Heat Island is the term used to describe the relative increase in temperature that is experienced within a city. Cities are always generally warmer than the surrounding rural areas for many reasons.
Waste heat from city buildings, cars, buses, and other vehicles all contribute to the heat within the city limits. Also, solar energy is trapped within a city because materials such as tar, asphalt, brick and concrete are good conductors of heat.
There are two different weather stations that report hourly observations in Austin. The average high temperature at Camp Mabry for the month of April was 70 degrees. However, according to the reports at Bergstrom International Airport, the average high was only 67.