Archives For relief rainfall

Good example of orographic rainfall this morning as a deep summer depression moves off into the North Sea and strong NW polar maritime winds bring heavy rainfall to the mountains of North Wales 23 June 2013. Note that the relief rain picks out the coast and the highest mountains where land rises steeply forcing more condensation and higher rainfall intensity. Note also the lower rainfall intensity in the valleys, Ogwen / Nant Ffrancon valleys can be clearly seen drawn out by lower rainfall!

28-12-2012 09-21-42 scotland

Check the heavy relief rainfall over NW Scotland! Relief rainfall is happening here because warm moist air from the SW is being forced to rise over the Scottish mountains, where it cools, condenses, forms clouds and then rains…heavily! The warm front which passed over Scotland overnight brought the moist SW tropical airstream and the hills of NW Highlands are continuing to “wring-out” the moisture.  Locations on the west coast are 10°C while those on the east coast are a chilly 5°C or less.  The Scottish west coast is warmed by the North Atlantic Drift (Gulf Stream), a warm ocean current.  Winter temperatures up on the NW Scottish coast are frequently warmer than those we get in SE UK which is often influenced by chilly easterly continental winds in winter.  At the moment, for example, Reigate is 8°C and the Isle of Skye is 10°C.
Look at the video and spot the obvious blob of relief rainfall in NW Scotland: what’s amazing is how the heaviest rain consistently matches the highest ground over the NW Highlands and the rainfall area also matches the outline of the coast almost exactly. Each mountain seems to do its own “orographic job” uplifting the moist air (which is inherently stable, so not happy to be forced up, it prefers to sink). The glens (valleys) also seem to appear as areas of lower rainfall.  A remarkable example of relief rainfall.  The relief rainfall also appears to “stay-put” while the frontal rainfall has drifted off into the North Sea.  This highlights the different mechanisms by which the two areas of rain are being formed: frontal air masses move, while mountains do not!  In contrast, us down in the SE UK saw only drizzly light rain as the fronts passed through. We have no significant high ground so no relief rainfall. On the rainfall radar you can also spot the RAINSHADOW effect over eastern Scotland where less rain is falling and there is even a dry spot.  In brief, this is because the air sinks, warms and dries out in the lee of the mountains, leaving a clearer, drier area -sometimes literally a “hole” in the clouds.