5G is planning to exploit millimeter wave (mmWave) channels to achieve the massive capacity increase needed for future applications. Unfortunately, wireless signal attenuation and noise in this high frequency band is very sensitive to atmospheric conditions, such as temperature and humidity. Given that such atmospheric conditions at any given location could vary throughout the day, mobile communication systems seeking to exploit this band must expect some sort of diurnal variations in attenuation and noise. In this paper, we study the extent of diurnal variation in mmWave channels in three largest cities of Australia by accessing the hourly air quality and weather data over 12 months in 2015. We find that all planned mmWave bands, with the exception of 60 GHz, experiences significant diurnal variation in attenuation and noise. The attenuation and noise generally drops in the middle of the day when temperature rises and humidity falls, but remains high during the night. The diurnal variation is found to be more significant in summer compared to winter. The 60 GHz band remains stable throughout the day as this band is mainly affected by the amount of oxygen in the air, which does not fluctuate much for a given location.