[mmaimcal] wind thresholds

[Simon Radford]

Prompted by John Conway, I have prepared four figures showing what
fraction of the time the wind is below threshold values of 6, 9, 16, and
20 m/s for each hour of each month. The data cover 56 months, 1995 April
to 1999 November. As we know, the wind is generally higher during the
winter and during the afternoon. In detail:

1) The antenna stow threshold, 20 m/s, is exceeded only rarely.

2) The threshold to halt antenna transport, 16 m/s, is exceeded often
only in the winter daytime. During the windiest times, this threshold is
exceeded 30-40% of the time.

3) The specified nighttime pointing condition, 9 m/s, is seldom exceeded
at night, but frequently during the day. During the windiest month on
record, 1997 June, this threshold was exceeded at night about half the
time.

4) The specified daytime pointing condition, 6 m/s, is frequently
exceeded during the afternoon in all seasons. During most afternoons,
the wind is almost always stronger than this.

The figures show UT. As a reminder, local Solar time is UT - 4.5 h.
Chilean civil time is, I believe, UT - 4 h in the winter and UT - 3 in
the summer.

I welcome your comments.

Simon

http://www.tuc.nrao.edu/alma/site/Chajnantor/overall/threshold-windspeed-6.gif
http://www.tuc.nrao.edu/alma/site/Chajnantor/overall/threshold-windspeed-6.ps
http://www.tuc.nrao.edu/alma/site/Chajnantor/overall/threshold-windspeed-9.gif
http://www.tuc.nrao.edu/alma/site/Chajnantor/overall/threshold-windspeed-9.ps
http://www.tuc.nrao.edu/alma/site/Chajnantor/overall/threshold-windspeed-16.gif
http://www.tuc.nrao.edu/alma/site/Chajnantor/overall/threshold-windspeed-16.ps
http://www.tuc.nrao.edu/alma/site/Chajnantor/overall/threshold-windspeed-20.gif
http://www.tuc.nrao.edu/alma/site/Chajnantor/overall/threshold-windspeed-20.ps