Current Condition (CHATHAM, ON)

Saturday, Apr. 04, 2020


Today’s Forecasted LOW: °CHIGH: °C

Weather Products

Forecast Data

Subscribers to WIN can also receive site-specific forecasts. By providing your latitude and longitude you can receive up to 8 parameters of forecast data, unique to your location. The information is posted daily, and both hourly and daily forecasts are available.

Forecast data is currently available for Ontario, Quebec, Michigan, and Manitoba, with continual expansion into new locations.

Available Forecast Parameters:

  • Temperature
  • Precipitation
  • Relative Humidity
  • Solar Radiation
  • Wind Speed
  • Wind Direction
  • Soil Temperature
  • Soil Moisture

Sample Forecast results


Crop Heat Unit (CHU)

Crop Heat Units are calculated on a daily basis, using the maximum (Tmax) and minimum (Tmin) daily air temperatures, measured from midnight to midnight, in degrees Celsius. The Crop Heat Unit (CHU) system is used to assist farmers to select the most suitable hybrids and varieties for their area.

The following equation is used to calculate daily CHU:

Daily CHU = (Ymax + Ymin) ÷ 2


Ymax = (3.33 x (Tmax-10)) - (0.084 x (Tmax-10.0)2)

(If Ymax < 0, set Ymax = 0)


Ymin = (1.8 x (Tmin - 4.4))

(If Ymin < 0, set Ymax = 0)

The Crop Heat Unit ratings are based on the total accumulated crop heat units (CHU) from the season start date to season-ending date.

A standard start date (date to start accumulating CHU) of May 1 is used across the province of Ontario. Read More

The season-ending date (date to stop accumulating CHU) is the earlier date of either:

  1. the first occurrence of -2 °C; or
  2. the date when the 30-year average daily mean air temperature dropped to 12°C (or lower).

Maps between May 1 and Oct. 15:

  1. Current CHU Accumulation - The CHU accumulated to today, from a start date of May 1.
  2. 10 Day Forecast of CHU Accumulation - The predicted CHU accumulation for 10 days from now.
  3. Predicted CHU Seasonal Accumulation - The predicted CHU accumulation at the end of the season. The season-ending date (date to stop accumulating CHU's) is the date when the 30-year average daily mean air temperature dropped to 12°C (54°F) or lower.
  4. CHU Deviation - Current vs. Normal - The difference between the current CHU accumulation and the 30 year normal for the current date.

Sample CHU results


Growing Degree Day (GDD)

Growing degree days are a measure of how many heat units are accumulated in a single day or over time. They are calculated on a daily basis using the daily maximum (Tmax) and minimum (Tmin) temperature. This indexing system is useful in determining how the season is progressing, whether the crop is ahead or behind and which varieties are suitable to a particular location

The following equation is used to calculate daily GDD:

Daily GDD = ((Tmax + Tmin) ÷ 2) - Tbase


Tbase = the GDD base temperature for the organism being monitored

Season GDD are generally accumulated from from Apr. 1 to Oct. 31.

Sample GDD results



Evapotranspiration (ET) is a measure of the amount of moisture lost to the atmosphere through evaporation from the ground and transpiration from plants. ET values can be used for determining irrigation required by estimating how much moisture is being lost.

There are several methods for calculating ET, some requiring more parameters than others. WIN calculates ET using the Priestley-Taylor method which uses a minimum number of parameters.

To access the historical ET Maps for more regions, please visit


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