The University of Alabama at Huntsville has a valuable summary of day by day global, and northern and southern hemispheric, average values of the lower tropospheric temperatures. Since the Earth is at different distances from the Sun during the year, and the two hemispheres have different amounts of land and ocean, this is an interesting analysis in its own, in addition to its use to compute the deviation from the averages at any time of the year.
The entire day by day set of values is at http://vortex.nsstc.uah.ed/data/msu/t2lt/tltdayac7998_5.2 . Here I present just the Julian Day 1 and Julian Day 183 to illustrate the large variations that occur. The data on their website are in units of 0.001K but I have rounded here.
Julian Day 1
Global Average= 267.8K
Northern Hemisphere Average= 265.6 K
Southern Hemisphere Average = 270.0K
Julian Day 183
Global Average= 270.1
Northern Hemisphere Average= 274.7
Southern Hemisphere Average = 265.6
This is quite an enlightening presentation of data as it shows that the Earth’s lower troposphere goes through a warming and cooling of over 2 K each year. The variation in the Northern Hemisphere is also larger than the Southern Hemisphere due to the larger fraction of land (a large portion of which is covered with snow in the winter). Also, despite that the Earth is further from the Sun in July than in January, the global average lower tropospheric temperature is higher in July because of the greater fraction of land in the Northern Hemisphere. [also, see More Background Information On Diagnosing Global Warming and Cooling for more discussion on the variation of radiative forcing at the top of the atmosphere during the year].
From the climate perspective, the variation of lower tropospheric temperatures during the year further illustrates why a global average trend in the annual value is not very informative. We need values for months (and seasons) as well for each hemisphere and globally. Then these values should be quantitatively compared with the predictions of the IPCC climate projections.