One of the primary excuses used by NOAA to hide the decline in the US temperature record, is Time Of Observation Bias (TOBS.) The theory is that people used to be incredibly stupid and reset their max/min thermometers only once per day – in the afternoon. This would cause double counting of some high temperatures, and result in historical temperatures which were too high.
This theory is easy to test. Half of the stations in Missouri took their min/max readings during July 1936 in the afternoon, and the other half took them at morning or night during that month. I separated those groups out and found that it makes no difference to summer temperature trends.
The group of morning/night stations has been cooling at almost exactly the rate as the group of all stations. Also note that summer temperatures in the 1930’s are essentially identical in the two groups. No matter what tricks NOAA uses to hide the decline in US temperatures, it doesn’t change the facts.

Morning/night stations.
APPLETON CITY MO USC00230204
CLINTON MO USC00231711
FARMINGTON MO USC00232809
JEFFERSON CITY WTP MO USC00234271
LAMAR 2W MO USC00234705
LEBANON 2W MO USC00234825
LEES SUMMIT REED WR MO USC00234850
LEXINGTON 3E MO USC00234904
MEXICO MO USC00235541
MOBERLY MO USC00235671
MTN GROVE 2 N MO USC00235834
SPICKARD 7 W MO USC00237963
UNIONVILLE MO USC00238523
Afternoon stations
BOWLING GREEN 1 E MO USC00230856
BRUNSWICK MO USC00231037
CARUTHERSVILLE MO USC00231364
CONCEPTION MO USC00231822
DONIPHAN MO USC00232289
LOCKWOOD MO USC00235027
MARBLE HILL MO USC00235253
NEOSHO MO USC00235976
ROLLA MISSOURI S T MO USC00237263
STEFFENVILLE MO USC00238051
SWEET SPRINGS MO USC00238223
TRUMAN DAM RSVR MO USC00238466
WARRENTON 1 N MO USC00238725

Another important test is the percentage of days over 100 degrees, comparing morning and afternoon stations. The result of this interesting – the morning/night stations actually have a higher frequency of hot days than the afternoon stations. This is the opposite of what TOBS theory would predict, and most likely due to the fact that the morning stations are generally further southwest than the afternoon stations.

There is no indication that TOBS has a serious biasing effect, and no indication that the NOAA adjustments being done are legitimate.