Archives For mid latitudes

Polar Continental (Pc) air is most common in winter as HIGH pressure forms over cold northern continental interiors and pushes out freezing air to mid-latitudes.  In summer, when it does occur, Pc brings dry stable and warm conditions to the UK as the continents warm up.  Pc has been an unusually frequent visitor this March and effectively reversed our usual south westerly prevailing wind. As Spring sunshine warms the surface and Atlantic LOW pressure systems edge closer to the UK next week dragged by a more northerly migrating jetstream, we can be assured that moist maritime air will be making a return and any remaining incursions of polar continental air will increasingly lose their frequency and ferocity, Russia has to warm up sometime!
air masses UKMOBefore we bid “farewell” to the freezing Polar Continental air until next winter it is worth remembering the good times.  Pc has occasionally brought crystal clear skies with excellent visibility and dramatic views of the sky both day and night (as anyone staying up to see the ISS will testify). The long picture series shows Cumulus Congestus building over Stratford on Avon last week and an unusual Pileus Altocumulus Lenticularis veil forming over the dramatic rising thermals. Pileus is a fleeting, ephemeral cloud type and forms as convective up-draughts in the cumulus force upper winds over the rising congestus, just like air being forced to rise over a mountain range.  Moisture in the air condenses, or sublimes into ice, and forms a beautiful veil called Pileus.  The photos were taken over just two minutes and then the Pileus melted away.  Pileus is a beautiful cloud but has a darker side because it sometimes forms above rising nuclear mushroom clouds and volcanic eruptions.

Cloud streets, lines of Stratocumulus, were also a feature of the easterly winds: where an isothermal “cap” (temperatures staying the same with increasing height) kept a lid on rising thermals and clouds remained flat and formed lines in the airstream.  Cloud streets seem to urge us to follow them, pointing the way to something important over the horizon.  Finally, the “sundog” (mock sun) was another fleeting feature of polar continental air, though not exclusive to it: apparently only 5/100 people have ever seen a sundog, so here is a picture of one in case you haven’t caught one yet.  They occur as low-angled sunlight refracts through hexagonal ice crystals.

Pc air wasn’t all as beautiful as this of course: freezing grey blankets of dull stratocumulus dominated the weather for days in the south east and deposited icy snow grains right through to Friday.  Nevertheless, I do hope you had the time to look up and admire the best of the Polar air show this March.  So, Polar Continental may crack our cheeks and rage and blow but we’ll kind-of miss it… won’t we?  “Adieu, adieu, adieu… remember me.” Exit Ghost of Pc! (The photos above were all taken along the Stratford canal last week, the statue is William Shakespeare in Bancroft Basin).

n.b. March summary for Reigate coming soon!

Update 23/04/2013: UK Met Office now backtrack on loss of Arctic Sea Ice as a possible cause. Read it here. Nevertheless, Arctic Sea ice loss could be a future cause of climate change so we will leave it here as a suspect!

Scientists have been monitoring the shrinking mass of sea ice which covers the Arctic Ocean.  Arctic sea ice shrank by 2.7%per decade between 1979-2006 and continues to do so at a faster rate now.  The 2013 ice maximum was reached on March 15 and was the sixth lowest on record at 15.13 million sq km, 733,000 sq km below the 1979-2000 average.  The lowest maximum extent occurred in 2011.  Sea ice naturally expands during winter as more sea freezes over. By March it is at its maximum size, extending furthest from the North Pole. The ice naturally shrinks to a minimum area around the Pole by late summer / Autumn which is when scientists usually take another measurement of sea ice extent.  The Arctic has warmed TWICE as fast as the rest of the northern hemisphere (Arctic Amplification) and the Arctic sea ice has shrunk in area.  In summer 2007 the North West Passage north of Canada became ice free for the first time. Sea ice has a high ALBEDO which means it is bright and white and reflects solar energy (the Arctic gets 24 hours of sunlight during the summer). Most sunshine is reflected back into space off the ice and does not heat the atmosphere or the sea, the energy is more or less “lost” from the earth-atmosphere system.   Less sea ice means more dark ocean water is exposed and, with a lower albedo, this readily “absorbs” solar energy i.e. warms up.  The Polar atmosphere is warmed up by this extra heat flux from the ocean. The “warmer” atmosphere contains more energy and water vapour and this could potentially deliver more humid Polar air carrying more snow to mid-latitudes.
There is a final twist to this theory: warmer Arctic air reduces the difference between Polar temperatures and those further south. This reduced “thermal gradient” weakens the jet stream which feeds off big differences in temperature. A weaker jet stream is less able to BUST the blocking HIGH pressure built by Polar air.  All this adds up to slower moving weather systems in the mid-latitudes which prolongs any extreme weather: the cold gets further south and lasts longer.  There are many factors which control climate but loss of Arctic sea ice and the almost prophetic predictions made by scientists studying the effects it would induce, seems to keep Arctic Sea ice as a key suspect! The video below concerns all types of ice loss in the Arctic: land based and sea ice.

http://www.theweathernetwork.com/news/storm_watch_stories3&stormfile=Winter-time_Arctic_ice_cover_was_fifth_lowest_on_record__NASA_07_04_2013?ref=ccbox_homepage_topstories

Climate Cluedo!