This thing not only goes to show the main process that dictates the climate of Nepal, but is also a good way to figure out how cold it is going to be while you are trekking in Nepal. The altitude here basically squeezes the climate of an entire hemisphere from the equator to the poles into a 200 kilometer wide cross section. To put that in perspective, the variety that comes from 10,000 kilometers of expanse or 90° of latitude is squeezed here within 200 kilometers or 2° of latitude. Hence if you are in Nepal, it is possible to walk from the tropical heats of the equator to the frigid cold of the poles in a matter of a few days.There are three major reasons for the variety of climates that are found in Nepal: altitude, altitude and altitude. Temperature goes down by about 5.5°C for every 1,000 meters climb when the air is humid, and by about 10°C for every 1,000 meter climb when the air is dry. The average is about 6.4°C/1,000 meters. So between the lowest point in Nepal at 70m and the highest point at 8,848 meters, there will be a temperature difference of 56°C!!! Okay that was a bit extreme but even when you go from Kathmandu (~1,400 meters) to Everest Base Camp (5,380 meters) there is a difference of 25°C!!
It is also interesting to note how while most of the world has four or less seasons, Nepal actually has six. The season here starts with the promise of Spring (Basanta) which gives way to the scorching dry heat of Summer (Grishma). Respite comes in the way of the Monsoon (Barsha) to give way to more pleasant temperatures of the Autumn (Hemanta). Before the land gives in to the harsh Winter (Shishir) there is a short period of dry, clear and pleasant Pre-winter (Hemanta).