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Every day, atmospheric balloons (often referred to as “soundings”) are launched all over the world to help determine the state of the atmosphere and to provide forecast models with much needed data. For simplistic sake, you can think of these as large and expensive pibals. Worldwide, there are nearly 900 sites which sample the atmosphere regularly. A network of sites which launch balloons are scattered across the United States, separated by approximately 300 miles. In the continental US, 69 National Weather Service Offices launch these balloons a minimum of twice a day. A few offices in the Midwest that makes these launches include La Crosse WI, Davenport IA, Minneapolis MN, Omaha NE and Aberdeen, SD. Each balloon costs nearly 300.00 to launch, which is why there are specified locations which perform these launches. Balloon launches across the United States are performed at as close to the same time as possible, to provide an accurate picture of the atmosphere at a specific across the United States. Each balloon is made out of a stretchy latex material, and is most generally filled with hydrogen. Initially, the balloon that is launched is nearly six feet in diameter, but by the time the balloon bursts, the atmospheric pressure around the balloon has reduced significantly so that the balloon has expanded to a size of approximately thirty feet in diameter. During the balloons flight, three main types of weather observations are made directly by an instrument pack (called a “rawindsonde”) carried by the balloon. These observations are then transmitted back to the launch site in nearly real time. Ten observations per minute are made of pressure, temperature and relative humidity. Wind information is also computed using a simple geometry. A satellite dish is used to track the radio signal from the rawindsonde, and using the position of the balloon, an accurate wind speed and direction can be computed. Each balloon ascends at a rate of around 1000 feet per minute, and rises to a height around 100,000 feet before bursting. At this height, the balloon has traversed the entire depth of the troposphere, and has generally sailed into part of the mesosphere (where the temperature increases with height). When the balloon bursts, the rawindsonde falls back to the earth slowly with the help of a parachute. During a flight, radiosondes can drift more than 125 miles from the original launch site. A mailbag also accompanies the rawindsonde in case it is recovered after a flight. The rawindsonde may then be able to be sent in for reconditioning, and then used again on a future flight. By now, you may be wondering what type of information can be obtained, especially at that kind of price tag. The rawindsonde may be the single most important meteorological tool available, and can be used for many purposes including aviation and for verification of satellite observations. For example, one of the most important pieces of information is a temperature profile of the entire troposphere (where most to all of the weather as we know it occurs). This allows for analysis of the atmosphere that helps determine the stability of the various layers and the depth and strength of an inversion to name just a few. Wind information (both speed and direction) can be helpful in determining the location of the jet stream and in determining storm motion prior to the storm formation. Moisture information for a large depth of the troposphere is also recorded, which along with the wind data and temperature data, can be helpful in determining the type of storms that would occur if they were to form. In the next article, we will begin to learn how to read and interpret the data from a sounding.

Thermals Typically, as balloonists, you wouldn’t consider flying in less than optimal conditions. However, one weather phenomenon that is not necessarily visible and therefore could sneak up on you are thermals. While thermals can make some aviation activities possible, many times they can mean trouble and result in a decision to end a flight prematurely. Although no two thermals are exactly alike, there are some commonalities that you can look for to help avoid being caught in one. Thermals are warm bubbles of air generated by the sun’s heating of the earth’s surface. Early in the morning, the sun warms the earth which in turn then warms the air adjacent to the surface. Throughout the day, a mixed layer grows through turbulent thermals originating at the surface. At times, this motion may result in the development of cumulus clouds, creating a self limiting process as the clouds shield the earth’s surface from the sun. Of course, if the clouds are scattered in nature, thermals will continue bubble. This process will continue until either the solar source is cut off from the surface or until the suns angle becomes low enough where the sun can no longer heat the surface of the earth effectively. Once this occurs, the stronger winds from aloft are no longer mixed down to the surface and the overall wind speed and gusty conditions at the surface will likely decrease. This usually occurs within a few hours of sunset. Therefore, flying within a couple hours within sunset not only allows the winds at the surface to decrease but also decreases your potential of encountering a thermal. The strength of each thermal varies, but because of simple physics, the same amount of displacement upward needs to be equalized by the equal amount of downward motion. Usually, the upward motion is much more concentrated than the areas of downward motion, so being aware of where thermals form at the surface can help you avoid these areas while in flight. Areas that absorb energy and warm relatively quickly are favored areas for thermal development. Some areas that are favored include: areas covered by rock (since rock heats relatively quickly), urban development with substantial areas of concrete, and dark areas of ground cover (recently plowed fields). Time of day can also influence thermal development in a particular area. If one area is exposed to more direct sunlight than another, it has a better chance of producing thermal activity. For example, if a dark colored hillside faces east it is more likely to develop thermals in the morning hours under the influence of direct sunlight. Additionally, you can use signs mother nature to identify the presence of a thermal. If you see two flags pointing towards each other, there is likely a thermal between them since the air is flowing toward a common point and can’t go into the ground. If you see something like this, do what you can to avoid the area. As alluded to earlier, thermals can at times cause cumulus clouds to develop. These types of clouds are usually scattered throughout the sky, but the taller they are, the more unstable the atmosphere is, the stronger the thermals are, or a combination of the two. Cumulus clouds form at the top of thermals, but dissipate relatively slowly. If a cloud is fairly crisp looking, the cloud is fairly fresh and thermals are likely present in the area. Likewise, if the cloud is fuzzy on the edges there were likely thermals earlier in the day, but they have since weakened or dissipated. Don’t solely rely on the formation of clouds for identifying a thermal though, since a lack of clouds does not necessarily imply a lack of thermals. Time of year also plays a significant role in the development of thermals. In the spring and summer, the sun’s angle results in more direct and rapid warming of the surface of the earth. However, in the late fall and winter much less energy reaches the surface of the earth, and some of the incoming energy may be reflected away by snow cover. Therefore, thermals are often times weaker or nonexistent. If you are planning for an upcoming flight, you may be wondering how quickly thermals develop/subside? Unfortunately, there is no concrete answer, as the atmosphere is constantly changing; however, there are clues that will let you know how the atmosphere is expected to behave. For example, you may be able to use the dew or frost to determine how quickly thermals develop. The more overnight cooling, the greater the potential for dew. Ideal conditions for dew are clear skies and light winds, similar to conditions for the development of an inversion. If you remember from last month’s article, inversions are very stable layers of air in the atmosphere and actually suppress the development of thermals. Therefore, the heavier the dew, the greater the likelihood that a fairly strong inversion exists. As the sun rises and attempts to warm the earth, some of the solar energy is sacrificed to the process of evaporation. Therefore, as a general rule, thermals do not develop until the dew has begun to burn off. In the evening, you can use wind gusts as a clue to when the development of thermals decrease. In the Upper Midwest, thermals often times transport stronger winds from aloft down to the surface. As the thermals decrease in intensity, the wind gusts also decrease. Thermals, which are ultimately powered by the sun, vary in both time as space as the atmosphere changes. The strongest thermal activity typically occurs in the late morning and afternoon hours, times at which balloons typically are not flying. However, thermals can be present at other times during the day as well. Knowing where thermals are likely to form at the surface, and keeping track of Mother Nature’s warning signs will help make your flight safe! Over the past several months, I have received several questions about weather web sites. Several new sites have popped up over the last year or two, and in the next article we will discuss some of these sites. In the meantime, hope you have smooth, thermal-free sailing.

Low –level Jet During a morning flight, you may have noticed that the winds were quiet on the surface, but screaming just a few hundred feet above the surface. You have probably noticed this phenomenon particularly on morning flights in the late spring to early summertime, when the frequency of what is referred to as the low-level jet reaches a maximum. The low-level jet often peaks in intensity at night right above the surface. There are several complex explanations as to why the low level jet forms, but the primary reason is caused by fluctuations in the wind that are created when the low levels of the atmosphere are no longer influenced by the daytime surface heating. The low-level jet forms because the daytime mixing caused by surface heating is not sustained overnight. The low-level jet generally peaks in intensity between 400 to 750 meters (1300 to 2500 feet) above there surface, but its effects can be found much closer to the surface. This is a problematic area in meteorology, because many of the instruments that we use do not measure winds aloft. Instrumented balloons, which are launched twice a day from various sites across the US, record wind, temperature and moisture information at various heights through the atmosphere. Because the cost of each one of these balloons is around $ 500.00, the network where each of these launches occur is sparse. Another tool located in the Midwest is in the wind profiler. As mentioned in a previous article, this instrument does not take a measurement below one-half of a kilometer above ground level, which can be above the maximum peak in the low-level jet. To aid with this sampling problem in Central Iowa, the Central Iowa National Weather Association has considered instrumenting a 2000 foot radio tower located just north of Des Moines. The idea was to place various weather instruments at various heights on the tower that would record various weather information including temperature, dew point and wind speed and direction. This would likely be a costly endeavor as each instrument pack would cost around $800.00. The sustainability of the instruments due to damage from birds and icing became an issue, and this project has been placed on hold indefinitely. However, wind data at these levels would be extremely helpful for balloonists and meteorologists alike in determining the presence and strength of the low-level jet and other meteorological features.

Inversions are meteorological phenomena which balloonists encounter on a regular basis. While inversions can affect the performance of a balloon in flight, they can make flights possible even when strong winds aloft are present. Inversions form under a variety of conditions, and over the next few minutes we will explore a few of the most common types and how they affect balloons in flight. In the lowest portion of the atmosphere (the troposphere), which extends to around 30,000 feet above mean sea level, the temperature typically decreases with height. However, within the troposphere there may be layers where the temperature warms with height. These layers are inversions, which are strictly defined as departures from the typical increase or decrease in temperature throughout the layers of the atmosphere. As we have discussed in previous articles, air associated with high pressure systems flows from the upper levels of the atmosphere towards the surface. As this air sinks, the air is compressed and in response warms in response. This compressional warming caused by sinking air and has thus been termed a subsidence inversion. Because high pressure systems typically have light winds and move relatively slow, inversions formed under these conditions can potentially lead to a series of days with smog. Another type of inversion and the most common type which balloonists encounter on a regular basis is a radiation inversion. As the surface of the Earth is heated by radiation from the Sun, the surface in turn warms the atmosphere. At night, under clear skies and light winds, the surface cools. The layer of air above the surface remains warmer than the air next to the surface, and an inversion develops. Inversions often separate layers of stronger winds from weaker winds. During the morning hours, the solar radiation begins to warm the surface. Eventually, the surface warms to the point where the inversion is unable to suppress mixing and the inversion is overcome. This is often noted on the surface as stronger winds from aloft are mixed down and the overall speed and direction of the surface wind are altered. In the vicinity of fronts (boundary dividing air masses), inversions are commonplace. Fronts not only exist at the surface, but extend well into the atmosphere. These fronts are not typically stacked vertically, but instead slope horizontally with height. A change in air mass sloping with height results in an atmospheric profile with at least one inversion along the boundary(ies) of the different air masses. Finally, the last type of inversion we will discuss forms solely due to the horizontal transport of air. This type is called an advection inversion as air is moved horizontally from one location to another. These inversions can be found near coastlines as sea breezes transport cool air inland from over the water. This cool air near the surface undercuts the boundary layer forming an inversion. These can also be found in the Midwest under strong southerly flow. Under this weather pattern, air is advected from the Mexican Plateau northward. This type of setup may initially suppress thunderstorm development, allowing the atmosphere to destabilize. Late in the day the inversion sometimes weakens or is overcome by mixing and allows thunderstorms to develop rapidly. As with all types of inversions, the warm layer of air relative to the ambient air acts as a lid preventing air from rising, thus suppressing mixing. As one encounters an inversion in a balloon (either through rising or falling), there will be a hesitation before penetrating through the layer. When flying through an inversion, it is best to ascend or descend at a moderate rate as wind shear often accompanies inversions. Low level jets, which form under complex conditions, can be found on the high side of inversions. Therefore, it is important to know what conditions exist above an inversion before flying. Inversions help keep the atmosphere stratified, separating the stronger winds aloft from the weaker winds near the surface. Without them, balloonists would be stuck on the ground waiting for light winds throughout much of the lower atmosphere. In the next article, we will explore a topic that affects weather patterns all over the world, El Niño and La Niña. Radiative Inversions While attending balloon races all across the Midwest this summer, it seemed as though the weather was more marginal for flying than in years past. Many times it seems like the winds were too strong especially in the evening hours, keeping us grounded. One thing that can influence wind speeds at the surface is the strength of what is referred to as a radiative inversion. There are various factors that affect the development and strength of this type of weather feature, which we will explore below. Inversions are simply defined as an increase in temperature with height. In the lowest 30,000 to 50,000 feet in the atmosphere, the temperature decreases with height. When the temperature deviates from the norm and increases in height over a layer, that layer of the atmosphere is referred to as an inversion. In the evening, as the sun sets and the angle of the sun becomes low, the amount of incoming energy that heats the surface of the earth and in turn heats the atmosphere decreases. Air near the earth’s surface cools, while air several hundred feet above the surface remains warm. As the night progresses, the air temperature will warm with height and a radiative inversion develops. During the morning hours, the solar radiation will begin to warm the surface. As the surface warms during the day, it will once again become warmer than the air above. This is often noted on the surface as stronger winds from aloft are mixed down and the overall speed and direction of the surface wind are altered. There are many factors that affect the strength of the inversion. Clear skies and light winds allow the surface to cool as efficiently as possible increasing the strength of the inversion. Surface cover also affects the strength of an inversion. Obviously, snow cover can affect the strength of an inversion in a positive way, but other surface features can lead to a more rapid destruction of an inversion. Surface pavement and freshly plowed fields are fairly dark absorbing much of the incoming energy from the sun and thus warming fairly efficiently. As these areas warm more rapidly than surrounding areas, such as bodies of water and fields covered with vegetation, the strength of the inversion weakens. Just as the sun angle influences the development of a radiative inversion, it also affects the depletion of one. During the late spring and early summer months, the sun angle reaches its peak, resulting in a fairly rapid destruction of any inversion that develops. The amount of night time hours is relatively low this time of year as well, only allowing inversions a few hours to develop. Similarly, the sun angle is relatively low in the winter months, and with daylight hours at a premium and a low sun angle and the potential for snow cover, some radiative inversions remain intact throughout the day. This is one of many reasons why winter months are preferred for long jumps as the inversion keeps the stronger winds aloft from reaching the surface. Once the inversion is overcome, the overall speed and direction of the surface wind is altered. A weakening inversion is similar to a pot of water of the stove that is on the verge of reaching its boiling point. As the water continues to warm, bubbles of water vapor begin to develop on the bottom of the pan but do not reach the surface of the water until the pot has reached a full boil. Once this is accomplished, the water particles in the pot become fairly well mixed, much like atmosphere. The strength of a radiative inversion can be represented by the difference in temperature between the surface and the top of the inversion. The greater the difference, the stronger the inversion. As alluded to earlier, inversions allow the atmosphere to develop into several unique layers, much like layers of a cake. Just as layers of cake can be radically different, the layers in the atmosphere can differ and can result in light to moderate wind shear (the change in wind direction and/or speed over a small distance). As one encounters an inversion in a balloon (either through rising or falling), there will be a hesitation before penetrating through the layer. When flying through an inversion, it is best to ascend or descend at a moderate rate through any potential wind shear. Low level jets, which are favored around sunrise, can also be found on the warm side of inversions. Therefore, it is important to know what conditions exist above an inversion before flying. In the next article we will discuss thermals, the meteorological feature responsible for the destruction of inversions.

Throughout the winter months, nature provides a show of its own. From the mystical magic of the aurora borealis to the chilling appearance of halos, sundogs, and pillars, conditions in the winter months provide favorable environments for these phenomena. Halos are visible throughout the year, but generally occur in the upper levels of the atmosphere because they are caused by refraction of light through ice crystals. Ice crystals, which are in the shape of pillars, refract the light twice, once when it enters and once when it leaves the crystal. The light is refracted 22 degrees, producing a ring of light around the original light source. If the ice crystals are oriented in such a manner that the light enters or exits through the ends of the crystal, a ring of light at 46 degrees is created around the light source. Sun pillars can be viewed most commonly at sunrise or sunset as light is reflected off of small ice crystals falling towards the surface. Generally these ice crystals are high up in the atmosphere, but in frigid conditions with temperatures below 0, these ice crystals can form near the surface. Often referred to as diamond dust, these thin ice crystals can reflect light off of sources near the surface such as street lamps creating pillars of light. Aurora Borealis occur because of interaction between the Earth’s magnetic field and charged particles originating from the Sun. The Earth’s magnetic field is strongest at the poles and weakest near the equator. Charged particles, which are in constant motion around the Earth, are guided by the magnetic field. At times, these particles become energetically charged by a fast moving stream of particles from the Sun. When these particles become charged and then collide with gas atoms in a layer of the atmosphere called the Ionosphere, they give off light. The stronger the magnetic field, the less intense the stream of particles originating from the Sun needs to be to be seen. Although auroras occur throughout the year, they are most commonly viewed in the northern hemisphere in the winter months during the long nights. The light can appear in various colors, including green, blue, red and purple. To help determine where there is an abnormally large number of charged particles originating from the Sun headed for the earth, an index has been devised based on the amount of geomagnetic activity measured in Gottingen, Germany. The index, which updates every three hours, varies from 0 to 9, with 9 being extreme activity. The latest values of this index, called the KP Index, can be found at http://www.sec.noaa....lots/kp_3d.html . In addition, information on the amount of upcoming solar activity can be found at http://www.spaceweather.com Auroras can be viewed on a semi regular basis across the Upper Midwest. When looking for an aurora display, be sure to wait for a clear night and avoid light pollution from near by towns. Auroras are best viewed on cold nights when there is little moisture in the atmosphere. To view pictures of recent aurora events in the Central Iowa area, please go to: http://www.nightskyevents.com/. Unfortunately, my pictures of sun pillars and halos are packed right now, but there are numerous great examples on the web. A few good ones I have found include http://www.extremein...om/03-02-16.htm , http://antwrp.gsfc.n...d/ap011107.html and http://apollo.lsc.vs...pter4/halo.html . In upcoming articles, we will explore tools used in the forecast process and brush up on severe weather terminology.

If you are an avid weather enthusiast, you’ve probably heard a lot about El Nino in recent years. It seems to get blamed for any kind of unusual or bad weather, especially along the West Coast. But up until February 2005, El Nino and its counterpart La Nina have been loosely defined across North America. Even with the lack of a concise definition, it has been know for nearly a century that sea surface temperatures in the equatorial Pacific have a drastic effects on weather patterns. Over time, scientists have learned sea surface temperatures affect tropical rainfall. That starts a domino affect that alters the jet stream and much of the weather globally, enough to have social effects certain parts of the world. The National Oceanic Atmospheric Administration, Meteorological Service of Canada and the National Meteorological Service of Mexico came together to formally define El Nino as “a phenomenon in the Pacific Ocean characterized by a positive sea surface temperature departure from normal…greater than or equal 0.5 degrees Celsius in magnitude.” These events are restricted within 120 to 170 W longitude, and within 5 degrees of the equator. La Nina was defined in a similar manner with negative sea surface temperature departures of a half a degree Celsius or more. To officially qualify as an El Nino or La Nina, the temperature anomaly needs to be sustained for three months or more. El Nino was named after the Christ Child by fisherman off the coast of South America as it most often appears in late December. El Nino events produce rains over the coastal desert brings periods of growth called, anos de abundancia (years of abundance). The establishment of an El Nino pattern creates higher pressure and drought conditions in Darwin, Australia, while lower pressure along the international dateline results in heavy rainfall. La Nina, El Nino’s counterpart, was named for abnormal cooling of the equatorial waters for the first time in 1986. During La Nina phases, trade winds are stronger and the water brought to the surface off the coast of South America is initially cooler that normal. In the western Pacific, waters are warmer than normal causing greater amounts of evaporation leading to heavy rainfall events. The formation of these phenomena begins as water along the South American coast is drawn up from below the surface. This water, which initially starts out around 68 degrees Fahrenheit gradually warms to around 80 degrees as it flows west. The layer which warms due to solar radiation is fairly shallow, usually less than 100 meters (330 feet) deep. Extremely warm temperatures are tough to obtain because of the process of evaporation. As the water continues to warm, more water is lost due to evaporation, thus resulting in a net cooling of the surface. This process makes it difficult to obtain sea surface temperatures greater than 30 degrees Celsius (86 degrees Fahrenheit). However, when above normal water temperatures develop; drastic effects are felt on the fishing industry. The warm water is not as nutrient packed as the cooler water, and the amount of plankton decreases under such an event. This decrease in available food causes a significant decrease in the number of available anchovies, thus resulting in a net loss of fish available for fisherman and birds. These events often lead to widespread starvation which may result in death. El Nino is only part of a weather pattern that brings drastic changes in pressure and winds to large parts of the world. With the impact that these two phenomena have on the global circulation, it is important to forecast strength and duration of an El Nino or La Nina event. El Nino and La Nina are cyclic but not periodic phenomena, meaning on average they occur once every four to five years but can vary from every other year to 10 years between episodes. This makes studying them very difficult because an episode may not be realized until they have formed. Unfortunately, because these events have been discovered in the recent past and are not periodic in occurrence, the cause of these events has not been determined. Scientists thus far have only determined characteristics that are common with each phenomenon. Hypotheses on the cause of El Nino have ranged from bursts of magma along the ocean floor to snowfall amounts over Asia, but have yet to be proven. Until then, atmospheric scientists will continue to determine the role and the drastic effects both El Nino and La Nina play in the global earth system.

Directional wind shear, or the change in wind direction with height, plays an important role in weather and affects everything from the sport of ballooning to storm behavior. A change in wind direction with height that is clockwise as viewed from the surface generally means warm air is moving in. Conversely, winds that change counter-clockwise with height means cooler air is headed your way. Directional wind shear, or the change in wind direction with height, plays an important role in weather and affects everything from the sport of ballooning to storm behavior. A change in wind direction with height that is clockwise as viewed from the surface generally means warm air is moving in. Conversely, winds that change counter-clockwise with height means cooler air is headed your way. In ballooning, directional shear (commonly referred to as “steerage”) is generally most evident in the morning when lower layers of the atmosphere are not well mixed. However, with the help of solar heating from the sun, turbulent mixing causes the stratification of the various wind layers to mix into one layer. This layer, which contains similar characteristics, grows throughout the day until the late afternoon hours when the atmosphere begins to decouple from the surface. Research has shown that the depth of this layer is dependant on the square root of heat transport from the surface to the atmosphere. Some other important parameters include the square root of both the length of daytime heating and the initial amount of stratification. Directional wind shear also plays an important role in storms. If moderate to strong directional wind shear exists in the atmosphere, storms can begin to rotate much easier, increasing the threat for severe weather, especially tornadoes. One of the first ingredients in tornado formation is a rotating updraft, and a change in wind direction helps the thunderstorm updraft rotate. In fact, fairly recent research has shown that directional shear in the lowest kilometer of the atmosphere is a very important factor in tornado formation. If directional wind shear is too weak, then the storm may not be able to rotate efficiently enough to produce a tornado. Directional wind shear can be observed using various instruments. One of the simplest is a pibal, which is nothing more than a helium balloon that is tracked by sight through the lower levels of the atmosphere. This type of observation is commonly made in Africa. Here in the US, we use something called a rawindsonde to try and get an understanding of the atmosphere at a given point. Rawindsondes, which are instrument packs carried aloft by a hydrogen filled balloon, are generally launched twice a day and take a large sample of data through the atmosphere. These instruments record temperature, dew point, wind direction and speed as they ascend through the atmosphere. As the instrument rises through the atmosphere, the atmospheric pressure becomes less and less and the balloon expands till it bursts. The instruments then fall back to the earth under the guidance of a parachute. These measurements are then used to determine aspects about the atmosphere related to temperature, moisture and stability. Because of the expense involved in launching this type of instruments, they are only launched on a regular basis in 92 locations across the United States. A few locations where these are launched on a regular basis in the Midwest include Green Bay, Omaha, Davenport, and Minneapolis. Once the data is gathered for a particular site, it is input into the forecast models and is plotted on a chart called a skew T log P. These charts can be found on the internet daily at http://www.rap.ucar.edu/weather/upper . On the same web page, there is also data that is taken in hourly intervals by wind profilers. Wind profilers are a form of radar that take measurements solely in the vertical. Instead of focusing on precipitation as radar does, wind profilers focus strictly on wind. Wind profilers are prone to errors due to migrating birds. Another drawback to profilers is that they don’t sample the winds below one-half of a kilometer above the ground level. As you can see, the change of wind direction with height can be observed in many different ways. In the next article, we will discuss land sea breezes and how this affects the sport of ballooning.

A few years ago, I found out that it possible to arrive at a good estimate of the wind speed at any height in the boundary layer (the layer near the surface) based on one surface observation. I am sure you are asking yourself, just as I did, how accurate is this? Believe it or not, this is fairly accurate based on a few assumptions that you make. To understand how this equation works, we first need a crash course (or a friendly refresher) in boundary layer meteorology. The boundary layer is a well mixed layer that develops near the surface throughout the day due to the heating from the sun. In a strict definition, the boundary layer is the layer of air that is in communication with the earth’s surface through the process of turbulent transfer. Early in the morning when the sun rises, the amount of solar radiation that reaches the earth’s surface is small. However, as the sun rises, the amount of solar radiation reaching the surface increases. As the amount of energy intensifies, the surface of the earth heats the air from below. The more intense solar energy this is, the faster the boundary layer grows. The boundary layer increases in height throughout the day through turbulence and mixing. Turbulent eddies distribute both heat and moisture throughout the boundary layer. The depth of the boundary layer is dependant on many quantities including cloud cover, stability of the atmosphere and time of year. The depth of the boundary layer can vary from a few hundred meters (~700 ft) to several kilometers depending on these varying quantities. Because of turbulent eddies driven by solar heating that are constantly redistributing the air, certain properties of the atmosphere are constant throughout the mixed layer. Other properties, such as the winds speed are not constant through the mixed layer, but have been studied and are well understood. The measured wind speed is dependant on the roughness of the surface in the area around the observations. As the surface becomes increasingly rough, we would expect winds to be less in the lower part of the boundary layer due to friction. With an increase in height, the wind speed increases similar to that shown in figure 1. Through research, it was found that the shape of the wind profile in the boundary layer is similar to the graph of an exponential function. Therefore, if we can arrive at an estimate of the surface roughness, we can estimate the wind speed in a well mixed boundary layer. This method is good for determining the wind speeds aloft in the afternoon and evening hours after a well mixed layer has developed. Because mixing does not take place overnight, this method would not be able to capture various features such as the low level jet and thus would not be useful for estimating wind speeds for morning flights. In the next issue, I will present the equation that can be used to find the wind speed at any altitude in the mixed layer along with a few examples. Fig.1 Wind speed increases with height as an exponential function. In the last issue, we saw that the wind speed with respect to height in a well mixed boundary layer can be approximated using an exponential function. In this article, we will explore an equation that can be used to find the wind speed at any height in the boundary layer (mixed layer). The formula itself is fairly simple and straight forward: where u* is called the friction velocity, z is the height (in meters) of the observation or level you would like to compute the wind at, zo is called the roughness parameter that is based on the surface characteristics(table 1) and U(z) is the wind speed in meters per second. The friction velocity is constant throughout the entire boundary layer. The natural log function in this equation comes from the fact that the wind speed is exponential with height. The inverse of an exponential function is a logarithmic function. We need to use the logarithmic function because we are solving for the wind speed and not for the height. Let’s apply this formula to a few examples: On a warm sunny July day, the 5 PM news reports a wind speed at O’Hare International Airport of 9 mph (4 meters per second). What is the wind at 300 meters (~1000 ft) in the atmosphere assuming the boundary layer extends higher than this? It is important to note that most surface wind measurements are taken at 10 meters. First, we need to estimate the type of surface found at and around the observation. O’Hare is surrounded by a very urban setting since it lies in the middle of downtown Chicago. Because it is such an urban setting, you might think we should use a value near the upper threshold for an urban setting. However, the observing equipment is located out in the middle of the tarmac, and is therefore more exposed than equipment would be located next to the base of the Sears Tower. To account for this, we will use a roughness parameter of 2.7(see table 1). Now, we need to use the wind speed to determine the friction velocity. Using the equation from above we get: where the 4 on the left hand side of the equation is our wind speed in meters per second, the 10 in the natural log term comes from the observation height (10 meters) and 2.7 is from the assumptions we made about the surface near the airport. Therefore we need to rearrange this formula so we can solve for u*. First we move the 0.4 to the other side of the equation: Then, we need to move the natural logarithm on the right hand side of the equation to the left hand side. If we divide each side of the equation by , then we get the following: Plugging this into a calculator, you get u* = 1.222 meters per second. Since this value is constant throughout the boundary layer, we can use the information we have to solve for the wind at any height. Therefore, if we wanted to find the wind speed at 300 meters, we get: where u* = 1.222 meters per second, z = 300 (the height of the wind speed in meters that we are interested in) and zo= 3.1. Therefore we get, , which equals 14.391 meters per second or about 32.19 miles per hour. Not exactly ideal ballooning conditions! Let’s suppose instead that the wind speed was recorded by a school net station. The height above ground level at which the school net station makes measurements varies because most (if not all) the school net weather stations are on the roof of the school. To keep things simple, I will assume the height of the school to be 10 meters. For this case, we will also assume a hot, sunny day in July. Therefore, the only thing changing in this problem will be the surface roughness parameter due to the terrain around the school. Most schools are located in the town or city, so we would expect the surface roughness parameter to be higher than it would be in open country. To arrive at an estimate of the surface roughness parameter, let assign a specific school to this problem. Since Indianola, Iowa is a popular place to fly, let choose the school there. The only schoolnet station in Indianola is located at Emerson Elementary School. Emerson Elementary School is located in a sparsely populated residential area. Therefore, we could assume a roughness parameter of around 1.1. Assuming this roughness parameter, we can calculate the wind speed at any height in the boundary layer. In this case, as in the last, we will assume a wind speed of 9 mph was measured. Plugging these values into our equation for the various values, we get: Solving for u*, we get meters per second Solving for the wind at 300 meters (~1,000 ft), we get meters per second which is approximately 22.73 mph. These examples show how important it is to arrive at an accurate estimate of the surface roughness parameter. The surface roughness parameter can change both due to location and season. This also shows how dependant the strength of the wind is based on the surface below. Once you arrive at an estimate of the surface characteristics, using this simple equation will give you an estimate of the wind speed at any height in the mixed layer near the surface.