We report results on the influence of relative humidity on the propagation speed, the intensity of the emitted light, the energy and the gas temperature of a pin-to-plane nanosecond pulsed discharge at atmospheric pressure in synthetic air. The discharge is generated under very high overvoltage (several tens of kV) so that it propagates with a voluminous, diffuse, and stable pattern. It is shown that the water vapor content has a strong impact on the discharge dynamics for gas mixtures with high relative humidity and for the highest electric field values. In particular, for voltage pulse amplitudes higher than 65 kV and relative humidity higher than 30%, the propagation abruptly slows down and the light intensity profiles show a stronger emission at the pin which weakens in the rest of the gap. The electric energy is slightly lower in humid air, independently of water vapor concentration. Also, time and spatially resolved gas temperature measurements carried out for different voltages show a late and significant heating at the pin whatever the water vapor content. An evaluation of the energy consumed in fast heating processes is proposed, showing an increased energy consumption at the pin in highly humid air. Besides, the hypotheses allowing for the consideration of the rotational temperature of the second positive system of nitrogen (N2(SPS)) as the gas temperature under high electric field conditions are discussed.