Clouds Forecast For Astronomical Purposes

These cloud forecast images are produced twice a day, around 17:30 UTC and 05:30 UTC (Universal coordinated time). To convert UTC time to local time, you have to take into account the time difference between your time zone and Greenwich time zone as well as daylight time if necessary. Thus,

• 18:00 UTC = 14:30 Newfoundland standard time (winter) = 15:30 Newfoundland daylight time (summer)
• 18:00 UTC = 14:00 Atlantic standard time (winter) = 15:00 Atlantic daylight time (summer)
• 18:00 UTC = 13:00 Eastern standard time (winter) = 14:00 Eastern daylight time (summer)
• 18:00 UTC = 12:00 Central standard time (winter) = 13:00 Central daylight time (summer)
• 18:00 UTC = 11:00 Mountain standard time (winter) = 12:00 Mountain daylight time (summer)
• 18:00 UTC = 10:00 Pacific standard time (winter) = 11:00 Pacific daylight time (summer).

Note: Saskatchewan does not use Daylight Saving Time.

A few words regarding the colors on these images.

1. White areas represent overcast cloud cover.
2. Light blue areas to indigo represent a partly cloudy sky condition.
3. Dark blue areas represent clear skies.

These images do not provide information on the height or the type of clouds. The only information provided is about the amount of cloud forecast for the next 48 hours.

Caution

1. Cirrus (thin cloud in high altitude)

   Numerical models are very good at forecasting the cirrus preceding a weather system. However, you might get the impression that the cirrus is over-forecast . There is a distinction between the opacity and the extent of the clouds. The model could produce an area of 9/10 of cirrus cover which has an opacity of 3/10. In other words, the sky can be covered with a layer of cirrus thin enough to permit visibility of the sky or the stars through it. Thus, under certain conditions, you will have the impression that forecasts predict too many clouds whereas only a thin veil of cirrus blocks the sky. On the other hand, these conditions will not be favourable for astronomical observation, except for planetary observation.

2. Thunderstorms

   Thunderstorms are a small scale weather phenomenon below the resolution of numerical weather prediction models. During the warm season, thunderstorms are a source of difficulty for the weather models. They sometimes tend to be over or under forecast depending on the weather conditions. Thunderstorms can have great vertical extension, the cloud tops often reaching the jet stream level (strong winds), so the clouds are spread large distances downstream. Given the temporal and spatial uncertainty in predicting thunderstorms, the associated cloud deck could be forecast in a corridor further north or south than it should be. During heat waves, please be advised that the forecasts of clouds may not be at their best.