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 1 Aviation Weather Weather plays a pivotal role in each and every flight. It is important as the pilot, that you understand how weather systems and storms move and develop and how to read and interpret weather charts and reports in order to make your go/no-­‐go decision before and during any flight. This lesson will cover: •
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The Layers of the Atmosphere and Basic Weather Circulation Air Masses and Fronts Stability and Hazards And finally, Weather Briefings and Weather Products Layers of the Atmosphere We’ve learned about the composition of the atmosphere in previous lessons, but did you know the atmosphere has several layers with very different characteristics? The atmosphere is made up of 5 layers which contain all the gasses that protect us from the dangers of outer space. The lowest layer of the atmosphere is called the troposphere. This is where almost all weather occurs. The troposphere begins at the Earth’s surface and extends to anywhere from 4 to 12 miles high. The varying height gives this layer more of an egg shape with a height of 4 miles at the poles and closer to 12 miles at the equator. In the troposphere, density decreases with height, meaning, as you go up in altitude, the temperature decreases and the air becomes thinner. Just to give you an idea of temperature change, by the time you reach the top of the troposphere, or, tropopause the temperatures will decrease to about -­‐60° Fahrenheit or -­‐51° Celsius. The stratosphere extends from the tropopause up to around 31 miles from the Earth’s surface. In this layer, temperature increases with height due to the creation of Ozone that’s present in this layer. The increased temperatures make the stratosphere a very stable atmospheric layer and give anvil clouds their famous flat tops. The next layer of the atmosphere is called the mesosphere. The mesosphere extends from 31 miles to 53 miles above the surface of Earth. This layer is where most of the meteors burn up in the atmosphere. Temperatures once again decrease as altitude increases, similar to what happens in the troposphere. The thermosphere is next and extends from 53 miles up to 375 miles above the Earth. Being one of the outermost layers, the air here gets bombarded with ultraviolet and x-­‐ray radiation from the sun, which, in turn, causes a large temperature increase…as high as 3,600° Fahrenheit at the top. The last layer of the atmosphere is called the exosphere and is the outermost layer. It extends from the top of the thermosphere to 6,200 miles above the Earth. In this layer, atoms and molecules of the atmosphere escape into space. 2 Basic Weather Circulation The atmosphere is important but we need to also look at the big picture. Here is Earth. The sun heats the Earth, but it does not get heated evenly. At the equator, the sunlight is more direct and heats the Earth the most. Out toward the poles, the sunlight is spread out over a larger area, so that area won’t heat up as much. This creates an area of rising air at the equator, which in turn creates surface flow from the poles to the equator. This simplified circulation pattern is just a concept. The rotation of the Earth changes everything. The Coriolis force, a force created by the rotation of the Earth, causes the circulation of air to flow and break up into three distinct cells. In the northern hemisphere, the Coriolis force acts to turn the wind to the left. The circulation pattern is what gives rise to the westerly flow over much of the US from 30N latitude to 60N latitude and the easterly flow from the equator to 30N latitude. There are several other factors that can also affect the circulation of air. Things like: seasonal changes, topography, and friction from objects on the surface. The Earth circulation patterns always try to maintain a balance. This means that air flows from areas of high pressure into areas of low pressure. This causes both horizontal movements of air, called wind, and vertical movements of ascending and descending air. In the northern hemisphere, a low-­‐pressure system will have air circulating counter-­‐clockwise as it flows into the center. A high pressure system will circulate clockwise as it flows away from the center. Air masses and Fronts Air masses are large regions of air that have similar characteristics from the surrounding area. Air masses can form over places like warm tropical oceans, cold dry polar-­‐regions, or even dry deserts. As air masses move over different regions, they will gradually take on the characteristics of the new region and begin to change. As air masses move across the Earth they will eventually run into each other. The boundaries between these two air masses are referred to as fronts. Fronts are labeled in reference to the relative temperature of the air that is approaching and replacing the existing air mass. There are four types of fronts: warm fronts, cold fronts, stationary fronts, and occluded fronts. A warm front occurs when a warm air mass overtakes an air mass of a colder temperature. Warm fronts usually move slowly, around 10 to 25 miles per hour, and have a gradual slope that will slide over the top of a cooler air mass and move it out of the area. Warm fronts usually contain high humidity and have more “stratoform”, or layered, clouds along the frontal boundary. Drizzle and decreasing visibility will be present as temperatures rise. Once the front passes, rain showers are possible, followed by a gradual clearing. A cold front occurs when cold, dense air overtakes warmer air. Cold fronts move faster than warm fronts, averaging around 25 to 30 miles per hour, but can go as fast as 60 miles per hour. Cold fronts stay close to the ground and have a steeper angle than a warm front. They will move across the ground and lift existing air at a very rapid rate. This rapid upward motion will cool the air and form cumulus 3 clouds, which could eventually develop into thunderstorms or even form a squall line. Once the front passes, the weather will clear and improve with general wind flow from the west-­‐northwest. A stationary front occurs when two air masses meet, but neither one moves the other out of the way. Stationary fronts can have a combination of weather from both a warm and cold front. These fronts can oscillate, but will generally linger around for several days before one of the air masses pushes the other air mass out of the way. The last type of front is called an occluded front. Occluded fronts form when a fast-­‐moving cold front catches up to a slower-­‐moving warm front. The cold front will then lift up the warm front and eventually interact with air that is in front of the warm front. This stacking of fronts can give rise to a variety of weather conditions from fog to thunderstorms. Stability Stability is how the atmosphere will react when there is vertical motion of air. There are three types of stability: stable, neutral, and unstable. A stable atmosphere is one where, when the air is lifted, that air will return to its original position. In a neutral atmosphere, air that is moved will remain in its new location. Finally, an unstable atmosphere is one that when air lifted, will continue to rise freely up in the atmosphere. An unstable atmosphere is a key ingredient in the formation of large cumulonimbus clouds and thunderstorms. Hazards While poor visibility and low clouds can be a hazard to pilots, one of the most dangerous weather events is a thunderstorm. Thunderstorms need three main ingredients to form: sufficient moisture, an unstable atmosphere, and a lifting mechanism (like a front) to start it all. Thunderstorms contain several of the biggest hazards to aircraft, including such things as turbulence, wind shear, hail, tornadoes, lightning, and icing. Some of these hazards can occur outside thunderstorms as well. [Turbulence] Air doesn’t always flow smoothly and can develop into areas of flow that can be rough to fly through. Areas of severe turbulence can damage aircraft and injure occupants due to the abrupt movements. [Wind Shear] Wind shear is any change in wind direction or speed over a short distance. This can cause turbulence in the air and is especially dangerous near the ground. As an airplane is taking off or landing, the sudden change in wind direction or speed can suddenly affect the performance of the airplane. A sudden shift from a headwind to a tailwind can cause an abrupt and dangerous drop in airspeed and could cause the airplane to stall. If there is not enough altitude to recover, the airplane could impact the ground before stall recovery occurs. [Icing] Icing is a very dangerous condition that can drastically affect the performance of the aircraft. Only approved and properly-­‐equipped aircraft can fly into areas of known icing conditions. Icing occurs 4 when an airplane flies through an area of visible moisture, like rain or a cloud, where the temperature is below freezing. These water droplets are called “supercooled droplets” because they exist as liquid water at a temperature below freezing and will freeze upon impacting the surface of the airplane. Ice can form on any surface of the airplane, from the wings, to the pitot tube, and even the windshield and propeller. The addition of ice to an aircraft will increase the overall weight of the airplane and the ice on the leading edge of the wings will affect the airflow over the wing and create drag, both of which negatively affect the airplane. Aircraft use a variety of methods to combat inflight icing. Smaller propeller-­‐driven aircraft can use pneumatic inflatable boots on the front of the wing to “pop” off the ice once it forms. Propellers and windshields can be heated to remove and prevent ice from forming. Some airplanes will spray alcohol on the surfaces to keep the ice from sticking to the airplane. Larger aircraft that use jet engines will actually divert heat from the jet exhaust to heat the surfaces of the aircraft to melt and keep ice from forming. There are three different types of icing: rime ice, clear ice, and mixed ice. Icing detail The size of water droplets determine the kind of ice that will form. Rime ice forms when small water droplets impact and freeze on the airplane. These smaller water droplets are usually found in stratus or layered clouds with no vertical development. Rime ice is rough and whitish in color and will form protrusions on the front edge of the aircraft surfaces. In extreme cases, rime ice can form “horns” on the leading edge of the wing that interfere with the production of lift. Clear ice is formed when larger water droplets impact and freeze on the airplane. These larger water droplets are found in cumulous or cumulonimbus clouds where there is a lot of vertical development. When these large water droplets impact the airplane they freeze and spread out over the surface. This gives clear ice a clear color and smooth finish. Clear ice is most commonly found on the leading edge of the wings where it will conform to the curve of the wing. This will interfere with the airflow over the wing but its main hazard is the additional weight that is added to the airplane. The third kind of icing is called mixed icing. Areas with mixed icing are simply areas that have a combination of both rime and clear icing. Visibility Obstructions to visibility can also pose a hazard to aircraft. Low visibility can make it difficult to see runways when landing, ground checkpoints while navigating on a cross country, and even other traffic inflight. Haze, mist, fog, smoke, and dust can all get into the air and reduce the visibility. Any visibility less than three miles requires aircraft to perform instrument approaches to land at the airport. In extreme cases, the visibility can drop to zero, making it nearly impossible to land in most aircraft. There are many different weather hazards in aviation to be aware of, but even simple things like the wind direction can play a big role in flight planning. In fact, Federal Regulations require that “Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight, including weather reports and forecasts.” 5 Weather Briefings There are many different ways to obtain weather information before a flight. The FAA maintains a nationwide network of Automated Flight Service Stations to serve the weather needs of pilots. An official weather briefing can be obtained by calling 1-­‐800-­‐WX-­‐BRIEF. Another way is using an online system called DUAT, which stands for Direct User Access Terminal Service. This is a program funded by the FAA to allow any pilot to use a computer to access weather information, or even file flight plans. There are two companies that provide this service: CSC, which is found at duats.com, or DTC, found at duat.com. Weather information can also be obtained from the Aviation Digital Data Service, or ADDS. This is found at aviationweather.gov/adds. Make sure, no matter where you get your weather information, that on every product or image, you check the date and time it was published to make sure it’s the most up-­‐to-­‐
date information you can get. There are several types of weather products that pilots can use to make their go/no-­‐go decision before a flight. Many of the weather products contain abbreviations and codes, which give pilots a lot of information in a little bit of space. If you need help decoding the reports, try checking out the latest copy of the FAA’s Aviation Weather Services Advisory Circular from the FAA’s website. Weather Products Here is just a quick overview of some of the weather products. We’ll start with weather products that tell us what is happening right now. METAR The basic surface observation of weather conditions is called a METAR. A METAR is an hourly weather report that includes the airport identifier, time of observation, wind, visibility, runway visual range, present weather phenomena, sky conditions, temperature, dew point, and altimeter setting. There will also be some “remarks” added to the end of the report to give additional information. METARS are created once an hour, usually at 53 minutes after the hour. If the weather changes significantly, a special METAR called a SPECI is produced to make sure that pilots have the latest weather information. PIREPS A PIREP, or pilot report, is a type of weather report made by pilots themselves, recording actual weather conditions that he or she is experiencing. Pilots should report any observation, good or bad, to assist other pilots with flight planning and preparation. If conditions were forecasted to occur, but not encountered, a pilot should also report this inaccuracy. There are two types of PIREPs: Routine and Urgent. The Required elements for all PIREPs are the message type, location, time, altitude, type of aircraft making the report, and at least one other element describing the reported weather conditions. Some of the items that can be reported are the sky condition and cloud levels, flight visibility and weather, air temperature, winds aloft, turbulence level, icing conditions, or any other remarks the pilot wishes to add. 6 Satellite Imagery There are several satellites in orbit around Earth that give us imagery of the clouds around the Earth. There are three types of imagery commonly used by pilots: visible, infrared, and water vapor. Each image is updated about every 15 minutes. Visible satellite images display reflected sunlight from the Earth’s surface, clouds, and particulate matter in the atmosphere. It creates black and white images that are only useful during daylight hours. This image uses the highest resolution and approximates what is seen with the human eye. You can even see features like snow or bodies of water. Infrared (IR) images display temperatures of the Earth’s surface, clouds, and particulate matter. The warmer an object is, the more infrared energy it emits. The satellite sensor measures this energy and the higher something is in the atmosphere, the colder it is, while the lower something is in the atmosphere the warmer it is. This information can then be displayed on a map either in color code or in black and white. The colors help enhance the image and make it easier to see the temperature changes. The water vapor imagery displays the quantity of water vapor in the air. This generally only measures water vapor located in the middle and upper troposphere between FL100 to FL390. Water vapor imagery does not really "see" clouds but "sees" high-­‐level water vapor instead. The water vapor imagery is very useful to see the motion of the upper atmosphere and features like fronts, highs and lows, for example. Radar Imagery Radar images are graphical displays of precipitation and non-­‐precipitation targets detected by weather radar. Under the dome, a radar station is composed of a rotating dish that sends pulses of radio waves out into space. When the pulse hits a target such as rain, airplanes, or any solid object, it is scattered in all directions, even continuing through the rain away from the radar. However, a small amount of energy does return back to the radar station. By determining the length of time it took for the signal to return to the dish, the amount of energy returned (in other words, its reflectivity) and the position of the radar, computers can generate an image to display precipitation. Once a full 360 degree revolution is complete, the radar dish will tilt up and scan the area again. The station will do this a few times, which accounts for the delay in updated radar images. The radar is able to make different images to show different parameters of storms. The most common radar images are base reflectivity and composite reflectivity. Base Reflectivity shows radar returns at the lowest radar scan level above the horizon. This image is useful for seeing the precipitation that is lower to the surface. Composite Reflectivity shows the maximum echo intensity or reflectivity detected within a column of the atmosphere above a location. The station scans through all of the elevation slices to determine the highest value in the vertical column then displays that value on the product. When compared with Base 7 Reflectivity, the Composite Reflectivity can reveal important storm structure features and intensity trends of storms. The maximum range of a single radar site is approximately 124 nm. This means you may have to look at multiple radar sites to cover a longer flight. It’s also a good idea to check out any available radar mosaics. A radar mosaic is an image composed of multiple single-­‐site radar pictures on one map to get a large-­‐scale view of where precipitation is falling. It’s also always best to animate the images too; that way, you can see trends of motion, and if storms are building or deteriorating. Charts Surface Analysis Charts are analyzed charts of surface weather observations. The chart shows sea level pressure in millibars, the positions of highs, lows, the location and type of fronts, and various boundaries such as drylines, outflow boundaries, sea-­‐breeze fronts, and convergence lines. Freezing level graphics show where the temperature is below freezing and can give pilots an idea of where to possibly expect icing conditions. Be aware that there can be multiple freezing layers of air over a given point. This usually implies that different air masses are stacked on top of each other. These charts are issued hourly and are based off of computer models. AIRMETS, SIGMETS, and Convective SIGMETS Airmen’s meteorological information, or AIRMET, is a concise description of the occurrence or expected occurrence of specified en route weather phenomena which may affect the safety of aircraft operations. AIRMETs are intended for dissemination to all pilots in flight to enhance safety, and advise pilots of weather potentially hazardous to all aircraft, such as icing conditions, turbulence, or locations where there are IFR conditions or mountain obscuration. These AIRMETS are valid for 6 hours. Significant meteorological information, or SIGMET, is a weather advisory which is disseminated when weather phenomena which may affect the safety of aircraft operations is expected or occurring en route. SIGMETS are issued for conditions of severe or greater turbulence, severe icing, a widespread dust storm, or a widespread sandstorm. These SIGMETs are valid for 4 hours. For thunderstorm-­‐related activity, Convective SIGMETs are issued. These will be disseminated for such things as: thunderstorms, lines of thunderstorms, embedded thunderstorms, tornados, hail greater than or equal to ¾ of an inch, or when wind gusts of greater than or equal to 50 knots (at the surface) are reported. Any Convective SIGMET implies severe or greater turbulence, severe icing, and low level wind shear conditions. Convective SIGMETs are only valid for 2 hours. All airmets, sigmets, and convective sigmets are shown on maps with boxes that show the different types of weather conditions and the altitudes that are affected. Now that we’ve taken a look at the most popular weather reports, let’s look at some Forecast products, which help determine what the weather will be in the future. 8 TAF A Terminal Aerodrome Forecast, or TAF, is a forecast of the expected weather conditions within five statute miles of the center of the airport. However, not every airport has a TAF. These are only available at the larger airports around the country. TAFs use the same weather codes found in METAR weather reports and are valid for 24 or 30 hours depending on the airport. TAFs will forecast for wind direction and speed, visibility, significant weather, and cloud types and heights. AREA FORECAST (FA) The Area Forecast, abbreviated by the letters FA, is a weather forecast for a large area, the size of several states. It can be used to determine weather conditions that can be expected during flight, and to help determine weather conditions at an airport where a METAR or TAF does not exist. Area forecasts are divided into 6 geographic areas. They are issued three times a day and are valid for 18 hours. Area forecasts give similar weather information to a TAF. Wind and Temperature Aloft Forecasts (FB) Wind and Temperature Aloft Forecasts are computer prepared forecasts of wind direction, wind speed, and temperature at specified times, altitudes, and locations. Wind forecasts are not issued for altitudes within 1,500 feet of a location's elevation. Temperature forecasts are not issued for altitudes within 2,500 feet of a location's elevation. Prog Charts Short-­‐Range Surface Prognostic Charts, or Prog charts, provide a forecast of surface pressure systems, fronts and precipitation locations in the future. The forecast charts cover the 48-­‐contiguous states and are divided in to four forecast periods, 12-­‐, 24-­‐, 36-­‐, and 48-­‐hours. Each one of the prog charts depict a “snapshot” of weather elements expected at the specified valid time. Conclusion Weather is a complex topic to fully cover in a short video. It is important that pilots understand all the weather products available to them and how weather patterns and storms move and develop. Pilots need to know how to obtain and use those products to make a competent and correct go/no-­‐go decision before every flight. Remember to always call a Flight Service Station Weather Briefer or get a DUAT weather briefing before each flight and use reputable weather websites such as ADDS to get all the information you need before you head into the blue skies.