Since I fly in the jetstream all the time, I can provide some personal experiences with it. First off, it’s not always moving at 80-100 mph. Sometimes, it can be flowing at only 10 mph and that’s often determined by the season, although occasionally it’s just driven by local conditions. (It’s only 10 mph over Oregon as I write this.) The “jet” runs at high altitudes where local geographic features like mountains and valleys have less effect on the flow. Over the US, the jet runs at altitudes from the high 20’s up to around 45,000’—near the top of the tropopause for mid-latitudes. Remember that half of the atmosphere lies below 18,000’ so the refractive index of the high altitude air is pretty low, which means that the optical effect of the air up there is a lot less than for lower altitude air.
Most of the time, jet stream flow is laminar and conditions are glass smooth. If the wind speed transitions through the atmosphere are smoothly varying, you are likely to experience excellent seeing—even if the wind speed is pretty high. Poor seeing results from turbulence, which is most commonly caused by wind shear conditions. Wind shear occurs when the wind speed changes rapidly with altitude or position, or when the jet stream changes direction in a “short” distance. Another, less common condition in the jet called “mountain wave” is the result of vertical waves within the river of air. The wavelength of mountain wave can be quite long but if it can cause density variations in the air that results in slower variations in seeing, which won’t be serious, but if you see the condition going from worse to better to worse over periods of a few minutes, that’s likely due to mountain wave.
You can often judge the effects of the jetstream by looking at high altitude wind charts (https://www.aviation...v/windtemp/plot) to check the speed and “straightness” of the wind barbs. If the wind is strong and the flow is strongly curved over your site, the seeing is likely to be poor. You also want to look at the wind speed gradients. Strong gradients will indicate turbulence and poor seeing. BTW, this is the same aviation chart that I saw on the computer screens when I visited the LBT observatory. Pro-astronomers pay attention to this stuff as closely as aviators!
In Bend, the Cascades are a big driver of seeing conditions. Prevailing strong winds between 12-18,000’ out of the west will drive downwind turbulence over Bend and I’ve seen incredibly bad seeing under those conditions. Interestingly, Bend also gets some absolutely spectacular seeing. It doesn’t happen often but I’ve also achieved 1.0” arc-sec subs using 20 minute exposures while imaging from the Bend airport.
John
Edited by jhayes_tucson, 13 June 2019 - 05:43 PM.
