Rohtang pass during may. Its not possible to reach upto there in any other time of the year. The roads will be covered with now. the places is a 50km 'bumpy' ride from Manali. If you dont start early to the place you'd have to climb up in proportion to the hours you're late. We reached there around 7:30 am as i remember. The wind was chilly, its better to carry a leather gloves and something to cover your face if you're not accustomed to such a climate. But at the end of the day, i can say, its worth all the effort!! The place is awesome. The sky is amost dark blue! No clouds and you'll have lots of space to roam around over there.