Updated: May 1, 2019
Indeed, haze can be problematic for pilots not flying on an instrument flight plan. Add nightfall or flight over a large body of water and you can easily become disoriented when the visibility drops below 7 statute miles. The biggest concern is forward visibility...not being able to see what's directly in front of you. Obstructions like towers and mountain peaks may not be visible until you are right on top of them...especially if you get distracted. Other aircraft are essentially invisible at your flight level once the haze restricts visibility to below six miles. Traffic sensors and ATC radar advisories are critical when flying VFR in hazy conditions.
Haze as it appears in the METAR is just an obstruction to visibility. That is, obscurations are not directly measured by the Automated Surface Observation System (ASOS). Instead, they are inferred by the reported visibility. The other three obscurations are FG (fog), FZFG (freezing fog) and BR (mist). What about smoke, dust and sand? Other obscurations, such as DU (dust), FU (smoke), and SA (sand) are not automatically reported by the ASOS, but may be augmented by a human observer.
To understand the reporting of obscurations, here's how the ASOS automatically determines what to report. Once each minute, the obscuration algorithm checks the reported visibility. When the visibility drops below 7SM the current dewpoint depression (temperature-dewpoint spread) is checked to distinguish between FG, BR, and HZ. If the dewpoint depression is less than or equal to 4°F (~2°C), then FG or BR will be reported. Visibility will then be used to further differentiate between FG and BR (another subject of a future e-Tip). When the dewpoint depression is greater than 4°F and no precipitation is reported, then HZ is reported as the obscuration. When precipitation is reported, HZ is not provided as the obscuration.
Even on the clearest of days, the atmosphere contains significant concentrations of micron and submicron size particles (1,000 microns = 1 mm). These are very tiny and are essentially invisible to the naked eye. A small cloud drop is about 10 microns in diameter and the average human hair is about 100 microns in diameter for comparison. Some of these particles become wetted at a relative humidity less than 100% and typically account for the haze we see that impedes visibility. More hygroscopic particles, called condensation nuclei have an affinity for water and serve as centers for condensation and may grow to cloud drop size as the relative humidity increases or as the air rises, expands and cools to eventually reach saturation.
Negative lapse rates, also known as a temperature inversion, will typically increase the concentrations of particles trapping them in the unstable air below the inversion, thus increasing the haze in the layer below. The inversion prevents these particles from "mixing up" into the air above the inversion keeping the air a bit cleaner aloft. The picture to the left shows the haze between the cumuliform clouds that is trapped in the mixed layer. The key is to identify these inversions and determine their approximate height. Flying above the haze layer does two things. It keeps you in stable (smooth) air and as you can see in the picture, it keeps you above the haze with nearly unlimited visibility. When cumliform clouds like this exist, flying above them is one way to identify the clear and smooth air. However, on days where the sky is cloudlesss, it's actually very easy to find these inversions using a thermodynamic chart called a Skew-T log (p) diagram (see below how you can learn to read this powerful diagram).
More particles due to dirtier air caused by pollution, forest fires, construction, farming, burning, etc. and higher moisture (higher relative humidity) will typically increase the haze. A stronger inversion will also increase the intensity of the haze especially when it is confined to a thin layer just above the surface. One of the reasons the visibility is worse in the morning hours near the surface is due to a nocturnal inversion and typically higher values of relative humidity. The nocturnal inversion is normally very shallow.
One particular inversion that traps pollutants is called a subsidence inversion that occurs in areas of high pressure. As air subsides within the high pressure, it compresses and heats up a bit. This produces an inversion aloft. This subsidence inversion can often occur well into the teens (15,000 feet, for example). Often the water vapor satellite image is a also a good source determine how "dry" the air is aloft. Dryer air aloft as shown in red in this color-enhanced water vapor satellite image on the right generally leads to a less hazy atmosphere when a subsidence inverson exists.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS research meteorologist