The magnitude, spatial extent, growth, and seasonal and diurnal behaviors of the urban heat island of Houston, Texas are characterized using both in situ air temperature and remotely sensed surface temperature data. Between 1990 and 2000, the air temperature heat island of Houston had an average magnitude of 1.25 K at night but was largely absent during the day. This behavior is reflected in a survey of extreme temperature events, which reveals a dramatic increase in the number of extremely warm nights relative to the surrounding rural areas. Thermal satellite imagery acquired between 1985 and 2001 demonstrate a surface temperature heat island of approximately 3 K at night and up to 10 K during the day. Climatological analysis reveals an inverse dependence of air temperature heat island magnitude on rural temperature. Conversely, daytime surface temperature heat islands grow with rural temperature, while nighttime surface temperature heat islands show no relationship to rural temperature. Examination of temperature maps reveals an urban heat island area of 1200 km2 at night and 2100 km2 during the day. Comparison of satellite imagery taken twelve years apart exhibits a growth in the nighttime heat island of 0.8 K in magnitude and 650 km2 in area. High-resolution temperature data are also examined and show an urban temperature dependence on population density.