2. Turning with the
Wind.
Coriolis explained and why steering is so variable.
3. The Nocturnal Low-Level
Jet.
What, when and where.
4. Watching Winds Aloft.
How to read weather maps and upper air charts.
-----------------------------
By Don Portman, Sr.
Associate Member:
Emeritus Professor
Balloon Federation of America
of Atmospheric Science
SouthEastern Michigan Balloon Association
The University of Michigan
Member: American Meteorological Society
Questions, corrections, comments,
suggestions?
Email: donport@umich.edu
Copyright 2004
Sooner or later, almost every balloon
pilot is going to have a high-wind landing.
If it's just you in the basket, it
will be pretty exciting. If there are passengers along
with you, which is the case most of
the time, it will not only be exciting, but could be
very dangerous. Tom Reusse in
Ballooning, March 1996.
Introduction.
A total of 88 fatalities were reported in more than 1400 government accounts of hot-air balloon accidents, from 1964 to 2001. The reports were examined by the authors of Reference 1who stated that "Many of the landing accidents occurred late in the morning in wind or thermal activity." For a similar study (Reference 2), but for the period 1964 to 1995, 495 crashes were examined in which there were a total of 92 fatalities and 384 serious injuries. Of all crashes described in the latter report, 63% happened between 5 and 10 in the morning, 22% between 4:30 and 8 in the late afternoon, and 15% at other times. Nearly half (46 %) happened during landings. Data for 421 of the 495 crashes showed that 85 % of them could be accredited to pilot error (or incapacitation) with greatest errors being inappropriate flight planning (28 %) and misjudging wind (19 %).
Anyone familiar with the difficulty in forecasting the onset of daytime winds after sunrise, would not be surprised to find that a high percentage of the crashes occurred in morning landings. There appears to be an important lack of application of wind knowledge by balloonists.
.
Boundary-layer wind described in this tutorial is restricted primarily
to conditions having minimal influence from the atmosphere above. They
are the conditions popular for balloonists: cloudless or nearly cloudless
skies and overall light winds. These are also the conditions most intensely
examined by boundary-layer meteorologists. The most complete and detailed
measurements, in addition, have been made over extensive flat terrain,
far from trees, buildings and other topographic features that would distort
the flow. Data from such investigations are used here to illustrate
basic boundary-layer wind behavior that is also especially relevant to
hot-air balloon operation. Winds influenced or caused by valleys, mountains,
trees, bodies of water and other topographic features are to be treated
in following installments.
This installment has the following parts:
1. Average Wind-Speed Changes with Time-of-Day and with Height.
2. Near-Surface Wind-Speed
Changes in Transition Periods.
(Transition periods = sunrise and sunset times)
3. Boundary-Layer Turbulence and Mixing.
4. Mixing and Stability.
5. Stability and Heat Exchanges at the Ground.
6. Temperature Profiles and Wind Profiles.
a) Day/Night Differences.
b) Sunset Transition Times.
c) Sunrise Transition Times
7. Departures from Transition-Time Patterns.
8. Summary.
All figures have been carefully selected to illustrate fundamental processes or significant features for hot-air ballooning, or both. Figures 1 through 6 and Figure 11 show how things change with time. The others show how temperature and wind change with height. Height changes are graphed in a standard meteorological scheme: height on the vertical scale and things dependent on height on the horizontal scale. For the fullest comprehension of the text, it is important to examine each figure and diagram.
Reading and understanding the material in this brief tutorial will not enable you to forecast the weather. Hopefully, however, it will give you an awareness of your flying environment, an appreciation of a weather forecaster's problems, and an ability to prepare for failed forecasts.
1. Average Wind-Speed Changes with Time-of-Day and with Height.
A weather forecast for only two-hours, a time period that would include most hot-air balloon flights, might be expected to be reliable and routinely available from either government or private weather services. Unfortunately this is not the case. Atmospheric conditions at times safest and most popular for hot-air ballooning are also among the most difficult to forecast, especially in the detail required. These are times when the boundary layer is undergoing physical changes from day-to-night or night-to-day. The most important result is the change in wind speed. Even the most casual observer knows that wind near the ground is generally stronger in the daytime than at night.
This fact is illusrated by the data analyzed for Figure 1. They show the frequency of occurrence of wind speeds less than 10.5 mph (9.1 knots) at different times of day as measured at the Lansing, MI, airport, averaged for five July months, 1949-54. During the night, from sunset to sunrise, the observed speeds averaged less than 10.5 mph about 90% of the time. After sunrise the frequencies decreased to nearly 50 % by midday, then increased back to about 90 % after sunset.
The changes are such that speed differences between the lowest level and
the highest are much greater during the night than during the day.
Almost always, in addition, the change in speed per unit height is greater
at lower levels than at higher ones.
1. Near-Surface
Wind-Speed Changes in Transition Periods.
Measurements made during a balloon competition at Battle Creek, MI, July
1988, illustrate daily transition-time winds for sunrise (Fig.3) and sunset
(Fig.4). They are five-minute average speeds measured at a height of 33
feet on a mast at the edge of the airfield. The graphs show typical wind
patterns in sunrise and sunset transition periods, when it is not very
cloudy or windy, popular flying
conditions.
The wind speed increase following sunrise may progress rapidly,
or slowly, to an unacceptable landing value. A flight taking off at sunrise
in calm conditions can easily be threatened by a gusty wind speed of 15
knots or more, less than an hour later. Toward the end of the day, on the
other hand, wind may begin to decrease long before sunset. But it
is often erratic, with periods of teasing low speeds alternating
with higher speeds associated with passing thermals. If, as usual,
it is necessary to land at sunset, or before, many different situations
can make the wind forecast difficult.
Of course, typical changes in speed at transition times, are not
always experienced. Many different patterns are possible. Wind data are
shown in Figure 5 for an early June day in southeastern Michigan. Again,
these are five-minute averages about 33 feet above ground. It was overcast
throughout the 24-hour period and the wind, ranging between 2 and 12 knots,
shows little of the pattern seen in Figures 3 and 4. The lowest speeds,
in fact, are at midday and the highest during the night.
The difference between the patterns represented in Figures 3 and 4 and
that in Figure 5 is due mainly to the difference in cloudiness. This important
relationship is most easily understood by a look at the basic nature of
turbulence in the boundary layer, along with how wind speed and temperature
change with height, and how these changes are caused by the presence or
absence of solar heating. (Note that the differences in time intervals,
6, 8 and 24 hours, represented in Figures 3, 4 , and 5, respectively,
can account for the different appearances of the five-minute speed averages.)
3. Boundary-Layer Turbulence and Mixing.
Figure 6 is a graph of wind speed during a cloudless day, recorded at the same location as were the data for Figure 5. The location is several hundred feet from the nearest trees or other obstructions to flow and, in this case, the ground was uniformly covered with snow. The speeds are one-minute averages, measured 33 feet above ground. They are shown with heat from the sun measured simultaneously.
The minute-by-minute fluctuations of the wind speed seen in Figure 6 illustrate an important characteristic of the wind in the air layer the ground. Even in the absence of obstacles, it is almost always turbulent, more so during the day than during the night. It is made up of fluctuations of speed in all directions, both horizontal and vertical, including smaller ones that can't be measured by a standard anemometer. These rapid fluctuations may not be felt while piloting your balloon but they keep the air stirred-up most of the time.
As the wind blows horizontally across the ground in response to large-scale
pressure differences, as shown on a weather map, the air nearest the ground
is held back by friction. The reduction in speed is transmitted to higher
layers by the air's molecular viscosity and by turbulent fluctuations,
themselves acting as a much more effective viscosity. The turbulence can
be thought of as being caused by speed layers adjacent to each other, the
higher speed being in the upper layer. The shear between the two layers
causes small parcels to be moved in all directions. Thus shear may be thought
of as causing turbulence which, at the same time, works to eliminate shear
by its mixing ability. Except at times of calms, the average horizontal
motion keeps the process alive.
Ground roughness elements add to the friction effect. Because of their
wide range of shapes and sizes, from grains of sand and blades of grass
to trees and buildings, their effects range widely. A flat and level snow-covered
field has one of the least influences. The data in Figure 6 were obtained
over such a surface, but as can be seen, turbulence at 33 feet height is
clearly evident.
4. Mixing and Stability.
The day/night differences in wind characteristics, both in average speed and level of turbulence, cannot be explained by friction and shear effects. The differences, instead, are rooted primarily in the day/night differences in heat exchange between the air and the ground. As described below, these differences result in how height-changes in average air density and temperature, themselves, change from day to night. The effect is most easily seen when they are averaged over several minutes to exclude rapid turbulent fluctuations. If more-dense (heavier) air underlies less-dense (lighter) air, turbulence, caused by shear or roughness elements is inhibited. In fact, it can be totally suppressed if density decrease with height is large enough.
Average density change with height can be determined from average temperature change with height, a temperature profile, if the fall of pressure with height, always present and essentially constant, is taken into account. Fortunately this is easily done by comparing a temperature profile with the Dry Adiabatic Lapse Rate. The latter represents the fall in temperature required to compensate for the fall in pressure with height that would otherwise require a fall in density. (The word adiabatic means "no heat exchange" and, here, the modifier "dry" means no evaporation or condensation of water.) The DALR would exists in a layer of air thoroughly mixed and isolated from any sources or sinks of heat.
In this hypothetical situation there would be no change of average air density with height and turbulence would be neither enhanced nor suppressed. It is an important reference to separate conditions of inhibited turbulence, when the density decreases with height, from those with enhanced turbulence, when the density tends to increase with height. The DALR is very close to 5.4 degrees Fahrenheit per 1000 feet (1 degree Celsius per 100 meters) and does not change significantly with height.
Basic characsteristics of vertical temperature profiles are shown in Figure
7. Superadiabatic refers to a lapse of temperature greater than the DALR.
It is commonly found only at the ground on sunny days and can be maintained
only if the rate of heat transfer to the air is great enough to offset
the turbulent mixing it is enhancing. Meteorologists describe a layer as
having neutral (static) stability , or simply being neutral, if its lapse
rate is the same as the DALR; turbulence is neither inhibited nor
enhanced. (At times, however, a dry-adiabatic layer is described as being
unstable because of the absence of restraining affects of an inversion.)
A layer is called stable if the temperature increases at a rate greater
than the DALR. The latter includes inversions and isothermal layers as
well as the average tropospheric lapse rate (3.6 Deg. F. per 1000 ft.),
as shown by the blue lines.
(There is ambiguity in the meteorological profession in use of the word
inversion. It sometimes refers, as used here, only to a temperature increase
with height. At other times it refers to any temperature increase greater
than the DALR. In the latter case it would include all the stable layers
in Figure7.)
5. Stability and Heat Exchanges at the Ground.
The sun, of course, is the major component in heat exchange processes at the ground. On average about 85% of the sun's heat passes through a cloudless atmosphere . At the ground 5 to 30 percent, or more, is reflected back through the atmosphere. The remainder (1) heats the soil and vegetation, (2) evaporates water from soil and from transpiring plants or melts snow and, (3) heats the air. The last Item (3) controls the vertical distribution of temperature near the ground. Heat is transferred from ground to air mainly by physical contact.
The ground, like all substances above absolute zero temperature, is always emitting heat by infrared radiation so that, in the absence of the sun at night, the ground cools, and cools the air next to it. Water may condense as dew, releasing heat required to evaporate it during the day, but the overall result is usually a reversal of the vertical temperature distribution in the air near the ground. An inversion replaces the daytime decrease with height.
Clouds have a large influence on amounts of heat exchanged at the ground.
They reflect and absorb solar radiation and emit infrared radiation. An
overcast layer of low clouds, such as the common stratocumulus, has the
overall effect of reducing, or effectivey eliminating, the difference
between night and day in the temperature structure of the lower atmosphere.
The cloudless atmosphere also emits (and absorbs) infrared radiation but
in amounts much smaller than those of a cloudy one.
6. Temperature Profiles and Wind Profiles.
Clearly showing the difference between day and night are the temperature and wind profiles shown in Figure 8. The morning temperature profile has a characteristic nocturnal inversion and the midday profile has a superadiabatic layer. Both features, as usual, are based at the ground and, in this case, extend up to around 800 feet.
The 45-knot maximum is known as a nocturnal low-level jet (LLJ),
a misleading term because its physical dimensions are more like those of
a sheet, than those of a jet. It may extend horizontally many tens
of miles, but with a thickness, as shown here, of only a few hundred feet.
To help understand diurnal wind speed changes at the ground, it should
be noted that the LLJ is characterized by a diurnal oscillatory behavior
related to the turning of the earth. It has a distorted period due partially
to the daily asymmetrical alternating of stable and unstable layers near
the ground.
Apart from the oscillatory behavior of the LLJ, the most significant process
responsible for these important changes in the wind profile is the development
and subsequent growth of the ground-based inversion caused by the reversal
of heat exchanges at the surface described above. As the inversion develops,
turbulence is severely restricted and the speed from above can no longer
be mixed effectively to the ground. The momentum of the air, now restricted
in its downward motion, appears in its forward motion as the low-level
jet. Such jets are not observed every night but average data, as seen in
Figure 2, reveal nighttime speed increases above a few hundred feet to
suggest that the jets occur in a range of intensities. One can expect relatively
strong wind shears during the night, and into the morning, necessarily
associated with such high speeds not far above the ground.
b) Sunrise Transition
Time. A look at wind and temperature profiles changing
in the morning transition period can help anticipate when the nighttime
low wind speed at the ground will change to the higher daytime speed. Morning
transition profiles are shown in Figure 10. They are for the morning of
the cloudless night following the sunset transition period just described.
The 6:30 AM wind-speed profile displays a strong shear, the speed increasing
from 12 knots near the surface to 30 knots at 328 feet, a low-level jet
with a maximum speed of 36 knots at 984 feet. Speed at the lowest level,
13 feet, increased from 12 to 17 knots in the warm-up period from 6:30
to 10:30. Above that level the speeds decreased (at least below 2300 feet),
the jet disappeared and the wind speed became nearly constant with height.
Two important processes were at work: 1) As the lowest air layer became
unstable, turbulence within it was enhanced and mixed wind speed downward
allowing momentum to be lost at the ground, and 2) The low-level jet disappeared
after sunrise, reacting to the diurnal change in near-ground intensity
of turbulent mixing.
It is apparent that knowledge of the time of breakup of the inversion and the development and growth of an unstable layer at the ground would help anticipate the time of speed increase at the ground. The time of changeover in stability at the ground, in turn, could be estimated from knowledge of:
1) Wind and temperature profiles just before sunrise;
2) Factors in the surface heat balance, especially heat from the sun, and
3) The nature and behavior of the LLJ during this period.
Items 1) and 2) have been used by forecasters to predict the times in the
mid-morning when sizes and strengths of thermals are sufficient to sustain
sail-plane (glider) flight. Usually small aircraft are used to measure
wind and temperature profiles up to several thousand feet just at sunrise,
or before. The procedure involves analysis with a thermodynamic diagram,
such as a skew-T diagram or with a computer, to calculate the heat required
to change the ground-based layer from an inversion to an adiabatic or superadiabatic
condition. With a forecast of degree of cloudiness, and estimation of moisture
in soil and vegetation and, of course, knowing latitude and day of year,
it has been possible for experienced forecasters to achieve reliable results.
7. Depatures from Transition-Time Patterns
Clouds and Winds Aloft . The above descriptions of wind patterns and temperature profiles have emphasized day/night differences to focus on conditions during sunrise and sunset transition periods. Diurnal changes are most pronounced in cloudless conditions when the differences between day and night heat exchanges are greatest. When it is overcast in the daytime the clouds restrict heat available at the ground; when overcast at night, clouds radiate to the ground and reduce the net loss of heat to space. Temperature profiles are then less extreme and the consequent wind distributions are less dependent on time of day.
Wind above the boundary layer also plays an important role in wind patterns near the ground. Near the center of an anticyclone (a HIGH on a weather map) the winds are light and diurnal changes as described above dominate. There are usually few clouds in these conditions so that the effect is enhanced. The other extreme is cyclonic flow around a LOW center which is usually windy and cloudy so that this combination minimizes diurnal boundary-layer changes.
The separate influences of wind and cloudiness are uniquely shown in Figure
11 by a compilation of two years of observations taken at Brookhaven National
Laboratories, Long Island, NY. Temperature differences between heights
of 360 and 36 feet on a meteorology tower were sorted according to clear
or overcast conditions for three different wind speed categories. One important
feature is that the 360 foot minus 36 foot temperature differences average
about +6 degrees on clear nights for wind speeds 0 to2 knots, but only
-0.6 degrees for wind speeds 12-17 knots. Many other features of this figure
are worth careful examination.
Thermals and
Thunderstorms. Thermals can influence sunset transition-time
ballooning by showing up unexpectedly before they completely collapse in
the late afternoon. Thunderstorms, perhaps nowhere in sight, can cause
a sudden, possibly disastrous, increase in wind speed as the outflow from
downdrafts spreads sometimes over tens of miles.
Topographic Effects. Most examples in the foregoing were taken from observations made over flat terrain void of trees, buildings or other obstructions to flow. In other words, topographic effects were not considered. Wind patterns influenced or caused by, valleys, mountains, trees, bodies of water and other common features of the landscape have not been described here.
The important characteristics of such influences will be treated in following
installments in this series and, along with others, a summary of stability
influences on steering in the boundary layer.
8. Summary.
A high percentage of balloon crashes, especially those with fatalities and injuries, are apparently related to high-wind landings. Difficulty in forecasting wind speed at landing time can be expected if the landing is during the changing wind conditions in transition times, i.e., a few hours following sunrise and a few before sunset. Winds near the surface are often changing between day and night regimes: weak during the night, but much stronger during the day.
A few hundred feet above the ground, however, they are stronger during the night and weaker during the day. Day/night wind differences in the atmospheric boundary layer are mainly due to different intensities of turbulent mixing between day and night. At night the ground cools and cools the adjacent air. The air becomes stable with warmer air overlying colder air thereby inhibiting turbulent mixing. In the daytime solar energy reverses the heat balance at the ground, the situation changes and turbulent mixing is enhanced. Nighttime stability gives rise to large increases of wind speed with height; neutral and unstable daytime conditions cause the air to be well mixed with little wind speed change with height. Through their control of turbulent mixing, temperature profiles, in comparison to the Dry Adiabatic Lapse Rate, nicely show how the ground surface heat balance influences wind profiles.
Cloudiness reduces day/night differences and thereby reduces the changes experienced during transition periods. Knowledge of wind and temperature profiles at the beginning of transition periods and the nature of the changes to be expected can help anticipate time and strength of the onset of daytime winds after sunrise and their rate of decay before sunset.
The foregoing generalizations are valid only for uncomplicated homogeneous
topography without flow obstructions and without significant changes in
wind patterns aloft. However, they illustrate basic physical principles
always at work even if obscured by many possible natural complications.
REFERENCES
Reference l: Stockwell, Brent and Shery Larson, 2003: A Look at Hot-Air Balloon Accidents. Balloon Life, 18, May 2003.
Reference 2: Cowl, Clayton T, M.P.Jones, C.F.Lynch, N.L Sprince, C.Zwerling, and L.J Fuortes, 1998: Factors Associated with Fatalities and Injuries from Hot-Air Balloon Crashes. Journal of the American Medical Association, 279, 1011-1014.
SOURCES OF FIGURES AND DATA
Figure No. Source
1. Data from U.S. National Weather Service, Capital City Airport, Lansing, MI.
2. Crawford, Kenneth C. and Horace R. Hudson, 1973: The Diurnal Wind Variation in the Lowest 1500 ft in Central Oklahoma: June 1966- May 1967. J. Appl. Meteor., 12 , 127-132.
3. Data logged by computer in field forecast unit operated by the author's students.
4. Same as Figure 3.
5. Data logged by computer at the author's automatic weather station.
6. Same as Figure 5.
7. Created by the author.
8. Data from: Clarke, R.H., A.J. Dyer, R.R. Brook, D.J. Reid, and A.J. Troup, 1971: The Wangara Experiment: Boundary Layer Data. Division of Meteorological Physics Technical Paper No. 19, Commonwealth Scientific and Industrial Research Organization, Australia.
9. Data from: Lettau, Heinz H. and Ben Davidson, Ed., 1957: Exploring the Atmosphere's First Mile, Volume II, Site Description and Data Evaluation, Pergamon Press.
10. Same as Figure 9.
11. Slade, D. H., Ed., 1968: Meteorology and Atomic Energy 1968,
U.S. Atomic Energy Commission, Division of
Technical Information.
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FLYING TIMES AND WINDY TIMES
THE NOCTURNAL LOW-LEVEL JET
Questions, corrections, comments,
suggestions?
Email: donport@umich.edu