Oftentimes, meteorologists tell folks that a spell of hot and humid but rain-less days are ahead because of an, "upper air high pressure." What does that mean? You have to see the atmosphere is three (really, four) dimensions. It's like a pool of water covering the surface of the globe, and over time its moving eddies, currents, and cooler or warmer spots bring us weather changes. When a portion of the atmosphere is blocked from moving much and piles up, its builds into a high pressure area. That extra weight of atmosphere compresses, sinks and heats up. It is the compression that increases heat, and the sinking that prevents showers and thunderstorms from developing. Even if there is moisture, it cannot generate showers and thunderstorms because of the sinking air, so you end up with hot and humid conditions instead.