Obviously this is because you're going from day to night; a natural cycle that doesn't contain useful information about underlying warming or cooling. (skepticalscience.com)
Apparently, after taking into account the effects of the NAO, there's a substantial underlying warming trend. (realclimate.org)
If you take away the transient cooling in 1983 and 1992 caused by two major volcanic eruptions in the preceding years, the remaining underlying warming trend in the bottom eight kilometers (almost five miles) of the atmosphere was 0.096 C (about 0.17 ° Fahrenheit) per decade between January 1979 and June 2017. (co2coalition.org)