blocking highs.

Discussion in 'Alpine & Snow' started by james.dutschke, May 20, 2006.

  1. james.dutschke

    james.dutschke First Runs

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    what is the influence that promotes the northward migration of the sub polar high pressure belt.

    Im guessing its got alot to do with ocean temps...

    anyone care to inform me better?
     
  2. Falls expat

    Falls expat One of Us

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    I am assuming that you mean the sub-tropical high pressure belt???

    This is caused by all the air rising over the inter-tropical convergence zone (ITCZ). This reaches the 'ceiling' of the troposphere an inversion called the tropopause and has no where to go other than spread out polewards. This air cools and decends around the 15-30 deg latitude either side of the equator depending on the time of year. High pressure systems are caused by decending or very cold dense air and lows are generally areas of rising air.

    The subtropical high pressure belt is closely linked with the location of the seasonal movement of the ITCZ when it moves north in our winter and southwards in the southern hemisphere summer. At the moment it is moving northwards with the ITCZ.

    Strengthening of the high pressure belt may be associated with regions of stronger convection at the tropics pumping more air away from the poles or linking up with transcient high pressure systems that occur in the mid-latitudes (roughly 35-60deg).

    Sea temperatures do make a difference where colder sea temps can cause locally colder dense low level air and thus higher pressure. In contrast warmer sea temps can allow more convection and thus lower pressure.

    This year the high pressure belt seemed to have weakened early during April and early May, but it may be a false dawn. Traditionally the high pressure belt keeps most snow bearing fronts away until mid to late June. Sometimes this occurs earlier and sometimes later. I think if anyone could confidently predict exactly when they will recede far enough northwards to consistently allow polar fronts to make it to the Alps then they would be championed here.
     
    #2 Falls expat, May 20, 2006
    Last edited by a moderator: Nov 17, 2013