These are busy figures - concentrate on just the two thick black lines. The one on the right is the air temperature and the one on the left is the dew point temperature, and the bottom scale is degrees Celsius while the left scale is height in terms of pressure in millibars. These graphs are cut off at a low height of only 700mb (roughly 3200m elevation, as shown on the graphs), but data is acquired up to much higher elevations, assuming the balloon does not pop before it should.
Why do the temperature and dew point curves begin at almost 900mb rather than around 1000mb which is a typical sea level pressure? Boise is at a high enough elevation that its surface pressure is almost 900mb, so that is where the readings start.
Note that there is not much variation in the dew point readings. The values at 700mb are about -5°C in both soundings, and the values at the surface range from about 2°C in the morning to 7°C in the afternoon. What does this say about the moisture content of the air on this day? The moisture content of the air remains about the same, with a slight increase near the surface in the afternoon compared to the morning.
Note that between 825mb and 700mb there is virtually no change at all in air temperature. Both in the early morning and afternoon the temperature at 825mb is about 27°C and at 700mb is about 13°C. Above the surface several hundred meters the air temperature remained almost unchanged.
However, from the surface to about 825mb, and particularly closer to the surface there is an extreme change in air temperature. In the early morning the surface temperature is about 19°C, but in the afternoon it is about 38°C. Above the surface several hundred meters the air temperature remained almost unchanged.
In which case is the near-surface (say the lowest 100mb) air stable? The morning case with the inversion (temperature increasing with height) is stable. Colder air is on bottom and warmer air is on top, so the column is "happy". Any air pushed upward from the surface would cool and become even colder than the warming-with-height surroundings and want to sink back down. Likewise a parcel from say 850mb pushed down would warm and become even hotter than the cooling-with-height surroundings.
The air right at the surface in the afternoon case is unstable. There is a temperature decrease of about 3°C of a height range of about 15mb. You may recall that around sea level pressure decreases by about 1mb for every 10m. Thus the environmental lapse rate for that thin layer is about 3°C/150m, which equals about 20°C/km. This is much greater than the dry adiabatic lapse rate, so that air is unstable. It will spontaneously convect to mix heat upward, trying to make the lapse rate stable. The environmental lapse rate may remain unstable because solar energy continues to heat the surface.
Further above the surface the lapse rate is smaller. The temperature decrease from 850mb to 700mb is about 12-15°C, over a distance of about 1700m (as determined from the height values given on the chart). Thus the lapse rate for that layer is ~14°C/1700m, which is ~8.5°C/km. Since the environmental lapse rate is smaller than the dry adiabatic lapse rate and greater than the moist adiabatic lapse rate, this layer is conditionally unstable, meaning it is stable to unsaturated air and unstable to saturated air. However, with such low dew point values air would not be able to cool to saturation and form clouds unless lifted quite high, something that would not happen spontaneously to unsaturated parcels in that conditionally unstable environment.
