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Rookie Snow Question


shark17

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Posted
  • Location: Reculver, North Kent Coast (1 mile inland)
  • Location: Reculver, North Kent Coast (1 mile inland)

    :D I've seen some weather I've never seen before today, so wondered if anyone on here could explain for me?

    Last night (just after midnight) the temp here (sea level, North Kent coast) was 2 degrees C when the front hit. We got around one minute of snow, then heavy rain.

    Today, I left for London in Brilliant sunshine, temp 6 degrees. Within one mile the temp was 7 degrees and I was in the middle of a blinding snowstorm! The snow was horizontal and really cold (bouncy not sticky!) and blowing about.

    I was soon back in the sun, but all the way to London and back I saw regularly 'cold/bouncy' snow at 7 degrees!?!

    Can anyone explain to a Rookie how you can have good 'real' snow at such high temperatures?

    Thanx

    T :-)

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    Posted
  • Location: Abingdon - 55m ASL - Capital of The Central Southern England Corridor of Winter Convectionlessness
  • Weather Preferences: Winter: Snow>Freezing Fog; Summer: Sun>Daytime Storms
  • Location: Abingdon - 55m ASL - Capital of The Central Southern England Corridor of Winter Convectionlessness

    Cold air gets dragged down with the precipitation from the top of the snow showers, causing rapid drops in temperature. Sure someone can give you a more techinical explanation.

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    Posted
  • Location: Sutton, Surrey (approx 65m asl)
  • Location: Sutton, Surrey (approx 65m asl)

    It also depends on the moisture content of the air, or Dew Point (there is also the wet-bulb temp, but I haven't a scooby what that is about - similar I think :) ) The dew point is the temperature at which the air would become saturated. If there is a high moisture content in the air, then any falling snow will melt quickly before making it to the ground. If the air is dry (as you would get after a cold front has passed) then the snow will not melt whilst falling so easily. That is why it is possible to have falling snow at about 5c, whilst on occassions you get rain at barely above freezing.

    As Enforcer said, it also helps if the ppn is intense as that drags down colder air, with evaporative cooling, which is the cooling effect on the air when its energy is used up trying to evaporate the falling ppn.

    Those are my interpretations (from a complete novice), so anyone able to put me right?

    SS

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    Posted
  • Location: Upper Tweeddale, Scottish Borders 240m ASL
  • Location: Upper Tweeddale, Scottish Borders 240m ASL

    Hi SS

    Try looking in the N-W Guides:

    http://www.net-weather.co.uk/forum/index.php?showforum=5

    Popping this over to the Learning Area :)

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    Posted
  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

    This would be my thinking on the subject ,but I am sure one of the others will be able to explain it more clearly.

    Rain and Snow usually begin as snow higher up in the atmosphere in the clouds where temperatures are lower.As the snow falls, if the temperature is below zero all the way to the surface then we get snow.If at some point in the atmosphere on the way down the temperature rises above zero then the snow begins to melt.Whether the snow turns completely to rain depends on how high up in the atmosphere the point where temperatures rise above freezing are (zero degree isotherm), how quickly the snow falls (i.e. are they pellet like) and the temperature at which they are formed.

    My guess is the 7C was a fairly shallow layer of warmth, but the critical feature was that the snow was pellet like (similar to hail) and fell very quickly preventing it from melting. The reason for this is a combination of the coldness and dryness of the air above ,with moisture forming (cloud formation) from the atmosphere at low temperatures due to low dewpoints. This means the clouds contained very cold snow, which takes longer to melt.

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    Posted
  • Location: Reculver, North Kent Coast (1 mile inland)
  • Location: Reculver, North Kent Coast (1 mile inland)
    This would be my thinking on the subject ,but I am sure one of the others will be able to explain it more clearly.

    Rain and Snow usually begin as snow higher up in the atmosphere in the clouds where temperatures are lower.As the snow falls, if the temperature is below zero all the way to the surface then we get snow.If at some point in the atmosphere on the way down the temperature rises above zero then the snow begins to melt.Whether the snow turns completely to rain depends on how high up in the atmosphere the point where temperatures rise above freezing are (zero degree isotherm), how quickly the snow falls (i.e. are they pellet like) and the temperature at which they are formed.

    My guess is the 7C was a fairly shallow layer of warmth, but the critical feature was that the snow was pellet like (similar to hail) and fell very quickly preventing it from melting. The reason for this is a combination of the coldness and dryness of the air above ,with moisture forming (cloud formation) from the atmosphere at low temperatures due to low dewpoints. This means the clouds contained very cold snow, which takes longer to melt.

    Thanx Brickfielder, that might indeed go part way to explaining it?

    However, by 'bouncy' snow, I didn't mean pellet like.

    The snow that fell on and off all day (at 7 degrees!!! in showers on an otherwise sunny day) was small and light, blowing about in the wind, and not even melting on the bonnet of the car at 5pm when I was queueing for the Blackwall tunnel.

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    Posted
  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

    Apparently there are different Crystal types which make up snow depending on the temperature that the snow was formed at. I am guessing that what you saw might be rimed plate or column crystals .

    Star

    Star crystals are born at temperatures near -15 degrees C, and are among the most common type of snowflakes. They are as delicate as they look, and superstars are rare, because large flakes tend to become broken by wind and midair collisions with other crystals. Under ideal conditions several stars my join to form a larger snowflakes. The largest snowflake on record was reported to be a whopping 8" by 12" (about the size of a sheet of typing paper). It was reported to have fallen, probably with a thud, in Bratsk, Siberia in 1971.

    Dendrite

    Dendrites are stars with attitude. Essentially, they are three dimensional star crystals with branches growing on more than a single plane. Branches (or arms) connect randomly to a central structure. These complex critters form under extremely cold conditions (-20 to -25 degrees C) when high levels of atmospheric moisture are present.

    Columns

    Columns are produced when the air is dryer. They are generally smaller, have a higher density than star crystals, and form over a wide range of temperatures (15 to -25 degrees C).

    Plate

    Plates are wanna-be stars that are essentially moisture starved. They form at temperatures of -10 to -20 degrees C when there isn't enough atmospheric water vapor available to form the delicate arms of a classic star.

    Column capped with plates

    Capped columns are composite flakes formed when the particle of snow passes through different temperature and moisture zones on its journey to the ground. The columns form first, usually at higher and dryer regions of a cloud, and combine with star flakes as they fall through lower and wetter cloud elevations.

    Needles

    Needles are formed at the upper end of the temperature spectrum, usually when ground temperatures are at or near the freezing point. To grow, these crystals need an air temperature in the -5 to -10 degrees C range. Needles tend to produce a dense, stiff snow pack which can produce an avalanche under the right conditions.

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    Posted
  • Location: Derbyshire Peak District 290 mts. Wind speed 340 mts
  • Weather Preferences: Rain/snow, fog, gales and cold in every season
  • Location: Derbyshire Peak District 290 mts. Wind speed 340 mts
    Thanx Brickfielder, that might indeed go part way to explaining it?

    However, by 'bouncy' snow, I didn't mean pellet like.

    The snow that fell on and off all day (at 7 degrees!!! in showers on an otherwise sunny day) was small and light, blowing about in the wind, and not even melting on the bonnet of the car at 5pm when I was queueing for the Blackwall tunnel.

    The official definition of this type of snow is snow pellets, otherwise known as soft hail. They are very light, a bit like polystyrene balls, and break up easily under pressure, unlike true hail which is hard and icy.

    T.M

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    Posted
  • Location: Taunton, Somerset
  • Location: Taunton, Somerset
    The official definition of this type of snow is snow pellets, otherwise known as soft hail. They are very light, a bit like polystyrene balls, and break up easily under pressure, unlike true hail which is hard and icy.

    T.M

    The stuff that was falling yesterday was soft hail.

    If you look back you might have noticed that the clouds were great towering efforts – almost cumulonimbus. All that was lacking was the thunder and lightning.

    So moisture was being convected to great heights where it became icy and where - almost certainly because of an extremely cold upper atmosphere – the drops crystallised to form snow; then were carried even higher where they froze into hail.

    What you might call ‘proper’ snow usually comes from cumulus clouds.

    Think of summer storms and rain showers.

    Rain showers come from small cumulus clouds – hail comes from thunder clouds.

    Not a truly scientific answer but it may help. B)

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    Posted
  • Location: Rugby, Warks
  • Weather Preferences: Dangerous
  • Location: Rugby, Warks

    As an example...today, with temperatures reaching around 5/6c or so. The cumuliform clouds developed during the daytime and when the precipitation-producing Cbs finally arrived at 2 p.m.ish - the precipitation started off as this "bouncy pellet" type stuff. Temperatures then rapidly dropped as the ppn became more intense and the evaporative cooling of the air kicked in. As the temps dropped off sharply, the change to heavy snow was almost instant.

    This situation wouldn't occur from frontal cloud. Purely the wonders of convection :)

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    • 8 months later...
    Posted
  • Location: Yorkshire Puddin' aka Kirkham, Lancashire, England, United Kingdom
  • Weather Preferences: cold winters, cold springs, cold summers and cold autumns
  • Location: Yorkshire Puddin' aka Kirkham, Lancashire, England, United Kingdom
    Cold air gets dragged down with the precipitation from the top of the snow showers, causing rapid drops in temperature. Sure someone can give you a more techinical explanation.

    A similar process (Icy downdrafts) to this must have allowed a thunderstorm that traveled northeast from North Wales to Northeast England to dump some snow at Hedon, East Yorkshire on the 21st August 2000!

    In The Mirror Newspaper on the 22nd August 2000 there was a picture of snow on the ground and a snowball fight! There is also a picture of a snowman!!!! :D

    Elsewhere on that August day there was six inch thick accumulations of hail on the North York Moors in addition to flooding from an hour long "monsoon deluge".

    York railway station was hit four times by lightning causing disruption to train services whereas a 50 foot wide waterspout formed off the Lincolnshire Coast near Cleethorpes which then swirled over near the mouth the Humber.

    However a sunny heatwave continued on Bournemouth Beach! :D

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    Posted
  • Location: Derbyshire Peak District 290 mts. Wind speed 340 mts
  • Weather Preferences: Rain/snow, fog, gales and cold in every season
  • Location: Derbyshire Peak District 290 mts. Wind speed 340 mts

    A similar process (Icy downdrafts) to this must have allowed a thunderstorm that traveled northeast from North Wales to Northeast England to dump some snow at Hedon, East Yorkshire on the 21st August 2000!

    This would almost certainly be hail of some description rather than snow. I seem to remember reading an article in 'Weather' about that storm in which the error of the media in proclaiming it was snow was corrected.

    As far as I'm aware lying snow has never been reliably recorded in August, away from the highest ground, in the last 300 years or so.

    T.M

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    Posted
  • Location: Yorkshire Puddin' aka Kirkham, Lancashire, England, United Kingdom
  • Weather Preferences: cold winters, cold springs, cold summers and cold autumns
  • Location: Yorkshire Puddin' aka Kirkham, Lancashire, England, United Kingdom

    As BBC weatherman Paul Hudson said back on 22nd August 2000: "I've never seen anything like. This was a mother of a storm". B)

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