Letter received dated 11pm 7th January
Hi Ken
Large hail stones in Masterton and outlaying district. Some orchardists
wiped out. Nobody forecasted for this.
Comments please.
Richard
Two days ago I had mentioned that snow in central North Is. for the 7th
was predicted in my Weather Almanac; but then when none came you'll recall
I apologized! But that VERY night - hailstones as big as golfballs in the
Wairarapa. I guess there's not much difference between snow, sleet and
hail.
Disturbed westerlies followed by southerlies will be soon affecting the
South Island. I did not want to wait a week, as by then the rain will have
nearly arrived. So seeing some more thunderstorms and hailstorms are due
in some areas over the next few days I thought it might interest some to
know - just what IS rain and hail?
Whether or not it will rain depends on the upsetting of the balance
between rising water-laden air and gravity.Ý Before a cloud can hold
sufficient moisture to burst, an updraft is usually necessary. Raindrops
fall at a maximum speed of 27.36km per hour (17mph). If an updraft is
climbing at the same speed as the raindrops are trying to fall, the
raindrops will become stationary enabling the cloud to continue to gather
further moisture until it becomes super-saturated.Ý This higher speed
causes the clouds to break down and form mists.
Such a cloud is easily recognisable, remaining almost stationary over an
area with a dirty-cloud dark appearance. If the updraft speed is above the
27kph, the raindrops will rise higher and form hail in the colder air
above and so remove the danger that the cloud will burst. But when heavy
enough, the hail will fall to the ground.
The updraft requires heat to propel it. The heat can only come from the
sun shining onto the ground for a minimum time beforehand. When the
updraft ceases through loss of this heat from the ground and any coolness
from space brought about by the lower atmospheric tide,Ý the cooled super-
saturated cloud will be unable to hold its excess moisture and will
subsequently drop its contents in one short fall.
Hail is generally associated with towering cumulo-nimbus clouds, sometimes
called 'tower' clouds, which reach 10 miles high into the troposphere, to
where temperatures are well below freezing point. Strong updrafts cause
rising moisture to form hail at the higher altitudes. Cumulo-nimbus clouds
are unstable.Ý After gaining weight and then falling to lower levels, hail
may be repeatedly carried aloft into the freezing temperatures where extra
moisture collected at lower altitudes will freeze further adding more
layers, so forming larger hailstones. This will continue until gravity
overcomes updraft.
Ordinary rain
What makes what we call rain is the same process but not progressing as
far. Hail occurs at times of low atmospheric tides. Therefore when the
Moon is overhead in the sky, the freezing level is necessarily at a higher
altitude, which is therefore too high for raindrops to reach to in the
updraft. Gravity will then take over before the raindrops freeze,
resulting in rain; and even if they do freeze then the hail will melt
again into raindrops which will still fall as normal rain.
Reminder: next weekend's weather event around 14th
14th: Rain likely over most of the South Is and south and west of the North Is. Heavier in Westland, Otago and Southland. Floods a possibility.
Don't forget that the second biggest perigee for the year occurs on January 10th and in some areas we are already starting to feel some of its effects. The Australian coast and Otago have been getting flash floods and a cold wet front is already hitting Fiordland and Southland.
This is what the experts say..
(from NIWA)
Climate Outlook for Southland and Otago: December to February -
Atmospheric pressure patterns in the New Zealand region suggest generally
settled conditions over the south of New Zealand, with weaker westerly
winds to the south of the country. Below average rainfall is expected for
the south of the South Island, and average to above average temperatures
are likely. River flows are expected to be below average with 50% chance
of below average flows. (posted December 2000)
Contact
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