PRO xytowindcoordinates,wdir,x,y,xwind,ywind

;wdir; degree

; transform wind angle to angle in normal x-y coordiante system

wdirtemp=270.0-wdir
;print,wdirtemp
rad=wdirtemp*3.1415926/180.0


; find a sign mistake in the coordinate transfoamtion in the code of
; /davis/s2/wang/d2wwg/code/fortran/footprintpaper/code/integral-flux.pro
; maybe not, becsaue the different definition of dx

xwind=x*cos(rad)+y*sin(rad)
ywind=-x*sin(rad)+y*cos(rad)

return
end