# Horizontal helicityΒΆ

The example to calcluate horizontal helicity.

```print 'Open data files...'
f_uwnd = addfile('D:/Temp/nc/uwnd.2011.nc')
f_vwnd = addfile('D:/Temp/nc/vwnd.2011.nc')

print 'Calculate average wind field from 850 to 600 hpa...'
tidx = 173    # Jun 23, 2011
t = f_uwnd.gettime(tidx)
level = '850:600'    # 850 - 600 hPa
lat = '15:55'
lon = '70:135'
uvar = f_uwnd['uwnd']
vvar = f_vwnd['vwnd']
uwnd = uvar[tidx,level,lat,lon]
vwnd = vvar[tidx,level,lat,lon]
uc = uwnd.ave()
vc = vwnd.ave()
sp = magnitude(uwnd, vwnd)
speed = sp.ave()
#speed = magnitude(uc, vc)
direc = atan2(vc, uc)
cdirec = direc-40./180.*3.14159
if cdirec < -3.14159:
cdirec=cdirec+3.14159*2
cuc = speed*cos(cdirec)
cvc = speed*sin(cdirec)

print 'Calculate horizontal helicity...'
Hrs = 0.
zHrs = 0.
hh = 6
while hh <= 10:
zHrs = (uvar[tidx,hh+1,lat,lon]-cuc)*(vvar[tidx,hh,lat,lon]-cvc)- \
(uvar[tidx,hh,lat,lon]-cuc)*(vvar[tidx,hh+1,lat,lon]-cvc)
Hrs = Hrs+zHrs
hh += 1
Hrs = Hrs[::-1,:]

print 'Plot...'
axesm()
geoshow('country', edgecolor='black')
layer = contourf(Hrs, 20)
title('Horizontal helicity (' + t.strftime('%Y-%m-%d') + ')')
colorbar(layer)
```