Diurnal variation in water vapor over North America and its implications for sampling errors in radiosonde humidity

At Lamont, specific humidity in the free troposphere is significantly higher in the early morning (0000–0008 local solar time (LST)) than during the day (0800 - 1800 LST). This diurnal variation changes little from -4 to 16 km above the ground. Near the surface, specific humidity tends to be lower in the morning than in the afternoon and evening in all seasons except summer.

This near-surface diurnal cycle propagates upward through the lower troposphere (up to -4 km). Errors in seasonal mean humidity due to undersampling the diurnal cycle with twice-daily synoptic soundings (at 0000 and 1200 UTC) are generally small (within ±3% or ±0.5 mm for PW), but it can easily reach 5 - 10% if there is only one random sounding per day. Several physical processes are proposed that could contribute to the diurnal variations in atmospheric water vapor.