Lesson 2: Avoiding Winter-Weather Hazards in the Terminal Area

 This lesson is about avoiding winter weather hazards in the terminal or airport area. While we will cover a lot, we won't cover it all. For our international students, this lesson is about winter weather detection, reporting, and forecasting in the National Airspace System (NAS) of the United States. Several weather organizations from around the world will join us later on in the course to supplement this material with the weather services in other countries.

How do we as aviators and operations personnel know about winter weather hazards in the terminal or airport area? In this lesson will cover the basic ways an aviator will be told about the weather either in written or verbal form.

All aviation weather begins at the airport.

The first step in the process of creating aviation weather is the observation. This report is also one of the first clues that the airport you are using has a thunderstorm impacting it. Before we even worry about the encoding or decoding language, we have to understand the things a observer looks for when an observation is taken. Only then can we understand exactly what the observer is trying to communicate when he or she mentions gusting winds, falling pressures, and the other thunderstorm clues. The following sections come directly from the Federal Meteorological Handbook Number 1, the Weather Observers ultimate guide.


5.4.1 Wind Direction. The wind direction shall be determined by averaging the direction over a 2-minute period. When the wind direction sensor(s) is out of service, at designated stations, the direction may be estimated by observing the wind cone or tee, movement of twigs, leaves, smoke, etc., or by facing into the wind in an unsheltered area.

5.4.2 Variable Wind Direction. The wind direction may be considered variable if, during the 2-minute evaluation period, the wind speed is 6 knots or less. Also, the wind direction shall be considered variable if, during the 2-minute evaluation period, it varies by 60 degrees or more when the average wind speed is greater than 6 knots.

5.4.3 Wind Speed. The wind speed shall be determined by averaging the speed over a 2-minute period. At designated stations, Table 5-1 shall be used to estimate wind speeds when instruments are out of service or the wind speed is below the starting speed of the anemometer in use.

5.4.4 Wind Gust. The wind speed data for the most recent 10 minutes shall be examined to evaluate the occurrence of gusts. Gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knots or more between peaks and lulls. The speed of a gust shall be the maximum instantaneous wind speed.

5.4.5 Peak Wind Speed. Peak wind data shall be determined with wind speed recorders. The peak wind speed shall be the maximum instantaneous speed measured since the last routine observation.

5.4.6 Wind Shifts. Wind data shall be examined to determine the occurrence of a wind shift. A wind shift is indicated by a change in wind direction of 45 degrees or more in less than 15 minutes with sustained winds of 10 knots or more throughout the wind shift.

5.5.5 Peak Wind Data. The peak wind shall be reported in the remarks section whenever the maximum instantaneous speed in knots (since the last observation) is greater than 25 knots

Visibility Parameters

The visibility parameters are:

Prevailing visibility. The visibility that is considered representative of visibility conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.

Sector visibility. The visibility in a specified direction that represents at least a 45 degree arc of the horizon circle.

Surface visibility. The prevailing visibility determined from the usual point of observation.

Tower visibility.
The prevailing visibility determined from the airport traffic control tower (ATCT) at stations that also report surface visibility.
Table 6-2. Summary of Visibility Observing and Reporting Standards and Procedures
Type of Station
Surface Represents 10-minutes of sensor outputs Visual evaluation of visibility around horizon
Variable Reported when the prevailing visibility varies by 1/2 mile or more and the visibility is less than 3 miles. 
Tower Augmented Reported at stations with an ATCT
Sector Not reported Reported at all stations

Present Weather

Drizzle. Fairly uniform precipitation composed exclusively of fine drops with diameters of less than 0.02 inch (0.5 mm) very close together. Drizzle appears to float while following air currents, although unlike fog droplets, it falls to the ground.

Rain. Precipitation, either in the form of drops larger than 0.02 inch (0.5 mm), or smaller drops which, in contrast to drizzle, are widely separated.

Hail. Precipitation in the form of small balls or other pieces of ice falling separately or frozen together in irregular lumps.

Small Hail and/or Snow Pellets. Precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm).

Snow. Precipitation of snow crystals, mostly branched in the form of six-pointed stars.

Snow Grains. Precipitation of very small, white, and opaque grains of ice.

Ice Crystals (Diamond Dust). A fall of unbranched (snow crystals are branched) ice crystals in the form of needles, columns, or plates.

Ice Pellets. Precipitation of transparent or translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2 inch (5 mm), or less.

There are two main types:

(1) Hard grains of ice consisting of frozen raindrops, or largely melted and refrozen snowflakes.

(2) Pellets of snow encased in a thin layer of ice which have formed from the freezing, either of droplets intercepted by the pellets, or of water resulting from the partial melting of the pellets.

Unknown Precipitation. Precipitation type that is reported if the automated station detects the occurrence of light precipitation but the precipitation discriminator cannot recognize the type.

Present weather qualifiers fall into two categories: intensity or proximity and descriptors. Qualifiers may be used in various combinations to describe weather phenomena.

Intensity/Proximity. The intensity qualifiers are: light, moderate, and heavy. The proximity qualifier is vicinity.

Intensity of Precipitation. When more than one form of precipitation is occurring at a time or precipitation is occurring with an obscuration, the intensities determined shall be no greater than that which would be determined if any forms were occurring alone.

The intensity of precipitation shall be identified as light, moderate, or heavy in accordance with one of the following:

Intensity of Rain or Ice Pellets. The intensity of rain and ice pellets shall be based on the criteria given in Table 8-1, Table 8-2
Table 8-1. Intensity of Rain or Ice Pellets Based on Rate-of-Fall
Light Up to 0.10 inch per hour; maximum 0.01 inch in 6 minutes
Moderate 0.11 inch to 0.30 inch per hour; more than 0.001 inch to 0.03 inch in 6 minutes.
Heavy More than 0.30 inch per hour; more than 0.03 inch in 6 minutes.

Table 8-2. Estimating Intensity of Rain
Light From scattered drops that, regardless of duration, do not completely wet an exposed surface up to a condition where individual drops are easily seen.
Moderate Individual drops are not clearly identifiable; spray is observable just above pavements and other hard surfaces.
Heavy Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to height of several inches is observed over hard surfaces.

Intensity of Snow and Drizzle. The intensity of snow and drizzle shall be based on the reported surface visibility in accordance with Table 8-4 when occurring alone.


Table 8-4. Intensity of Snow or Drizzle Based on Visibility
Intensity Criteria
Light Visibility > ½ mile.
Moderate Visibility > ¼ mile but < = ½ mile
Heavy Visibility < = ¼ mile.
Descriptors. Descriptors are qualifiers which further amplify weather phenomena and are used with certain types of precipitation and obscurations. The descriptor qualifiers are: shallow, partial, patches, low drifting, blowing, shower(s), thunderstorm, and freezing.

Beginning/Ending Times of Precipitation
. At designated stations, the time precipitation begins or ends shall be reported to the nearest minute. The beginning and ending times shall be reported in the next observation after the event. Beginning and ending times for separate periods shall be reported only if the intervening time exceeds 15 minutes

Low Drifting. When dust, sand, or snow is raised by the wind to less than 6 feet, "low drifting" shall be used to further describe the weather phenomenon.

Blowing. When dust, sand, snow, and/or spray is raised by the wind to a height of 6 feet or more, "blowing" shall be used to further describe the weather phenomenon.

Freezing. When fog is occurring and the temperature is below 0°C, "freezing" shall be used to further describe the phenomena. When drizzle and/or rain freezes upon impact and forms a glaze on the ground or other exposed objects, "freezing" shall be used to further describe the precipitation. THINK OF SLEET AS FZRA!


Pressure Change (Rising/Falling).
At designated stations, the pressure calculated for each report shall be examined to determine if a pressure change is occurring. If the pressure is rising or falling at a rate of at least 0.06 inch per hour and the pressure change totals 0.02 inch or more at the time of the observation, a pressure change remark shall be reported

Remarks about snow:

Snow Increasing Rapidly (SNINCR_[inches-hour/inches on ground]). At designated stations, the snow increasing rapidly remark shall be reported, in the next METAR, whenever the snow depth increases by 1 inch or more in the past hour. The remark shall be coded in the format, SNINCR [inches-hour/inches on ground], where SNINCR is the remark indicator, inches-hour is the depth increase in the past hour, and inches on ground is the total depth of snow on the ground at the time of the report. The depth increase in the past hour and the total depth on the ground are separated from each other by a solidus "/". For example, a snow depth increase of 2 inches in the past hour with a total depth on the ground of 10 inches would be coded "SNINCR 2/10".

Snow Depth on Ground (4/sss). At designated stations, the total snow depth on the ground group shall be coded in the 0000 and 1200 UTC observation whenever there is more than a trace of snow on the ground. It shall be coded in the 0600 and 1800 UTC observation if there is more than a trace of snow on the ground and more than a trace of precipitation (water equivalent) has occurred within the past 6 hours. The remark shall be coded in the format, 4/sss, where 4/ is the group indicator and sss is the snow depth in whole inches using three digits. For example, a snow depth of 21 inches shall be coded as "4/021".


Icing is indicated by the 6 leading number in the remarks section, which is followed by five trailing digits. Digit 2 determines the kind of icing with the following codes: (0-none or trace; 1-light mixed; 2-light rime; 3-light clear; 4-moderate rime; 5-moderate-clear; 6-severe mixed; 7-severe rime; and, 8-severe clear). The next three digits tell the base of the icing in hundreds of feet AGL. The final digit is the thickness in thousands of feet.
So, these are the criteria for reporting weather phenomena associated with winter weather. So how will the observer make the observation? And how will the observer report the weather to you?

There are actually several different ways.

Many of us in the civilian world of the United States have an automated observing system (AOS) at our airport. Of course there are different systems out there and there are different uses for these systems. But no matter which type, there are many with limitations right now for sensing thunderstorms in the terminal area.

The FAA and NWS have several different types of AOS. There are AWOS and there are ASOS.
Go to the FAA automated observing website and find out what each of these systems can or can't do about thunderstorms. Both AWOS and ASOS are covered here. What kind of system do you have at your airport?
For more information on the ASOS and its capability to report the weather, look at the Pilots' Guide to ASOS by the NWS. You can also see a picture of the sensor package of the ASOS.

Not every ASOS produces the same kind of observation. At the most congested and the ones most impacted by weather, humans provide the missing information like some of the detail on freezing precipitation. There are four levels of augmentation. What are the levels and what kind of augmentation is at your airport? Also check out this link. You can also read about it in the FAA's AIM in the Resource Menu.

For the military (but this is changing) and some small airports, human observation is still the only report you can get. Did you ever wonder how a machine compares to a human? So did the pilots unions, industry, and the FAA. Here are the results of one study. Some important differences between just machine and just human...

The human observes the whole sky dome and reports on it. The machine with a "cloud height" indicator ONLY looks up every minute to describe the sky. The human looks at a distant object for visibility, the machine uses light scattering over two foot distance to determine prevailing visibility on the whole airport. The human requires 20 minutes to do a whole observation while the machine can do one in several minutes.

Now lets give you a chance to review or learn the written weather code that comes from an observation, and is used to describe the forecasted weather for an airport.

Before we begin, please let me address the question of why pilots need to know how to read METAR/TAF code. Granted that the original reason for encoded weather reports was because of limited ability to transmit observations by teletype. Yes, the computer has made that reason obsolete when it comes to giving you the information you need to fly. But there is still limited bandwidth on the lines that collect the observations from all over the United States and the world. In the foreseeable future that will require METAR code for weather folks. Even more important to you, the pilot, is the fact that we will soon be data linking weather information directly to aircraft. Almost all commercial carriers in the United States use the ACARS radio system which can provide digital character messages to airborne or taxiing flights. Right now the United State's air traffic control system is preparing to provide flight-information services (FIS) like weather via digital means. The USAF will be datalinking weather information in the future. There is only so much radio bandwidth available and costs of these services are based on the number of characters sent. If you need one more reason, how about the fact that reading encoded observations is the most efficient means of gleaning a word-picture of the status of the airport. Take it from someone who reads digitally presented ATIS messages for a living, it is a lot easier to find out one bit of information (like the code SN in its specific place) if it is presented in a logical sequence with the minimum number of characters rather than have to read words until you come across the word "thunderstorm" imbedded in the message. By the way, if you want to get a United States Air Transport Pilot (ATP) rating, the Federal Aviation Regulation (FAR) says that you will need to be familiar with "all codes in the Federal Meteorological Handbook."

Okay, you get to choose. Please read one or more of the following presentations of the METAR/TAF codes and look at the ways that are used to describe the phenomena we learned about earlier in this lesson. When it comes to reading the TAF forecasts, PLEASE use the study questions on the left for this lesson to make sure you know some of the finer details of the code.

You can read about it in the Pilot's Handbook of Aeronautical Knowledge. Look in Chapter 11.

You can look in the Federal Meteorological Handbook.

You can review the quick reference on METAR/TAF codes.

Alright, can you decipher these codes?

If not, go back to the quick reference card.

SN or SG
GR or GS
PL (This is a change from PE which was deleted several years ago...)

By the way, what criteria will change an observation? In the US there are several...

Ceiling changes between the values of 3,000 - 1500 - 1000 - 500 - Lowest approach mins
Volcanic eruptions
Sky condition that obscures descends below 1000 ft
Aircraft mishap
Wind shift (do you remember the definition?)
Visibility changes between the values of 3 miles - 2 miles - 1 - lowest approach min (1/2 mi)
RVR (runway visibility range) goes above or below 2400 feet visibility
Tornado seen or disappears
Thunderstorm begins or end
Precipitation (hail, freezing precip, ice pellets) begin or end

INTERESTING FACT: The ASOS CANNOT make a change to the observation (make a SPECIAL OBSERVATION) for any reason between 46 and 53 minutes past the hour. This is the time it is doing a regular observation. Even if you had a tornado heading straight for the airport or on the airport, there is no way for anyone to override this process to update the observation or broadcast the fact over the ATIS.

So now the observation is made and augmented. At FAA controlled towers it is sent for review and addition of the airport information (when controller workload allows) and added to the ATIS for your consumption. For advanced students who ever wondered why an ASOS at an uncontrolled tower can update its weather every minute, while at controlled airports the ATIS is only updated every hour--the answer is the desire of the FAA tower to control the weather information we get around the airport. There can only be ONE OFFICIAL AIRPORT WEATHER and ATIS wins.


Forecasts come from National Weather Service (NWS) forecast offices located in the fifty states. Most are NOT on the airport.

Please link to the NWS Organization Page to learn how the service is divided into regions and centers. Find out which region you fly in.

Now go to the region homepage for your area at the NWS main website. Find your forecast office. Some of them offer online tours. Take the tour and see all the products that come from your forecast office. Its not just for aviation.
If you can't find a tour for your office, take the tour at mine, Washington DC/Baltimore MD.

Airlines that are designated as EWIN (Enhanced Weather Information) certified are able to make their own forecasts based on observations. Most of the times the offices making these forecasts are away from the actual airport.
Use the Quick Reference Card again to review the TAF code. (Note: link is not active)

TAF code informaton has forecasts usually for 24 hour periods. There is the main body forecast which will describe the weather which must exist for at least half the time and have a greater than 50 percent chance of happening. THIS ONLY COVERS FIVE MILES AROUND THE AIRPORT.

Operationally significant weather should be forecasted including thunderstorms, wind shear, freezing precipitation, moderate or greater rain, accumulating snow, winds, and wind changes and gusts.

There are also conditional statements which can describe weather changes or intermittent conditions.

TEMPO - Greater than 50 percent chance of happening for less than one hour and cover less than half the forecast period

PROB - 40 or 30 indicates numerical chance of occurring. This is sometimes translated as "chance" and "slight chance."

Can you read a TAF?

Let's change formats for a while and look at the bigger picture. Now that the observations are done and the forecasts for the terminal area are made, someone is creating warning messages about thunderstorms for you to pay attention to. Let's learn who they are.

USAF and US Army personnel taking this course will want to link to a lesson that teaches where their weather information comes from.

Now, let's discuss two hazards while on the ground at an airport. The first is the wind chill. Did you know that the United States and the rest of the world developed a NEW wind chill chart for this winter season. You can read all about it at a National Weather Service web site explaining the changes.

Wind Chill:It is based on the rate of heat loss from exposed skin caused by the combined effect of wind and cold. As the wind increase, heat is carried away from the body at an accelerated rate, driving down the body temperature. Animals are also affected by wind chill. The term wind chill was coined by Paul Siple. He was a pioneer in determining the relationship between heat loss to wind and temperature. During the 2nd Byrd Expedition to Antarctica in 1939-40, Siple exposed water-filled plastic cylinders to various temperatures and wind speeds. He then recorded the time that it took for the water to freeze in the cylinders. With this data and the assistance of his colleague Charles Passel, he developed a formula for calculating wind chill. The new equation and wind chill temperature chart is:

In order to determine the wind chill using the above chart, one must know both the wind speed and temperature. For example, if your temperature is 10oF and your wind speed was 25 mph. One would follow down the 10oF column until it intersects with the 25 mph row. Doing this, one gets a -11oF Wind Chill.

How about taxiing and traveling on runway surfaces? There is a real hazard in the winter. The most common mishap in winter is NOT STAYING ON THE RUNWAY (overruns). NASA is conducting research on this area. You can read about it.

This lesson we've covered some important aspects of knowing that winter weather is impacting an airport.

Thank you for continuing to participate in the National Weather Association's "Winter Weather and Flying" internet course. We're halfway through right now and have two more lessons to go. Please take a moment to provide some feedback if you can.
Study Questions:
1. What is the name of the book that describes weather observations and their codes?
2. How long must wind be measured before the direction and speed is known?
3. What is the definition of variable wind?
4. How long ago was a gust reading recorded?
5. How long must an automated sensor be used to determine visibility?
6. Is sleet an officially observed weather condition?
7. What is the "official" definition of Heavy snow?
8. What is the difference between low drifting and blowing snow?
9. What is freezing precipitation?
10. What is sleet officially?
11. What is the level of ASOS augmentation or other automatic weather observations at your airport?
12. Define these codes:
13. What is the wind chill for 30 degrees Fahrenheit with 25 mph winds?