A Guide to Ireland's Air Masses

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.

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. 


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.  

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. 


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.


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. 


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. 


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.