Sky Transparency Forecast For Astronomical Purposes

Regional model, sky transparency forecast for North AmericaFootnote

Observing deep sky objects such as faint galaxies and nebulae requires excellent sky transparency. Astronomers evaluate sky transparency with the faintest star visible to the unaided eye. In semi-desertic regions such as Arizona, one can see stars as faint as faint as 6.5-7.2 magnitude. At mid-latitudes and in the more humid eastern regions. most of the time sky transparency is limited to the 5.5-6.5 range in the countryside. Sky transparency also varies with airmass type. With a humid airmass the transparency is reduced significantly. With a continental airmass from the arctic, relatively cold and dry conditions prevail, allowing the sky transparency to be at times be as good as in the semi-desertic regions. Good forecasts of such rare starry evenings will clearly be useful to the amateur astronomer.

The goal of this new product is to inform you about expected sky transparency conditions in your region.

The following images are sky transparency forecasts for North America. Moisture is the only element affecting sky transparency which can be both measured and forecast all across the globe. It is often the most important factor in reducing sky transparency. A muggy summer day with a whitish sky is the best example of this moisture effect.

Industrial pollutants and light pollution affect the sky transparency as well, but only locally. Aerosols such as volcanic ash, pollen, sea salt and smoke from forest fires also contribute to reduced sky transparency. The same can be said for auroras. None of these factors is integrated into our transparency forecast images.

So the experimental forecast is only an integration of moisture throughout the air column with a distribution emphasizing the humidity near the surface and near the tropopause. The quality of the sky transparency is represented by 4 colour levels. The darkest shade of blue represents the best sky transparency. The white zones are areas where the weather models forecast clouds.

Dark blue: excellent sky transparency.
Medium blue: above average sky transparency.
Light blue: average sky transparency.
Pale blue: poor sky transparency.
Grey: very poor transparency.
White: cloudy sky.



T+hh refers to the forecast’s time interval (hh), in hours, from the hour T of the model’s initial run time. The hour T is indicated, in Universal Time, at the top of each column.

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