Rhododendron fulgens Hook. f., Rhododendron Sikkim Himalaya 3: 27, pl. 24 27 1851.; 

rhododendron arboreum, from ericaceae family

rhimbhik phata, singalila national park
in full bloom on mist covered slopes

second week of april this year

I think all belong to Rhododendron fulgens, never R. arboreum

beautiful flowers. We have seen lots of trees during our last trip to Uttarakhand, but the flowering season was over and we have to enjoy the beauty of the trees. It is the State flower of Uttarakhand. The flower is known as buransh and they preserve the flowers in sugar syrup to use later in sharbats, which is good for health.