The past few days have been rather remarkable for July with record lows being set over parts of the midwest. Each day the sun heats the ground, which warms the air near to the ground. This warmed air vertically mixes growing what is known as the planetary boundary layer. The air temperature we experience near the ground is a result of the strength of the surface heating and the mixed air down from above. The featured chart shows a climatology of air temperatures at the 850 hPa pressure level for the sounding site near Minneapolis for its 12 UTC (6/7 AM CT). This level is typically included in the planetary boundary layer. For this recent event, the temperature at this level (2.2 centigrade) was the coldest on record for the site in July.

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