Climate and Average Weather Year Round in Cornwall Ontario, Canada

In Cornwall, the summers are long, warm, and partly cloudy and the winters are freezing, snowy, and mostly cloudy. Over the course of the year, the temperature typically varies from 6°F to 80°F and is rarely below -14°F or above 88°F.

Based on the tourism score, the best time of year to visit Cornwall for warm-weather activities is from late June to early September.

The warm season lasts for 3.9 months, from May 21 to September 17, with an average daily high temperature above 69°F. The hottest month of the year in Cornwall is July, with an average high of 79°F and low of 60°F.

The cold season lasts for 3.3 months, from December 3 to March 12, with an average daily high temperature below 36°F. The coldest month of the year in Cornwall is January, with an average low of 8°F and high of 25°F.

In Cornwall, 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 Cornwall begins around May 25 and lasts for 5.0 months, ending around October 27.

The clearest month of the year in Cornwall is August, during which on average the sky is clear, mostly clear, or partly cloudy 63% of the time.

The cloudier part of the year begins around October 27 and lasts for 7.0 months, ending around May 25.

The cloudiest month of the year in Cornwall is January, during which on average the sky is overcast or mostly cloudy 69% of the time.

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

The wetter season lasts 6.7 months, from April 9 to October 31, with a greater than 32% chance of a given day being a wet day. The month with the most wet days in Cornwall is July, with an average of 12.8 days with at least 0.04 inches of precipitation.

The drier season lasts 5.3 months, from October 31 to April 9. The month with the fewest wet days in Cornwall is February, with an average of 6.4 days with at least 0.04 inches of precipitation.

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 Cornwall changes throughout the year.

Rain alone is the most common for 9.2 months, from March 8 to December 12. The month with the most days of rain alone in Cornwall is July, with an average of 12.8 days.

Snow alone is the most common for 2.8 months, from December 12 to March 8. The month with the most days of snow alone in Cornwall is January, with an average of 4.2 days.

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. Cornwall experiences significant seasonal variation in monthly rainfall.

Rain falls throughout the year in Cornwall. The month with the most rain in Cornwall is June, with an average rainfall of 3.2 inches.

The month with the least rain in Cornwall is February, with an average rainfall of 0.5 inches.

Snowfall

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

The snowy period of the year lasts for 5.9 months, from October 27 to April 25, with a sliding 31-day snowfall of at least 1.0 inches. The month with the most snow in Cornwall is December, with an average snowfall of 10.0 inches.

The snowless period of the year lasts for 6.1 months, from April 25 to October 27. The least snow falls around July 21, with an average total accumulation of 0.0 inches.

The length of the day in Cornwall varies significantly over the course of the year. In 2024, the shortest day is December 21, with 8 hours, 46 minutes of daylight; the longest day is June 20, with 15 hours, 37 minutes of daylight.

The earliest sunrise is at 5:11 AM on June 14, and the latest sunrise is 2 hours, 28 minutes later at 7:39 AM on November 2. The earliest sunset is at 4:17 PM on December 9, and the latest sunset is 4 hours, 33 minutes later at 8:49 PM on June 26.

Daylight saving time (DST) is observed in Cornwall during 2024, starting in the spring on March 10, lasting 7.8 months, and ending in the fall on November 3.

The figure below presents a compact representation of key lunar data for 2024. The horizontal axis is the day, the vertical axis is the hour of the day, and the colored areas indicate when the moon is above the horizon. The vertical gray bars (new Moons) and blue bars (full Moons) indicate key Moon phases.

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.

Cornwall experiences some seasonal variation in the perceived humidity.

The muggier period of the year lasts for 3.3 months, from June 4 to September 15, during which time the comfort level is muggy, oppressive, or miserable at least 6% of the time. The month with the most muggy days in Cornwall is July, with 7.3 days that are muggy or worse.

The least muggy day of the year is February 18, when muggy conditions are essentially unheard of.

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 Cornwall experiences significant seasonal variation over the course of the year.

The windier part of the year lasts for 4.6 months, from December 4 to April 24, with average wind speeds of more than 6.2 miles per hour. The windiest month of the year in Cornwall is February, with an average hourly wind speed of 7.9 miles per hour.

The calmer time of year lasts for 7.4 months, from April 24 to December 4. The calmest month of the year in Cornwall is August, with an average hourly wind speed of 4.6 miles per hour.

The predominant average hourly wind direction in Cornwall is from the west throughout the year.

To characterize how pleasant the weather is in Cornwall 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 Cornwall 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 Cornwall for hot-weather activities is from early July to mid August, with a peak score in the last 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.

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 Cornwall typically lasts for 5.0 months (153 days), from around May 5 to around October 5, rarely starting before April 17 or after May 23, and rarely ending before September 19 or after October 21.

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 Cornwall should appear around May 1, only rarely appearing before April 18 or after May 10.

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 month of the year in Cornwall is June, with an average of 6.5 kWh.

The darker period of the year lasts for 3.5 months, from October 27 to February 13, with an average daily incident shortwave energy per square meter below 2.3 kWh. The darkest month of the year in Cornwall is December, with an average of 1.2 kWh.

For the purposes of this report, the geographical coordinates of Cornwall are 45.018 deg latitude, -74.728 deg longitude, and 187 ft elevation.

The topography within 2 miles of Cornwall contains only modest variations in elevation, with a maximum elevation change of 154 feet and an average elevation above sea level of 182 feet. Within 10 miles also contains only modest variations in elevation (279 feet). Within 50 miles contains only modest variations in elevation (3,763 feet).

The area within 2 miles of Cornwall is covered by artificial surfaces (38%), trees (35%), and water (19%), within 10 miles by trees (48%) and cropland (24%), and within 50 miles by trees (54%) and cropland (29%).