Nepal is situated on just about the same latitude as the US State of Florida and thus in the northern hemisphere. Because of the latitude temperatures are generally pleasant, even at higher altitudes. Due to the southern location the tree- and snow border in Nepal are high.
The climate varies enormously by area and especially by height. In the flat plains on the southern border with India there is a hot, subtropical climate. High in the mountains, there is a climate similar to the Alps. All the variations in weather are possible, depending on location and altitude.
The rainy season (monsoon) in Nepal lasts from June to September, although that is changing due to climate change. During these months humidity is high and there are massive daily downpours that you mostly can see coming from miles away. Unlike in the Netherlands, the monsoon rain is not accompanied by changes in temperature. When the weather is dry, everyone continues doing what they where doing before.
The capital Kathmandu is located in a basin between the mountains and hills, the Kathmandu Valley. In the summer months it easily gets up to 30/35 ºC in the city. A refreshing shower at times is not to bad. Especially because most downpours fall at the end of the day and at evening or night fall. So very practical.
Only downside is that roads and sidewalks – which are already difficult when dry and full of obstacles, potholes, uncomfortable steps – change in mud pools water slides and pools. You’ll be well advised to leave your good shoes at home. A few cheap plastic flip flops or sandals in your backpack is a good idea. This way at least you can wade through ankle high water. Because that is sure to happen to you at this time.
In January, the temperature in Kathmandu varies during from around 8/10°C during the day to around or under the freezing point after sunset. It rarely really freezes. But since most houses in Nepal have no heating, it is important to dress well against the cold. In July it is generally 25 to 30°C, but peaks at 35°C certainly occur.