Predicting The Atmo Calms

The popular concept of an astronomer's perfect night is one where a cold front has swept through the observer's location and a crystal-clear sky full of twinkling stars can be seen. In fact, this is a nightmare scenario for the planetary observer. A cold front passing through a region may well reduce the moisture content of the air and is great for looking at deep sky objects and comets, but it leaves the air in a very unstable state and, invariably, the hotter ground radiates its daytime heat into space, further increasing the chaos in the atmosphere. At high powers, the Moon and planets will be a wobbling, distorting mess. What is required for the best planetary views is stability, and this invariably comes from the presence of a high-pressure system anchored over the region. In a high-pressure system, especially one that has been around for a few days, the air becomes hazy (and polluted in cities) but it is very stable. The ultimate in atmospheric stability is often achieved when mist or fog is forecast. Of course, this invariably leads to a heavy dew settling on every surface and, when mist turns to fog, the planets become too faint to image. However, just before the mist turns to fog, exquisite planetary views can be seen. As a keen planetary imager I get pretty excited when there is a high-pressure system over the U.K. and fog is forecast. Time and time again, my best views have been acquired just before the planet faded away as the fog thickened. It is not surprising that this should be the case. Fog is the ultimate proof of atmospheric stability. A high-pressure system is not essential for planetary imaging, but low wind speeds at all levels in the atmosphere seem to be crucial. This is the experience of almost every planetary observer. Of course, sitting in the middle of a high-pressure system reduces wind speeds to zero, but so does being in a "Col," that is, sitting between two high-pressure and two low-pressure systems where little is changing and winds are low.

The Earth's lower atmosphere is only half the problem, however. There is another issue that does not appear on national TV weather forecasts. This is the issue of the Earth's upper atmosphere jet stream. The jet stream is, without a shadow of a doubt, the "fine planetary detail" wrecker. It might seem incredible that an area of the Earth's atmosphere that is 8 to 10 kilometers above the surface and where the pressure is only 300 millibars could possibly affect our view of planets so badly. However, the light from the planets has to pass through the jet stream altitudes and, in extreme cases, the jet stream wind speeds can be as high as 500 kilometers per hour. When high jet stream winds are over your observing site, they will appear, in a defocused planetary image, like a river of water streaming through the eyepiece. This will result in extremely "fast" seeing, with no hope of even a webcam freezing the planetary details. Under such circumstances, my advice is to go to bed. Nothing useful can be achieved!

Fortunately, via the Internet, various companies publish jet stream weather forecasts, principally for aviators. Many of these forecasts originate from data compiled at the U.S. National Center for Atmospheric Prediction (NCAP), and predictions are issued twice daily for sites all around the world. Any Internet search engine should be able to find a site predicting jet stream activity for your location without too much trouble. Such forecasts can save the planetary observer a lot of wasted effort. The Unisys Aviation pages (currently located at http://weather.unisys.com/aviation) are the best pages currently on the web for studying the wind speeds at various atmospheric heights all over the Earth's surface (see Figure 3.1). The 300-mb pressure pages are the ones that the planetary imager should study in detail, and that altitude is not covered by TV weather forecasters. Obviously, one is looking for the jet stream equivalent of a lower atmosphere high pressure system centered on the observer's site (i.e., low wind speeds). Quite often this will occur when a standard high-pressure system has been in place for a few days, but not always. At high northern temperate latitudes, winter often brings a polar jet stream down from the arctic, even when a high-pressure system is not far away. The jet stream forecasts, compiled for the aviation industry, are the nearest thing that amateur astronomers have to a nightly "seeing" forecast. High jet stream winds can wreck seeing even when a high-pressure system is nearby. However, after a sunny day, turbulence can be poor regardless of other factors, as the hot ground can radiate rapidly. There is considerable evidence that a stable state forms in all levels of the atmosphere (even in the jet stream) immediately after sunset. There appears to be a delay before temperatures plummet, in which good planetary images can be secured. Often, this means finding the planet in bright twilight, only half an hour after sunset. Another calm period exists in dawn twilight, too, when the nighttime cooling has slowed down to a minimum.

Figure 3.1. Jet stream weather maps. Purple = low winds; yellow = high winds. Image: Unisys.

Apart from the jet stream sites, two other web addresses may be of interest. Firstly, a huge amount of relevant data regarding wind speeds from sea level to a height of 9 kilometers is available at Meteoblu at http://pages.unibas.ch/geo/ mcr/3d/meteo/index.htm. There is also a weather chart archive, where you can retrospectively check the conditions on a given night at http://www.meteoliguria.it/ archivio21.asp.

Damian Peach cross-checked his best U.K. seeing nights against this page and came up with the following table:

Date

Seeing

Sea-Level Pressure

3GG-mb Wind Speeds

1999/10/06

Pickering !

-9

High 1028 hPa

12 m/s

2000/10/13

Pickering !

-9

Low 1002 hPa

12 m/s

2003/09/29

Pickering !

-9

High 1016 hPa

20 m/s

2003/12/16

Pickering !

-9

High 1030 hPa

18 m/s

2004/03/01

Pickering !

-9

High 1036 hPa

20 m/s

2004/04/14

Pickering !

-9

High 1018 hPa

12 m/s

2004/10/01

Pickering !

-9

High 1020 hPa

17 m/s

2004/12/11

Pickering !

-9

High 1026 hPa

12 m/s

2GG5/G1/13

Pickering !

-9

High 1032 hPa

14 m/s

All of these nights featured low wind speeds at sea level and 300-mb jet stream wind speeds of 20 meters per second or less. The September 29, 2003, "event" was one I well remember. The U.K. was under a "Col," not, strictly, a high-pressure system, just a region of inactivity between high- and low-pressure systems.

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