Precipitation generally decreases from east to west with increasing distance from the Bay of Bengal, source of the summer monsoon. Eastern Nepal gets about annually; the Kathmandu area about and western Nepal about . This pattern is modified by adiabatic effects as rising air masses cool and drop their moisture content on windward slopes, then warm up as they descend so relative humidity drops. Annual precipitation reaches on windward slopes in the Annapurna Himalaya beyond a relatively low stretch of the Lower Himalayan Range. In rainshadows beyond the high mountains, annual precipitation drops as low as .