Plots of the variation in temperature over the year for 1998 and 2000 are shown below. Individual measurements are plotted as small green dots and nightly averages as larger blue dots. The blue vertical bars are termed ‘error bars’ and they indicate the uncertainty of the average temperature measurement. Results from the MSISE-90 computer model of the middle atmosphere are shown as a thin orange line. This is based on historical measurements of wind and temperature.
Annual variations in the mesopause region are characterized by an extended warm (206 ± 4 K) winter period, with a gradual decline (0.04 K/day) over the interval Day Of Year 106 (mid-April) to DOY 258 (mid-September) and including episodic 10–20 day (planetary scale) variations of amplitude up to 30 K.
Equinoctial transitions from cold summer temperatures show a sharp rise in autumn (1.2 K/day; DOY 49–80) and a more gradual spring decline (-0.65 K/day; DOY 275–296). The autumn transition occurs earlier, and the spring transition later than MSISE-90 model predictions.
Note that each of the years shown above have similar characteristics but that there are also some large differences between them. For example, the 10–20 K cooling of the mesosphere that occurred around DOY 170 of 1990 is not present in 2000.