We have a hot, tropical climate here, sun beating down for about 50 weeks of the year. March - June expect sunny weather with temperatures around 25-30 C/ 77-86 F, the hotter days around 35C / 95F. July & August is the hottest time of the year averaging 35C / 95F by day, occasionally peaking nearer 40C / 104F on some days.
January & February temperatures average nearer 25C / 77F with high humidity, often windy and this is when it's most likely to rain here in the south. Parts of the island (usually the north or central areas) flood & dry beds become rivers as reservoirs overflow. Fortunately, a rainy spell rarely lasts more than a day and is usually followed perfect, holiday weather the day after!
As well as the rain we have the occasional sandstorm caused by the "calima" - a cloud of sand blown in from the Sahara. They're not as bad as they sound. It's something to be seen rather than avoid and there's no problem spending the day on the beach enjoying the sun that shines through them. Imagine them as kind of a fog that doesn't spoil your sunbathing. The hot wind that blows from the Sahara is known as the "siroco" and also arrives this time of year.
Here's the average maximum temperatures on the beach from personal experience over the last several years!
JANUARY 23C / 71F
FEBRUARY 24C / 74F
MARCH 25C / 77F
APRIL 27C / 81F
MAY 30C / 86F
JUNE 32C / 90F
JULY 35C / 95F
AUGUST 35C / 95F
SEPTEMBER 32C / 90F
OCTOBER 30C / 86F
NOVEMBER 27C / 81F
DECEMBER 26C / 79f
Thanks to the rains around January, the island becomes very fertile and green for the early part of the year. The north of the island has a different climate, so if you're getting weather information relating to the capital, Las Palmas, you'll find it's always 5C or 10C (approximately) cooler up there. It also receives more rain and clouds than the south, as do the rest of the Canary Islands. The south of Gran Canaria gets the sunniest and hottest weather throughout the Canaries.
The mountains in the centre of the island have a different climate altogether. Often it's quite cool with fresh winds, most of the island's rain falls here and during winter it's not that unusual to find snow whilst the south of the island still has hot, sunny weather.
By night the temperature stays around 15C / 59F or a little more all year round other than July and August where it can vary quite a bit from around 15C / 59F up to 25C / 77F some nights.
During the summer months, swimming pools and the ocean warm up and it's a relief from the heat to get in but during the winter, though everyone's sunbathing most of the day, the water gets pretty cold. People are still swimming in the ocean though it's generally a quick in and out and if your complex has a large pool, make sure it's heated!
On one of our other websites' forum there's a thread which is usually updated daily from people here already letting us know what the weather is like at the moment - you'll find it here