Hello fellow astronomers,
Today I'm happy to announce the release of an enhanced Transparency forecast on Astrospheric. Read on to learn about the new model and how to opt-in. This is one of the first updates that falls into the bucket of "the more people you tell, the better we can make the model for everyone", so please share!
Transparency (similar to visibility) is a measure of the amount of contrast which can be discerned as you look through the atmosphere towards your target. To view low contrast objects (nebula, galaxies, etc), one requires excellent transparency. Transparency is a measure up and out of the atmosphere, whereas visibility is generally a measure horizontally through the atmosphere.
The current Transparency model produced by Allan Rahill at the CMC is based on the amount of water vapor in the column of air above you. Generally, the higher the amount of water vapor, the worse the transparency, limiting the magnitude of stars and DSOs that can be observed. More info on this model available here.
Astrospheric's enhanced Transparency model improves upon the current water vapor based model by increasing the scale and resolution, taking elevation and surface pressure into account, taking all cloud cover into account, and increasing the sensitivity of lower level water vapor in the model. These changes look to improve the accuracy of the overall Transparency model.
Overall, this model works well because
- Water vapor is a well forecasted variable in our atmosphere. It's crucial to other aspects of the forecast and can be validated with spectral imaging from satellites.
- Water vapor is the primary aerosol in the North American atmosphere that impacts transparency.
- There are certainly other factors (those who endured the forest fire burnt skies this past summer can attest), but they tend to be difficult to model by standard numerical weather models, are relatively short lived, and rely on multiple complex variables. For example, to model the impact of smoke on transparency, you need to know with high certainty variables such as particle size, wind speed at various elevations, and the total output of the fire creating the smoke (highly variable). The Aerosol Optical Depth forecast available on Astrospheric attempts to model aerosols like smoke, dust, salt, etc, but has a markedly lower resolution and accuracy.
- The higher the elevation, the less atmosphere there is between you and what you're observing.
- While it's possible to get great transparency at sea level, it's much more common to find a transparent sky on a mountain top.
Here are some examples of how the models are similar and different. Note that the legends are the same
The 30,000 mile high view
From up here, the large structures in the forecast are quite similar. This is due to the fact that the primary variable influencing the model is water vapor from the top of the atmosphere down to the floor. One difference immediately visible is that the new Transparency model produces a gradient of colors instead of a five specific colors. The legend is still the same, but now there is more data between values.
California's Central Valley
One area that best shows the impact of taking elevation and pressure into account is in California's Central Valley, which is nearly at sea level. The new model shows the impact of more atmosphere being taken into account at sea level versus the high elevation mountain ranges that enclose the valley floor. In general this holds true, the higher you are in the atmosphere, the better the transparency will become.
Coasts and altitude
The effects of elevation and pressure are visible along coast lines and mountain ranges across the continent. In general this will lead to the new model showing an overall lower forecasted transparency at low elevation vs the old model. In the example below, cloud and low elevation are leading to an overall worse transparency forecast on the east coast. It is possible to have excellent transparency near the coast, just less likely than at higher elevation.
The South West
Both models (and most all humans) agree, the American Southwest is the place to be for a transparent window into our universe
A note on clouds
Unlike the old transparency model, the new model takes all cloud cover into account. In the past it was possible to have excellent transparency and a thin layer of cloud cover, which was confusing. If this were a measure of horizontal transparency (visibility), then it makes sense. But, since our goal is to look out of the Earth's atmosphere, cloud cover is crucial in the forecast. In the images above you can see the new transparency model is showing a high level cirrus cloud cutting through the center.
How to view the new Transparency model
For now, the new Transparency model is opt-in, which you can do in a couple of ways
- If you simply want to view the differences between the two Transparency models, use the map to switch between the layers
- If you want to integrate the new Transparency model into your forecast, do the following
If you opt-in using #2 above, then you may send corrections to help improve the model over time. In fact, the more folks you know who can provide meaningful corrections to the model, the better it will become. To send a correction, simply tap the transparency icon and select a value. You can also turn off the enhanced transparency model by selecting the transparency icon and opting out.
That's it for now, please don't hesitate to send questions and comments my way. Astrospheric is a community driven project, and while I can't always respond, I really appreciate the feedback and do read all of it.
Clear (transparent) Skies!