Yesterday, driving between Farnham and Guildford along the Hogs back
(A31) about Noon, I stopped at a lay by and was easily able to see the
Wembley Arch 28 miles away with unaided vision. Further on near
Guildford I was able to see the "Gherkin" and the London Eye. Never
before have I seen these from that location but never before have I
searched for them.

My question is, was the visibility exceptional yesterday. Judging by
the distance I saw I would think so. To save driving to the same
location only to get a poor view, what weather parameters (and where
to find them) do I need to check for great visibility. I assume low
humidity and no precipitation are necessary but is that all?

Thanks for reading

Very generally speaking, visibility is good in air from the polar regions,
and so-so or poor in air from the south. Having said that, air from the
polar regions often doesn't reach us from that direction. For example, it
often swings round the west side of an Atlantic depression and then comes up
to us from the SW, so you really need to look at a weather map and try to
figure out where the air at your location has come from. Not always easy!

Moreover, visibility is better when there is cumulus cloud and/or showery
weather, as opposed to layer cloud, because then convection allows exchange
with cleaner air above. Other generalities: visibility is often good behind
cold fronts and poor behind warm fronts; better in windy weather than in
stagnant conditions; better in low humidity than in high; better down-sun
than up-sun.

But you sometimes find the phenomenally good vis where you least expect it.
The best visiblity I ever encountered was behind a cold front in the Yemen,
of all places, despite all that sand!

Ian Bingham,
Bieldside, Aberdeen.




"MD" wrote in message
...
Yesterday, driving between Farnham and Guildford along the Hogs back
(A31) about Noon, I stopped at a lay by and was easily able to see the
Wembley Arch 28 miles away with unaided vision. Further on near
Guildford I was able to see the "Gherkin" and the London Eye. Never
before have I seen these from that location but never before have I
searched for them.

My question is, was the visibility exceptional yesterday. Judging by
the distance I saw I would think so. To save driving to the same
location only to get a poor view, what weather parameters (and where
to find them) do I need to check for great visibility. I assume low
humidity and no precipitation are necessary but is that all?

Thanks for reading

MD wrote:

Yesterday, driving between Farnham and Guildford along the Hogs back
(A31) about Noon, I stopped at a lay by and was easily able to see the
Wembley Arch 28 miles away with unaided vision. Further on near
Guildford I was able to see the "Gherkin" and the London Eye. Never
before have I seen these from that location but never before have I
searched for them.

My question is, was the visibility exceptional yesterday. Judging by
the distance I saw I would think so. To save driving to the same
location only to get a poor view, what weather parameters (and where
to find them) do I need to check for great visibility. I assume low
humidity and no precipitation are necessary but is that all?

Thanks for reading
Thanks to both of you for the info. haven't been along that way since
but I'll keep a eye on the conditions you advised to watch for.