Cloud Cover

The cloud cover forecast shows how obsured the sky will be be at a given hour by clouds. Darker blue indicates clearer skies, and light blues to white indicate increased total cloud cover.

A note of caution from the Canadian Meteorological Centre on forecasting cloud cover

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.

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.

Further information on Cloud Cover can be found on the web here.

Forecast data provided by the Canadian Meteorological Centre

Transparency (Enhanced)

The enhanced transparency model shown on Astrospheric is based off of a similar model used by Allan Rahill's, but Astrospheric's model takes smoke, elevation, and surface pressure into account differently to try and produce a more accurate forecast.

Sky transparency uses darker blues to indicate excellent transparency and lighter blues and whites to indicate poor transparency.

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 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.

Seeing

The seeing forecast indicates the level of atmospheric turbulence. Similar to cloud and transparency, the darker the blue, the better the conditions.

Observing planets, planetary nebulae or any celestial object with details at high power requires excellent seeing conditions. When we look at planets, we need high power to see all the fine details but most of the time we are limited by turbulence occurring in the telescope (local seeing) and/or in the atmosphere. During a night of bad seeing we are usually limited to see only two bands on the Jupiter disc and we can hardly use power over 100-150x. On excellent seeing conditions we can use high power and see many bands, white spots, festoons and details in the great red spot.

Further information on Seeing can be found on the web here.

Forecast data provided by the Canadian Meteorological Centre

Wind

The wind forecast (along with temperature) can help in deciding what to wear to stay warm or cool. However, higher winds can make observing or astrophotography difficult, especially with larger telescopes.

Along with speed, Astrospheric also shows the direction of the wind. The triangle within a hour's wind circle points in the direction the wind is heading. It's easiest to think of the circle as a compass of sorts, when the triangle points towards the top, then the wind is blowing towards the North.

Note that the actual 10km resolution model used to produce these forecasts has cold bias and weak wind bias on top of mountains.

Further information for Wind can be found on the web here.

Forecast data provided by the Canadian Meteorological Centre

Temperature

The temperature forecast (along with wind) can help in deciding what to wear to stay warm or cool. Large temperature changes may cause tube currents in some scopes as the internal conditions cool (or warm) to match ambient temps. Also, temperature decreases during high humidity can lead to dew. Please note, the temperature shown on Astrospheric does not include wind chill (or heat index). This is so it can be directly compared to the dew point temperature to make the best decisions for dew protection. Because of this it may feel colder (or warmer) and will differ from values seen on other civilian forecasts.

Note that the actual 10km resolution model used to produce these forecasts has cold bias and weak wind bias on top of mountains.

Further information for Temperature can be found on the web here.

Forecast data provided by the Canadian Meteorological Centre

Dew point

The dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air (such as a telescope), water will condense on the surface.

If the dew point temperature turns orange on the forecast, it means heavy dew or fog depending on other conditions. For example, if there is low wind speed, you're at lower elevation, and the sky is clear, then watch out for fog!

Using the dew point forecast on Astrospheric will allow you to determine how much dew protection you may need during a night of observing. The closer the temperature and the dew point temperature are, the higher the humidity, and the more you may need to use dew shields or heaters to keep your primary objective clear from dew.

Remember, even though the air may be warmer than the dew point, your telescope will continue to decrease in temperature on a clear night through radiation. That, coupled with the fact that water vapor needs a solid on which to condense, make your telescope a good place for dew to form. A slight breeze, dew protection, and/or setting up at a higher altitude away from a valley can help combat the formation of dew.

Forecast data provided by the Canadian Meteorological Centre