It is often difficult to plan for bitter cold winds and snow while walking past banana trees in the hot sun. In the main trekking seasons in the spring and Fall, the weather is often stable, and even the high passes may be free of snow and relatively easy to traverse at times. Those trekkers who encountered an easy day at altitude may spread the word that boots and warm clothing are not required. Sudden storms can occur at any time, dumping one or two meters of snow on the passes. At that point, anyone with simple running shoes will not be able to proceed, and may even be stranded for a number of days.
Frostbite is a constant risk if one walks in snow at high altitude.
If you trek in the winter, you must be prepared for cold and snow. If you trek in the monsoon you might be faced with slippery trails and difficult river crossing but there are many less people on the trail. You are heading into the world's highest mountain range. Be prepared for changes in temperature and weather. If your trekking route will go over 4000 meters, take a warm sleeping bag, boots adequate for snow, a warm jacket and good quality sunglasses.