Kathmandu's weather and temperature varies widely according to the season, with very cold and harsh winters and mostly cool summers. The area's high elevation, around 1300 meters, means that it never quite gets as warm as other locations at the same latitude. Kathmandu's summers see a great deal of rainfall and are also very humid. The period from May to September sees most of the city's rainfall for the entire year, more than triple the amount of what the other months receive combined. The city's wet summers create the lush, green countryside which surrounds it, an aspect which has lent Nepal the nickname "The Switzerland of Asia".
Visitors generally come to the city during the summer months, when they can best enjoy the city's attractions and historic sites. However, some people are attracted to the region during the winter months for its skiing and snowboarding. For an up to date weather forecast on Kathmandu, visit the BBC's Weather Centre.
To add some more information from a frequent visitor .. summer is not the best time sadly - unless you enjoy hot monsoon. Quite mystified by the references to skiing and snowboarding - really go to Switzerland for that - what you will not find in Switzerland is the Himalayas!!
During the winter months people come to Kathmandu for the good daytime climate. In November through mid February nighttime temperatures are quite low - and there is no central heating in hotels etc. Daytime temperatures are quite pleasant to warm. No skiing and snowboarding in Nepal! Within the Kathmandu valley you can take time out to hike in the countryside or visit many cultural attractions and World Heritage sites within the three cities - Kathmandu, Patan and Bhaktapur. Visits to Pokhara (a short flight from KTM or several hours on a bus) will lead you to trekking opportunities in the Annapurna mountains and around - or from Kathmandu you can access Everest treks. There are many opportunities for sports in Nepal including trekking, rafting/kayaking, paragliding, mountain biking and many many more - most of them extreme! - except skiing and snowboarding - they really don't have that facility. They do have national parks for wildlife safari - down in the south at Chitwan among others - they do have amazing wilderness to explore. They do have an amazing cultural experience. Winter is fine if you bring warm clothes for the evening/night - top seasons are October to mid December (brilliant clarity in the Himalayas) - then March - mid May - best climate in these seasons in summer is monsoon so get ready to be rained on and very warm and humid!
In short - Nepal is a most beautiful and challenging country to visit - a must see - put it on your bucket list. Then go. When you have been you will know why. Life changing.