When we speak of upper highs, it really means only one thing in August:
H O T!
It looks like a big bubble on a weather map:
Notice that there are some stormy-looking areas around the edge of the high, but places closer to the middle (like Texoma in this example) don’t have much cloud cover.
The red arrows show the clockwise flow around the high. Sinking air inside the core warms up, and “upper highs” in the summertime are often responsible for heat waves like the one that cooked Texoma in 2011, and the brutal heat wave in the desert southwest in June of this year. Here's a 3-D view of the air motion:
So when we talk about upper highs, we’re talking about sinking, heating air, and we can all look forward to September when it begins to break up. The first “decent” fall fronts usually roll in about the 3rd week of September. By then, highs fall to around 90 degrees on average, and rain chances pick up nicely. Not bad compared to August!