One of
the most notable and talked about features of the Irish weather is how rapidly
it can change on a daily - or even hourly – basis.
Due to
its mid-lattitude location of between 52’ and 58’ north, Ireland lies in the zone
where polar and tropical air masses often meet and clash which results in the
formation of various types of weather fronts which affect the country on a
regular basis giving us our varied climate.
Below is
a basic guide to the most common types of air masses that affect Ireland. Descriptions
are based mainly on the idealized scenario of each but should serve to give a
good idea as to the weather we can expect under each one on them.
TROPICAL MARITIME: (mT / Tm)
By far
one of most frequent influences on Ireland’s weather, tropical maritime air
usually has its origins over the Azores or sub-tropical Atlantic and usually
takes quite a bit of time to reach Ireland. Because of its long journey over
the relatively warm waters of the Gulf Stream, this warm, tropical maritime air becomes increasingly moist and damp but increasingly stable. Tropical maritime air masses often
reaches Ireland in the form of a warm front approaching from the southwest.
The most
observable sign that a tropical air mass is approaching Ireland is a general
increase in cirrus cloud which eventually thickens and lowers to cover the entire the sky. The actual warm
front is likely to bring rain across the country, heaviest in coastal and
upland locations. Once the warm front has passed damp, drizzly gloomy
conditions usually set in with little variation in temperature.
If the
cloud does break (and this is more likely to occur on the lee side of hills and
away from exposed coasts), temperatures will be raised further, especially during
the summer months and more especially in the east and southeast of the country. Moderate to poor visibility is also a common characteristic of a tropical maritime air mass.
RETURNING TROPICAL MARITIME (rTm)
A
returning Tropical maritime air mass is essentially a modified version of the
more frequently occurring Tm air mass discussed above. Returing tropical
maritime air is a basically a tropical maritime air mass that is diverted
northwards by an intensifying ridge of high pressure to the west or south west
of Ireland before it is diverted south-eastwards over the country once again. Returning tropical
maritime air masses when over Ireland retain most of the parent Tm characteristics
though with subtle modifications. These include slightly cooler temperatures
and cloud cover with a slightly higher altitude base which may even out into
expansive sheets of stratocumulus clouds(Sc) with any rainfall tending to be very light.
POLAR MARITIME (mP/Pm)
Together
with the tropical air mass as discussed above. Polar maritime
(Pm) air is another of the more frequent influences of Ireland’s weather.
A Pm air mass generally follows on from a Tm in the form of a cold front. Polar
Maritime air is sourced more to our northwest, usually around the seas south of
Greenland. Polar maritime air is cooler and less moist than that of a tropical maritime air mass but is more unstable (become cooler with height) which can result in the formation of showers as the air flow passes over the relatively warm waters of the north Atlantic.
The typical characteristics of a Polar maritime air mass over Ireland is for moderately
sunny weather with occasional (sometimes heavy) showers that tend to affect
more windward coasts in the autumn and winter which can be experienced
countrywide in the late spring and summer season. Temperatures will tend to be close
to or slightly below average overall but can be contingent on actual wind
speed which normally is from of a westerly or north-westerly direction.
RETURNING POLAR MARITIME (rPm)
A Returning
Polar Maritime is an Polar maritime air mass that has been diverted southwards
to the west of Ireland before it is ‘returned’ back north-eastwards towards the
country. A returing polar maritime
therefore is modified a little although retains many of its Pm characteristics as
described above.
Like its
parent Pm air mass, a rPm air flow over Ireland tends to bring bright weather
with temperatures that tend to be a little warmer than that of the Pm. Showers
can still occur and may be thundery if there is strong enough toughing
contained within the flow.
ARCTIC MARITIME (Am / mA)
A less
frequent visitor to Ireland is the Arctic Maritime air mass (Am) which, as prefix suggests, is
sourced more in Arctic regions. Although much colder than the Polar maritime air
mass, the Arctic maritime is similar in that the air starts off as dry but
picks moisture as it passes over warmer seas on its journey southwards towards
Ireland. This decreases the stability of the flow allowing showery troughs to
develop which can occasionally affect the country. Significant wintry
precipitation can occur as a result, more especially over the northern
half of Ireland.
In the
main however, vivid sunshine and crisp temperatures are the general characteristics
of the Arctic air mass. Some of the coldest temperatures recorded in
Ireland have occurred when Arctic air has settled over the country
during the winter months.
During
the late spring and summer period, the Arctic maritime airflow tends to bring
cooler than average temperatures and depending on the stability of the flow,
can bring heavy convective showers and even thunderstorms to more inland parts
during the afternoons especially.
TROPICAL CONTINENTAL (cT / Tc)
Of
all the various types of air masses that affect Ireland, the Tropcial
continental air mass is the least frequently occurring of them all. As the name suggests, the tropical
continental air mass has its origins over the European continent or more
rarely, the North African land mass. Tropical continental air is normally
drawn up over Ireland by the combination of low pressure systems to the
southwest or south of Ireland and a building ridge of high pressure over
Eastern Europe. Because of its long land track, a Tc tends to bring extremely
favourable conditions to Ireland with relatively low humidity values.
Skies
under a tropical continental air mass tend to be either partly cloudy or a stunning
blue which can be obscured to an extent by a blue pearly haze. Winds are mostly
light and can range between southerly and easterly in direction and mostly
light. During the summer months, vigorous, and occasionally violent
thunderstorms can sometimes break out if the air air mass becomes unstable
enough.
Tropical continental air masses more likely to occur during the summer months and into
the early autumn. As a result some of the warmest temperatures ever to be
recorded in Ireland have occurred under the influence of Tc air mass. If occurring
during the late autumn and winter months, the air mass will bring no more than
average temperatures by day while nights can be quite crispy and cool, contingent on cloud cover.
POLAR CONTINENTAL (cP / Pc )
Like the Tropical continental air mass, the Polar continental is a rare visitor to Irish shores
and tends to be exclusive to the winter and spring seasons, although can
occasionally occur in summer when it may, depending on length of its sea track,
take on the characteristics of the tropical continental type. Unlike the cT, a Polar continental air mass will have its source over
north-eastern and occasionally, eastern Europe. The Polar continental air mass
when over Ireland can bring very cold temperatures over the country in the
winter months, and depending on the length the sea track it may have taken, can
bring rain, sleet and snow showers to eastern and southern coasts especially.
In the main, however, the polar continental air mass will bring vivid clear blue skies
over the bulk of the country although haze at both surface and upper levels can
occur if the air mass is of the shorter sea track variety.
In the
late spring and summer months, the polar continental air mass tends to bring fine weather with
warm to very warm day time temperatures away from exposed coasts although
occasional, sometimes heavy showers may break out during the afternoon.
