Calculate the humidex

humidex.rb

#!/usr/bin/env ruby

# estimate the dewpoint temperature
def dew_point(temp, rv)
  e = 6.1121 * (10 ** (7.502 * temp / (237.7 + temp)))
  p = (rv * e) / 100.0
  dp = (-430.22 + 237.7 * Math.log(p))/(-(Math.log(p)) + 19.08); 
end

# estimate the wet bulb temperature
def wet_bulb(temp, rv)
  ex = 0.6108*Math.exp((17.27*temp)/(temp+237.3))
  ex_h = ex * rv / 100
  d_p = 237.3*(((Math.log(ex_h/0.6108))/17.27)/(1-((Math.log(ex_h/0.6108))/17.27)));
  prem = (4099*ex)/((temp+237.3)**2)
  seco = (4099*ex_h )/((d_p+237.3)**2)
  third = (prem + seco) / 2;
  wb = temp - ( ex - ex_h ) / (third + 0.00066 * 101.325)
  wb + 1
end

temp = ARGV.shift.to_f
rv   = ARGV.shift.to_f

puts "Estimates:"
puts "\tDew point : #{dew_point(temp, rv)}"
puts "\tWet bulb  : #{wet_bulb(temp, rv)}"


# TODO parse opts for this
motion    = 0    # air motion, 0 = none, -0.2 some, -0.4 fair
radiation = 2.1  # heath radiation, 0 = none, 2.1 = some, 4.5 = strong, 
                 # 3.1 = direct sunlight
clothing  = 0    # 0 = summer clothing, 3.5 overall, 5 double overall

black_globe = temp + radiation + clothing + motion
wbg_temp    = 0.7 * wet_bulb(temp, rv) + 0.3 * black_globe

puts "\tWBGT      : #{wbg_temp}"


metabolism = 1  # 0 light work, 1 = moderate work, 2 = heavy work,
                # 3 = extra heavy work
acclimated = 0  # 0 = not acclimated, 1 = acclimated


conditions = case acclimated
when 0
  case metabolism
  when 0
    { 27.5 => "OK",                  29 => "75% work - 25% rest",
      30   => "50% work - 50% rest", 31 => "25% work - 75% rest" }

  when 1
    { 25 => "OK",                  26.5 => "75% work - 25% rest",
      28 => "50% work - 50% rest", 29   => "25% work - 75% rest" }

  when 2
    { 22.5 => "OK",                  24.5 => "75% work - 25% rest",
      26.5 => "50% work - 50% rest", 28   => "25% work - 75% rest" }

  when 3
    { 25   => "50% work - 50% rest", 
      26.5 => "25% work - 75% rest" }

  end
when 1
  case metabolism
  when 0
    { 29.5 => "OK",                  30.5 => "75% work - 25% rest",
      31.5 => "50% work - 50% rest", 32.5 => "25% work - 75% rest" }

  when 1
    { 27.5 => "OK",                  28.5 => "75% work - 25% rest",
      29.5 => "50% work - 50% rest", 31   => "25% work - 75% rest" }

  when 2
    { 26   => "OK",                  27.5 => "75% work - 25% rest",
      28.5 => "50% work - 50% rest", 30   => "25% work - 75% rest" }

  when 3
    { 27.5 => "50% work - 50% rest", 
      29.5 => "25% work - 75% rest" }

  end
end

condition = nil
conditions.keys.sort.each do |index|
  if wbg_temp <= index
    condition = conditions[index]
    break
  end
end

condition ||= "STOP WORK"

puts "\nConditions:\n\t#{condition}"