Professional Pilot, June 2019

Temperature in degrees Fahrenheit Amount of barometric change in the past 3 hours in tenths of mb Sign showing whether pressure is higher or lower than 3 hours ago 147 28 Example of a station model The filled circle in the middle represents an airport with overcast skies An empty circle on the other hand means the sky is clear Other commonly displayed features include wind speed and direction temperature visibility and types of clouds PROFESSIONAL PILOT June 2019 43 nearby contour lines one can infer a value at a desired location But normally only one or two variables can be contoured on a map before it becomes impossible to read due to the sheer number of different lines For variables that are not necessarily continuous such as cloud cover or precipitation map makers may simply draw boundary lines around areas where those conditions are present One good example is the scalloped areas on some surface analysis charts that indicate IFR weather conditions While these graphical notations help to identify where those conditions are present or absent they do not quantify the conditions such as ceiling height cloud type or rainfall rate Station model Although not as graphically interpretable as contours or shaded regions on the map the station model has long been used by chart makers to convey a great deal of information about what was actually observed at a location and to do so in a very compact way A single station model may contain several dozen bits of information of importance to pilots so its worthwhile being able to read and understand these graphical notations The basic structure of the station model is a circle over the airport The circle is filled in according to the sky cover present An empty circle signifies clear skies while 1 4 1 2 3 4 or full shading represents scattered to overcast skies A single line means few clouds an X means the sky is obscured and an M is used when the observation is missing The 2nd most commonly displayed feature of a station model is wind speed and direction This is shown as a flag pole pointed in the direction from which the wind is blowing and barbs and flags to note the wind speed in knots A short barb is 5 kts and each long barb is 10 kts Strong winds may necessitate the appearance of a flag each of which is 50 kts To determine sustained wind speed simply add up the flags and barbs values When winds are less than 3 kts they are classified as calm This is indicated by a 2nd circle around the first Unfortunately the station model does not have the capacity to report gusty winds Beyond wind and sky cover More information is distributed around the circle Below the circle you may find a symbol that indicates low cloud type Above the circle there is space for 2 cloud type symbols Mid level cloud type will be closest above the circle with high cloud type above it If there are no clouds at a certain level the space for the cloud type is left blank These symbols are also often omitted from station models especially when it would make it difficult to read the other parts of the model You can differentiate no cloud at a given level from omitted information by whether the circle itself indicates the presence of clouds While fog is technically low clouds it is not reported as low cloud but rather is reported in the present weather notation The left of the circle is where you will find from top to bottom temperature Fahrenheit in the US Centigrade elsewhere a symbol to indicate present weather and dew point Although some weather maps will not include it if visibility is reported it appears to the left of the present weather symbol Visibility is given in statute miles on US weather maps and in meters elsewhere These notations are important to pilots because they can give an indication of the saturation of the air as well as the likelihood of freezing rain or icing While these are surface observations it is not difficult to estimate the altitude of the freezing level from surface temperatures The atmosphere cools by around 35 F 2 C per 1000 ft 300 m of altitude while rising unsaturated air cools at 54 F 3 C per 1000 ft until it saturates cools to the dew point To estimate the lowest cloud base divide the dew point depression surface air temperature dew point by 54 F 3 C and multiply that by 1000 ft 300 m To estimate the freezing level subtract 32 F 0 C from the surface temperature divide the answer by 35 F 2 C and multiply by 1000 For example if the temperature is 41 F 5 C with a dew point of 38 F 3 C the freezing level will be around 2500 ft 762 m The base of the lowest clouds would likely be around 550 ft 170 m This information can be helpful if you want to understand when you will break out of the clouds the likelihood of encountering icing conditions and whether you might have room to maneuver if you must descend below the freezing level due to ice accretion at a higher altitude The present weather symbol is also highly useful to pilots This notation indicates the presence of a host of weather conditions that are potentially dangerous to aviation There are many symbols for present weather most of which deal with Wind speed 18 to 22 knots Visibility 3 4 mile Dew point in degrees Fahrenheit Height of cloud base 300 to 599 ft Part of sky covered by lowest cloud seven or eight tenths Direction of wind from the northwest Present weather continuous slight snow in flakes Cloud type low fractostratus and or fractocumulus Cloud type middle altocumulus Barometric pressure at sea level Initial 9 or 10 is omitted 10147mb Barometric tendency in the past 3 hours rising Weather in the past 6 hours rain Amount of precipitation in last 6 hours Cloud type high cirrus Total amount of clouds sky completely covered 34 45 32 2 6 3 4