Why does the windward side of a sand dune have a gentle slope while the slip face has a steep slope?

northern hill zone (nhz)

western himalayan regions of j& k (except jammu and kathua distt.); h.p. (except una and paonta valley); uttaranchal (except tarai area); sikkim and hills of west bengal and n.e. states

north western plains zone (nwpz)

punjab, haryana, delhi, rajasthan (except kota and udaipur divisions) and western up (except jhansi division), parts of j& k (jammu and kathua distt.) and parts of hp (una dist. and paonta valley) and uttaranchal (tarai region)

forms a sonnet: c on e2020