An excellent article written by Gene Donald Lamont:
The island of Tiree in the Inner Hebrides was formed from Lewisian gneiss, the oldest rock in Britain, which forced its way upwards through the earth’s crust about 3,000 million years ago. Tiree and its neighbors, Coll and Skerryvore, are highpoints along a ridge of this same rock formation. This rock, upon which Tiree and the others stand, is immensely hard, much harder for example than the softer granite of their neighbor Mull. The island is situated nineteen miles northwest of Iona and twenty-two miles west of the nearest point of Ardnamurchan on the mainland. The ancient Gaels named the island Eileann Thiriodh, or land of corn. A play on its name twists it to Tir barr fo Thuinn, or ‘land beneath the waves’, which it must appear to anyone approaching it in a small boat, since three quarters of it lie below sixty feet. It is shaped somewhat like a war club, being a little more than ten miles in length and about five miles in width at its widest point. It covers an area of 13,000 acres. The winds that buffet Tiree ensure that it is devoid of even a single treee, and with the exception of three small hills, none higher than 462 feet, it is entirely flat. Its coastline, forty six miles in length, is indented by a number of magnificent bays, many lined with beautiful silvery shell sand. An Trágh Mhòr, the big strand, is over two miles long, but unfortunately Tiree lacks a truly good anchorage.
The latitude of Tiree is the same as the southern part of Alaska, but the island is warmed by the Gulf Stream and does not have the severe cold of the latter. The maximum temperature on average in the summer is just under 20º C. (68º F.), with the highest recorded being 26.3º C. (79º F.) in July of 1991. The lowest temperature recorded in winter was –7º C. (19º F.) in December of 1985. Tiree is known to have bright early summers and a record amount of sunshine for the British Isles, which is said to be about 1400 hours a year. It averages 47 inches of rainfall a year, which is about a third of the rainfall of its neighbor, Mull. The winds that are such a part of Tiree are generally from the southwest and reach gale speed about 34 days a year. The highest wind speed ever recorded on Tiree was 118 miles per hour in January of 1968.
The icy gales of winter often isolated Tiree from the rest of the world for months at a time in the old days, and travel to it was even hampered in the summer by its lack of a good harbor. In 1792 there was a regular ferry sailing between Tiree and Coll, but no such service between Tiree and Mull, although this would have been much more needed. Tiree’s main advantage has always been its fertility, which it owes to the wind that blows across it at an average speed of seventeen miles per hour. This almost constant wind brings a myriad of sand particles to the island that regularly replenishes its soil. This wind blows sand inland to form the basis of the fertile machair, a flowerrich grassland, that flourishes on the island. This machair can be thought of as forming an outer ring around a middle section of dark, rich cultivatable earth. The center of the island, however, is wet, peaty ground termed sliabh. Townships were often divided so as to have a section of each sort of ground. The sliabh and small hills, which held their moisture, were used for summer grazing, while the machair provided grazing in wetter months.