An outdoor air cooler with two stages includes a first stage for cooling and humidification with constant wet bulb temperature. The air cooler has a first stage pump circulating water over the first stage evaporative cooling media operating at conditions having a constant adiabatic wet bulb temperature line. The first stage tank is supplied with water from external source. The second stage includes a second set of evaporative cooling media using water from the first stage media, which is equal to or lower than outdoor air wet bulb temperature. The temperature of air leaving the second stage evaporative media follows the adiabatic wet bulb temperature line toward the second stage tank water temperature thereby cooling humidifying the treated air to stable wet and dry bulb temperatures.