail injures only a few Canadians each year.

Downbursts

Downbursts are another hazard of large thunderstorms. Downbursts are the downdrafts that usually accompany rain or hail. They can plunge to the ground at speeds of over 200 km/h, the speed of an EF2 tornado.

In fact, people often confuse downbursts with tornadoes, believing that only tornadoes can generate such damaging winds. Derechos (pronounced day-RAY-cho) comes from the Spanish word “straight ahead,” while tornado comes from the Spanish word for “turn.”

Derechos are long-lasting winds associated with lines or clusters of thunderstorms. They can damage an area several kilometres wide and several hundred kilometres long in a single or series of swaths of 90 km/h winds.

A microburst is a form of downburst that is less than 4 km wide. Microbursts have caused aircraft to crash and capsized sailboats.

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