On some days the visibility outside is just magnificent. This magnificence includes crystal blue skies, the ability to see objects far in the distance and the air smelling fresh. These clear days have a few things in common. Several of these will be discussed. a. LOW DEWPOINT and LOW TROPOSPHERIC RH- With a low relative humidity throughout the troposphere, water vapor can not condense (even on condensation nuclei which can support condensing when RH is somewhat less than 100%). This relative low abundance of water vapor and lack of condensation takes the white hue out of the sky. It makes the sky appear bluer and increases horizontal and vertical visibility. RH can lower when a deep layer of subsidence occurs. Sinking air decreases in relative humidity over time. If air near the surface is blowing from a continental source, that will also help keep the troposphere dry when subsidence occurs. b. LACK OF PARTICULATES- Air with less particulates will be clearer. Particulates include smog, sea salt, pollen and dust. Particulates will make the sky appear milkier and will decrease the visibility. Areas near large cities, humid ocean regions and windy dry areas are prone to particulates. Interior vegetated rural areas often have enhanced visibility on days with subsidence in the troposphere. c. CONTINENTAL COLD FRONT- Continental cold fronts bring drier air into a region. The CAA (Cold Air Advection) behind the front can enhance subsidence. This results in a dry troposphere at a certain distance behind the frontal boundary if only a subsidence pattern is in place. The cool air also mixes down to the surface during the day. This fresh clean air of the middle troposphere significantly reduces the number of particulates in the lower troposphere once it mixes down to the surface to replace the previous air. |