- the red line is the
(environmental lapse rate)
line represents the dewpoint against height.
Thus at 10,000 feet,
the dewpoint is -22°C.
At the baseline, it is
8°C, ie surface dewpoint 8°C
The dewpoint is the temperature
at which the air can hold no more water vapour, ie it is saturated.
Surface dewpoint is of
On the real sounding on the
next page, the dewpoint line is not green, but is a the solid line on the
two arrows point to brownish/red lines .
These indicate the mass
of water the air can hold at various temperatures.
For example the arrowed
example on the right can hold 12 gms of water/kg of air.
In the colder air on left,
it is much less at 3 gm/kg air.
At the surface dewpoint (green
line intercepting base), the mass of water the air can hold at that temp
As that air rises in a thermal,
it expands, cools (3°/1000) until it becomes saturated and cloud forms.
But - and this is important
- the total water the air is holding has not changed from that it held
at the surface.