Blown in largely from the south to southwest, the generally mild moist airstream deposits 2 metres (approx 80 inches) of rainfall on the Garden. Although generally windier and wetter through the autumn and winter, when damp periods can be more persistent, refreshing rains can be experienced at any time, with some precipitation recorded on two thirds of the days each month. In spring and summer sustained dry spells of several days, or even weeks, can occur – when irrigation of young plantings becomes advisable. On average over 60% of days are overcast , whereas brightly sunny or partly cloudy days prevail a third of the time. Snowfall is sporadic and rarely lingers, occurring anytime from December to early April. Some years barely a dusting falls in the garden and is gone in twenty four hours; now and again a more fulsome deposit of perhaps two inches can fall, cloaking the landscape and plants in a graceful white mantle for a few memorable days. In general the coastal fringe around Arisaig catches the best of the weather, as the wildest winds, clouds and rains are drawn beyond into the higher mountainous hinterland. Very often, work party participants will emerge from a snowy or rain-lashed day at Fort William and Glenfinnan and descend into clear conditions as they reach the west coast.
Exceptionally fierce storms do periodically blast in from the North Atlantic and wreak havoc as their associated winds, which have been known to exceed 100mph, roar through the garden. Most winters there will be a few fallen specimens and broken tree limbs. On the west-facing slopes a number of the large, but very shallow-rooted beeches lie recumbent – but continue growing. The early January hurricane of 2012 devastated pockets of trees and shrubs, most notably a massive oak just off the main drive, and a complete row of shelter-belt Sitka spruce at the more exposed south eastern end of the Garden.
Hot and Cold:
With excellent shelter, adequate soil and generous moisture levels, it is ultimately the prevailing temperatures that enable such a range of exotic plants to thrive at Larachmhor. Ground frosts are infrequent, with sub-zero conditions descending on perhaps 4 or 5 nights between November and February. Air frosts, however, can occur on clear nights anytime from October to April or even early May. Such late spring frosts are damaging to young foliar flush and above all to the fine flowering of the rhododendrons that peaks across this period. The consequences can be seen as colourful shadows of fallen blooms that carpet the ground beneath the more vulnerable large specimens – a disappointing but delicious sight! Mean monthly temperatures at Larachmhor rise from 3-5ºC in the depth of winter to 12 to 14ºC in summer. Sun-soaked blue sky days can occur in any month and from April through to September, if a high pressure area settles across the west coast for several days, temperatures may briefly soar to beyond 25ºC. Then the lure of the nearby renound white sand and coral beaches can prove irresistible - although those taking a refreshing dip will discover that the summer sea temperatures only reach a rather breath-taking 10 to 13ºC, from their winter minimum of 7 to 8ºC thanks to the Gulf Stream.