Climate Variables

The Climate Atlas of Canada allows you to display and explore many aspects of climate. This page describes all the various climate variables and indices provided on the map.

Coldest Minimum Temperature

The very coldest temperature of the year.

Cold weather is an important aspect of life in Canada, and many places in Canada are well adapted to very cold winters. It is especially important to know how our winters will change in the future, because cold temperatures affect our health and safety, determine what plants and animals can live in the area, limit or enable outdoor activities, define how we design our buildings and vehicles, and shape our transportation and energy use.

Technical description:

The minimum minimum temperature of the year.

Cooling Degree Days

Cooling Degree Days (CDD) are equal to the number of degrees Celsius a given day’s mean temperature is above 18 °C. For example, if the daily mean temperature is 21 °C, the CDD value for that day is equal to 3 °C. If the daily mean temperature is below 18 °C, the CDD value for that day is set to zero.

Cooling Degree Days are often used to estimate how much air-conditioning is required in a year. If a location shows an increase in projected CDD values, this implies that it will experience hotter or longer summers.

18 °C is the temperature at which air conditioning is required to maintain a comfortable temperature inside buildings. A place that gets many days with average temperatures above 18 °C or that gets mean temperatures much higher than 18 °C will require a relatively large amount of energy (and thus money) to cool buildings for comfort and safety.

Technical description:

Annual sum of the number of degrees Celsius that each day’s mean temperature is above 18 °C.

Corn Heat Units

Corn Heat Units (CHU) is a temperature-based index often used by farmers and agricultural researchers to estimate whether the climate is warm enough (but not too hot) to grow corn.

One of the common climate indices used to assess the viability of growing a crop in a region is average annual CHUs. The CHUs expected in a region’s growing season are used to assess whether corn, or a particular variety of corn, is likely to fully mature in that region. Very generally, at least 2200 CHUs are required to mature most varieties of corn.

Note that this index is only based on temperature and does not take into account the availability of water to grow the crop.

Technical description:

An index of growing-season heat available to grow corn. It is the annual sum of daily corn heat units (CHU) derived from daily maximum and minimum temperatures, taking into account the temperatures that are optimal for corn growth. The starting date for the annual accumulation of CHUs is estimated as the last day of three consecutive days with a mean daily temperature greater than or equal to 12.8 °C; the ending date for CHU accumulation is the first date with a minimum temperature less than or equal to -2.0 °C.

 

Date of First Fall Frost

The date of the first fall frost, which marks the approximate end of the growing season for frost-sensitive crops and plants.

The arrival of frost marks the end of the growing season and announces the imminent return of winter. If projections show a later Date of First Fall Frost, then the seasonal transition from warmer to colder weather is happening later in the year.

Changes in the length and timing of the frost-free season affect plant and animal life, but also our social, psychological, and physical experience of the changing seasons.

The growth of most plants and crops is limited by the temperature of the air and soil. Since crops and plants need time to mature, the later in the fall they experience freezing temperatures, the more likely it is that they will be able to mature to their full potential. Of course, the time available for growth, maturity and productivity of these plants is also determined by the Date of Last Spring Frost that, together with the Date of First Fall Frost, determines the overall length of the frost-free season.

Technical description:

The first date in the fall (or late summer) on which the daily minimum temperature is equal to or less than 0 °C (Tmin ≤ 0 °C).

Because this variable is largely used to assess the end of the growing season in southern Canada, the earliest possible date for a fall frost was set as July 15; that is, the date of the first fall frost in any given year is determined by an assessment of minimum temperatures, one day at a time, forwards from July 15.

Date of Last Spring Frost

The date of the last spring frost, which marks the approximate beginning of the growing season for frost-sensitive crops and plants.

The final frost in the early spring marks the beginning of the growing season and announces the imminent return of summer. If projections show an earlier Date of Last Spring Frost, then the transition from colder to warmer weather is happening earlier in the year.

Changes in the length and timing of the frost-free season affect plant and animal life, but also our social, psychological, and physical experience of the changing seasons.

For most crops and plants, emergence and growth in the spring is limited by freezing temperatures in the air and soil. Since crops and plants need time to mature, the earlier in the spring they start to grow without the occurrence of frost, the more likely it is that they will grow to their full potential. Of course, the time available for growth, maturity and productivity of these plants is also determined by the Date of First Fall Frost that, together with the Date of Last Spring Frost, determines the overall length of the frost-free season.

Technical description:

The spring date after which there are no daily minimum temperatures during the growing season less than or equal to 0 °C (Tmin > 0 °C). Because this variable is largely used to assess the beginning of the growing season in southern Canada, the latest possible date for a spring frost was set as July 15; that is, the date of the last spring frost in any given year is determined by an assessment of minimum temperatures, one day at a time, backwards from July 15.

Freeze-Thaw Cycles

This is a simple count of days when the air temperature fluctuates between freezing and non-freezing temperatures. Under these conditions, it is likely that some water at the surface was both liquid and ice at some point during the 24-hour period.

Freeze-thaw cycles can have major impacts on infrastructure. Water expands when it freezes, so the freezing, melting and re-freezing of water can over time cause significant damage to roadways, sidewalks, and other outdoor structures. Potholes that form during the spring, or during mid-winter melts, are good examples of the damage caused by this process.

Technical description:

A freeze-thaw cycle occurs when the daily maximum temperature (Tmax) is higher than 0 °C and the daily minimum temperature (Tmin) is less than or equal to -1 °C. The minimum temperature of -1 °C (rather than 0 °C) is used as the threshold for freezing to raise the likelihood that water actually froze at the surface.

Freezing Degree Days

Freezing degree days (FDD) begin to accumulate when the daily mean temperature drops below freezing: if a day’s mean temperature is -21 °C, for example, it increases the annual FDD value by 21. Days when the mean temperature is 0 °C or warmer do not contribute to the annual sum.

High FDD values are associated with relatively cold conditions: places with high FDD values likely get many days with temperatures significantly below freezing. If projections show a decrease in FDDs, then that location is likely to experience shorter or less severe winters.

High FDD likely imply greater snow and ice accumulation, which is an important consideration for winter activities such as skiing, the building of winter roads, municipal and rural snow clearance, and many other aspects of winter life. Such places would likely require a relatively large amount of energy for heating.

Technical description:

Annual sum of the number of degrees Celsius that each day’s mean temperature is below 0 °C.

Frost Days

A frost day is one on which the coldest temperature of the day is lower than 0 C. Under these conditions frost might form at ground level or on cold surfaces.

The number of frost days is an indicator of the length and severity of the winter season. A location with a large number of frost days is also likely to have a short growing season, since frost is harmful to many plants.

Cold weather is an important aspect of life in Canada, and many places in Canada are well adapted to very cold winters. It is especially important to know how our winters will change in the future, because cold temperatures affect our health and safety, determine what plants and animals can live in the area, limit or enable outdoor activities, define how we design our buildings and vehicles, and shape our transportation and energy use.

Technical description:

A day on which the daily minimum temperature (Tmin) is below 0 °C.

Frost-Free Season

The Frost-Free Season is the approximate length of the growing season, during which there are no freezing temperatures to kill or damage plants.

Changes in the length and timing of the Frost-Free Season affect plant and animal life, but also our social, psychological, and physical experience of the changing seasons.

The average length of the growing season (and its year-to-year variability) is an important consideration when selecting or predicting what plants might grow well in a region. A longer Frost-Free Season means plants and crops have a longer window to grow and mature.

If projections show an increase in the length of the Frost-Free Season, then the annual growing season will be longer, and the period of cold weather correspondingly shorter.

Technical description:

The number of days between the date of the last spring frost and the date of the first fall frost, equivalent to the number of consecutive days during the ‘summer’ without any daily minimum temperatures equal to or below 0 °C.

Growing Degree Days (Base 10 °C)

Growing Degree Days (GDD) provide an index of the amount of heat available for the growth and maturation of plants and insects. Different base temperatures (5, 10 and 15 °C) are used to capture results for organisms that demand different amounts of heat.

Growing Degree Days (GDDs) are often used to determine whether a climate is warm enough to support plants and insects with temperature-dependent growth rates.

GDDs accumulate whenever the daily mean temperature is above a specified threshold temperature. Generally, 5 °C GDDs are used for assessing the growth of canola and forage crops; 10 °C GDDs are more appropriate for assessing the growth of corn and beans; and 15 °C GDDs are used to assess the growth and development of insects and pests.

Technical description:

Annual sum of the number of degrees Celsius that each day’s mean temperature is above a specified base temperature (Tbase).

Growing Degree Days (Base 15 °C)

Growing Degree Days (GDD) provide an index of the amount of heat available for the growth and maturation of plants and insects. Different base temperatures (5, 10 and 15 °C) are used to capture results for organisms that demand different amounts of heat.

Growing Degree Days (GDDs) are often used to determine whether a climate is warm enough to support plants and insects with temperature-dependent growth rates.

GDDs accumulate whenever the daily mean temperature is above a specified threshold temperature. Generally, 5 °C GDDs are used for assessing the growth of canola and forage crops; 10 °C GDDs are more appropriate for assessing the growth of corn and beans; and 15 °C GDDs are used to assess the growth and development of insects and pests.

Technical description:

Annual sum of the number of degrees Celsius that each day’s mean temperature is above a specified base temperature (Tbase).

Growing Degree Days (Base 5 °C)

Growing Degree Days (GDD) provide an index of the amount of heat available for the growth and maturation of plants and insects. Different base temperatures (5, 10 and 15 °C) are used to capture results for organisms that demand different amounts of heat.

Growing Degree Days (GDDs) are often used to determine whether a climate is warm enough to support plants and insects with temperature-dependent growth rates.

GDDs accumulate whenever the daily mean temperature is above a specified threshold temperature. Generally, 5 °C GDDs are used for assessing the growth of canola and forage crops; 10 °C GDDs are more appropriate for assessing the growth of corn and beans; and 15 °C GDDs are used to assess the growth and development of insects and pests.

Technical description:

Annual sum of the number of degrees Celsius that each day’s mean temperature is above a specified base temperature (Tbase).

Heating Degree Days

Heating Degree Days (HDD) are equal to the number of degrees Celsius a given day’s mean temperature is below 18 °C. For example, if the daily mean temperature is 12 °C, the HDD value for that day is equal to 6 °C. If the daily mean temperature is above 18 °C, the HDD value for that day is set to zero.

Heating Degree Days are a measure of how much heating is required in a year. 18 °C is the temperature below which heating is required to maintain a comfortable temperature inside buildings. A place that gets many days with average temperatures below 18 °C or that gets mean temperatures much below 18 °C will require a relatively large amount of energy (and thus money) to heat buildings for comfort and safety.

If a location shows a decrease in projected HDD values, this implies that it will experience shorter periods of cold weather, or that it will experience less severe cold.

Technical description:

Annual sum of the number of degrees Celsius that each day’s mean temperature is below 18 °C.

Heavy Precipitation Days (10 mm)

A Heavy Precipitation Day (HPD) is a day on which at least a total of 10 mm (or 20 mm) of rain or frozen precipitation falls. Frozen precipitation is measured according to its liquid equivalent: 10 cm of snow is usually about 10 mm of precipitation.

What counts as a Heavy Precipitation Day (HPD) depends on where you live: some places in Canada rarely get 10 mm of rain in a day, whereas others often do. The Climate Atlas offers two versions of HPD: days on which either 10 mm or 20 mm or more of rain (or its frozen equivalent, usually snow) falls.

Heavy rainfall events can create many challenges. In cities and towns, heavy rainfalls can overwhelm storm drains and cause flash flooding. They can also cause problems in rural areas by drowning crops, eroding topsoil, and damaging roads.

Heavy snowfall events can disrupt ground transportation, and very heavy snowfall events can cause damage to buildings if their roofs become overburdened.

Technical description:

Heavy Precipitation Days are defined in two ways: either as a day on which at least a total of 10 mm of rain and/or liquid-equivalent frozen precipitation is deposited at the surface, or a day with 20 mm.

Heavy Precipitation Days (20 mm)

A Heavy Precipitation Day (HPD) is a day on which at least a total of 10 mm (or 20 mm) of rain or frozen precipitation falls. Frozen precipitation is measured according to its liquid equivalent: 10 cm of snow is usually about 10 mm of precipitation.

What counts as a Heavy Precipitation Day (HPD) depends on where you live: some places in Canada rarely get 10 mm of rain in a day, whereas others often do. The Climate Atlas offers two versions of HPD: days on which either 10 mm or 20 mm or more of rain (or its frozen equivalent, usually snow) falls.

Heavy rainfall events can create many challenges. In cities and towns, heavy rainfalls can overwhelm storm drains and cause flash flooding. They can also cause problems in rural areas by drowning crops, eroding topsoil, and damaging roads.

Heavy snowfall events can disrupt ground transportation, and very heavy snowfall events can cause damage to buildings if their roofs become overburdened.

Technical description:

Heavy Precipitation Days are defined in two ways: either as a day on which at least a total of 10 mm of rain and/or liquid-equivalent frozen precipitation is deposited at the surface, or a day with 20 mm.

Icing Days

An Icing Day is a day on which the air temperature does not go above freezing (0 °C).

The number of days on which the air temperature does not rise above freezing is a good indicator of the length and severity of the winter season.

Cold weather is an important aspect of life in Canada, and many places in Canada are well adapted to very cold winters. It is especially important to know how our winters will change in the future, because cold temperatures affect our health and safety, determine what plants and animals can live in the area, limit or enable outdoor activities, define how we design our buildings and vehicles, and shape our transportation and energy use.

Technical description:

An Icing Day is a day on which the daily maximum temperature (Tmax) is less than or equal to 0 °C.

 

Maximum Temperature

The highest temperature of the day.

The temperature range we expect within a season or year is a very important aspect of climate. Changes in average and extreme temperatures can dramatically affect our everyday lives as well as a wide range of planning and policy decisions.

The average highest temperature is an environmental indicator with many applications in agriculture, engineering, health, energy management, recreation, and more. The maximum temperature is also used to compute a number of other maps and indices in this Atlas.

Technical description:

The daily maximum temperature (Tmax).

Mean Temperature

The average temperature of the day.

The temperature range we expect within a season or year is a very important aspect of climate. Changes in average and extreme temperatures can dramatically affect our everyday lives as well as a wide range of planning and policy decisions.

The average temperature is an environmental indicator with many applications in agriculture, engineering, health, energy management, recreation, and more. The average temperature is used to compute a number of other maps and indices in this Atlas.

Technical description:

The average of the daily maximum temperature (Tmax) and the daily minimum temperature (Tmin).

Minimum Temperature

The lowest temperature of the day.

The temperature range we expect within a season or year is a very important aspect of climate. Changes in average and extreme temperatures can dramatically affect our everyday lives as well as a wide range of planning and policy decisions.

The average lowest temperature is an environmental indicator with many applications in agriculture, engineering, health, energy management, recreation, and more. The average temperature is used to compute a number of other maps and indices in this Atlas.

Technical description:

The daily minimum temperature (Tmin).

P-Days

This is a temperature-based index often used by farmers and agricultural researchers to estimate whether the climate is warm enough (but not too hot) to grow potatoes. It does not take into account the availability of water to grow the crop.

All plants have a temperature range conducive to optimal growth; temperatures that are too cold or too hot will inhibit their ability to grow and mature. Potatoes are no different. They are sensitive to temperatures that get too cold or too warm. The P-Days index allows us to assess whether potatoes are likely to fully mature in a region.

 

Importantly, P-Days values do not indicate whether the crops would have access to adequate levels of soil moisture.

Technical description:

An index of growing-season heat available to grow potatoes derived from daily maximum and minimum temperatures, taking into account the temperatures that are optimal for potato plant growth.

P-Days accumulate when the minimum temperature is not less than 7.0 °C and the maximum is not higher than 30.0 °C.

We use the commonly used P-day equation developed by the Institute of Agriculture and Natural Resources at the University of Nebraska-Lincoln (http://cropwatch.unl.edu/potato/physiological_days).

Precipitation

The total amount of rain, drizzle, snow, sleet, etc. Frozen precipitation is measured according to its liquid equivalent: 10 cm of snow is usually about 10 mm of precipitation.

Precipitation patterns are critical for many important issues, including water availability, crop production, electricity generation, wildfire suppression, snow accumulation, seasonal and flash-flooding, and short- and long-term drought risk.

Technical description:

The depth of liquid and/or frozen water deposited on the surface, with frozen precipitation converted to its liquid water equivalent.

Summer Days

A Summer Day is a day when the temperature rises to at least 25 °C.

High temperatures are important. They determine if plants and animals can thrive, they limit or enable outdoor activities, define how we design our buildings and vehicles, and shape our transportation and energy use. It is useful to know how high summer temperatures are likely to become in the future, to make sure that our cooling and air-conditioning systems can reliably deal with these extremes.

When temperatures are very hot, people - especially the elderly - are much more likely to suffer from heat exhaustion and heat stroke. Many outdoor activities become dangerous or impossible in very high temperatures. In general, Canadians are not used to extremely hot summers, and further warming will bring new and unusual risks as well as a very different experience of the summer season.

High, persistent temperatures increase the risk of drought, which can severely impact food production and increases the risk of wildfire. High temperatures can also lead to more thunderstorms, which means increased risks of flash flooding, lightning, hail and perhaps even tornadoes.

Technical description:

A Summer Day is a day with a maximum temperature (Tmax) greater than or equal to 25 °C.

Tropical Nights

A Tropical Night occurs when the lowest temperature of the day does not go below 20 °C.

Hot summer days are physiologically stressful, especially if overnight temperatures do not provide cooling relief. Many people are at risk from suffering heat exhaustion or heat stroke when nighttime temperatures fail to drop below 20 °C. Elderly people, the homeless, and those who live in houses or apartments without air conditioning are especially vulnerable during these heat events, particularly if they last for more than a few days.

High temperatures are important. They determine if plants and animals can thrive, they limit or enable outdoor activities, define how we design our buildings and vehicles, and shape our transportation and energy use. It is useful to know how high summer temperatures are likely to become in the future, to make sure that our cooling and air-conditioning systems can reliably deal with these extremes.

When temperatures are very hot, people - especially the elderly - are much more likely to suffer from heat exhaustion and heat stroke. Many outdoor activities become dangerous or impossible in very high temperatures. In general, Canadians are not used to extremely hot summers, and further warming will bring new and unusual risks as well as a very different experience of the summer season.

High, persistent temperatures increase the risk of drought, which can severely impact food production and increases the risk of wildfire. High temperatures can also lead to more thunderstorms, which means increased risks of flash flooding, lightning, hail and perhaps even tornadoes.

Technical description:

A Tropical Night occurs when the daily minimum temperature (Tmin) is greater than or equal to 20 °C.

Very cold days (-30°C)

A Very Cold Day is a day when the temperature drops to at least -30 °C.

Very cold days are an indicator of winter severity.

Cold weather is an important aspect of life in Canada, and many places in Canada are well adapted to very cold winters. It is especially important to know how our winters will change in the future, because cold temperatures affect our health and safety, determine what plants and animals can live in the area, limit or enable outdoor activities, define how we design our buildings and vehicles, and shape our transportation and energy use.

Technical description:

A Very Cold Day is a day with a minimum temperature (Tmin) less than or equal to -30 °C.

Very hot days (+30°C)

A Very Hot Day is a day when the temperature rises to at least 30 °C.

Very Hot Days are an indicator of summer heat.

High temperatures are important. They determine if plants and animals can thrive, they limit or enable outdoor activities, define how we design our buildings and vehicles, and shape our transportation and energy use. It is useful to know how high summer temperatures are likely to become in the future, to make sure that our cooling and air-conditioning systems can reliably deal with these extremes.

When temperatures are very hot, people - especially the elderly - are much more likely to suffer from heat exhaustion and heat stroke. Many outdoor activities become dangerous or impossible in very high temperatures. In general, Canadians are not used to extremely hot summers, and further warming will bring new and unusual risks as well as a very different experience of the summer season.

High, persistent temperatures increase the risk of drought, which can severely impact food production and increases the risk of wildfire. High temperatures can also lead to more thunderstorms, which means increased risks of flash flooding, lightning, hail and perhaps even tornadoes.

Technical description:

A Very Hot Day is a day with a maximum temperature (Tmax) greater than or equal to 30 °C.

Warmest Maximum Temperature

The highest temperature of the year.

High temperatures are important. They determine if plants and animals can thrive, they limit or enable outdoor activities, define how we design our buildings and vehicles, and shape our transportation and energy use. It is useful to know how high summer temperatures are likely to become in the future, to make sure that our cooling and air-conditioning systems can reliably deal with these extremes.

When temperatures are very hot, people - especially the elderly - are much more likely to suffer from heat exhaustion and heat stroke. Many outdoor activities become dangerous or impossible in very high temperatures. In general, Canadians are not used to extremely hot summers, and further warming will bring new and unusual risks as well as a very different experience of the summer season.

High, persistent temperatures increase the risk of drought, which can severely impact food production and increases the risk of wildfire. High temperatures can also lead to more thunderstorms, which means increased risks of flash flooding, lightning, hail and perhaps even tornadoes.

Technical description:

The maximum maximum temperature of the year.