Flight Training - May 2012 - 41

Weather	interacts	with	all	scales,	and	 weather-wise	pilots	should	know	at	least	 a	little	about	each	of	them,	especially	the	 microscale,	storm	scale,	and	mesoscale	 sizes—which	most	directly	affect	flights.	 Synoptic-	and	planetary-scale	weather	 sets	the	stage	for	the	smaller	phenomena.	 For	example,	an	instrument-rated	pilot	 planning	a	flight	north	of	the	surface	position	of	a	warm	front	shown	on	a	synoptic	 map	is	most	interested	in	storm-scale	 details,	such	as	the	altitudes	of	potential	 icing	aloft	along	a	possible	route	through	 the	area,	where	warm	air	is	cooling	as	it	 rises	over	cold	air	at	the	surface.	 FAA	weather	resources	for	students	and	 instructors	don’t	cover	all	of	weather’s	 scales.	These	materials	describe	at	least	 one	microscale	event:	runway	turbulence	 caused	by	winds	blowing	nearby	around	 buildings	or	trees.	FAA	materials	introduce	some	storm-scale	thunderstorm	 types,	and	also	briefly	describe	synopticscale	storms	and	their	fronts,	as	well	as	 planetary-scale	jet	streams.	FAA	materials	 generally	ignore	mesoscale	events	except	 for	squall	lines.	
DERECHOS AND BOW ECHOES. Some	 squall	lines	produce	both	derechos	and	 bow	echoes.	The	NWS	has	been	using	 derecho—deh-RAY-cho—since	the	late	 1980s	to	describe	a	squall	line’s	straightline	winds	(as	opposed	to	the	rotary	winds	 of	tornadoes)	of	57	mph	or	faster,	that	hit	 several	places	along	a	path	that’s	at	least	 240	miles	long,	over	a	period	of	hours. Squall	lines,	especially	derechos,	sometimes	form	bow	echoes—named	for	the	 bow	shape	of	the	line	of	thunderstorms	 seen	on	weather	radar.	"Bow	echo"	should	 alert	pilots	to	the	possibility	of	dangerous,	gusty	winds	because	the	squall	line	 winds	that	do	the	most	damage	are	often	 associated	with	bow	echoes.	Weather	 researchers	and	forecasters	have	been	 talking	about	these	since	the	late	1970s;	 you	will	occasionally	hear	TV	meteorologists	talk	about	a	bow	echo	showing	up	on	 weather	radar.	 MESOSCALE CONVECTIVE SYSTEMS.

TYPE
BOW ECHO

DESCRIPTION
Bow-shaped line of thunderstorms.

IMAGE

DERECHO

The straight-line winds of a squall.

MESOSCALE CONVECTIVE SYSTEMS

The size of—and the up-and-down air motion of—showers and thunderstorms.

weather	for	pilots	is	found	in	groups	of	 thunderstorms	known	as	“mesoscale	 convective	systems”	(MCS).	The	word	 mesoscale	tells	you	the	size	of	these	 events.	“Convective”	refers	to	the	up	and	 down	air	motions	found	in	showers	and	 thunderstorms.	“System”	tells	us	the	atmosphere	has	organized	the	thunderstorms;	 they	don’t	just	happen	to	be	occurring	in	 the	same	area. One	reason	it’s	a	mesoscale	convective	system	is	that	cool	downdrafts	from	 mature	thunderstorms	spread	out	when	 they	hit	the	ground	to	trigger	new	thunderstorms	by	pushing	up	warm,	humid	air.	 Mesoscale	systems	can	form	in	moreor-less	straight	lines	(squall	lines)	or	as	 circular	areas.	In	addition	to	showers	and	 thunderstorms,	these	areas	can	include	 stratiform	rain	that	falls	steadily	over	 relatively	large	areas,	creating	instrument	 meteorological	conditions.	In	addition	to	 being	large,	MCSs	last	for	hours.	
MESOSCALE CONVECTIVE COMPLEXES.	 A	mesoscale	convective	complex	(MCC)	 is	one	kind	of	MCS.	Meteorologists	didn’t	 recognize	MCC	as	organized	systems	 until	the	late	1970s	and	early	1980s.	 Robert	Maddox,	a	former	U.S.	Air	Force	 weather	forecaster,	and	other	researchers	 used	then-new,	nighttime	color	infrared	 satellite	photos,	which	indicate	cloud-top	 temperatures,	to	conclude	that	what	had	 seemed—from	the	ground—like	unorganized	batches	of	thunderstorms	and	other	 rain	really	were	organized	systems	with	 very	high,	and	thus	cold,	clouds.	

Potentially	the	most	dangerous	mesoscale	

To	be	considered	an	MCC,	the	system’s	cloud-top	temperatures	of	minus	 25	degrees	Fahrenheit	or	colder	need	to	 cover	at	least	38,627	square	miles,	an	area	 roughly	the	size	of	Iowa.	A	center	area	 of	cloud-top	temperatures	of	minus	62	 degrees	Fahrenheit	or	colder	covering	at	 least	19,313	square	miles	also	is	needed.	 To	be	this	cold,	clouds	have	to	top	out	 between	40,000	to	50,000	feet	above	the	 ground.	Thunderstorms	reaching	this	 high	into	the	sky	are	powerful.	These	 high	clouds	also	must	be	nearly	circular	 in	shape,	distinguishing	the	system	from	a	 squall	line.	Finally,	the	system	must	hang	 together	for	at	least	six	hours.	 MCCs	grow	and	persist	after	the	sun	 goes	down,	shutting	off	the	solar	energy	 that	helps	power	most	thunderstorms,	 because	low-level	jet	streams	bring	in	 warm,	humid	air	from	over	the	Gulf	of	 Mexico.	These	are	fast-moving	streams	of	 air	1,000	to	2,500	feet	above	the	ground.	 The	wind	begins	blowing	when	the	ground	 south	of	the	thunderstorms	cools	after	 sunset.	As	this	happens,	an	inversion—air	 aloft	that’s	warmer	than	the	air	at	ground	 level—forms	because	the	air	aloft	doesn’t	 cool	as	fast	as	air	next	to	the	ground.	 The	inversion	shuts	down	the	upand-down	air	motions	that	make	flights	 on	sunny	days	bumpy	at	low	altitudes.	 Meteorologists	say	that	an	inversion	 decouples	the	upper-altitude	winds	from	 the	surface.	Without	slow-moving	air	 rising	from	below	and	some	of	the	fastmoving	air	aloft	sinking,	the	air	aloft	is	no	 longer	“connected”	to	slow-moving	air	

MAY 2012 FLIGHT TRAINING

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Flight Training - May 2012

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