At 22:43 10/10/2005, Graeme wrote:
Robert Hart wrote:
In response to several questions, I have written an article that
shows how to obtain and interpret (in a gliding friendly way) the
forecast atmospheric soundings available from the US National
Oceanic and Atmospheric Administration (NOAA).
The article is at
http://www.hart.wattle.id.au/alice/gliding/SkewT-logPexplain.html
I hope this is useful to other people.
Robert,
Thanks for your article. I assume that to find the origin of the dry
adiabatic line to slide up, it is necessary trace down the red lines
from the red numbers on the righthand edge to their intersection
with the 1050 mBar line and extrapolate the position of the required
DALR black line. If so, what are the black numbers across the top
and down the left side, ie 440, 430, .....260, 250. on the ends of
the DALR lines? Am I missing something?
Thanks,
Graeme
Yes you are, Graeme.
For some odd reason, the SALR lines are labelled with their CELSIUS
temperatures whilst the corresponding DALR lines are labelled with
their KELVIN temperatures. (At least you know from the label what
kind of line it is - little numbers for SALR and big numbers for DALR.)
Because they are based on the International Standard Atmosphere, the
label temperatures are for the 1013.2 hPa (millibar)
level. Following up the lines from bottom to top shows how a parcel
of air will cool if it is caused to rise and no heat is gained or
added from outside the airmass itself (this is what 'adiabatic' means).
To predict the cooling of any given mass of air, you start from the
conditions you have at your starting point (temperature and
pressure). From the temperature trace line (red 'squiggly') you
follow parallel to the nearest dry adiabatic line (up and left) until
you meet the short black mixing ratio line (up and right from the
dewpoint line at your starting level). Where these two lines meet
gives the condensation level (temperature and pressure, hence
height). Further upwards movement causes the air to cool along the
SALR line through the intersection point. Water vapour condenses
from the air as it rises, and so cloud forms.
However, if the only cause of the air rising in the first place is
that it has been heated, it will only rise as far as the point where
it stays above the red squiggly line (plus a little bit further that
its momentum will carry it). Once it reaches the red line it is no
longer buoyant, so it stops rising.
I'll stop here and let somebody else explain cloud tops, spread-out,
rain formation and the several other things that can be deduced from
the graphs.
However, just a reminder that the sounding only gives a valid
prediction in the same airmass - so if there is a new airmass moving
in, don't expect an early morning temp trace to predict the
conditions in the airmass that was different to when you measured its
temperature and humidity!
Wombat
_______________________________________________
Aus-soaring mailing list
[email protected]
To check or change subscription details, visit:
http://lists.internode.on.net/mailman/listinfo/aus-soaring
