Contents:
Flying in weather; ...Weather
Training; ...VFR Weather; ...Effects of density altitude; …Computing
Altitudes; ...SVFR; ...Visibility;
...Haze; ...Go-No
Go Decision; ...VFR into IFR;
...Weather Planning; ...Low
Visibility effects; ...Enroute Decisions;
….PIREP; ..Turbulence;
...Avoiding Convection; …What
to do in convection; …Summer
Thunderstorms; …Turbulence Flying;
…IFR Weather; …Weather
Items; …Fog; …About
Severe Weather; …Tornado Safety;
…Avoiding a Thunderstorm; …Riding a Thunderstorm; …Flying
in Convection; …
Preflight must include pre-heating the engine and oil, fingertip examination of flying surfaces, and free movement of controls is essential. Don't hand prop to start on a slippery surface. Deicing is best done inside a hangar. Some deicing fluids may adhere to the flying surfaces and act just as ice would in destroying lift. Some aircraft are powerful enough to takeoff with out tires rolling. Check tires for their ability to roll.
It is a practical understanding of weather that keeps aviators safe. Any trip of 750 miles is sure to encounter below VFR weather. Anytime the weather gives you doubtful feelings it is best to exercise the judgmental skill that cancels the flight. By knowing the weather you can know what you would do before the need to make a change in plans. Safety lies in knowing how to deal with the weather within the operating limitations of your aircraft. It is not a "failure" to cut your potential losses by turning back. On the other hand, you will fly in weather if the utility of flying is important.
Pilots and passengers die every year from exposure to the weather elements. Usually, death is a result of failure to anticipate the possible weather and dressing accordingly. Hypothermia is the culprit and it takes only wind, not cold to bring death. Wind plus being wet accelerates the onset and ultimate result.
The most dangerous weather is that of which you are unaware. The signs of adverse weather are always available if we look for them. Windshear is never inadvertent. There is a ratio between safety and aircraft utility. This ratio varies with every flight. By taking the time to study the probabilities, keeping the final briefing inside the hour of departure, and (ideally) getting a look at the radar depiction you will not be surprised.
Begin your cr0ss-country flight planning four or five days before the day of the flight. Continue your planning right up to the day of the flight. You make adjustments, including cancellation, based upon any information that will not fit into your model of what it takes to make a safe flight. In the Nino year past experience must be set aside and forecasts as well as plans will be flexible even to the last moment.
You are looking at the jet stream locations, frontal movements, and airflow around pressure centers. You are looking to atmospheric instability that will cause weather you do not want to encounter. You don't want rising air and condensation. A good pilot must apply what he has learned to what he sees happening. The good pilot will insist on at least one standard briefing before the flight. Inform the briefer of your total planned flight as well as any options you can think of. VFR you don't need an alternate but you should have an alternate plan anyway. Follow up with abbreviated briefings just before departure and stay in contact with Flight Watch. Accept VFR flight not recommended (VNR) and any mention of icing as sufficient reason to re-think making the flight at all.
Daylight Savings Time changes the way we fly. We do not fly as frequently so the accident frequency declines and the influence of weather on the occurrence of accidents increases. This is a time of more night when those accidents that do occur tend to be more serious. Accident rates at night tend to occur more often the less experience at night the pilot has. Night flying difficulty is directly related to what VFR pilots see outside and what IFR pilots see inside. Aircraft and ATC system failures are more critical at night. The transition from instrument to visual by the single IFR pilot is a killer.
There is no FAR that requires a pilot to make in-flight weather checks but thanks to Flight Watch this is easy to do from 6 a.m. to 10 p. m. local time any where in the United States. There is only one nationwide frequency of 122.0 that is available almost anywhere above 5000' AGL. For situations where 122.0 won't work consider the high-level frequencies of 135.7 or 134.52 frequencies usually reserved for high altitudes. The process of contacting Flight Watch only requires that you include the name of the closest VOR along with your identification. You will know you have a weather problem when you experience turbulence and cumulus clouds.
So called Judgment errors cause most aviation accidents. Terrain is a frequent corollary for judgment errors, actually nearly 30 percent. Another 70 percent are related to VFR weather into IFR weather, fuel complexities, exceeding aircraft capabilities and social pressures .
1996 weather related accidents was 68.2% with 58.9% being VFR into IMC. It is my opinion that proper exposure to weather conditions in familiar areas will enable a pilot to maintain situational awareness and make sensible avoidance decisions. To teach this requires that we fly into weather. Never fly into deteriorating weather and always fly into improving conditions.
Gradual exposure into the conditions along with the limitations of VOR navigation and radio communications is important. I feel that only exposure to these conditions in an instructional situation can overcome the emotional and intellectual paralysis that accompanies a virgin entry into such conditions. I have so exposed every student I have ever taught and have empirical evident that it is the way to go.
Every lesson is a weather lesson. We look at the windsock, the clouds, and the winds before getting the plane. After solo I deliberately plan a SVFR flight back and fourth between neighboring airports. Living on the West Coast makes finding this situation relatively easy. These are airports that we have flown to before but under better weather conditions.
It is only by actually experiencing such a flight can a pilot
can fully understand just what is meant by remaining within 700'
or 1200' AGL while remaining clear of clouds. Counting 5000' runways
for each mile visibility
can be difficult. It helps if you are so familiar with the airport
checkpoints that you can give an exact building, bridge, intersection
or common visual checkpoint when ATC asks for a position report.
This kind of flying is not recommended in unfamiliar territory.
The advent of El Nino has made for some strange weather but when
traditional patterns return it becomes possible to make more realistic
projections micro weather areas.
Determined by visibility, cloud clearance, and ceiling
Standards for VFR below 10,000' and night:
Visibility of three miles
500 below, 1000 above, 2000 lateral of clouds
Ceilings 1000' AGL
(Night exception 1/2 mile of runway below 1200')
Class G Airspace Operations;(700/1200 AGL)
Visibility 1 mile daytime
Clear of clouds
Heated molecules increase in speed and need for a greater space. The distance between them increases and the density decreases since for every unit of space there are fewer molecules. Lifting surfaces rely on the weight of passing air molecules to produce lift. Altitude density varies from sea level to space. A wing produces the most lift at sea level because of the density (closeness together) of the molecules. In space a wing does not produce lift since there are no molecules.
In the atmosphere, the air molecules near the surface of the earth are held closer together through molecular attraction and gravity than those molecules higher up in the atmosphere. The higher up you go in the atmosphere, the fewer the molecules. Density decreases as you go up in altitude.
Factors are altitude, temperature, humidity and low barometric pressure. Effects are wings lose lift, propellers lose efficiency, and nonturbo engines lose power. Results are that higher true airspeeds are necessary taking longer runs before reaching rotation speed. The climb will be more shallow even at higher true airspeeds.
A 2000' altitude above sea level will result in about 25% reduction in takeoff and climb performance. A sea level rotation of 70 knots ground speed will be 80 knots ground speed at 8000'. Even turbo performance only helps engine power without any effect on propeller efficiency or wing lift. You should double all book(POH) numbers because there are complicating factors such as the following.
True airspeed is only varied by about 5% by leaning. Speeds
will be faster the cooler it is. This extra speed is reduced somewhat
by the drag of denser air.
--The time of day that you fly makes a great difference.
--The lighter you fly the better your performance. you may be
well advised to make two flights with half load instead of one
flight that doesn't get you where you are going. A 20% reduction
in load gives a 36% performance increase.
--Learn to manage the mixture. Lean for altitude, takeoff and
climb. Without an EGT lean for peak rpm. You can lean at less
that 75% power even for takeoff. With an EGT you can use the EGT
setting of a cruise altitude as a takeoff reference setting. Except
for full power rich mixtures are unnecessary. Keep the mixture
leaned during descent to keep the engine warmer.
--The best way to learn flying at density altitude conditions
is to fly with an experienced pilot several time into different
conditions. This will relieve anxieties you probably have that
only knowledge can overcome. Personally I flew across the Sierras
three times with an instructor before attempting the flight by
myself. I have never regretted the time, cost or experience in
doing those flights.
--Headwinds require the use of more power to mitigate the percentage
effect adverse winds have on ground speed. Fly faster into headwinds.
The slower you go the more important it is to use fuel planning.
Pressure altitude
Set 29.92 in the Kollsman window of your altimeter and read the
altitude directly.
Get the altimeter setting from ATIS. Take the difference between
that and 29.92. If the altimeter setting is lower than 29.92 add
to the airport elevation 100' for every 1/10 difference. For a
setting is above 29.92, the pressure altitude is lower
than the airport elevation.
From ATIS:
Current Pressure = 29.98 inches
Current temp at 4500 feet = 18 degrees C
Cruise Altitude = 4500 feet
To get PRESSURE ALTITUDE:
(Current Pressure)
-29.92 (Standard Pressure)
------
= FEET
(Intended Indicated Altitude)
+ (Actual altitude adjustment; add/subtract pressure
-----
(PRESSURE ALTITUDE)
*****************************************
To get DENSITY ALTITUDE:
Temp (degrees C)
----------------
(Standard Sea Level Temp)
(Standard lapse for intended altitude = 2 degree per 1000ft = 2C x )
----
(Standard at intended altitude)
(Current at intended altitude)
----
(Actual temperature at altitude)
Use pressure altitude and temperature with E6b to get density altitude.
Density altitude is the effect of air pressure, temperature and humidity on performance. Each one affects the engine's power, the propeller's thrust and the wing's lift. Density altitude charts use pressure altitude and temperature to get performance factor. Performance factor is the total change in performance due to loss of engine power, propeller thrust, and wing lift. These losses affect ground roll distance on takeoff, climb performance after takeoff and ground roll on landing.
SVFR
When you are flying VFR in Classes C, D and E airspace and in
radio contact when weather conditions become below VFR minimums
you can request get a SVFR clearance.
Requires clearance at airport
1 mile flight visibility
Clear of clouds
Night: IFR airplane and pilot
Even under the new METAR system visibility is usually given in the U.S. as statute miles. Ground visibility is reported at airports as distance over at least 50% of the horizon. The pilot is bound by the reported ground visibility and must have the required visibility to operate in Classes B, C, D, or E airspace. See FAR 91.155(d) However, the pilot will be using flight visibility while flying. Flight visibility is the distance you can see in the direction you are flying. When visibility is not reported you can fly using flight visibility. See FAR 91.155(a). Touchdown zone visibility is measured by a transmissometer and is given as visual range in feet or fractions of miles.
Haze can be a serious problem where spatial disorientation can follow lack of situational awareness. You can out-fly your visibility and checkpoints. Haze is caused when the air contains relatively high amounts of water vapor and dust particles to the point that visibility is diminished. An airport at less than five miles may be reported by you to be up to fifteen miles away because it is so hard to see. You can see down but the slant range visibility is an illusion and deceptive. The position of the sun low on the horizon either morning or evening that requires you to look into it is particularly hazardous. In this case my recommendation is to over fly the airport and come in with the sun behind you. The difference in visibility is dramatic.
Go-No
Go Decision:
--VFR flight into IFR has only 10% of the accidents but 82%
of the fatalities.
--The most difficult forecast is ceiling and visibility.
--Any flight into marginal conditions is more likely to be in
error. Error either way that is.
--The greatest errors occur in time. The weather will happen but
betting on when is a crap shoot. Don't bet on improving
weather at the time forecast.
General Cautions
1. Reduced visibility and haze may require instrument flight.
2. Morning flights are smoother.
3. Approach ridges at 45-degree angle
4. Fill tanks on landing to avoid night condensation.
5. Always carry water and have it available.
Learn to watch trends. A rising altimeter setting indicates
improving weather. If the temperature/dewpoint spread is getting
wider then things are improving. If in the air, don't fly over
an undercast unless you know you can reach VFR. Don't fly
under a ceiling that is forcing you ever lower. Winds that are
different than forecast are advanced warnings that other parts
of the forecast are going to be wrong.
1. NoGo if known icing
2. NoGo if ice forecast at flight altitude
3. NoGo if forecast conditions may be worse. Don't fly into deteriorating
weather conditions.
VFR into
IFR
Unintentional VFR into VMC can happen to anyone. The ability
to forecast just when a weather change is going to occur
is still for the future. Even the most careful pilot cannot always
accurately time his weather avoidance.
I live and fly on the West Coast and never cease to be surprised
with the way weather seems to evade accurate prediction.
I try to be selective in my weather flying by taking my students
into marginal but improving conditions. Improvement seems
to progress more slowly than deterioration. I have had
fog close my home field from full VFR to zero-zero during the
time of preflight. We get both avection and radiation fog
often in consecutive days depending on conditions. Many of the
S.F. Bay valleys have micro-climates that defy prediction.
I do believe that a 'California' instrument rating is a very
valuable flying asset but extremely difficult to keep current.
I believe
that both the private and instrument ratings are best acquired
during the late fall and early spring since it gives the trainee
exposure to the entire weather gamut available over a period of
four or five months. Woe is to the pilot who learns during
the summer because they are doomed to remain on the ground for
many months before summer comes again. A pilot
needs exposure to the vagaries of coastal weather that cycles
from sea to central valley and back again every eight days.
It has taken me many years of study and observation to evade the
mistakes of the weather forecasters.
California coastal weather is always changing. Often the status
of good or bad is contradictory depending on the desires
and intentions of the pilot. I have lately cancelled a number
of flights because the weather has not been IFR enough for
what I wanted to teach. At the same time I have been unable to
do a number of SVFR lessons because the weather
improved too rapidly.
The field of weather prediction has still to accurately time
the arrival of weather consistently. The coast has a lack of
airports and a corresponding lack of PIREPS. A PIREP is real-time
weather for a specific place and time. This is weather
you can believe. Be cautious in reliance on ASOS and AWOS. My
home field has had one for nearly two years and it
is yet to be certified for use.
The weather patterns are a changing worldwide. When people
from all over the world come to San Francisco they must
be prepared to unusual weather. July can be quite cold; last December
was the third warmest on record. Once a year thunderstorms are
now occurring monthly. Storms line up on the jet-stream and march
through with only two-day breaks. Occasionally, a storm will pause
and drop rain for three days before moving on.
You can avoid weather problems by staying on the ground. You can fly in certain 'bad' weather conditions if you always have a backdoor escape route and know exactly where you are. Getting on the ground is always a desirable option. Get down and settled before dusk. Any weather will be worse at night. Be ready to change any preconceived plans. Be prepared to wait-out the weather.
Visibility is the most obvious weather factor likely to abort
a flight.
Most weather factors can be flown under, over, or around. Not
so easily with low visibility.
Low Visibility
effects:
--Avoidance of embedded weather is difficult.
--Reliance on pilotage and VOR navigation becomes uncertain.
--Disorientation is a real possiblity. Use GPS if possible.
--CFIT (controlled flight into terrain) becomes a hazard.
--Avoid being caught on top.
--Don't try to out climb a rising cloud. You can't.
--Live with rain. When it exists avoidance except for improved
visibility is unlikely.
--Don't fly under dark clouds that may be the base of an embedded
thunderstorm.
--Avoid turbulence. Don't fly into forecast turbulence or ice.
--Note how winds affect your adherence to route and planned speed.
Know where the severe weather is.
Know where to go to avoid problems.
Check the weather often on 122.0.
PIREPS are your best friends.
Don't expect to influence the weather.
You won't go wrong making the safe decision
PIREP
Begin by just calling up and the name of the nearest VOR and
relationship to a prominent checkpoint if possible. You can help
the system of giving the PIREP work better by giving the information
in the correct order. Use navaids for location not local checkpoints.
Give your aircraft identification, type, altitude, flight type,
departure point, destination, and route.
UA or UAA Urgency of information
OV Where
TM Time
FL Altitude
TP Type aircraft
SK Sky cover and altitude
WX visibility, precipitation
TA Temperatrue in C
WV Magnetic wind and velocity
TB Turbulence
IC Icing
RM Remarks
UA(A), OV, TM., FL, TP, SK, WX, TA, WV, TB, IC, RM
Design a ten word sentence:
Unless Old Timers Fly The Skies weather takes wing
Over Time Flying Types the skies weather tempers; the bumps I see remain.
PIREPS are retained for only one hour.
Preparation:
1. Secure cockpit
2. Light touch
3. Fly attitude not altitude enroute
4. Bumpy clouds are bumpy top and bottom.
5. Never enter a cloud without knowing what's inside.
6. Slow down before entering a potential turbulent area.
7. On approach go to smooth air alternate.
8. Turbulence is distracting...use your checklists.
9. LLWAS Low Level Wind-shear Alert System at airports
Proximity to the ground allows little time to recognize and respond to turbulence-induced control difficulties. Accident cause is usually, "The pilot's failure to maintain control of the aircraft." Turbulence accidents usually occur in high drag configuration and low speeds. Knowledge is the only known antidote for overcoming concerns related to turbulence. Knowing what causes it will allow you to avoid it. Science is working on a turbulence detection system but 90% of effort is to devise a method that pilots can trust.
Where you have great changes in air velocity you will have turbulence. This can occur as a ventrui effect in a mountain pass or over an obstacle. Air is much more affected than water because it is light and has little viscosity.
Fly early in the day before sun causes thermals or late afternoon when sun's effect is lower. Carry extra speed on any approach experiencing turbulence, diversion to another airport is a good option. If turbulence affects your ability to remain on the approach heading and altitudes, execute the missed.
Avoiding
Convection
Watch the weather for a few days before a flight. Get the
area forecast (FA)(TAF) which will give state by state hazardous
weather. Radar reports (rareps) and severe weather outlooks (AC)
will give real-time updates of storms, intensity, and movement.
TAFs exclude temperature, turbulence and icing forecasts.
ACs show storm area by intensity and area of coverage. ACs come
out at 12Z and are accurate and reliable indicators of things
to come. Rareps tell what is now. Rareps can be over three-hour
old. Text type convective sigmets tell about level-four or greater
storms of over four-tenths coverage. Since briefers often fail
to cover these, be sure to ask for any convective sigmets.
If the forecast indicates convection problems, leave early in
the morning, get on the ground before 11 A. M., stay visual with
flight following. File IFR but request vectors around convective
activity. Don't let yourself be forced low in mountains. Use Flight
Watch. If in doubt, do a 180 and land. Avoid all storms by 10
miles or more. Don't fly so that the storm blinds any radar coverage.
Wind velocity changes of 15 knots and vertical velocity of 500-fpm
or greater are hazardous. Vertical velocities of over 4000 fpm
and wind speed changes over 90 knots have been recorded. Every
downdraft that reaches the ground sends out horizontal winds in
all directions just as spilled water does.
It is easier to get ATC help in convection (thunderstorm) avoidance
once you are away from terminal areas. It is harder for ATC to
give you an altitude change than a course change because of the
coordination required with other sectors. ATC radar can only detect
two levels of precipitation. Light rain is shown with ////slashes
and heavy rain with HHHHs. ATC cannot see thunderstorms.
ATC can give departures more help than arrivals because departures
tend to be more spread out. Make a decision to divert early. It
is easier to change airports before the IAF than being committed
and having to do it
from the missed approach. The more time you give ATC the better
they can help you.
Basic truth is that updrafts are not rising so much as being displaced
below by cold air. The movement of cold air causes the warmer
air to be displaced upward.
Avoidance
--Weather briefings
--Flight Watch updates
--Virga presence is warning
--Thunderstorms within 20 miles
--Get on the ground
--Extra airspeed on approach
What
to do in convection:
1. Tighten everything
2. Pitot heat
3. Slow down, drop gear
4. Interior lights on high
5. Use only instruments
6. Don't turn
1) Avoid
Clues:
--PIREPS
--Thunderstorms, cumulus or virga in area
--Temperature over 80o
--Temperature/Dew Point spread more than 30o
--Blowing dust, dust devils, gust fronts
--Extreme variations in wind direction/velocity
--Being downwind of virga or thunderstorms
2) Longest runway
Full power
Watch for sudden changes in airspeed/performance
Don't change configuration
Issue PIREP
3) Add 10-20 kts to approach speed
Clues
First clue is increase in airspeed followed by stagnation and
loss of altitude.
Apply maximum power
Maintain configuration
Accept low airspeeds
Don't lower nose-best pitch climb attitude
Hang on, the greatest danger is in over-control by the pilot
4) Avoidance
1. Fly at least 7000' above terrain if equipment (oxygen) allows.
2. Fly early and be on the ground by noon
3. Slow down and stay out of the yellow airspeed arc
4. tay above 1.5 stall speed else a bump may cause a stall
5. Set your power for turbulence before you get into it.
6. Lower landing gear
7. Ride with what ever altitude you get. Advise ATC of the problem
and get a block altitude which lets you ride up and down between
altitudes.
Emergency Landing
Cows face into the wind ???True??
Sheep face away from the wind
Windmills face into the wind
Major highways have frequent straight sections
Towers come in bunches but never right over the road.
Summer
Thunderstorms
1. Avoid cells by 20 miles and more if the cells are moving
faster than 20 knots.
2. Cells at the south end of a line are worse.
3. Strong T-storm indicators are (1) surface dew point above 50
F with over 30 degree spread to temperature.
4. Check AC convective outlook. Warning area is very dangerous.
5. Winds at 18000 that are from the southwest are T-storm indicators.
6. Flying in the a.m. is best.
7. Don't depart with T-storms within 20 miles.
What if the weather had deteriorated? I was unfamiliar with
the airport and severe weather would have required me to do the
following.
1. I would keep the wings level, reduce speed to Va for my lighter
weight
2. Would request vector to nearest or 180 as applicable
3. Would ask about minimum altitudes and ask for a block altitude
referenced to it.
4. Would double check freezing level and try to stay under it.
5. It could be that flying to the 'light' does not always work
in cumulus clouds but does in stratus clouds.
IFR
Weather
What do you do if your personal minimums don't go down to
500 and one? Any approach or takeoff with this weather demands
that the pilot have some skill and experience operating in this
level. Even then being familiar with the airport layout, terrain
and obstructions gives some additional comfort. It would be nice
if you have 'home field' advantage.
Take all of these things away your survival option is to head for an alternate. Don't even 'take a look'. The pilot who takes off from a runway and airport to which a legal return approach cannot be made has no right to take others with him. Even by yourself you should have a takeoff alternate.
One dot off the localizer is 300' at the outer marker. At the middle marker one dot is 100 feet. The one dot at the outer marker for the glide slope is 50 feet. The glide slope at the middle marker has only 8' for one dot variation. The glide slope never reaches the runway. It flares at 50' for a ways and then rises. This could be a problem for an autopilot-coupled approach. Exact values will vary.
Low IFR conditions are right at the line where the MDAs of
non-precision approaches exist.
Even at home, you will be better off going for an ILS somewhere
else. My part of California has periods of large areas of low
IFR conditions that mean a C-172 would have a hard time getting
to an alternate. However there is usually a VFR escape on local
mountaintop airports. It behooves the pilot to know where these
airports are. The more options you have the safer the flight.
Weather
Items
Ceiling is reported as AGL when give locally.
Area ceilings are given as MSL unless prefixed with C or CIG
Cloud tops are given as MSL
The average total time of pilots involved in icing accidents
is over 3000 hours.
Icing accidents of experienced pilots occur at airports and are
usually survived.
Icing accidents of inexperienced pilots are usually off-airport
and fatal.
It is the vertical air currents inside the storm that creates turbulence and lightning.
Fog
When planning a flight we research weather and become focused
upon areas of concern. This focus may cause us to miss side issues
that await nearby. An emphasis upon weather fronts may miss freezing
levels or fog potential. Unlike thunderstorms, fog is created
when temperatures and dew points merge but are not lifted.
It is the existence of condensation nuclei in a minimum density
of over 75,000 per square inch that makes fog possible. Below
that level the water vapor will attach to the nuclei but it takes
hygroscopic nuclei to cling to the moisture in sufficient numbers
and droplet size before the moisture becomes visible and able
to reflect light. The point of visible moisutre is critical and
slight changes in wind or heat will make it appear and disappear.
When the relative humidity is about 60% it is called haze. At
100% humidity we have fog.
Radiation fog is predictable and occurs close to the groun
in calm stable air conditions. The cooler air and layers of fog
are closer to the ground. This reversal of hot and cold air is
called an inversion. Waterside airports are often subject to radiaton
fog that may stay all day. An aircraft over head may be able to
see the airport but an airplane on approach may be unable to
find the airport due to slant range visibility. The more moisture
on the ground the longer will the fog persist.Radiation fog will
only disappear when the moisture evaporates from the nuclei and
the relaltive humidity reaches about 50%.
Avection fog arrives as a layer and expands into a layer several hundred feet thick which can cover entire regions. Avection fog may be initially very close to the ground but as the day progresses the base will rise to a thousand feet before it thins and disappears. Avection fog will lift and gradually dissapate as the sun rises.
About
Severe Weather
The U.S. experiences more severe storms and flooding than
any other country. We have 10,000 thunderstorms every year along
with 1000 tornadoes and several hurricanes. Two out of five hurricanes
will be severe. 45 million U.S. Citizens now live in hurricane
country.
Storm probability are forecast four times a day. Probability is given as a percent that the center of the storm will pass within 65 miles of some 44 selected points from Brownsville, TX to Eastport, Maine. Places between two points have a probability of their average.
Tornado
Safety
--62% of U.S. tornadoes are classified as weak.
--Weak tornadoes have winds below 100 mph but kill 3% of the people
killed.
--1/3 of the tornadoes have winds of 200 mph, are 200 yards wide
travel about nine miles and are 'strong'.
--30% of the nations tornado deaths are caused by 'strong tornadoes.
--2% of the tornadoes are 'violent'. Winds exceed 300mph, can
be a mile wide, and travel over 25 miles.
--Violent tornadoes cause 70% of the deaths.
--Tornadoes travel 30 mph.
--A thunderstorm near large hail occurs with a tornado.
--Most tornado deaths are caused by flying debris
--The higher the tornado, the stronger the winds.
--A tornado sounds like a passing train or large aircraft.
--Most tornadoes occur in the afternoon and early evening.
--Most tornadoes occur between March and July
Avoiding
a Thunderstorm
…Early flights
…VFR and 20 miles clear of T-storms
…Don't fly through gaps
…Don't fly over
…Don't fly under
…Get good briefing on weather
…Turn back and land when cells lie ahead.
Riding
a Thunderstorm
As soon as you read or hear the word instability in your weather
briefing you should realize that staying on the ground may
be your best option. Stable air and unstable air differ in terms
of temperature and humidity. The gusty winds and turbulence
that exists in mildly unstable air takes very little change in
temperature and humidity to become the thunderstorm heat engine.
The driving force is the lapse rate which comes in two forms.
First is the environmental lapse rate. It is the rate at which
the temperature usually decreases with altitude in stable air.
This environmental lapse rate is the one than changes usually
for every thousand feet 3.5 F and 2.0 C degrees. At one time only
700 balloons were used twice a day to get this
information. Today, every airliner transmits the data to the world
satellite system.
The other lapse rate is called the adiabatic lapse rate. This
rate is used for air that is moving up and down. The word
adiabatic means that the air is heated or cooled without any outside
forces other than the vertical movement of the air.
Adiabatic air expands as it rises, as would a bubble. The pressure
drops and the expansion of the bubble requires heat.
The heat comes from the air molecules inside the bubble. This
cools the bubble. The standard adiabatic lapse rate is
5.5 Fahrenheit or 3 degrees Celsius for every 1000 feet of rise.
This rise requires that no condensation take place. At
some point the rising air temperature will reach the dew point
and condensation will occur. Condensation releases all
the latent heat. Now the bubble is cooling at the moist adiabatic
rate, which averages 3.3F, or 1.8 C per thousand feet.
It is these two lapse rates in conjunction that determines
the stability of the air. Air that tends to return from whence
it
came is stable. Air that stays warm and confutes to rise is unstable.
When condensation occurs inside the bubble the
temperature difference increases the rate of upward movement.
As this rise occurs the outer cool air descends. This
process is called convection. While stable air gives poor visibility
by allowing pollution to exist, convection clears the air.
These two conditions can exist at the same time. At times the
stable air exists above the insatiable in a situation called
an inversion. The warm air rests as a layer above the cooler air.
Flying
in Convection
--Hold as near level attitude as you can.
--Slow to Va
--Extend gear but not flaps
--No autopilot
--Activate all anti-ice
--Tighten belts and turn up lights
--Maintain only heading; ignore altitude and avid turning
--get block altitude from ATC
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