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Relating Overwater Visibility to Air Pressure or Dew Point Depression: A Case Study during Hurricane Isaac in 2012

S. A. Hsu
Coastal Studies Institute
sahsu@lsu.edu

Abstract

Overwater visibility measurements during Hurricane Isaac show that the visibility decreases almost linearly from 10 miles at approximately 1016hPa to 0.25 miles around 990hPa. The visibility remains at 0.25 miles when the air pressure is below 990hPa. It is found that between 990 and 1016hPa, the visibility is linearly related to the air pressure or dew point depression. For operational applications, formulas are provided for estimating the visibility from air pressure or dew point depression measurements.

According to the Glossary of Weather and Climate (see Geer, 1996), the visibility is the greatest distance from an observer that a permanent object of known characteristics can be seen and identified by unaided, normal eyes. Most often refers to horizontal visibility. According to the User’s Guide to the Automated Surface Observing System (ASOS) (see http://www.nws.noaa.gov/asos/pdfs/aum-toc.pdf ), visibility remains one of the most difficult elements to automate. Sensors can successfully measure atmospheric elements relating to visibility, but it is very difficult to relate these elements to the characteristics of human vision. The physical limitations of human eye and human subjectivity are greatly impacted by precipitation, day and night vision adaptation, contrast, physical obstructions, and perspective. This situation is particularly true during storms at sea because of additional effects by winds, waves, and currents on a moving vessel. Since visibility is not measured commonly as compared to atmospheric pressure or dew point depression (i.e. the difference between the air temperature and the dew point), it is the purpose this brief note to relate overwater visibility to air pressure or dew point depression.

According to Robbie Berg at the National Hurricane Center (see http://www.nhc.noaa.gov/data/tcr/AL092012_Isaac.pdf ), Hurricane Isaac in 2012 was a tropical cyclone that spent most of its life as a tropical storm but became a category 1 hurricane (on the Saffir-Simpson Hurricane Wind Scale) a few hours before making landfall in southeastern Louisiana. During Isaac measurements of visibility, air pressure, and dew point depression are available at an oil platform located at Main Pass 140B (KMIS or MIS, for location see: http://www.ndbc.noaa.gov/station_page.php?station=KMIS). The data are provided online at http://mesonet.agron.iastate.edu/sites/site.php?station=MIS&network=MS_ASOS . The period used in this analysis is from 2055UTC on 26 thru 1715UTC on 28 in August 2012.

Our results are presented in Figures 1 thru Figure 4. Figure 1 shows that the visibility decreases almost linearly from 10 miles at approximately 1016hPa to 0.25 miles around 990hPa. Since the visibility remains at 0.25 miles when the air pressure is below 990hPa, the measurements higher than 990hPa is analyzed in Figure 2, which shows that:

Visibility in miles = 0.45*Air Pressure in hPa–447 (1)

With the coefficient of determination (R2) = 0.91, meaning 91 per cent of the variation in visibility can be explain by Equation 1.

Visibility can also be estimated by the dew point depression as illustrated in Figure 3 and Figure 4. From Figure 3, we have:

Visibility in miles = 0.63* (T air - T dew) in degrees F - 1.15 (2)

With R2 = 0.76, and

Visibility in miles = 1.13 * (T air – T dew) in degrees C - 1.15 (3)

With R2 = 0.76. Note that T air and T dew are air and dew point temperatures, respectively.

On the basis of aforementioned analysis, it is concluded that:

(1) Overwater visibility decreases almost linearly from 10 miles at approximately 1016pHa to 0.25 miles around 990hPa;

(2) The visibility remains at 0.25 miles when the air pressure is below 990hPa;

(3) Between 990 and 1016hPa, the visibility is linearly related to the air pressure as shown in Equation 1 and to dew point depression in degrees Fahrenheit in Equation 2 and in Celsius in Equation 3, respectively.

References

Geer, I. W., Editor, 1996, Glossary of Weather and Climate, American Meteorological Society, Boston, MA.

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Page last modified: October 31, 2023