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Can you explain why as one travels east over the BC mountain ranges the climate gets progressively drier, but travelling from Vernon over the Monashee Mountains, you don't see the same effect?

Thursday, August 21, 2008 | 10:49 PM PT

Question submitted by John Campbell
(Vernon, B.C.)

Here's the full question:
"I am a bit confused about one thing. As you travel Eastward over BC mountain ranges the climate gets progressively drier on the Eastern side of the mountains. For example, Golden is much drier than Revelstoke, Princeton much drier than Hope. BUT, we you travel from here (in Vernon) over the Monashee Mountains over to Nakusp and Revelstoke, it gets much much wetter."

John, great question but you are essentially mixing up two local weather effects here.. the "rain catchment" effect and the "rain shadow" effect. Basically airmasses crossing over higher terrain are usually rained out on the windward side of the mountains and end up much drier on the leeward side of the mountains. But this is not always the case.. sometimes local weather effects override the climatological effect of the "larger scale topography".. and I believe that's what you're seeing here.

I was in California coast and was thinking about our west coast. Obviously because we are north we are much cooler but what makes California have so much sunshine year round, and BC coast have ALOT more cloud and rain.

Friday, August 8, 2008 | 06:00 PM PT

Question submitted by Dave Seminchuk
(Burnaby, B.C.)

Hi there Dave,

Welcome back to "cloudy ol' BC"!! Good question. Now part of the answer purely is perception. San Francisco is actually a rather foggy/cloudy city, thanks to the warm air from inland (where it gets quite hot in the summer) clashing with the cold air coming across from the surrounding Pacific Ocean and the SF Bay. When that happens, fog forms. But that's a rather local effect.

But in a more general context, BC's coastline is cloudier due in part, to the path of the jet stream. This ribbon of fast moving air aloft, carries with it weather systems (more specifically cloud bearing weather systems). Due to the latitude of Canada and the tilt of the Earth, we lie in a more temperate zone.. and the jet stream tends to feed these systems along our coastline for longer periods of time. Hence, on average, we end up with cloudier conditions for greater periods of time in the year than they do in, say, California.

Here are two great questions that I got, in regards to Environment Canada's UV Index and secondly in regards to the recording of "hours of sunshine".

Friday, August 1, 2008 | 12:19 AM PT

Question submitted by Jason Williams
(Burnaby, B.C.)

1) How do they forecast the UV index for all the different areas of BC
when there is only 1 or 2 UV monitoring stations for the entire
province?

2) I was looking back through the Environment Canada database and
noticed that there are currently no weather stations in the BC mainland
that record hours of sun. If I look 10 years ago or even 40 years ago
there were 20 to 30 weather stations that measure hours of bright
sunshine in the BC interior. Why is this the case?

For the answer, I went to Environment Canada. Here is their answer:

"UV radiation is very strongly seasonally and latitudinally-dependant.
The day-to-day variation due to the presence and strength of high
pressure systems in the upper atmosphere is important during the summer
but still very small compared to the seasonal and latitudinal influence
and of course whether the sky is cloudy or clear. Consequently the rough
value of the index for any location and time can be predicted far in
advance. Measurements of UV radiation (taken on Saturna Island) are
important for research and monitoring but not so critical to producing
the day-to-day forecast.

Regarding sunshine hours, the weather service is no longer officially
recording hours of sunshine.
There are several reasons but most important is that the instruments
used to record sunshine are outdated and crude. A close second reason is
the fact that NAV Canada has taken over the contracting for weather
observations at the airports where sunshine hours were previously
measured by Environment Canada staff or Contractors. Bright sunshine
hours are understandably of little value to flight operations and hence
Nav Can has not made their measurement a requirement at these airports
since they took over.

More sophisticated measurements of solar radiation are now possible and
we expect these measurements to eventually form the basis for
determining sunshine hours. There are apparently very strong
correlations between certain wavelengths of solar radiation and the
previously measured bright sunshine hours. This has implications for
calibrating the past bright sunshine hours to newly measured solar
radiation."