Daily Wind Changes in the Lower Levels of the Atmosphere

February 2014

NOAA’s National Weather Service

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Managing Editor:

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Editor: Nancy Lee

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General Aviation:

Identify and

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Hazardous Weather

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Daily Wind Changes in the Lower

Levels of the Atmosphere

Jeff Halblaub, Meteorologist, NWS Hastings, NE

On most days, winds change substantially between the surface

and the lowest few thousand feet above ground level (AGL). These

changes are part of the daily cycle driven by the sun. The atmosphere

behaves like a uid. The layer of uid in contact with an underlying

surface is called the boundary layer. The atmospheric boundary layer

moves through a daily cycle based on heat from the sun.

This cycle of daytime heating and nighttime cooling explains why,

under most circumstances, higher winds are conned above the surface

at night. As low-level temperatures warm during the morning hours,

those higher winds gradually drop down to the surface, resulting in

daytime gustiness.

A well-mixed boundary layer results in substantial turbulence. This

turbulence is magnied when cumulus clouds form posing challenges

to Visual Flight Rules (VFR) operation. When ying above the boundary

layer, which is generally much smoother, there is an abrupt increase

in turbulence during descent into the boundary layer. This turbulence

continues down to the runway.

Pilots frequently must deal with daytime gustiness during takeoffs

and landings. These gusty surface winds usually begin in the late morning

hours, peak in the afternoon, and end by early evening. Winds in the

low-levels become much more uniform at night and in predawn hours.

Departures into a strong temperature inversion can result in smooth

ight conditions. There is, however, the occasional threat of low-level

wind shear and turbulence at the transition between the cool surface

air and the higher winds just a couple thousand feet AGL. This threat

is especially strong if there is a sharp change in wind direction.

During the night, the loss of the sun’s radiation causes the earth’s

surface to lose the heat it builds up during the day. This cooling creates

a shallow, stable layer of air near the ground, resulting in a temperature

inversion. In an inversion, the temperature in the layer above the ground

is actually warmer than it is near the surface.

The increased stability limits the transfer of temperature, humidity,

and wind down to the surface from the rest of the atmosphere above.

The term for this is called decoupling, where this layer is no longer

“aware” of what is occurring above it. The winds above the inversion

can be strong while in the stable layer below, winds are calm or light.

This change in wind character can occur over a

very short depth of 100 feet or less.

After sunrise, the sun’s radiation begins

warming the earth’s surface. As this occurs,

parcels (or bubbles) of warm air begin to rise within

the cool layer. These parcels can be compared to

the initial stages of a pot of water on the stove.

As the pot becomes hotter, bubbles form at the

bottom. They become warmer than the rest of the

water and begin rising one by one to the top of the

water. As the water gets hotter, more bubbles form

until the water is boiling.

A similar event occurs in the lower atmosphere.

The rst few parcels rise into the warmer air left

from the previous afternoon. The parcels can only

rise so far before they reach equilibrium with the

temperature of the surrounding environment. Just

like the pot on the stove, as the sun continues to

warm the ground, the parcels become warmer,

allowing them to rise through the lowest few

thousand feet AGL and mix deeper into the lower

levels of the atmosphere (see gure above). This

process continues until the previous afternoon’s

warmth is completely replaced and deep mixing

occurs between the earth’s surface and a few

thousand feet AGL. This process results in an

evolution of the progressively expanding boundary

layer during the daytime hours.

As mixing occurs, the boundary layer becomes

coupled (or “aware”) to what is occurring in the rest

of the lower atmosphere. As a result, temperature,

humidity, and wind are thoroughly “mixed”

throughout the lowest few thousand feet AGL.

If there is sufcient moisture, cumulus clouds

form at the top of the boundary layer. These

clouds can further increase the magnitude of the

turbulence. Typically, cooler temperatures, drier

air, and higher winds mix downward to the surface.

This mixing is why winds are usually gustier at the

surface during the daytime. The result is called

thermally-induced turbulence, which extends

through the depth of the boundary layer. Thus,

when aircraft are ying above the boundary layer

and descend into it, there is an abrupt increase in

turbulence. This change also explains why winds in

the lowest 3000 to 5000 feet gradually make their

way to the surface during the late morning hours.

As the sun’s radiation wanes late in the day,

the temperature of the earth’s surface begins to

fall. Stability increases and cooling air parcels can

no longer rise into the warmer surroundings. A key

sign of this change is that any cumulus clouds that

might have formed begin to decrease in coverage

and eventually dissipate. Surface wind speeds drop

and gustiness comes to an end as the cooling layer

near the surface decouples from the remnants of

Stull, R. B., 1988:

An Introduction to

Boundary Layer

Meteorology.

the afternoon boundary layer. Without the mixing,

winds within this remnant layer gradually become

less turbulent and begin to behave like the rest

of the atmosphere, usually increasing in speed

with height AGL. With much lighter or even calm

winds within the newly formed boundary layer,

the interface with the increasing speeds aloft

can create wind shear, referred to as mechanical

turbulence.

There are days when other factors alter this

diurnal trend such as low pressure systems or a

strong pressure gradient. Extensive cloud cover

and temperature inversions near fronts can also

signicantly limit the depth of the boundary layer.

In areas with lush, green vegetation, the average

depth of the boundary layer is lower versus arid or

desert regions. Because the earth heats up more

over dry, rocky soil, the average boundary layer is

much higher, from 8,000 to 12,000 feet.

As a pilot, being aware of the local regime or

weather pattern (high pressure location/frontal

positions) and looking at the METARs over the past

24 hours will help you begin to anticipate the local

nuances for your preferred aireld. For example

let’s look at Avoca, Wilkes-Barre Scranton, PA.

KAVP, under high pressure, typically the winds

become calm toward sunset. Later in the evening

and overnight, the colder winds from the ridges to

the northeast, providing a steady overnight wind,

typically something like 05007 kts. Then with

sunrise, the northeast winds die off, followed by

a few hours of calm winds. By 9 am to 10 am, the

inuence from the winds aloft due to the heating

mechanics discussed above would set up the

afternoon wind direction and speed through the

rest of the day.

This is just one example for a local airport. By

taking time to associate the local weather pattern

with the METARs, you’ll gradually be able to better

anticipate typical wind patterns that might affect

your ight.

Reprinted with permission from the National

Transportation Safety Board

The overwhelming majority of aviation-related

deaths in the United States occur in general

aviation (GA) accidents. In 2011, there were 1,466

GA accidents, of which 263 were fatal; 444 people

were killed. The accident rate per 100,000 ight

hours remains substantially higher in GA than in

commercial aviation (6.51 for GA compared to 1.5

for on-demand Part 135 operations and 0.162 for

scheduled Part 121 operations).

Historically, about two-thirds of all GA accidents

occurring in instrument meteorological conditions (IMC)

are fatal—a rate much higher than the overall fatality

rate for GA accidents. IMC refers to meteorological

conditions expressed in terms of visibility, distance

from clouds, and ceiling less than the minimums

specied for visual meteorological conditions.

A frequent contributing factor to these

accidents is hazardous weather. For example, on

December 19, 2011, a Piper carrying the pilot and

four passengers impacted terrain following an in-

ight break up near Bryan, TX. NTSB investigators

determined the probable cause of the ve-fatality

accident was the pilot’s inadvertent encounter with

General Aviation: Identify, Communicate Hazardous Weather

severe weather, which caused the left wing to fail.

One of the issues identied in the investigation

was the presentation of weather radar data in the

cockpit, obtained through the pilot’s subscription

to satellite-based weather services.

The NTSB continues to examine the Federal

Aviation Administration’s (FAA) weather information

dissemination practices in recent investigations as

well as the consistency of NWS weather advisory

products for the aviation community.

While having weather information available to

pilots, air trafc controllers, and meteorologists

is crucial, misunderstanding and misuse of this

information can prove just as dangerous, if not

more dangerous, as not having that information at

all. Examples include pilots gaining a false sense of

condence that may lead them unknowingly into

adverse weather conditions, or air trafc controllers

not effectively using weather information they

have to assist pilots in avoiding such conditions.

What Can Be Done . . .

In the almost 50 years of NTSB accident

investigations, the Board has determined solutions

to weather issues fall into three broad areas:

 Ensuring pilot training and operations

 Creating weather information and advisories

 Collecting and disseminating weather

information, particularly by the NWS and the FAA

The rst line of defense in preventing a GA

weather-related accident are the pilots. Pilots

decide when and where to fly the aircraft.

Therefore, training on how to obtain and use

hazardous weather information is critical. In

addition, granting pilots, as well as FAA-contracted

weather briefers, access to real-time weather

information through weather cameras would

further enhance operators’ situational awareness.

Another key line of defense is air traffic

controllers, who provide weather data to pilots

prior to, and during ight. Pilots then use this

information to decide when and where to y.

To meet these needs, controllers must have

unimpeded access to critical information on key

weather scenarios, such as mountain wave activity

advisories and real time lightning data.

A mountain wave is the wave-like effect,

characterized by updrafts and downdrafts, that

occurs above and mainly to the lee of a mountain

range when rapidly flowing air encounters

the mountain range’s steep front in a near-

perpendicular fashion within a supportive vertically

stable atmosphere (referred to as a mountain wave-

supporting environment). Mountain wave activity

refers to these updrafts and downdrafts, their

associated turbulence, and other wind phenomena

that can occur in association with a mountain wave-

supporting environment, such as rotors, hydraulic

jumps, and down-slope wind events.

Controllers must also be trained and equipped

to transmit this critical information expeditiously.

Further, because controllers are the primary

recipients of pilot reports (PIREPs), the FAA must

have the infrastructure and protocols in place to

ensure such vital information is conveyed in the

national airspace system (NAS).

What is the NTSB doing?

In 2005, the NTSB conducted a safety study

to better understand the risk factors associated

with accidents occurring in IMC or poor visibility.

This report was the fth on weather-related GA

accidents since the NTSB’s creation in 1967. The

NTSB has also explored hazardous weather issues

in numerous GA accident investigations. Based

on this body of research, the NTSB has reached

out to the various operator and user groups to

engage more than 20 stakeholders across several

agencies, including the FAA, the NWS, the National

Air Trafc Controllers Association, the Aircraft

Owners and Pilots Association, Lockheed Martin

Flight Services, and the Air Line Pilots Association.

To date, stakeholders have held initial meetings,

and progress has been encouraging.

The NTSB continues to investigate and

research ways to enhance hazardous weather

communications. In early 2014, the NTSB intends

to complete additional work on mountain wave

activity and work on addressing NWS internal and

external communications and operational use of

PIREP information.

The NTSB is also examining the use of Light

Detection and Ranging (LIDAR) information in

Las Vegas and the structure of retrieving and

disseminating airport runway wind information in

the NAS. LIDAR is a method of remotely sensing

atmospheric wind information by laser technology.

Given the frequent role that weather plays in GA

accidents, the NTSB will continue to examine all

aspects of weather-related safety issues.

For more information, see the NTSB Most Wanted List.