def Atmosphere(alt):
    """ Compute temperature, density, and pressure in standard atmosphere.
    Correct to 86 km.  Only approximate thereafter.
    Input:
	alt	geometric altitude, km.
    Return: (sigma, delta, theta)
	sigma	density/sea-level standard density
	delta	pressure/sea-level standard pressure
	theta	temperature/sea-level std. temperature
    """

    REARTH = 6369.0		# radius of the Earth (km)
    GMR = 34.163195
    NTAB = 8			# length of tables

    htab = [ 0.0,  11.0, 20.0, 32.0, 47.0,
	51.0, 71.0, 84.852 ]
    ttab = [ 288.15, 216.65, 216.65, 228.65, 270.65,
	270.65, 214.65, 186.946 ]
    ptab = [ 1.0, 2.2336110E-1, 5.4032950E-2, 8.5666784E-3, 1.0945601E-3,
	6.6063531E-4, 3.9046834E-5, 3.68501E-6 ]
    gtab = [ -6.5, 0.0, 1.0, 2.8, 0, -2.8, -2.0, 0.0 ]

    h = alt*REARTH/(alt+REARTH)	# geometric to geopotential altitude

    i=0; j=len(htab)
    while (j > i+1):
	k = (i+j)/2
	if h < htab[k]:
	    j = k
	else:
	    i = k
    tgrad = gtab[i]		# temp. gradient of local layer
    tbase = ttab[i]		# base  temp. of local layer
    deltah=h-htab[i]		# height above local base
    tlocal=tbase+tgrad*deltah	# local temperature
    theta = tlocal/ttab[0]	# temperature ratio

    if 0.0 == tgrad:
	delta=ptab[i]*math.exp(-GMR*deltah/tbase)
    else:
	delta=ptab[i]*math.pow(tbase/tlocal, GMR/tgrad)
    sigma = delta/theta
    return ( sigma, delta, theta )