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.
âThereâs a debate about how cities affect rainfall,â Shepherd answers. âThere are several hypotheses about what is going on, but they primarily involve the urban land use and urban aerosols.â The first hypothesis deals with the urban âheat islandâ effect. Cities are made of heat-absorbing materials like concrete, steel, and asphalt.