New Seeing and Transparency Forecasts

Seeing and Transparency, along with light pollution and smoke, are two of the most important considerations for astrophotographers when deciding where and when to plan a shoot.

We’re pleased to be offering new Seeing and Transparency forecasts with Photo WX, our weather service for photographers. The new forecasts, based on data from Environment and Climate Change Canada, represent a very significant upgrade over the predecessor forecast.

And we’ve added our own spin by publishing a new combined seeing and transparency forecast map that allows you to assess conditions along both dimensions at the same time.

Why should I care about Seeing and Transparency?

Seeing is a measure of the apparent scintillation of stars as viewed by an observer on the ground. The greater the scintillation — the flickering, if you like — the worse the seeing. Why? Because the image of the star loses definition and spreads out over a larger area. Better seeing is associated with sharper stars and improved resolution of detail in your images.

Seeing tends to worsen when there is strong wind shear between layers of the atmosphere — seeing forecasts take into account wind speeds and other factors at multiple levels.

You can have excellent seeing, but at the same time poor transparency. Transparency measures the impact of clouds, haze and humidity on the ability to perceive stars in the night sky. Again, the transparency forecast looks at multiple levels in the atmosphere. Remember, you can have clear skies (no clouds) but still suffer from poor or average transparency.

What’s New?

The new forecasts are based on the Canadian RDPS model. Some major differences in the new forecasts are:

• Much higher resolution: 10 km vs. 35 km for the GEM Regional Model
• Much broader coverage: Central America and northern Europe are included
• Longer forecast range: the new forecasts extend out to 84 hours

For the first time, we’re able to offer Seeing and Transparency forecasts to photographers in the UK and Ireland, Scandinavia and northern continental Europe as far east as Ukraine. The previous GEM forecast (still available) covers only North America at a much coarser resolution.

New Colour Schemes

We previously followed the colour convention long established by Clear Dark Sky, where excellent conditions appeared dark blue and poor conditions appeared white. This had the benefit of leveraging “ colour memory ” for users familiar with that site.

However, the colour scheme has two disadvantages for use on maps:

1. We’re used to reading weather maps for things like clouds and rain: you see something on the map where it’s raining or cloudy, but otherwise the map is clear and unobstructed. The Clear Dark Sky convention inverts this (to be fair, it was not necessarily intended for use on maps — that was our decision).
2. The fact that both seeing and transparency use the same colours makes it impossible to combine the colours to produce a legible composite map.

To address these issues, we’ve changed things up:

• Clear means clear — if the map is unobstructed, conditions are excellent
• Seeing uses a cherry-pink scale: dark pink means poor seeing
• Transparency uses a teal scale: dark teal means poor transparency/clouds

Combined Seeing and Transparency

Armed with our new colour map design, we can now combine seeing and transparency into a single forecast map, so you can read both at once.

In the example above, you can see that for the night of 1–2 July in central Germany, conditions are looking very good. The map appears pale pink, indicating very good seeing and excellent transparency.

In stark contrast, conditions in the UK are mostly poor (dark plum colour), with only Northern Ireland showing clearer conditions with decent seeing.

The pink and teal colour schemes combine to produce unique colour combinations in a range of lilac/plum hues, so you can visually distinguish where seeing is good versus where transparency is good.

As with the GEM model, the forecasts grade seeing and transparency on a 0 (poor)–5 (excellent) scale. In the combined forecast, the colour legend shows the colours for the extremes:

• S5T5: excellent seeing and transparency — white/clear
• S5T0: excellent seeing, poor transparency/cloudy — dark teal
• S0T5: poor seeing, excellent transparency — dark pink
• S0T0: poor seeing, poor transparency/cloudy — dark plum

We’ve applied the new scheme to the GEM model as well, for ease of comparison.

Conclusion

We night-time photographers and astrophotographers in North and Central America — and now Europe — will enjoy the new forecasts; it’s a major improvement in coverage, resolution, and convenience.

The new Seeing and Transparency forecasts are available to Photo Ephemeris Web PRO subscribers today.