Temperature

The warm season lasts for 3.9 months, from May 21 to September 18, with an average daily high temperature above 68°F. The hottest day of the year is July 20, with an average high of 80°F and low of 61°F.

The cold season lasts for 3.3 months, from December 3 to March 11, with an average daily high temperature below 33°F. The coldest day of the year is January 29, with an average low of 6°F and high of 22°F.

The figure below shows you a compact characterization of the entire year of hourly average temperatures. The horizontal axis is the day of the year, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.

Voronezh, Russia (4,700 miles away) and Sunagawa, Japan (5,882 miles) are the far-away foreign places with temperatures most similar to Ottawa (view comparison).

Clouds

In Ottawa, the average percentage of the sky covered by clouds experiences significant seasonal variation over the course of the year.

The clearer part of the year in Ottawa begins around May 24 and lasts for 5.1 months, ending around October 26. On August 8, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 64% of the time, and overcast or mostly cloudy 36% of the time.

The cloudier part of the year begins around October 26 and lasts for 6.9 months, ending around May 24. On January 5, the cloudiest day of the year, the sky is overcast or mostly cloudy 70% of the time, and clear, mostly clear, or partly cloudy 30% of the time.

Precipitation

A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. The chance of wet days in Ottawa varies throughout the year.

The wetter season lasts 8.1 months, from April 9 to December 14, with a greater than 29% chance of a given day being a wet day. The chance of a wet day peaks at 38% on July 3.

The drier season lasts 3.9 months, from December 14 to April 9. The smallest chance of a wet day is 20% on January 29.

Among wet days, we distinguish between those that experience rain alone, snow alone, or a mixture of the two. Based on this categorization, the most common form of precipitation in Ottawa changes throughout the year.

Rain alone is the most common for 8.7 months, from March 14 to December 4. The highest chance of a day with rain alone is 38% on July 3.

Snow alone is the most common for 3.3 months, from December 4 to March 14. The highest chance of a day with snow alone is 16% on January 9.

Rainfall

To show variation within the months and not just the monthly totals, we show the rainfall accumulated over a sliding 31-day period centered around each day of the year. Ottawa experiences significant seasonal variation in monthly rainfall.

Rain falls throughout the year in Ottawa. The most rain falls during the 31 days centered around June 25, with an average total accumulation of 3.0 inches.

The least rain falls around February 6, with an average total accumulation of 0.5 inches.

Snowfall

We report snowfall in liquid-equivalent terms. The actual depth of new snowfall is typically between 5 and 10 times the liquid-equivalent amount, assuming the ground is frozen. Colder, drier snow tends to be on the higher end of that range and warmer, wetter snow on the lower end.

As with rainfall, we consider the snowfall accumulated over a sliding 31-day period centered around each day of the year. Ottawa experiences significant seasonal variation in monthly liquid-equivalent snowfall.

The snowy period of the year lasts for 5.9 months, from October 27 to April 24, with a sliding 31-day liquid-equivalent snowfall of at least 0.1 inches. The most snow falls during the 31 days centered around December 24, with an average total liquid-equivalent accumulation of 1.3 inches.

The snowless period of the year lasts for 6.1 months, from April 24 to October 27. The least snow falls around July 26, with an average total liquid-equivalent accumulation of 0.0 inches.

Sun

The length of the day in Ottawa varies significantly over the course of the year. In 2019, the shortest day is December 21, with 8 hours, 43 minutes of daylight; the longest day is June 21, with 15 hours, 40 minutes of daylight.

The earliest sunrise is at 5:13 AM on June 15, and the latest sunrise is 2 hours, 29 minutes later at 7:43 AM on November 2. The earliest sunset is at 4:19 PM on December 10, and the latest sunset is 4 hours, 36 minutes later at 8:55 PM on June 26.

Daylight saving time (DST) is observed in Ottawa during 2019, starting in the spring on March 10, lasting 7.8 months, and ending in the fall on November 3.

Humidity

We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.

Ottawa experiences some seasonal variation in the perceived humidity.

The muggier period of the year lasts for 3.3 months, from June 5 to September 16, during which time the comfort level is muggy, oppressive, or miserable at least 5% of the time. The muggiest day of the year is July 20, with muggy conditions 19% of the time.

The least muggy day of the year is December 3, when muggy conditions are essentially unheard of.

Wind

This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.

The average hourly wind speed in Ottawa experiences significant seasonal variation over the course of the year.

The windier part of the year lasts for 4.4 months, from December 10 to April 24, with average wind speeds of more than 5.5 miles per hour. The windiest day of the year is February 19, with an average hourly wind speed of 7.4 miles per hour.

The calmer time of year lasts for 7.6 months, from April 24 to December 10. The calmest day of the year is July 31, with an average hourly wind speed of 3.7 miles per hour.

The predominant average hourly wind direction in Ottawa varies throughout the year.

The wind is most often from the north for 1.2 months, from March 14 to April 20, with a peak percentage of 32% on March 16. The wind is most often from the west for 3.9 months, from April 20 to August 18 and for 5.2 months, from October 8 to March 14, with a peak percentage of 41% on July 8. The wind is most often from the south for 1.7 months, from August 18 to October 8, with a peak percentage of 34% on September 18.

Best Time of Year to Visit

To characterize how pleasant the weather is in Ottawa throughout the year, we compute two travel scores.

The tourism score favors clear, rainless days with perceived temperatures between 65°F and 80°F. Based on this score, the best time of year to visit Ottawa for general outdoor tourist activities is from late June to early September, with a peak score in the first week of August.

The beach/pool score favors clear, rainless days with perceived temperatures between 75°F and 90°F. Based on this score, the best time of year to visit Ottawa for hot-weather activities is from early July to mid August, with a peak score in the third week of July.

Methodology

For each hour between 8:00 AM and 9:00 PM of each day in the analysis period (1980 to 2016), independent scores are computed for perceived temperature, cloud cover, and total precipitation. Those scores are combined into a single hourly composite score, which is then aggregated into days, averaged over all the years in the analysis period, and smoothed.

Our cloud cover score is 10 for fully clear skies, falling linearly to 9 for mostly clear skies, and to 1 for fully overcast skies.

Our precipitation score, which is based on the three-hour precipitation centered on the hour in question, is 10 for no precipitation, falling linearly to 9 for trace precipitation, and to 0 for 0.04 inches of precipitation or more.

Our tourism temperature score is 0 for perceived temperatures below 50°F, rising linearly to 9 for 65°F, to 10 for 75°F, falling linearly to 9 for 80°F, and to 1 for 90°F or hotter.

Our beach/pool temperature score is 0 for perceived temperatures below 65°F, rising linearly to 9 for 75°F, to 10 for 82°F, falling linearly to 9 for 90°F, and to 1 for 100°F or hotter.

Growing Season

Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).

The growing season in Ottawa typically lasts for 5.4 months (162 days), from around April 30 to around October 10, rarely starting before April 14 or after May 17, and rarely ending before September 23 or after October 26.

Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.

Based on growing degree days alone, the first spring blooms in Ottawa should appear around May 4, only rarely appearing before April 23 or after May 15.

Solar Energy

This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.

The average daily incident shortwave solar energy experiences extreme seasonal variation over the course of the year.

The brighter period of the year lasts for 3.5 months, from May 5 to August 22, with an average daily incident shortwave energy per square meter above 5.6 kWh. The brightest day of the year is June 30, with an average of 6.7 kWh.

The darker period of the year lasts for 3.4 months, from October 28 to February 12, with an average daily incident shortwave energy per square meter below 2.3 kWh. The darkest day of the year is December 17, with an average of 1.2 kWh.

Topography

For the purposes of this report, the geographical coordinates of Ottawa are 45.411 deg latitude, -75.698 deg longitude, and 217 ft elevation.

The topography within 2 miles of Ottawa contains only modest variations in elevation, with a maximum elevation change of 131 feet and an average elevation above sea level of 209 feet. Within 10 miles contains only modest variations in elevation (1,050 feet). Within 50 miles also contains very significant variations in elevation (1,903 feet).

The area within 2 miles of Ottawa is covered by artificial surfaces (89%), within 10 miles by trees (40%) and artificial surfaces (30%), and within 50 miles by trees (65%) and cropland (20%).

Data Sources

0%

clear

20%

east

comfortable

mostly clear

south

60°F

40%

humid

partly cloudy

west

65°F

muggy

60%

mostly cloudy

70°F

oppressive

80%

75°F

overcast

miserable

100%