As part of an immune response to foreign pathogens, histamine is produced by basophils and by mast cells found in nearby connective tissues. Histamine increases the permeability of the capillaries to white blood cells and some proteins, to allow them to engage pathogens in the infected tissues.
Where vegetation is sparse, temperature is hot. (Image: NASA) An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surroundings. According to the EPA, many U.S. cities have air temperatures up to 10Â°F (5.6Â°C) warmer than the surrounding natural land cover.
1 57 cities had measurable urban heat island effects over the past 10 years. 2  Single-day urban temperatures in some metro areas were as much as 27Â°F higher than the surrounding rural areas, and on average across all 60 cities, the maximum single-day temperature difference was 17.5Â°F. Single-day urban temperatures in some metro areas were as much as 27Â°F higher than the surrounding rural areas, and on average across all 60 cities, the maximum single-day temperature difference was 17.5Â°F. 2  Cities have many more searing hot days each year.