Appropriate Weather Climate and Temperature to Visit Nepal

The Nepali Seasons 
The weather in Nepal is reasonably stable and the climate naturally forms 4 main seasons.
Winter (December, January, Febuary) 
The Nepali Winter sees predominately high pressure bringing clear skies and dry weather. The temperatures are warm in the day but as the sun sets the temperature falls quickly and nights are often very cold.
Pre-Monsoon (March, April, May, Mid-June)
During the Pre monsoon you can expect more variable weather but with predomininantly clear mornings and overcast afternoons. Both the days and nights will be warm and the temperature will gradually increase as the season progresses. Thunderstorms are quite common at this time of year and are sometimes combined with short bursts of heavy rain or hailstones.
Monsoon (Mid-June, July, August, Mid-September) 
The Monsoon season dominates Nepali climate and is a perio
d of low pressure which brings frequent rain showers and storms. The majority of the rain falls in the afternoon and night-time but there are also occasional days of continuous rain. During this season the temperature and humidity is very high during both the night and day.
Post-Monsoon (Mid-September, October, November)
The Post Monsoon Season brings a return to stable high pressure giving mainly clear skies, sunshine, warm days and cool nights.

The Best Season to Visit 
Each of Nepal's climatic seasons offer something different and visitors do come to Nepal all the year round. However, the weather that suits most people is during the Pre and Post Monsoon seasons. It is at this time that the majority of vistors come to Nepal.
These two main tourist seasons (Pre-Monsoon, March, April, May, Mid June and the post-Monsoon, Mid-September, October, November). Both tend to be fairly dry, and to provide pleasant conditions for trekking, rafting and sightseeing. During the Pre-monsoon visitors will have the benefit of flowering plants, but visibility is sometimes reduced with hazy conditions. Post-monsoon normally provides good visibility.
Trekking 
For trekking below 4000m, the day time temperatures in these seasons will normally allow visitors to be comfortable in a t-shirt and light sweater. Above 4000m a fleece jacket may also be needed. At altitudes above 3000-3500m, night time temperatures will be low (often down to around freezing), but a down jacket or staying tucked up comfortably in a warm tent or lodge will be enough to stay comfortable.
Trekking is still possible outside the main seasons and during the monsoon months (mid-June, July, August, mid-September) the rain-shadow areas of the upper Marsyandi and Kali Gandaki valleys, and Dolpo receive far less rain than the Khumbu or Annapurna areas. During the winter conditions can also be good although much colder, but be aware that on occasions in January, February and March the higher passes and valleys are sometimes unreachable owing to snow fall.
Rafting
Rafting is possible throughout the year in Nepal. Seasonal variations in temperature and rainfall affect the water levels in the rivers and determine which rivers are best run at any one time. Optimum seasons for running each river will be advertised by good operators on their websites. See the Things to Do section of Tripadvisor for listed companies.

Climate Figures for selected locations

Normal (Average) Maximum, Minimum Temperature (°C) and Rainfall (mm) through year
Months	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Annual	Monsoon	Period
JIRI
Max temp (°C)	13	15	19	22	23	24	23	23	22	21	17	15	19.9		1971-2000
Min Temp (°C)	1	1	3	7	11	15	17	17	15	10	4	0	8.3
Rainfall (mm)	16	19	43	84	179	374	590	560	289	73	17	15	2,232	1,791
JOMSOM
Max temp (°C)	10	10	15	18	22	23	23	23	21	18	14	12	17.6		1981-2000
Min Temp (°C)	-2.7	-2	1	4	7	11	13	13	11	6	1	-2	5.3
Rainfall (mm)	7.9	10.2	23.8	18.1	16.9	23.9	40.0	34.6	36.9	30.2	8.9	6.3	253.1	133.0	1973-2000
KATHMANDU AIRPORT
Max temp (°C)	18	20	24	27	28	28	28	28	27	26	22	19	24.8		1971-2000
Min Temp (°C)	2	3	7	11	16	19	20	19	18	13	7	3	11.7
Rainfall (mm)	13	17	30	56	116	262	356	319	186	56	8	15	1,439	1,125
NAGARKOT
Max temp (°C)	12	14	18	22	23	23	22	22	21	20	17	13	19.4		1977-2000
Min Temp (°C)	2	3	7	10	12	14	15	15	14	11	7	4	9.9
Rainfall (mm)	15	17	29	55	139	343	476	475	263	75	10	12	1,921	1,554	1971-2000
NAMCHE BAZAR
Max temp (°C)	7	6	9	12	14	15	16	16	15	12	9	7	11.5		1996-2005
Min Temp (°C)	-8	-6	-3	1	4	6	8	8	6	2	-3	-6
Rainfall (mm)	26	23	34	26	41	140	243	243	165	78	9	39	1,147	791
POKHARA AIRPORT
Max temp (°C)	19	21	26	29	30	30	30	30	29	27	24	20	26.2		1971-2000
Min Temp (°C)	7	9	12	15	18	20	21	21	20	16	11	8	15.2
Rainfall (mm)	22	32	59	121	369	699	940	849	637	174	20	24	3,951	3,126

Bear in mind that these temperature figures are 'Shade Temperatures' - sunny day times will be warmer. Remember also that temperature will decrease with altitude, at a rate of around 1 degree per hundred metres higher.
It can be useful for all travellers to take a look at Meteorological Forecasting Division of Goverment of Nepal here: http://www.mfd.gov.np/  to check the weather before travel.
These two links are very usefull to trekkers. The first one is a live web cam from the top of Kala Patthar, so you can see how is the weather like every minutes:
http://www.evk2cnr.org/WebCams/Pyrami...
The second gives you the temperature, the umidity, the wind, the pressure:
http://share-everest.it/SHAREEverest2...