Weather, clothing & packing

Are mosquitoes a problem in Greenland? Can I buy outdoor clothing in Greenland? Do I need hiking boots? Do I need sunglasses and sunscreen? Do I need waterproof clothing? Does it rain a lot in Greenland? How cold is Greenland? Is the weather unpredictable? What should I pack for a summer trip? What should I pack for a winter trip? What should I pack for Greenland?