Basically, when the body sweats, the perspiration evaporates, cooling the body. When the humidity is high, the sweat doesn't evaporate as quickly and the body doesn't cool itself as well. Therefore, the body feels warmer. The heat index represents the heat you're feeling outside.

Friday will be sunny and pleasant with lower humidity before the heat builds with the hottest days expected Monday to Wednesday.

High heat is in the forecast, and the National Weather Service offices in Pennsylvania have issued an Extreme Heat Watch for parts of the region. The watch goes into effect at 11 a.m. Sunday, June 22,