I am planning to go to Hanoi in December, can you please let me know whether the weather will be ok for sightseeing and walking outside at that time? I am from Washington D.C.

Welcome to Hanoi, Vietnam. December is in the winter in Hanoi. Therefore, it is cold but is still ok for travellers, especially those from temperate zone like you will not have a difficult time at all. The temperature is about 16 Celcius degree on average, with light rain and windy weather. You can go out for sightseeing comfortably, but make sure that you bring some warm clothes because Vietnam does not have central heating system like North America so it might be colder than it sounds.

Many Hanoians talk about having ice-cream and walking around the tree-lined Hoan Kiem Lake as a leisure activity during the winter days. Winter food in Vietnam is not about apple cider and fresh-from-the-oven brownies, but there are sure things to enjoy: sweet potatoes grilled on charcoal or toasted corn on the cob; chicken skewer and a hot bowl of pho will definitely give your stomach a satisfying feeling like you never feel before. 

Enjoy Hanoi!

Suggested Readings:
Hanoi Travel Guide
Hanoi weather in December