Sky Transparency Forecast for astronomical observations
The conditions of transparency of the sky vary with the air masses. In regions such as Arizona, it is possible to observe stars with magnitudes ranging from 6.5 to 7.2 under very dry skies. In the countryside, in more northern latitudes or in humid regions, the depth of the sky is generally limited to values between 5.5 and 6.5. Moisture-laden maritime air masses give rise to poor transparency conditions. On the other hand, continental air masses are generally dry and can in some cases give transparency conditions as good as those of the southwestern United States, hence the interest in predicting these magnificent and all too rare starry evenings.
The only measurable and predictable variable affecting the transparency of the sky is humidity. In fact, atmospheric humidity is, most of the time, the dominant factor causing a decrease in the transparency of the sky. A hot day with a whitish sky is the best example of its effect. Industrial pollutants and light pollution also play a role, but on a local scale. Aerosols, such as ash from volcanic eruptions, pollen, sea salt, sandstorms and smoke from forest fires are other elements that can contribute to a decrease in the transparency of the sky. However, they are not taken into account in the preparation of these forecasts. The same is true for the Northern Lights.
The forecast of transparency of the sky is essentially an integration of the humidity in the whole air column with an increased importance at the level of the troposphere, especially near the surface. The sky transparency index is represented by 5 levels or shades of blue. White areas are where the weather model predicts clouds. Dark blue indicates the best transparency conditions and gray indicates the worst conditions.
| Categories | Color | Transparency conditions |
|---|---|---|
| 0/5 | White | Zero transparency (cloudy sky) |
| 1/5 | Grey | Very bad transparency |
| 2/5 | Light blue | Poor transparency |
| 3/5 | Light blue | Medium transparency |
| 4/5 | Bright blue | Good transparency |
| 5/5 | Dark blue | Excellent transparency |
Regional model, forecast of sky transparency at the zenith for North America
The image forecast is updated four times a day.
| 00 UTC | 06 UTC | 12 UTC | 18 UTC |
|---|---|---|---|
| Animation | Animation | Animation | Animation |
| T+01 | T+01 | T+01 | T+01 |
| T+02 | T+02 | T+02 | T+02 |
| T+03 | T+03 | T+03 | T+03 |
| T+04 | T+04 | T+04 | T+04 |
| T+05 | T+05 | T+05 | T+05 |
| .... | ... | ... | .... |
| T+84 | T+84 | T+84 | T+84 |
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.
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