Volume 5, Number 3   


High or low, look out below?


     No, this isn't about altimeter errors.  It's a question that continues to be as ubiquitous as the "dive and drive" versus "stabilized approach" debate amongst instrument pilots.  In other words, when working your way between/around an area of convection or when convective potential is in the making along your route, is it better to be higher up or down low, perhaps picking your way around the rain shafts and cloud bases?  Keep in mind that what makes sense during daytime flight conditions may not be an advantage when it is dark.  Night flights within a convective environment can be very challenging in their own right, deserving a separate protracted discussion.  

Field of cumulus clouds     The playing field will be different for every single flight you may take.  So, there's not always a definitive answer in every case and general "rules of thumb" may not always apply.  First of all, being somewhere in between high and low is fine assuming there are no clouds present.  Moreover, you may be stuck right at the top of the haze layer making forward visibility challenging.  But when convection is anticipated during the afternoon, you may also find a broken field of cumulus clouds develop in the early afternoon similar the clouds shown on the left (click here to view a larger image).  There's little doubt that choosing an altitude that keeps you popping in and out of the clouds in this cumulus deck is not the sane choice. Sure, most of these clouds may be harmless, but who wants to be tossed around for an hour or more in and out of instrument meteorological conditions (IMC). Therefore, take your pick; climb up above this field of clouds or descend down below the cloud bases. Is one choice better than the other?
    Before we take this a single step further, the prime directive in convective flying is that you should endeavor to stay visual at all times. Staying
in visual meteorological conditions (VMC) will go a long way to keeping you out of the nastiest part of the storm, that is, in-cloud turbulence.  

     There's a lot of buzz these days about "flying IFR like the pros."  In a convective environment this is one suggestion you may want to completely toss, especially if you want to take the low road.  Many pilots don't have the experience or equipment to handle IFR when there are convective SIGMETs also proposed for your route. Depending on the local terrain and obstructions, frequent deviations around convective cells while negotiating the airways near the minimum en route altitudes (MEAs) is going to be a nightmare from an ATC perspective.  

     One possible alternative for flights below FL180
is to fly visual flight rules (VFR) instead.  The flight restrictions for VFR force you to remain well outside of the physical cloud boundary and it keeps you separated vertically and horizontally from any clouds.  Flying VFR also allows you the maximum flexibility if you need to maneuver around the weather.  Remember that pilots flying above 10,000 feet MSL under VFR need to be horizontally separated from clouds by one mile.  

Altitude and forward visibility are your friends
     Having altitude on your side is often a good choice in many aspects of flying.  In a convective environment a higher altitude means better much better forw
ard visibility and no thermal turbulence (smooth conditions) assuming you stick to the prime directive mentioned above - stay visual.  You need to climb high enough to be above theHaze within cumulus cloud deck haze layer that is frequently present during the late spring, summer and early fall.  This can range from 8,000 feet to 17,000 feet depending on the meteorological environment and location within the U.S.  The image on the right (click here to view a larger image) is a good example.  In a highly capped environment (small positive or negative lapse rate above the cloud tops), haze can linger between the cumulus cells and below the bases making forward visibility extremely difficult.  Often climbing above this haze layer will also keep you above most of the cloud tops.

     A higher altitude keeps you separated from terrain and obstacles.  The greater the separation from terrain the better.  When you are near the surface, it is easy to become distracted in higher terrain and miss the presence of the mountain peak ahead or tall radio antenna.  Having an obstacle database on board or terrain warning system is a wonderful asset, but be sure to keep those databases updated.  New cell towers are erected frequently across the U.S.   

     Forward visibility is a must if you want to see the buildups and know which ones to challenge and which ones to avoid.  When flying down low, it is similar to having a short stature while in the back of an audience watching a parade.  If you can't accurately identify the danger ahead you may easily stumble into something uncomfortable or require a hasty maneuver to avoid something nasty.

     The goal is to be above the clouds most of the time while en route.  However, flying higher doesn't guarantee that all clouds will be beneath you since there will be instances where the cumulus may tower well above your flight level.  That's okay.  Given the unlimited visibility at the higher altitude, it is easy to identify these congested clouds in the distance and plan an easy deviation around them.  You can easily assess the height of those taller clouds and know which clouds you need to keep at a safe distance and those you can literally "brush by" without any concern.  Keep in mind, that all of these clouds are moving within the prevailing wind and may be growing or dissipating in time.       

Icing is not your friend

     One of the greatest threats of flying high is the risk of structural icing.  During the summer, the melting level
(better known to pilots as the freezing level) ranges from 8,000 feet in the most northern parts of the U.S. to 17,000 feet in the southern tier of states. Of course, this will vary greatly from day to day and will usually be lower during the spring and autumn months.  A good rule of thumb in flying is to never depart unless you know the melting level regardless what the calendar says.  Keep in mind, that AIRMETs (now G-AIRMETs) are not issued to warn the pilot about this kind of convective icing.  If you are forced to penetrate a cumulus cloud at a subfreezing temperature this will almost certainly send you through a shower of supercooled liquid water with drop sizes that could easily exceed 50 microns in diameter.  Even momentary encounters with supercooled large drops (SLD) such as this can be devastating to many airframes.  Again, the prime directive must be strictly followed here - remain in VMC.       

Other factors

     Whether or not you are a low or high flier will depend on several limiting factors.  The first factor is the length of your flight.  On a short flight of 30 minutes or less, it may not make a lot of sense to climb to a high cruise altitude just to be able to see the mess that's ahead.  If you opt to fly low on a short flight, you must be absolutely prepared to map out your route before you depart.  This may not be easy if you are flying IFR within busy airspace.  Your ATC clearance may compromise a solution to minimize your exposure to those convective buildups. Demanding frequent deviations only complicates the controller's job and create a domino effect forcing the controller vector other aircraft.  

     The second factor is the performance characteristics of your airplane.  In order to fly on top you need at least two capabilities.  The first is oxygen (or pressurization) and the second is service ceiling and rate of climb.  If you don't carry oxygen you won't have the ability to climb above the taller buildups.  The last thing you want 
to do is get to 12,500 feet and discover that higher buildups won't allow you to stay on top defeating the number one prime directive.

     An aircraft that is not certified to fly well into the teens or has limited climb capability at higher altitudes will not have as much success flying on top.  A turbocharged or turbo-normalized aircraft will provide the most favorable opportunity to remain above most of the cumulus field of clouds.  

An IFR descent from 15,000 through taller cumulusStuck on top

     One of the worries that a few pilots have with flying high is being "stuck" on top with no where to go.  Certainly if you are flying under VFR this can be an issue if the cumulus cloud deck below is broken and you are not instrument rated and current.  Maintaining the proper cloud separation on descent may be impossible.  If you are instrument rated, a pop-up IFR clearance may assist you to drop down below the deck as you descend down toward your destination. It is rare, however, to have such a continuous cloud deck without sufficient holes to descend through.  Being higher before your final descent allows you to see over a greater region to assess where those holes might be.  In some cases it may mean overshooting your destination by a few miles before you can safely descend. If descending under IFR, normally 10 and 20 degree deviations are all that is necessary to maneuver around the big stuff as shown on the left with an IFR descent from 15,000 feet (click here to view a larger image).

     As a general rule, you don't want to attempt to out-climb a building cumulus cloud.  In situations with high instability (high convective available potential energy), developing cumulus can grow at rates as high as 6,000 feet per minute where 1,500 feet per minute is very common.  You'll likely find yourself climbing into serious icing conditions if you do not have the excess performance to climb on top in clear air.            

Multi-cellular convection and aircraft-induced lightningLine of thunderstorm cells in the distance

     When flying high, avoid flight between multi-cellular convection including towering cumulus.  It is not uncommon for a cumulus cloud in the distance between two larger cells to look pretty benign from the distance as can be seen from the cumuliform growth between two cells in the image on the right (click here to view a larger image).  These "in between" buildups can quickly explode into a nasty cell in just five to ten minutes.  This also applies to buildups that are at the "book-ends" of a single multi-cellular line of storms.  In this scenario, these end-cap congested cumulus can grow into thunderstorms in just a short period of time, ordinarily 10 to 15 minutes.     

     It is also hard to judge just how fast it is growing and if you will be able to "top it" when you reach that location.  If you are at an altitude above the melting level, you don't want to enter that kind of cloud.
 There's copious supercooled large drops (SLD) waiting for you...guaranteed.    

     Be careful flying too close to a thunderstorm or towering cumulus when flying at an altitude near the melting level.  You might
just find your aircraft can induce a lightning strike out of the blue.  This is especially true of precipitating towering cumulus as described in this e-Tip discussing aircraft-induced lightning strikes.        

Ugly side of low

     Most accidents attributed to thunderstorms happen when departing or approaching the airport.  Generally, we don't see aircraft break apart in flight at higher altitudes.  This is because flying down below or near the convective cloud bases means the potential of being exposed to the most dangerous aspects of a thunderstorm.  Convective wind shear is likely the worst hazard you will face and causes the most thunderstorm-related accidents.   All thunderstorms produce some kind of outflow boundary during their life cycle.   According to thunderstorm expert, Dr. Charles Doswell, III, "cold, stable air is the 'exhaust' of deep, moist convection descending in downdrafts and then spreading outward like pancake batter poured on a griddle."  After spreading outward, the leading edge of the outflow boundary is essentially a gust front. 
Some of these outflow boundaries are benign and some are extremely hazardous and often hard to detect visually.

     Another serious threat when ducking under the clouds is stumbling into a convective downburst or microburst.  Pilots are trained to keep their distance from giant supercell-type thunderstorms.  If you think this will also keep you from encountering a microburst, think again.  As described in this e-Tip, microbursts are frequently generated from relatively benign-looking cells and not necessarily a giant supercell-type storm.  High-based thunderstorms with an extremely dry environment below the bases doesn't seem all that threatening and may actually look very inviting to any pilot trying to fly under the higher cloud deck.  This is a perfect environment for a microburst encounter.  As rain falls into the dryer air below the bases, it begins to evaporate which causes a cooling effect.  Such cooling makes the air denser and negatively buoyant which can eventually unleash a very intense downburst with winds that exceed hurricane force over a distance the size of a long runway.  Of course, other hazards such as reduced visibility, low ceilings, hail, lightning and strong straight line winds can be found when flying at or below the bases in and around most organized convection.   

XM ground-based radar and lightningSo you've got satellite-delivered weather?

     Satellite-delivered weather is extremely useful when it comes to avoiding thunderstorm hazards.  But don't let that be a panacea for staying in visual conditions.  While not every dangerous situation will appear on your satellite-delivered weather display, many do. When you see an active cell with reflectivity values of 50 dBZ or greater enveloped in lightning, that's certainly a cell to work around.  Don't be fooled by the radar mosaic, however.  It is about three to eight minutes latent from reality assuming you didn't miss an update.  This seems like a fairly short time, but what looks like a harmless cell with very light precipitation on the radar mosaic can turn into a dangerous thunderstorm with reflectivity values exceeding 60 dBZ in 10 minutes as described in this e-Tip.  In the end, identifying what cells to avoid is much easier to ascertain while flying in visual conditions above the haze layer. Being low makes it difficult to characterize the cells visually.                  

Keep a bigger fuel reserve     

     When battling through a convective environment, you always want to maintain a close eye on your fuel consumption and plan a larger fuel reserve.  Without fuel, you greatly limit your options.  Adding 30 or 45 minutes to your one hour fuel reserve will allow you to keep a safe distance while deviating around a nasty area or line of convection.  Remember a climb to a higher altitude will also eat into your fuel reserves as will ATC delays.  If you start to approach your original one-hour fuel reserve, plan an early diversion to another airport to refuel.  Do not be tempted to think you can still make it to your original destination.  You may put yourself into a position of deciding to descend through a dangerous cell to avoid running out of fuel during a large deviation around that storm near your destination.

Did you know?

Hurricane Ivan in the Gulf of Mexico     Did you know that the FAA imposes a hard character limit on text products such as convective SIGMETs and the area forecasts (FA)?  The Aviation Weather Center (AWC) has a check program for the area forecast that counts the number of characters.  On busy weather days when these forecasters have a lot to mention in the forecast, they often bust the character count.  This forces them to cut from either the text of synopsis in order to make the size fit through the NADIN hubs.  Often the first casualty is the ellipsis (...).  They can use two periods instead of three.  Another casualty is the synopsis.  Of course, the last resort is to cut from the text.  

Here's what you might see in the synopsis section with a very busy FA:

     This has been a long standing problem that you won't soon discover in the AIM or FAA handbooks.  The AWC has asked the FAA many times to increase the character limit size.  In fact, according to NWS meteorologist Jim Roets at the AWC, "As late as last week we had an issue with the Convective SIGMET.  The character count was a little over 3,000, so the forecaster sent it anyway.  The product was kicked out of the FAA system."  He was able to combine outlook areas and cut some of the areal coverage on his convective SIGMETs a bit which allowed the forecast to go out.

     In general, the NWS Telecommunications Gateway in Silver Spring, Maryland allows them 15K per product.  But when that product goes from the Gateway to the NADIN hubs it has to be 3K or less.  Unfortunately, the character count issue goes back to the teletype days when data storage and transmission speed were small.  With the transmission speeds and data storage capacity that exists today, it is very difficult to understand why the FAA won't increase the size. 

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