Low 90s possible by tomorrow and Monday

Yesterday was the last day of cooler than average weather that we will see for at least several days, possibly longer. Although the month started out with pleasantly cooler weather, our high temperatures in Madison for the first eight days of July have averaged 78.5 degrees, still nearly two degrees above the 76.7 we saw in the first eight days of July last year. The next week will be significantly warmer.

Today the high will likely reach the upper 80s, although humidity values will remain low at least for one more day. Humid, hazy weather that we had so much of in June may hold off until at least Tuesday or Wednesday as the remnants of Dennis approach from the south. Our forecast for midweek still depends on the exact track of Dennis, with current indications pointing to rains remaining to our south in Illinois and hot weather prevailing here.

If the humidity remains low for tomorrow and Monday, temperatures could climb into the lower 90s, with possibly even some mid 90s. So, why is it that daytime temperatures are warmer and nighttime temperatures cooler when the air is dry?
Here's your scientific explanation: The specific heat capacity of water vapor is nearly 4,000 J/kg, whereas the specific heat capacity of dry air is around 1,100 J/kg, or more than three times less. This means that it requires much more energy to heat up air with a lot of water vapor, humid air, and with the higher heat capacity, it takes longer to cool down at night.
Here is an example:
New Orleans, Louisiana, has a humid climate because of its close proximity to the Gulf of Mexico. This time of year, it averages a high of 91 and a low of 74, a daytime range of 17 degrees.
Tucson, Arizona, on the other hand, which typically has much drier air, averages a high of 100 and a low of 73 this time of year, a daytime range of 27 degrees.
Humidity is not the only control on daytime temperature range; cloud cover and other factors can also be important.