The Weekend Weather

[IanL]

thats a great site ian

please could you explain something for me as im new to that site

i assume blue zero is good for clouds

the seeing index is 3 and mid green which i assume is also good

i dont understand seeing index2, arcseconds or the jetstream (assume red is bad ? )

also.. bad layers ? haha thanks mate

Okay, Going from top to bottom:

Time (BST): Local time for selected location, broken in to hour chunks.

High Clouds (%) 0% (Blue) = Clear, 100% (White) = Full cloud coverage

Mid Clouds (%) Same as above

Low Clouds (%) Same as above

(Note: It doesn't predict for ground-level fog or 'very low cloud' which I assume is almost fog. They have other forecasts on the site which include those).

Seeing Index 1: 1 (Red) = Poor Seeing, 5 (Green) = Good seeing

Seeing Index 2: Same as above but uses a different model to predict it. Seeing index 1 generally seems more optimistic than Seeing index 2.

Seeing arcSeconds: What it says on the tin.

Jet stream (m/s): Speed of the jet-stream overhead, in metres per second. Higher speeds are worse for seeing, so look for green rather than red colours.

2m Rel. Hum. (%): Relative humidity at head/telescope height ** See note below.

Bad Layers Top (km): Altitude of the top of the 'bad layers' in kilometres

Bad Layers Bot (km): Altitude of the bottom of the 'bad layers' in kilometres.

Bad Layers K/100m: The average change in temperature per 100 metres change in altitude in degrees Kelvin (same as Celsius except 0 deg K = -273.15 deg C). Anything where the temperature gradient is more than 0.5 degrees K per 100m is considered 'bad', basically because mixing of air/instability is more likely thus causing worse seeing. Really you just need to look for green in this row and avoid yellow/red, don't worry about the altitudes unless you have a plane with a flying telescope or something!

** Relative humidity is a measure of the amount of moisture in the air, which is a function of moisture content, temperature and barometric pressure. When the relative humidity reaches 100%, moisture will condense out of the air at the current temperature. The issue for astronomers is of course that the lenses and mirrors on our willcool to a temperature which is at or below the current dew point of the surrounding air, thus causing dew to form on the optics. The nearer the forecast dew point is to 100% the more quickly dew is likely to form as the scope cools down during the night.

[IanL]

The nearer the forecast dew point is to 100% the more quickly dew is likely to form as the scope cools down during the night.

Sorry that last bit should have read "The nearer the forecast Relative Humidity is to 100% the more quickly dew is likely to form as the scope cools down during the night."

[knobby]

Looks like the weather has won ! As they say on dragons den ... I'm out

[Marc1980]

Sat in my car on the north cornish coast at 1:11am. Still cloudy, blowing a hoolie..... Goodnight, maybe tomorrow night.

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