Sky Watchers Teachers’ Guide: chapter 3
Section 3 Precipitation
- Prior Knowledge
- Section Summary
- 3.1 Read with Understanding: Precipitation
- 3.2 Observe
- 3.3 Predict
- 3.4 Reflect
What do you know about precipitation and why it falls?
Large white snowflakes falling at night. Precipitation is the part of the water cycle that returns water back to Earth from clouds.
Precipitation is the part of the water cycle that returns water back to earth from clouds. Clouds release precipitation when their droplets of water or ice grow big enough to fall. The type of cloud and conditions determine whether precipitation will be drizzle, rain, freezing rain, ice pellets, snow or hail.
Modern automatic weather stations and radar are used to monitor precipitation. Rain gauges and snow boards are also useful.
Canadian summers and winters both involve difficult precipitation conditions that warrant warnings. Environment and Climate Change Canada (ECCC) issues severe weather warnings such as rainfall, snowfall, freezing rain, snow squall and general winter storm warnings to help folks prepare for unseasonable and/or extreme precipitation. Weather is broadcast in many ways to ensure that everyone, no matter what technology is available to them, can access the information.
Canadians design and plan the built environment to be ready for precipitation, sometimes even creating new inventions. When severe weather does arrive, remember to take it seriously for your health and safety.
3.1 Read with Understanding: Precipitation
Precipitation is Part of the Water Cycle
More than 5 trillion tonnes of precipitation fall on Canada each year. About 3 trillion eventually runs into lakes and rivers. The other 2 trillion evaporates from the earth’s surface or passes back through plants through the process known as transpiration.
The Pacific Ocean, Gulf of Mexico and Caribbean Sea are the primary sources of water for precipitation in Canada, but water also recycles itself several times between the air and the ground. The water evaporates from soil, lakes, and rivers, rises into the air as water vapour, forms clouds, and then falls elsewhere as rain, drizzle, freezing rain, snow or hail.
Collaboration, collecting data, technology... Ask students to make a list of all the different sounds they hear from precipitation, such as the sound of cars on wet pavement or rain on the roof. Record these and use the sounds to create a guessing game either in-person or online.
Rain droplets of various sizes covering a window pane. Precipitation is called rain when the water droplets are greater than 0.5mm in diameter.
First Clouds, Then Precipitation
Rain, snow, hail, and other forms of precipitation occur when water droplets or ice crystals grow until they are too heavy for the air currents in a cloud to support. One million tiny water droplets are needed to form an average rain drop, about 1mm in diameter. It takes more than 30 minutes for a raindrop to grow.
Drizzle: Precipitation is called drizzle when the water droplets are less than 0.5 mm in diameter, which is about the size of the head of a pin. Drops of drizzle fall at a rate of 1 to 2 m/s while raindrops reach speeds of 4 to 9 m/s.
Rain: Precipitation is called rain when the water droplets are greater than 0.5 mm in diameter. Some raindrops are as large as 10 mm across.
Art: Comparing Raindrops
- dark sheets of construction paper OR white paper and watercolour paints
- a rainy day
- Paint the plain white paper with watercolours and let dry.
- On a rainy day, go outside and hold the dark construction paper or painted paper in the rain parallel to the ground.
- Count to 10.
- Return to the classroom and observe the raindrops on the paper.
Why it Works
Raindrops leave marks of various sizes on the paper because it dissolves, dilutes and splatters the watercolour pigments. Raindrops are different sizes, some as small as 1mm, others as large as 10 mm.
Types of precipitation
Different types of clouds and conditions produce different precipitation. Precipitation comes in three forms: liquid, freezing and frozen. Precipitation may be small and fall as drizzle or light snow, or clouds may form larger droplets and release steady rain or snow. Other clouds have convective air currents in them, so they release rain in bursts or showers.
In the winter there can be as many as four different types of precipitation when a warm front passes: rain, freezing rain, ice pellets and snow.
The classic recipe for freezing rain is a layer of warm air hovering above a shallow layer of cold air. Raindrops fall from clouds in the warm layer of air, then pass through the cold layer where temperatures hover around freezing.
Here, raindrops cool to the freezing point or just below it, becoming super-cooled. These very cold raindrops freeze on contact when they hit a colder object such as an overhead electrical wire or the branch of a tree. This creates glaze ice.
Ice pellets form under the same conditions as freezing rain. The water droplets form in the higher, warmer layer of air and fall into the lower layer of colder air. In this case, though, the cold layer is deep enough to give the water droplets time to freeze before they hit the ground.
A drawing that shows precipitation falling from clouds influenced by both warm air and cold air. The precipitation types shown are rain, freezing rain, ice pellets and snow. In the winter, there can be as many as four different types of precipitation when a warm front passes.
Snow is precipitation of white or translucent ice crystals that are clustered together to form snow flakes. The shapes and sizes of snowflakes depend on the temperature and humidity in the cloud, and in the air below it.
Big, soggy flakes are conglomerations of hundreds of smaller snowflakes that have fallen through relatively mild air and stuck together. Some of these flakes have measured as much as 2 cm across. In contrast, dry snow tends to fall as small, single flakes that do not bind together as they fall through cold, dry air.
About 36% of Canada’s precipitation falls as snow, compared to the world average of 5%.
Sparkles on a layer of freshly fallen snow. About 36% of Canada’s precipitation falls as snow.
Hail forms only in cumulonimbus clouds when strong updrafts carry water droplets high into the upper reaches of the clouds. Here the water droplets freeze, even in summer. Layers of ice are added when the updrafts throw more water droplets upward, which then collide with the frozen particles. This process continues until the ice particles become too heavy for the updrafts to support. Then the ice particles fall as hail.
A hailstone of a few millimetres in diameter needs updrafts of more than 100 km/h to support it. In Canada, hailstones range in size from 5 mm (the size of a pea) to 114 mm (the size of a grapefruit).
Modern automatic weather stations use remote-sensing technology to measure current conditions.
The precipitation sensor measures the speed at which precipitation particles are falling. This, combined with air temperature, identifies the type of precipitation because droplets of different size and composition fall at different rates.
To measure the depth of snow, a high-frequency pulse of sound is reflected off the ground, indicating how far it is to the snow’s surface. Similarly, the reflection of a laser beam from a cloud determines its height.
Some stations are also equipped with video cameras so forecasters can see a digital picture of the weather at that location.
Meteorologists use conventional radars to determine the size, motion, and concentration of precipitation in clouds within a 200 to 400 km range. The radar transmits a burst of microwave energy. If there are water droplets, some of the microwaves are reflected back and are detected by the antenna. The greater the size or density of water droplets, the more microwave energy that returns.
ECCC has a Doppler weather radar network that covers areas of the country that are prone to severe weather. About 90% of the country’s population lives in these areas.
Doppler radar measures the intensity, speed and direction of precipitation within a 250 km range. This helps forecasters to identify early signs of severe weather, such as when clouds are starting to rotate into a funnel cloud, and could possibly become a tornado.
A rain gauge is a weather instrument used to measure the amount of rain that has fallen. To set one up, attach it to a post so that the rain gauge is level. Ensure that the gauge’s top extends above the post and that the post is located away from buildings, trees or any other structures.
To take a reading, look at the level of water in the rain gauge and record the amount in millimetres. Empty the gauge and dry it thoroughly with a clean cloth after every reading.
Building a Weather Instrument: Recycled Rain Gauge
- a plastic 2 L pop bottle with straight sides
- ruler at least 15 cm in length
- stones or large gravel
- clear tape
- Cut the bottle about 10 cm from its top. Save the top part.
- Place stones or gravel in the bottom of the bottle until they fill the bumps in the bottom and come up to the part of the bottle where the sides are straight. This will add weight to the gauge to make it more stable.
- Tape the ruler to the side of the bottle so that the zero mark on the ruler is a centimetre or two above the stones.
- Pour enough water into the bottle so that the water level is at the zero mark on the ruler.
- Take the top of the bottle (the part you cut off earlier), turn it upside down, and put it into the bottom portion so that it looks like a funnel.
- Set your gauge in an open area away from trees or buildings that may affect the amount of rain that falls into the bottle.
- When it has rained, take a reading using the ruler taped to the side of the bottle. Then pour out the excess water until the water level is once again at zero. (If you pour out too much water, simply add more until the water level reaches zero on the ruler.)
Why it Works
The rain gauge collects rain as it falls. This works as long as it is in an open area and it starts exactly at zero. Water will evaporate if you leave your rain gauge out in the sun, so check that the water level is at zero before each use.
Find a patch of undisturbed snow on flat open ground away from any trees or overhanging roofs. Try to avoid areas where the snow has drifted into piles or the wind has blown the fresh snow away.
Measure the depth of the snow in centimetres using a long ruler or a metre stick. Keeping the ruler straight, push it into the snow until the ruler hits the ground below. Do this several times in different spots, then calculate the average.
Ten centimetres of snow is approximately equal to 10 mm of rain, depending on whether the snow is wet or dry. If you have had both rain and snow on the same day, measure the total amount of liquid in the rain gauge in millimetres.
There are three ways to determine how much snow fell since your last observation.
Use a board that is about 40 cm long and 40 cm wide. It must be light enough to sit on top of the snow but heavy enough to stay in place on windy days. Push the board into the snow until the top is level with the snow’s surface. If the forecast calls for a heavy snowfall, mark the board’s location with a flag or stick so you can find it the next day. Measure the amount of snow on the board, clean it off and place it back in the snow.
Frozen pine branches dripping with drops of rain. If you have had both rain and snow on the same day, measure the total amount of liquid in the rain gauge in millimetres.
Footprint in freshly fallen snow. Snow may compact, sublimate, or melt between measurements.
After measuring the amount of snow that has fallen, shovel and sweep the area clean so it is clear for the next snowfall.
Measure the depth of all snow on the ground after each snowfall. The latest snowfall amount is the different between the current depth and the previous depth. It is not exact because snow may compact, sublimate or melt between measurements.
Weather Reports: Precipitation
Chance of Precipitation
The chance of precipitation is the likelihood that measurable precipitation (0.2 mm of rain or 0.2 cm of snow) will fall on any point of the forecast region during the forecast period.
A 30% chance of precipitation means that there is a 3 in 10 chance of precipitation in the area. In other words, there is a 30% chance that rain or snow will fall on the area and, therefore, a 70% chance that it won’t.
A low chance of precipitation does not mean a sunny day; it only means a day where the chance of rain or snow is low.
Precipitation Weather Warnings
ECCC issues a variety of severe weather warnings specifically related to precipitation.
- Issued when heavy or prolonged rainfall is sufficient to cause local or widespread flooding or flash floods.
- Issued when an unusually high amount of snow is expected to fall in a comparatively short period of time. In Vancouver, 5 cm of snow in 12 hours would be unusual, whereas in Ontario, a warning is only issued if 15 cm is expected in that length of time.
Freezing Rain Warning
- Issued when freezing rain is expected to last long enough for the accumulation to create hazardous walking and driving conditions or damage to trees and overhead wires.
Snow Squall Warning
- Issued for localized, limited duration, intense snowfall that reduces visibility significantly and may be accompanied by strong, gusty winds.
Winter Storm Warning
- A broader warning issued by ECCC in some regions because of serious combinations of different winter weather phenomena.
Getting the Message Out
ECCC uses a variety of delivery methods to ensure that everyone, no matter what technology is available to them, can access weather information.
ECCC has its own radio network, broadcasting continuous weather information 24 hours a day. Known as Weatheradio, this network uses VHF frequencies so that specially equipped receivers will automatically activate when warnings are issued for your area. To learn more about Weatheradio and find the transmitter location closest to you visit ECCC’s website.
Millions of people visit ECCC’s main weather Web site at weather.gc.ca to look at radar imagery or to check the forecast for any of the hundreds of towns available on drop-down menus.
The most popular source of weather information for Canadians is still their local media outlet-- radio, television or newspaper--and ECCC feeds weather information to them directly through wire services and a special website just for media.
Listening to Folklore
Most weather lore is fanciful, but some actually do make sense, linking weather indicators to upcoming weather.
Showers Before Seven, Fine Before Eleven
Showers in the morning usually do not last long--for good reason. If they formed during the night when it was cool, then when the sun comes up and heats up the day, the humidity drops, the clouds dry out and the rain ends.
Collaboration and Community...What weather lore or folk sayings can you collect from your community or family members? Are they good predictors of the weather?
What Does Precipitation Mean to Me?
Precipitation and the Built Environment
Precipitation affects how we design and plan our built environment. For example, contractors build adequate support for the maximum anticipated snow load expected in a region. This can prevent a roof from collapsing.
When severe weather events happen, it is not only the large-scale events that affect the economy, but even small ones. A sudden thunderstorm has a large impact to the farmer who recently cut hay or the contractor who poured $10,000 worth of concrete.
The Canadian economy absorbs not only the direct costs for property damage from bad weather, but also millions of dollars worth of indirect costs from reduced sales and cancelled events.
Canada’s weather, specifically winter, has brought out the best in some of the country’s more inventive minds. Canadians invented the snow blower, the snowmobile, and snow garments such as polar fleece. Not surprisingly, Canadians also invented insulation and frozen fish, and have perfected the art of making ice wine.
Using a Variety of Sources, Collaboration...Have your students list ten occupations greatly affected by the weather and identify which weather element is the most critical for each. Then, see if they can identify two occupations not affected in any way. Remind them that most jobs are dependent to some degree on travel conditions to get to work and the availability of electricity once they arrive.
When severe weather does arrive, remember to take it seriously for your health and safety. For example, freezing rain is a significant winter hazard in Canada, but can also occur in late fall or early spring. Freezing rain glazes trees, hydro lines, roads and sidewalks with ice. Buildups of ice can bring down branches and trees as well as overhead power and telephone lines. This can disrupt power supplies and communications for days. Even a small accumulation of ice may pose a risk to both pedestrians and drivers.
Ice storm is a term used to identify particularly severe freezing rain events. Most ice storms last a few hours, but some continue for up to three days. The ice storm that hit New Brunswick, Quebec and eastern Ontario in January of 1998 went on for six long days. The storm was directly or indirectly responsible for the deaths of 25 people. At its height, the storm left nearly three million people in Quebec and Ontario without electricity or heat. A week after the storm ended, nearly one million people were still without light or heat.
Winter Storm Safety Tips for Kids
- Stay indoors and wait out the storm.
- If you must go outside for a short period, dress in multiple layers of loose-fitting clothing.
- Outer clothing should be hooded, tightly woven and water repellent.
- Mittens are warmer than gloves.
- Wear a hat, because most body heat is lost through the head.
- If it is very cold, cover your mouth with a scarf to protect your lungs from the cold air.
- Never touch a power line that may have come down due to wind or ice buildup. It may still be “live” and you could be electrocuted.
If you become stranded while traveling in a vehicle, wait for rescue:
- Stay in the car -- you won’t get lost and the car will provide shelter.
- Turn the car off. Ensure that the exterior exhaust of your car is clear of snow so you don't get carbon monoxide poisoning in the car.
- Keep your seatbelt on and put on your hazard lights. Even if you are pulled over, people can still hit you.
Keep dry and warm. If you begin to sweat, remove your hat or one layer of clothing.
- Keep fresh air in the car by opening the window 1 cm or less on the side away from the wind.
- Exercise your arms and legs periodically to keep your hands and feet warm.
- Keep watch for traffic or for search parties.
Precipitation as Inspiration
Precipitation is often used in song, stories and art to create a mood.
Using a Variety of Sources, Collaboration...Collect children’s storybooks showing different precipitation and share them with others. Ask family members for a story or memory of an event involving precipitation.
Technology...Record 5 to 30 second clips of movies that use precipitation to create a mood. Use movie editing software to organize and weave the clips together, labeling them with scientific facts.
Snow covering the bill of a mallard duck. Precipitation is often used in songs, stories and art to create a mood.
Rainbow in the sky over fall foliage. When a rainbow forms, the water bends light into its component colours.
Demonstration: Make a Rainbow
- clear plain glass bowl with water in it
- sunlight or a full spectrum flashlight
- small flat mirror
- heavy white paper/cardstock
- Place the bowl of water on a desk or table near a blank wall or heavy white paper.
- Put the mirror in the water so that the mirror rests against the side of the bowl at a 45° angle.
- Standing behind the mirror, shine the flashlight straight down on the mirror.
- If using natural sunlight, angle the mirror until you see a rainbow.
- Although there is water and sunlight in the sky, there are no mirrors. How does a raindrop do this?
Light bends in water. Even though light looks white, it is actually made up of different colours. When the rainbow forms, the water bends the light into its component colours and the mirror reflects those colours back out of the water. This is also what happens to sunlight. It, too, is bent, revealing its colours when it enters the front of a raindrop. Then, if the angle is right, it is reflected by the back of the raindrop.
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