Volunteer "street scientists" take part in a campaign to map heat in Maryland's Montgomery County, which neighbors the US capital, in an effort to identify areas at particular risk to extreme heat, called urban heat islands. During the one-day campaign, volunteers drive along predetermined routes in the morning, afternoon and evening for about an hour each time with car-mounted sensors measuring temperature and humidity. "It's going to tend to be low-income communities and communities of color that are most impacted" by extreme heat in urban areas, says Gretchen Goldman, of the White House Office of Science and Technology Policy. IMAGES AND SOUNDBITES