In this paper, atmospheric pressure nanosecond pulsed discharges in pin-to-pin geometry are very easily reproducible by applying a positive overvoltage, and such discharge system is placed in a sealed chamber filled with high purity argon gas. A continuum radiation model for the atmospheric pressure discharges is proposed to diagnose the electron temperature of the nanosecond pulsed argon discharges. The high voltage and current probes monitor the voltage and current waveforms during the discharge, and the discharge pulse width is about 20 ns. The time-resolved emission spectra of the discharge column at different times (0<t<20 ns) are measured by the combination of optical systems, such as achromatic lens, monochromator and ICCD. The results indicate that the continuum emission intensity of the discharge increases with time during the period of 0<t<10 ns, and then decreases during 10 ns<t<20 ns. However, the intensity of argon lines always increases with time. As the intensity of continuum emission is positively correlated to the electron density, the electron density also increases firstly and then decreases, which has the same tendency as the discharge current. According to the continuum radiation model, the electron temperature during the discharge (0<t<10 ns) is measured to be (1.4±0.2) eV. As the driven voltage drops (10 ns<t<20 ns), the electron temperature decreases gradually to 0.9 eV. Our research suggests that the excited argon atoms are mainly populated by electron impact excitation during 0<t<10 ns, and thus their emission intensities increase with the electron density. Afterwards, due to the decreasing of electron temperature, the rate of ${Ar_{2}}^{+}$ ions recombination reaction increases dramatically. The production of excited atoms is governed by the electronion recombination processes, leading to increase their emission intensities further. The virotational spectrum of OH species is detected by adding 0.5% water vapor into the working gas. It is found that the production mechanisms of OH(A) make it deviated from Boltzmann distribution. In this work, a two-rotational temperatures OH(A-X) spectral model is employed to examine the gas temperature. During the discharge pulse, the gas temperature remains invariant around the value of 400 K. Moreover, the addition of water vapor causes the increase of the intensity of the continuum in the short wavelength range. It is analyzed that H₂ could be produced by the dissociation of H₂O in the discharge and then excited to the excited state H₂($a^{3} \sum^{+}_{g}$) by means of the energy transfer reaction from argon atoms in a metastable state. Finally, H₂($a^{3} \sum^{+}_{g}$) decays by spontaneous radiation to the repulsion state H₂($b^{3} \sum^{+}_{u}$) and emits the short-wavelength continuum emission. The electron temperature (T_e>1 eV) is very sensitive to the short wavelength response of the continuum spectrum. So even if the working gas contains a small amount of water vapor, it will greatly influence the electron temperature diagnosed by the continuum radiation.