Huddled in doorways and tucked into underpasses, tens of thousands of Americans sleep on bedding made of cardboard, plastic tarp and thin fabrics that offer little protection from the cold, hard concrete beneath. On a single night, more than 553,000 people are homeless across the United States. Some sleep in shelters, but almost 193,000 people do not. Hypothermia begins to set in at 50°F. In even the mildest U.S. cities, nighttime temperatures often plummet to below 20°F. All over the country, the story is the same: Insufficient shelter space leaves people no other choice but to sleep outside, at the mercy of the elements.