Cloud Forecast for astronomical observations

The state of the sky, as defined in the public forecast program, depends on the coverage and cloud opacity of all cloud layers forecasted by the weather model, but also on the height and type of clouds as well as obstructions to visibility such as fog, precipitation or smoke. These parameters are determined at a specific time for a given location. Cloudiness, or cloud cover, is the fraction of the sky covered by clouds of a certain kind, species, variety, layer, or combination of clouds. Total cloud cover is the fraction of the sky hidden by all visible clouds. Opacity is the vertical visibility through the clouds. Clouds can be thin and transparent like cirrus clouds or block light completely. Both are measured in octas, one eighth of the sky, or in tenths. 

Remarks:

1. Cirrus (thin cloud at high altitude)

Numerical models are very good for predicting cirrus clouds preceding a weather system. On the other hand, they tend to give us the impression that they forecast a lot of them. A distinction must be made between cloud opacity and cloud extent. The model can forecast a 9/10 range of cirrus while the opacity is 3/10. In other words, the sky can be covered with a layer of cirrus clouds thin enough to see the sky or the stars through. Then, under certain conditions, you will have the impression that the forecast predicts a lot of clouds when only a thin veil of cirrus clouds obstructs the sky. These conditions will still not be favorable for astronomical observation, except for planetary observation.

2. Thunderstorms

A thunderstorm is a point phenomenon of smaller size and structure than the resolution of numerical weather prediction models. During the warm season, thunderstorms are a source of difficulty for models since they are associated with a certain temporal and spatial uncertainty. Models also sometimes tend to forecast a greater or lesser number of thunderstorms depending on the ambient weather conditions. Thunderstorms have a strong vertical extension so that their tops can reach the jet stream, an area of strong winds located aloft. The associated clouds will then be propagated over great distances. So, during heat wave periods, remember that cloud forecasts are not optimal.

The color tones used for the cloud forecast are interpreted in a fairly natural way:

1. The whitish areas represent a cloudy sky.
2. Areas ranging from pale blue to indigo represent a partially overcast sky.
3. Dark blue areas represent a clear sky.

Regional model, cloud forecasts for North America

Image forecasts are updated four times a day.

T+hh refers to the forecast time interval (hh), in hours, from the model initialization time T. The time T is indicated, in universal time, at the top of each column.

To convert UTC (Coordinated Universal Time) to local time, you must calculate the offset between your time zone and Greenwich Mean Time and take into account Daylight Saving Time, if applicable.

For example,

• 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 Prairie Standard Time (winter) = 13:00 Prairie 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)

Please note that Saskatchewan does not use Prairie Daylight Saving Time.