def ddx(self,s,lat,lon):

    lonLen = len(lon)
    latLen = len(lat)
    dsdx = np.empty((latLen,lonLen))
    rearth = 6371221.3
    # Input data is oriented N-S. Hence invert it. It is oriented W-E.
    s = s[::-1,;]
    # Differential increment along X direction. Equal grid spacing
    # Hence is it just the difference in longitudes in radians.
    # Convert into meters by multiplying the radius of earth
    di = abs(np.cos(np.radians(0.0))*rearth*(np.radians(lon[1]-lon[0])))

    # GRID order for loop- S-N(OUTER)
    #                      W-E(INNER)
    # Loop east along a row then north
    for j in range(0,latLen):
        for i in range(1, lonLen-1):
            if (abs(lat[j]) >= 90.0):
                dsdx[j,0] = 0.0
            elif (s[j, i+1] > -999.99 and s[j,i-1] > -999.99):
                # ds/dx = s[east] - s[west]/2*di
                dsdx[j, i] = (s[j,i+1] - s[j,i-1])/(2.*di)
            elif (s[i+1,j] < -999.99 and s[j,i-1] > -999.99 and s[j,i] > -999.99):
                dsdx[j,i] = (s[j,i] - s[j,i-1])/di
            elif (s[j,i+1] > -999.99 and s[j,i-1] < -999.99 and s[j,i] > -999.99):
                dsdx[j,i]  = (s[j,i+1] - s[j,i])/di
            else:
                dsdx[j,i] = -999.99
    # Check if the data is global and compute difference at the beginning
    # and end of row J
    for j in range(0,latLen):
        if (abs(lat[j]) >= 90.0):
            dsdx[j,0] = 0.0
            dsdx[j,-1] =0.0
        elif (np.allclose(2*lon[0]-lon[-1],lon[1],1e-3) or np.allclose(2*lon[0]-lon[-1],lon[1] + 360.0,1e-3)):
            if (s[j, 1] > -999.99 and s[j, -1] > -999.99):
                dsdx[j, 0] = (s[j, 1] - s[j,-1]) / (2.*di)
            elif (s[j,1] < -999.99 and s[j,-1] > -999.99 and s[j,0] > -999.99) :
                dsdx[j,0] = (s[j,1] - s[j,-1]) / di
            elif (s[j, 1] > -999.99 and s[j,-1] > -999.99 and s[j,0] > -999.99):
                dsdx[j,0] = (s [j,1] - s[j,0]) /di
            else:
                dsdx[j, 0] = -999.99
            if (s[j,0] > -999.99 and s[j,-2] > -999.99):
                dsdx[j,-1] = (s[j, 0] - s[j,-2]) / (2. * di)
            elif (s[j,0] < -999.99 and s[j,-2] > -999.99 and s[j,-1] > -999.99):
                dsdx[j,-1] = (s[j,-1] - s[j,-2]) / di
            elif (s[j,0] > -999.99 and s[j,-2] < -999.99 and s[j,-1] > -999.99) :
                dsdx[j,-1] = (s[j,1] - s[j,-1]) / di
            else:
                dsdx[j, -1] = -999.99
        elif (np.allclose(lon[0],lon[-1],1e-3)):
            if (s[j, 1] > -999.99 and s[j,-2] > -999.99) :
                dsdx[j,0] = (s[j,1] - s[j,-2]) / (2. * di)
            elif (s[j,1] < -999.99 and s[j,-2] > -999.99 and s[j,0] > -999.99) :
                dsdx[j,0] = (s[j,0] - s[j,-2]) / di
            elif (s[1, j] > -999.99 and s[- 2, j] < -999.99 and s[0, j] > -999.99):
                dsdx[j,0] = (s[j,1] - s[j,0]) / di
            else:
                dsdx[j,0] = -999.99
            dsdx[j,-1] = dsdx[j,0]
        else:
            if (s[j, 1] > -999.99 and s[j,0] > -999.99) :
                dsdx[j,0] = (s[j,1] - s[j,0]) /di
            else:
                dsdx[j,0] = -999.99

            if (s[j,-1] > -999.99 and s[j,-2] > -999.99):
                dsdx[j,-1] = (s[j,-1] -s[j,-2]) /di
            else:
                dsdx[j,-1] = -999.99

    return dsdx