This week people along the east coast and northeast suffered through the hottest temperatures they've seen in over 10 years. The reason, High Pressure.
Summertime highs usually bring descending air with tends to heat up the lower atmosphere. As air is compressed it heats up. Just like a bicycle tire pump gets hot as you pump it.
Humidity also plays a part in how dangerous heat is. Low dewpoints and dry air allow the body to cool itself easier. The high humidity makes summer very dangerous.
Ed explains more and compares several different cities in this edition of Weather 101.