Summer Weather Hazards

Despite sayings such as a “lazy summer day” and the “dog days of summer,” flying during the summer months can have many challenges with respect to weather.

Summer is thunderstorm season and thunderstorms pose the greatest weather hazard to aviation, as they can contain nearly all serious weather hazards in one dark cloud. Severe icing and turbulence, microbursts, wind shear, IMC, hail and lightning can all be expected from thunderstorms.  Jeppesen provides pilots with the tools needed for summer  flying by supplying weather from worldwide sources and uses a staff of professional meteorologists to create the best aviation weather maps available. Jeppesen's graphic weather maps display fronts, lightning, NEXRAD and Echotops, icing, turbulence, precipitation accumulation, METARs, TAFs, winds aloft, AIRMETs and SIGMETs. You can also view chart change notices and alerts as well as free text weather reports at www.MyJeppesen.com.

But even with the help of technology it is important to remember the three ingredients that are required for thunderstorms to form:
1. Lift
2. Moisture
3. Atmospheric Instability

Thunderstorm development requires that moist air be lifted to saturation and then continue to rise. This implies we need a lifting mechanism, such as surface heating (thermals), orographic flow over mountains or a front (includes surface troughs, convergence lines and dry lines). The lifting needs to occur in a moist atmosphere which causes air being lifted to condense into a cloud. Thunderstorms also need an unstable atmosphere, which means as the air becomes saturated, the air continues to rise.
 
Spring may be the prime time for severe thunderstorms, but summer has more numerous thunderstorms across the entire country. Summer sees the predominance of air mass thunderstorms, which are caused by intense heating. Air mass storms develop during maximum heating in the afternoon and diminish once the sun sets. Air mass thunderstorms tend to move more slowly, generally in the 5-15 MPH range. Spring thunderstorms tend to develop along fronts and move at a much faster pace.  
 
The biggest threat to aviation from thunderstorms is wind shear and microbursts during takeoff and landing. We tend to learn from disasters in aviation, and when Pan Am Flight 759 crashed on takeoff from New Orleans on July 9, 1982 while flying through a thunderstorm, research began on the hazards, detection and avoidance of microbursts. Research was intensified after Delta flight 191 crashed on approach to Dallas – Ft. Worth on August 2, 1985. Research led to the development and deployment of the LLWAS-2 system and the Terminal Doppler Weather Radar system. These systems enhanced the ability to detect and alert on terminal wind shear and microbursts. The graphic below illustrates the hazards associated with flying under or through a mature thunderstorm with a microburst.
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There are additional hazards related to thunderstorms as well. There are additional hazards related to thunderstorms as well. Lightning can impact electrical system and damage the aircraft skin. Moderate to severe turbulence can be encountered in and near thunderstorms, as well as moderate to severe icing. Avoiding thunderstorms is usually the best course of action.
 
Some useful information to detect and predict where thunderstorms will be is the NEXRAD Doppler radar network. NEXRAD scans a large volume of air surrounding the radar site every 6-10 minutes and can provide up to date information on position and movement of storms. On-board radar can be used for tactical avoidance while en route. For prediction of thunderstorm areas, some new products can be quite useful, such as the Collaborative Convective Forecast Product (CCFP), the Thunderstorm Outlook and the new Corridor Integrated Weather System (CWIS) that provides a forecast of NEXRAD activity much like the Jeppesen MAPLE product. The old Skew-T chart is another good way to view the stability of the atmosphere near the radiosonde balloon site. The Skew-T diagram usually contains computed stability indices such as the Lifted Index, K Index and CAPE that help determine the probability and severity of future thunderstorms. Check out current Skew-T diagrams here.
 
The first diagram below shows the thunderstorm outlook for March 10, 2009. The brown line depicts where general thunderstorms are possible, the green line depicts where a slight risk of severe thunderstorms exists.
 
In the Skew-T diagram below for Little Rock, AR you can see a moist and unstable lower level with a mid level dry and stable layer. This situation requires a strong lifting mechanism to break the “cap." the mid level dry and stable layer, so that thunderstorms can develop vertically. Either strong surface heating or a surface front is required to provide enough lift of the moist, unstable air to penetrate the cap. In situations when thunderstorms do break through the cap, they have the potential to become severe.
 
SPC Day1 1630Z Outlook Categorical

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Summer is the season when we don’t expect much icing as freezing levels are high and turbulence outside of thermals and convective activity is limited since upper level winds are generally light. Smoke from wild land fires can cause some restrictions to visibility generally below FL120 and TFRs become more numerous over fire fighting areas. Coastal stratus along the West Coast is predominant, so IMC exists during much of the night and early morning hours along the immediate coast.
 
The best way to prevent hazardous weather encounters is to obtain a detailed weather briefing before each flight, and if you have any questions, get them answered by a certified briefer or professional meteorologist.

About the Author: Mike Cetinich is a Senior Product Manager at Jeppesen for various ground-based solutions including Weather, Runway Analysis, Flight Tracking, NOTAMs, Flight Planning and Navigation Services. Mike started at Jeppesen in 1983 after receiving a B.S. in Meteorology from San Jose State University, and now teaches Aviation Weather to local Denver area pilots in addition to his day job. Mike’s passion for weather has led to articles for the American Meteorological Society, of which he’s been a member since 1983. In his spare time, Mike enjoys cycling, cross country skiing, snow shoeing and hiking with his wife and pack members, as well as storm chasing each spring.


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