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Page last updated at 13:47 GMT, Monday, 13 September 2010 14:47 UK
Wild Weather: Fish showers and a Frost Hollow

Trai Anfield
Trai Anfield
BBC Look North Weather Presenter

Fish in the rain
Raining fish just has to be the fishyest of tails


We're used to wild weather, but there can't be anything stranger than the events of 24 August 1918 in Sunderland.

It rained fish! For 10 minutes frozen sand eels pelted the people of Hendon as they ran for cover.

We believe that the weird shower was caused by a waterspout - a tornado that happens over the sea, sucking up the water and anything else near the surface.

It must have been a big job unblocking the drains that day!

Talking of strange weather, we have a place that defies nature right here in Durham.

Frost Hollow

Houghall
Houghall takes on a frosty beauty

Normally we expect the tops of mountains to be the coldest part, but at Houghall everything is turned upside down.

Here, we actually feel colder on the valley floor than we do at the top of the hill. It's called a frost hollow.

After a warm day, the sky often stays clear overnight.

All the heat of the day then escapes back out to space.

If winds are very light this cools the ground plus the layer of air directly above it.

So at the top of the hill, that cold air in contact with the ground, becomes more dense than the air sitting adjacent to it over the valley.

Topsy-turvy

Thermometer

Gravity then forces the colder and denser air from the hill top to slide gently down the slope and collect in the valley bottom.

It sets up a topsy-turvy situation where the air at valley floor, becomes colder than the air above it.

This is called a temperature inversion.

At Houghall, on nights like this, it is a full 2ºC colder at the bottom of the hill than the top.

It can make for a frosty night in the valley, while the hilltops around stay frost-free.

It can also mean summer frosts: they've recorded -6ºC in June!

The lowest temperature recorded here is -21.1ºC on 5 January 1941, still an English record for that day of the year.

Plants at Houghall are more severely frost damaged than elsewhere - so if you can grow it at Houghall you can grow it anywhere in the region!




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