Welcome to the land of eternal spring. The average temperature on the island is 20º centigrade (68 Fahrenheit). The island does not have a large temperature range: between 26º and 16º centigrade. That is the main reason why we say that the island has mild weather the whole year round.
As far as weather is concerned, there are different climates due to:
The Azores anticyclone and the trade winds
The cold Atlantic stream, chilling out the temperatures
The nearness to Africa. Specially related to this one is the "calima" or "tiempo sur" (sanddust coming from the Sahara Desert).
La orografía insular, que determina la existencia de específicos condicionados por su altitud.
From the coastline up to 200 m, the average temperature is 20º centigrade. So, we have nice weather all year round.
From Mazo to Barlovento (East of the island), the weather is cooler while from Fuencaliente to Garafía (West of the island), it is drier and warmer. Over 500m temperature there is more seasonal variation. Above 2000m we can even find some snow in winter.
Winds are normally gentle the whole year around and it rains more often than in other Canary Islands and, at the same time, it keeps its yearly average temperature between 17º and 25º centigrades all the year around. Wet trade winds blow in the East and North of the island, causing a "mar de nubes" (sea cloud) which favours the lush greenery and the wide variety in flora species of the laurel forests.
La Palma makes honour to its two nicknames, "la isla bonita" and "la isla verde", to the effect of the wet trade winds.