Morocco was described in Michael Palin’s Sahara as a “cold country with a hot sun". This is not a bad generalization for much of the year (apart from probably the summer months when almost everywhere except the mountains and coast is hot to very hot) and if you are unlucky, and the sun is in, you need to be prepared. Having said that Morocco has a huge number of sunny days and even in winter warm sunny days are the norm. You should definitely take some warm clothing even if visiting during the summer months. The evenings can cool off once the sun goes down. The higher you go the cooler it will be.

The peak season is perceived as being from March to late June when the risk of rain has largely gone and before the full force of the summer heat arrives. However it is an ideal destination for the northern hemisphere winter months. November and December are usually months of clear blue skies and warm sunshine (average 21c (70f) in winter). A light pullover would probably be worn throughout the daytime in Marrakech and swimming would only be enticing in a well heated pool but it is an ideal time for sightseeing, especially in Southern Morocco (Taroudant, Ouarzazate, Erfound) which can be very hot in the summer months. Essaouira on the coast has a warm Mediterrean climate – protected from the fierce summer heat by the onshore winds, however it can be windy during the first few months of the year.
From mid July to mid September it can be very hot in the cities and southern Morocco. January and February are the months of highest rainfall.