Page 221 - Planning and Design of Airports

186 Airp o r t D e sign

Once the maximum permissible crosswind component is selected,
the most desirable direction of runways for wind coverage can be
determined by examination of the average wind characteristics at the
airport under the following conditions:

1. The entire wind coverage regardless of visibility or cloud ceiling
2. Wind conditions when the ceiling is at least 1000 ft and the
visibility is at least 3 mi
3. Wind conditions when ceiling is between 200 and 1000 ft
and/or the visibility is between ½ and 3 mi.

The first condition represents the entire range of visibility, from
excellent to very poor, and is termed the all weather condition. The
next condition represents the range of good visibility conditions not
requiring the use of instruments for landing, termed visual meteoro-
logical condition (VMC). The last condition represents various degrees
of poor visibility requiring the use of instruments for landing, termed
instrument meteorological conditions (IMC).
The 95 percent criterion suggested by the FAA and ICAO is appli-
cable to all conditions of weather; nevertheless it is still useful to
examine the data in parts whenever this is possible.
In the United States, weather records can be obtained from the
Environmental Data and Information Service of the National Climatic
Center at the National Oceanic and Atmospheric Administration
located in Ashville, N.C., or from various locations found on the
Internet.
Weather data are collected from weather stations throughout the
United States on an hourly basis and recorded for analysis. The data
collected include ceiling, visibility, wind speed, wind direction,
storms, barometric pressure, the amount and type of liquid and frozen
precipitation, temperature, and relative humidity. A report illustrat-
ing the tabulation and representation of some of the data of use in
airport studies was prepared for the FAA [15]. The weather records
contain the percentage of time certain combinations of ceiling and
visibility occur (e.g., ceiling, 500 to 900 ft; visibility, 3 to 6 mi), and the
percentage of time winds of specified velocity ranges occur from dif-
ferent directions (e.g., from NNE, 4 to 7 mi/h). The directions are
referenced to true north.

The Wind Rose
The appropriate orientation of the runway or runways at an airport
can be determined through graphical vector analysis using a wind
rose. A standard wind rose consists of a series of concentric circles cut
by radial lines using polar coordinate graph paper. The radial lines
are drawn to the scale of the wind magnitude such that the area
between each pair of successive lines is centered on the wind direction.