We report experiments on a pump-probe configuration to elucidate the formation of a plasma channel by the hydrodynamic evolution of air breakdown in laser focus. A stable air breakdown was produced by focusing a picosecond laser pulse to create a shock driven plasma channel in the laser focus for propagating a nanosecond pulse. A four fold increase in the lifetime of the channel estimated by monitoring the temporal evolution of the fluorescence of a spectral line at 504.5 nm of N+ transition 3p 3S-3s 3P0 is reported. Assuming plasma in local thermal equilibrium plasma temperature of ∼8.2 eV and an electron density of ∼1.4 × 1018 cm-3 were determined using a Stark broadening of 649.2 nm line of NII transition 3d 3D 0-4p3D in the channel. An enhancement in the electron density of the plasma channel was observed at the 7 ns delay of the nanosecond laser pulse relative to the picosecond laser pulse. © 2007 American Institute of Physics.