What was done
Using temperature and humidity radiosonde data from 795 locations across the globe, the authors calculated trends in tropospheric cloud base and cloud top boundaries for the months of January, April, July and October over the period 1964 to 1998.

What was learned
The global mean for cloud bases exhibited a statistically significant linear decline of 4.4 m per year, while the global mean for cloud tops exhibited a statistically significant linear increase of about 15.4 m per year.  Thus, ceiling heights or cloud bases in 1998 were approximately 150 m lower than they were three and an half decades earlier in 1964, while cloud tops in 1998 were approximately 525 meters higher than they were in 1964.  Examination of cloud frequencies revealed that it is cloudier in the bottom 0-2 km of the atmosphere at the end, as opposed to the start, of the record.  This trend also held for the middle (2-6 km) and high (6-10 km) atmospheric layers.  Averaged over the entire atmospheric layer (0-10 km), cloud frequencies revealed a linear increase of 0.17% per year, which corresponds to a 5.78% increase over the 34-year period.

What it means
As discussed by the authors, the increase in cloud coverage experienced across the globe over this period is "consistent with the decrease in diurnal temperature range evident over most of the globe."  Such an observation is not what the climate alarmists want to hear, for it is their contention that the rise in minimum temperature and subsequent decline in the diurnal temperature range is the result of CO2-induced global warming, not changes in cloud cover.