Ground-based GPS data in the Qilian Mountains in China were used to study the daily changes in water vapor during summer. The study found that water vapor levels varied significantly throughout the day, with more pronounced changes on non-precipitation days. The diurnal cycle, peaking in the evening, explained most of the variance in water vapor levels. The semidiurnal cycle, peaking in the early morning and late afternoon, was weaker but still noticeable. The timing of water vapor changes preceded changes in precipitation frequency, and factors like air temperature, humidity, and mountain-valley breezes influenced these variations.