Seeing Forecast For Astronomical Purposes

Observing planets, planetary nebulae or any celestial object with details at high power requires excellent seeing conditions. The seeing is the term used in astronomy to quantify the steadiness or the turbulence of the atmosphere. Seeing should not be confused with sky transparency, which is the terminology used to qualify the darkness of the sky. 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. Excellent seeing with high quality telescopes can also show details on the largest moon of Jupiter, Ganymede. What we are seeking is the best nights where we can boost our telescopes to their limits… which reach as high as 50X per inch diameter for quality telescopes… which means 500x for a quality 10-inch (25cm) instrument.

Regional model, zenith seeing forecast for North America
Images forecasts are updated twice a day at 5:00 and 17:00 UTC
UTC Tonight Following Night Outlook  
05:00 03h · 06h · 09h · 12h 24h · 27h · 30h · 33h · 36h 48h Animation
17:00 12h · 15h · 18h · 21h · 24h 36h · 39h · 42h · 45h · 48h   Animation

Please read on for more details.

Professional astronomers and more advanced astro-amateurs evaluate the seeing with a scale 1-10. Through a telescope, they measure the star diameter which usually ranges from bad seeing at 5-8 arcsec to excellent seeing at 0.5-0.2 arcsec. Astro-amateurs, can also use a qualitative way to measure the seeing. They look through their telescope at the zenith for a 2-3 magnitude star at about 30-40X per inch diameter (300-400x for a 10 inch telescope) and from the look of the diffraction pattern they estimate the seeing on a scale I-V.

Diffraction pattern

The seeing can be rated through astro-amateur telescopes with the following guidance
Categories Seeing in arc-second
V Perfect motionless diffraction pattern
IV Light undulations across diffraction rings
III Central disc deformations. Broken diffraction rings
II Important eddy streams in the central disc.
Missing or partly missing diffraction rings
I Boiling image without any sign of diffraction pattern

Of course, the diffraction pattern diameter is related to the aperture of the telescope. The diffraction pattern of a 4 inch telescope is twice as large as for an 8 inch instrument. So the seeing rating with this method will depend of the diameter of the telescope. An astro-amateur rating the seeing at 4/5 with a 6 inch telescope will certainly appear as a 3/5 with a 12-14 inch optical instrument. So it is important to understand or be aware of this difference. This forecast is based on the data accumulated with 11-14 inch telescopes during a four year period, so this study was done with the average modern astro-amateur telescope diameter. Astro-amateurs owning a smaller telescope may find the following forecast a bit pessimistic but you can adjust the colour index to your observations. Amateurs owning an 8-20 inch telescope should find this product quite useful and when the forecast shows a seeing 5/5 over an area… it should be the best planetary conditions for for any telescope diameter.

The seeing can be expressed in arc-second for each catogories
Categories Seeing in arc-second
V < 0.4"
IV ~ 0.4-0.9"
III ~ 1.0-2.0"
II ~ 3.0-4."
I > 4"

The goal of this new product is to inform astro-amateurs and professionals of the possible seeing conditions for the next 48 hours. The following images are seeing forecasts for North America. Weighted wind shears, flux momentum in the low levels and temporal surface temperature trends are the elements used for this first attempt to forecast the seeing. The seeing forecast may not be well tune over mountains but topography is one of the elements used in the forecast.

The quality of the seeing is represented by 5 colour levels.The darkest shade of blue represents the best seeing and the grey colour the worst seeing conditions. The white zones are areas where the weather model forecast clouds.

This table describes categories and colours associated to seeing conditions
Categories Colour Seeing Conditions
1. 1/5 Grey Bad seeing
2. 2/5 Turquoise Poor seeing
3. 3/5 Light blue Average seeing
4. 4/5 Medium blue Good seeing
5. 5/5 Dark blue Excellent seeing
6. white Cloudy Sky

These seeing forecast images are produced twice a day, around 16:30 UTC and 04:30 UTC (Universal time coordinate). 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)
    18:00 UTC = 15:30 Newfoundland daylight time (summer)
  • 18:00 UTC = 14:00 Atlantic standard time (winter)
    18:00 UTC = 15:00 Atlantic daylight time (summer)
  • 18:00 UTC = 13:00 Eastern standard time (winter)
    18:00 UTC = 14:00 Eastern daylight time (summer)
  • 18:00 UTC = 12:00 Central standard time (winter)
    18:00 UTC = 13:00 Central daylight time (summer)
  • 18:00 UTC = 11:00 Mountain standard time (winter)
    18:00 UTC = 12:00 Mountain daylight time (summer)
  • 18:00 UTC = 10:00 Pacific standard time (winter)
    18:00 UTC = 11:00 Pacific daylight time (summer).