Frequently Asked Aviation Weather Questions (FAQ)

© Peter Lester

Frequently-asked-questions (FAQs) and appropriate answers are found below. They are organized according to chapter of Aviation Weather, 3rd Edition, unless otherwise stated. In cases of overlapping material, some questions and answers may appear in two or more FAQ locations.

The Table of Contents for Aviation Weather is given below to help you sort through the broad topics of each chapter. Click on the Part or Chapter Number and you will go to the page with the appropriate list of questions for that part of the text.

Can’t find relevant FAQ? Email your query to me.

Table of Contents

Part I Aviation Weather Basics
Chapter 1 The Atmosphere
Chapter 2 Atmospheric Energy and Temperature
Chapter 3 Pressure, Altitude, And Density
Chapter 4 Wind
Chapter 5 Vertical Motion and Stability
Chapter 6 Atmospheric Moisture

Part II Atmospheric Circulation Systems
Chapter 7 Scales of Atmospheric Circulations
Chapter 8 Airmasses, Fronts, and Cyclones
Chapter 9 Thunderstorms
Chapter 10 Local Winds

Part III Aviation Weather Hazards
Chapter 11 Wind Shear
Chapter 12 Turbulence
Chapter 13 Icing
Chapter 14 Instrument Meteorological Conditions (IMC)
Chapter 15 Additional Weather Hazards

Part IV Applying Weather Knowledge
Chapter 16 Aviation Weather Resources
Chapter 17 Weather Evaluation for Flight

Appendixes
Appendix A: Conversion Factors
Appendix B: Standard Atmosphere
Appendix C: Dewpoint and Humidity Tables
Appendix D: Standard Meteorological Codes and Graphics for Aviation
Appendix E: Glossary of Weather Terms
Appendix F: Internet Resources and Printed References
Appendix G: Review Question Answers

Part II Atmospheric Circulation Systems

Chapter 7 Scales of Atmospheric Circulations

Chapter 8 Airmasses, Fronts, and Cyclones

1. Q. I fly on the West Coast (California) and it seems that warm fronts are rarely seen out here. Is that true?

   A. Extratropical cyclones (midlatitude lows with their associated fronts) are often occluded as they approach the west coast of the U.S. In these cases, warm fronts are typically weak and often not discernable from surface observations or satellite images, so they are not indicated on surface analysis charts. However, there is often evidence of warm fronts aloft. Forecasters may describe these patterns as “overrunning” (warm, moist air flows over cool air at the surface) and associated weather will be very much like that expected with a classical warm front.

2. Q. If an occluded front becomes stationary, how is it indicated on the Surface Analysis Chart?

   A. There are several possibilities that apply to all fronts (See Table A, page D-14 in the text). If an airmass discontinuity remains after the front becomes stationary, it would simply be indicated as a stationary front. Sometimes, the front is no longer discernable, but the associated pressure trough is; in this case, the position would be shown as a dashed line. Another possibility is that the analyst may choose to “wash the front out” out over time because it displays neither a temperature discontinuity nor a pressure trough. And finally there is a possibility that a weakening occluded front may be indicated as a broken line with occluded front symbols (not shown in Table A).

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Chapter 9 Thunderstorms

1. Q. The section on Thunderstorms is in Part II Chapter 9 while the related hazards are found in Part III. Why are they separated?

A. A problem that arises frequently when discussing aviation weather with pilots is that they often become focused on one hazard, the thunderstorm, and one simple model, the airmass thunderstorm. Furthermore, they have no knowledge of what large scale conditions set up thunderstorms (its always a “cold front!?). Ask them about “windshear” and they will tell you that the thunderstorm is “the” source; ask them about turbulence, IMC, and icing … again, they are most likely to describe the primarily thunderstorm hazard. Not good.

What I have tried to do in Part II of the text is to describe the large and small circulations (cyclones, jet streams, thunderstorms, mountain waves, mountain and valley breezes, etc.) that produce hazards of all types. Then, in part III, I discuss aviation weather hazards in detail. This way, the reader has a better understanding of, for example, exactly what turbulence is and how it can be produced under several DIFFERENT weather conditions … similarly for Windshear, IMC, etc. In my opinion, the reader carries away a much better understanding each phenomena and a broader view of flight weather hazards. The result is a better ability to deal with all hazards, both preflight and inflight.

2. Q. How do you interpret all the weather radar information that is out there?

   A. There is a LOT of weather radar data out there and interpretations are not always straight forward. Here are some useful Websites that will enhance and extend the information given in Chapter 9 of Aviation Weather.

   Oklahoma Climatological Survey (OCS): okfirst.ocs.ou.edu/train/materials/radar.html

   NOAA Radar Operations Center (ROC):
   www.roc.noaa.gov

   Note: these URLs were valid at the time this answer was posted.

3. Q. There seem to be several different units to describe the intensity of weather radar echoes. Why isn’t there one standard set of units?
   
   A. Different units are used for different applications. However, all units are related to “Reflectivity” (Z)*. Because Z varies over a very wide range, it is usually expressed in decibels (dBZ). Reflectivity is used to judge such things as precipitation rate, thunderstorm intensity, and turbulence (within a thunderstorm). Since all of these items are of importance for flight, as a pilot, you will be exposed to all of them. For example, radar echoes coded in terms of rainfall rate may be expressed as “light,” “moderate,” or “heavy” or in inches per hour (in the U.S). Another related item is “Echo Intensity Level” (sometimes called “VIP Level”) which is simplified to 6 categories of reflectivity. For precipitation range from they vary from 1 (Light Precipitation) to 6 (Extreme precipitation). More quantitatively, for convective precipitation, values range from 1 (< 0.2 inches per hour) to 6 (>7.1 inches per hour). For turbulence in thunderstorms, Echo Intensity Categories vary from 1-2 (light to moderate turbulence) to 5 or greater (severe and extreme turbulence). On the other hand, some pilots prefer specific dBZ values. For example, a reflectivity of 30 dBZ or greater is interpreted as a red flag ("stay away!") for some commercial carriers operating in regions of potentially strong convection. It is helpful to remember when trying to sort these items out that all radar intensity classification schemes are ultimately related to radar reflectivity. See details in Chapter 9 of the Aviation Weather text.

   *Note FYI: the actual units of Z are mm⁶/m³. The decibel scale is a logarithmic scale (dBZ =10LogZ). Remembering these particular pieces of information are not important for the interpretation of radar information for operational aviation use as long as you know the critical values of reflectivity in terms of dBZ, precipitation rate, thunderstorm and turbulence intensity.

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Chapter 10 Local Winds