A sounding is a plot of atmospheric information using data from radiosonde observations. They show the vertical profile of temperature, dewpoint temperature and winds above a fixed location. Soundings provide an important means for determining the stability of the atmosphere above a specific location.
They are particularly useful for forecasting the vertical extent of cloud, the type of precipitation and the likelihood of thunderstorms or air pollution. From these profiles numerous indexes are calculated and displayed on the right of the page. Forecast soundings on the website display a forecast of these profiles in 6 hour intervals out to 48 hours. The data is generated from the GFS model.
The red line is the environmental vertical temperature profile. The blue line is the dew point temperature profile and the grey line is the theoretical air parcel plot line. The horizontal axis of the charts is temperature and the vertical axis is pressure (equal to height). Where the grey line is to the right of the red line the atmosphere is unstable. If the grey line is to the left of the red line the atmosphere is stable. If the atmosphere is unstable then a parcel will accelerate upwards which is the primary means for cloud and thunderstorm development.
19:21 EST Parts of Sydney and the Blue Mountains have been blanketed by large hailstones as a severe thunderstorm moves across the area.