Meteorology - Chapter 1: Met TheoryTopics:1. The Atmosphere2. Clouds3. Pressure4. Wind5. Humidity6. Temperature7. Stability
Chapter 1 – 1: The AtmosphereAtmospheric Composition The Earth’s ATMOSPHERE is made up of: -78% nitrogen -21% oxygen -1% rare gases (argon, carbon dioxide, water vapour)
Chapter 1 – 1: The AtmosphereWater Vapour Of all of the ingredients that make up the atmosphere, the most important component for pilots is WATER VAPOUR. It is the water vapour that is responsible for the formation of clouds, fog, and precipitation. In other words, it is the water vapour that produces weather.
Chapter 1 – 1: The AtmosphereProperties of the Atmosphere A parcel of atmospheric air processes 3 properties, namely mobility, capacity for expansion, and capacity for compression: MOBILITY – like a body of water over a river bed, a body of air can move over the Earth’s surface.
Chapter 1 – 1: The AtmosphereProperties of the Atmosphere CAPACITY FOR EXPANSION – like a balloon, a parcel of air can expand. When it does, the parcel of air cools.
Chapter 1 – 1: The AtmosphereProperties of the Atmosphere CAPACITY FOR COMPRESSION – like a deflating balloon, a parcel of air can compress. When it does, the temperature rises.
Chapter 1 – 1: The AtmosphereDivisions of the Atmosphere The layer of atmosphere closest to the Earth is called the TROPOSPHERE. The Troposphere extends to a height of 28,000 feet at the Poles, and 54,000 feet at the Equator. In the Troposphere, air pressure, density, and temperature all decrease with increasing altitude. In other words, with increasing altitude, the air becomes lighter, thinner, and colder. Because almost all water vapour is found in the Troposphere, all active weather occurs in this layer.
Chapter 1 – 1: The AtmosphereDivisions of the Atmosphere Above the Troposphere is a layer called the STRATOSPHERE. In the Stratosphere, the temperature ceases to drop, and remains at -56oC.
Chapter 1 – 1: The AtmosphereDivisions of the Atmosphere The boundary layer separating the Troposphere from the Stratosphere is called the TROPOPAUSE.
Chapter 1 – 1: The AtmosphereDivisions of the Atmosphere In an earlier chapter on Flight Instruments, we talked about “ICAO Standard Atmosphere”. (Recall that ICAO stands for “International Civil Aviation Organization”). This “standard” atmosphere is based on averages of atmospheric conditions at 49o of Latitude. By definition, ISA (ICAO Standard Atmosphere) is: -Pressure = 29.92” Hg (inches of Mercury) -Temperature = 15oC at sea level -Temperature Decrease (or Lapse Rate) = 2oC per 1,000 feet above sea level
Chapter 1 – 2: CloudsHigh Clouds Clouds can be classified according to their altitude. HIGH clouds have bases starting from 16,500 feet to 45,000 feet. The name of these clouds are prefixed with “Cirro”.
Chapter 1 – 2: CloudsMid Level Clouds MID LEVEL clouds have bases starting from 6,500 feet to 23,000 feet. The name of mid level clouds are prefixed with “Alto”.
Chapter 1 – 2: CloudsLow Level Clouds LOW LEVEL clouds have bases starting at the Earth’s surface, or up to 6,500 feet. The name of low level clouds have no prefix attached to them.
Chapter 1 – 2: CloudsCumulus Clouds Types of clouds can be further classified according to their characteristics. Clouds of vertical development (i.e. puffy or cotton ball type clouds) are called CUMULUS clouds.
Chapter 1 – 2: CloudsStratus Clouds Clouds that form in horizontal layers or sheet are called STRATUS clouds.
Chapter 1 – 2: CloudsFractus Clouds Clouds that are windblown or broken are referred to as FRACTUS clouds.
Chapter 1 – 2: CloudsNimbus Clouds And finally, clouds from which precipitation falls are called NIMBUS clouds.
Chapter 1 – 2: CloudsCloud Names Now, if we combine any of the above classifications of clouds with a cloud’s characteristics, we can determine the full name of a cloud. For example, a high level cloud that is cotton- like is called CIRROCUMULUS cloud. Cirro = high Cumulus = cotton-like
Chapter 1 – 2: CloudsCloud Names A high level cloud that is made up of sheets or layers of clouds is called CIRROSTRATUS cloud. Cirro = high Stratus = horizontal layers
Chapter 1 – 2: CloudsCloud Names A mid-level cloud that is rounded and puffy is called ALTOCUMULUS cloud. Alto = middle level Cumulus = puffy
Chapter 1 – 2: CloudsCloud Names A mid-level grey cloud that covers the whole sky is called ALTOSTRATUS cloud. Alto = middle level Stratus = horizontal formation
Chapter 1 – 2: CloudsCloud Names Low level cloud that resembles a series of patches or rounded masses is called CUMULUS cloud. no prefix = low level Cumulus = rounded masses
Chapter 1 – 2: CloudsCloud Names A uniform layer of low level cloud is called STRATUS cloud. no prefix = low level Stratus = layer cloud
Chapter 1 – 2: CloudsCloud Names Low level layer cloud that is windbroken is called STRATUSFRACTUS cloud. Stratus = layer cloud (horizontal formation) Fractus = windblown
Chapter 1 – 2: CloudsCloud Names Heavy masses of vertically developed cloud from which precipitation is falling is called CUMULONIMBUS cloud. Cumulo (from Cumulus) = mass cloud (vertical development) Nimbus = precipitation
Chapter 1 – 2: CloudsCloud Names Review your Environment Canada Clouds Poster included with your Groundschool kit for further cloud names and descriptions.
Chapter 1 – 2: CloudsSky Condition The SKY CONDITION refers to the amount of sky that is covered by cloud, as observed from the surface up. The sky condition can be any one of the following: -SKC = ‘sky clear’ = no cloud -FEW = ‘few’ = >0/8 to 2/8 cloud coverage -SCT = ‘scattered’ = 3/8 to 4/8 cloud coverage -BKN = ‘broken’ = 5/8 to <8/8 cloud coverage -OVC = ‘overcast’ = 8/8 cloud coverage
Chapter 1 – 3: PressureAtmospheric PressureATMOSPHERIC PRESSURE is the weight of the airabove us.The greater the amount of air above us, or thegreater the density of the air above us, the greaterthe downward pressure the air will apply on us.Atmospheric pressure changes from location tolocation. If there is dense, heavy air over anarea, the pressure will be higher than under an areaof less dense air.
Chapter 1 – 3: PressureAtmospheric PressureThe pressure (whether it be “high” pressure or“low” pressure) is important to pilots because it: -affects our altimeters (as discussed in the chapter on Flight Instruments) -controls the wind (as we will learn in this chapter)
Chapter 1 – 3: PressureMercury Barometer For aviation purposes, pressure is measured with a Mercury Barometer. A simplified Mercury Barometer would be a dish filled with liquid mercury and an inverted test-tube held in the dish of mercury.
Chapter 1 – 3: PressureMercury Barometer As the weight of the atmosphere increases (i.e. an increase in atmospheric pressure), it will push down on the surface of the mercury, thereby forcing it to rise up in the tube.
Chapter 1 – 3: PressureMercury Barometer If we now measure how many inches the mercury rises in the tube, (e.g. 29.92 inches), then we can determine the altimeter setting. In this case, we would call the altimeter setting 29.92” Hg.
Chapter 1 – 3: Pressure Mercury BarometerIn aviation, we use inches of mercury (”Hg) to express atmosphericpressure.Other units used to measure pressure are millibars and kilopascals.
Chapter 1 – 3: PressureIsobars ISOBARS are lines drawn on a Weather Map that join places of equal atmospheric pressure. Isobars never cross one another, but tend to form circular patterns. Although we commonly use inches of mercury to express pressure in aviation, the Isobars on Weather Maps are presented in millibars.
Chapter 1 – 3: PressureLow Pressure Area If we examine the pattern that the Isobars form on this Weather Map, we notice that as we move from the center of the map to the upper right corner, the pressure continually drops. We therefore conclude that there is a “low” pressure area in the top right corner.
Chapter 1 – 3: PressureHigh Pressure Area Likewise, if we look at the pattern as we move from the center of the map to the bottom right corner, we notice that the pressure continually rises. This would indicate that there is a “high” pressure area in the bottom right corner.
Chapter 1 – 3: PressureHigh Pressure, Low Pressure Likewise, the pattern of Isobars would indicate a “high” pressure area in the upper left corner, and a “low” pressure area in the lower left corner of this Weather Map.
Chapter 1 – 3: PressureTrough A TROUGH is an elongated u-shaped area of low pressure. A trough is like a “valley” of low pressure.
Chapter 1 – 3: PressureRidge A RIDGE is a protruding neck of high pressure. A ridge is like a “mountain range” of high pressure.
Chapter 1 – 3: PressureCol A COL is a “neutral” area between two high pressure areas and two low pressure areas.
Chapter 1 – 3: PressurePressure Gradient PRESSURE GRADIENT is the rate of change of pressure over a given distance. The pressure gradient can be a shallow gradient (i.e. a small rate of change), or a steep gradient (i.e. a large rate of change).
Chapter 1 – 3: PressurePressure Gradient The pressure gradient (or the nearness of the Isobars) is an indication of the strength of the wind. Where there is a shallow gradient (i.e. where the Isobars are far apart), there will be light winds. Where there is a steep gradient (i.e. where the Isobars are close together), the wind will be strong.
Chapter 1 – 3: PressureWind WIND is simply air trying to move (as it wants to) from an area of high pressure to an area of lower pressure. Just like an inflated balloon, the air inside the balloon is under high pressure. The air outside the balloon is at a much lower pressure. The air wants to escape from the balloon. In other words, the air wants to move from the high pressure area to the low pressure area.
Chapter 1 – 3: PressureWind So, if we had the above Weather Map, the air would want to move from the high pressure area to the low pressure area. The wind would tend to blow from the high to the low. However, as we are about to see, it gets a little more complicated than this…
Chapter 1 – 3: PressureWind Because the Earth is not stationary, but is rotating beneath the atmosphere, the wind does not move in a straight line (relative to the Earth’s surface) as it attempts to move from a high pressure area to a low pressure area. It becomes influenced by a force called CORIOLIS FORCE. In the Northern Hemisphere, Coriolis Force causes the air movement to be deflected to the right (in relation to the Earth’s surface), causing it to flow parallel to the Isobars.
Chapter 1 – 3: PressureWind If we had a high pressure area on either side of a low pressure area, we know that the wind would want to blow into the low (from high to low).
Chapter 1 – 3: PressureWind However, Coriolis Force says that in its movement, the wind gets deflected to the right.
Chapter 1 – 3: PressureWind This pattern shows how the wind tends to blow clockwise around a HIGH.
Chapter 1 – 3: PressureWind It also shows how the wind tends to blow counter-clockwise around a LOW.
Chapter 1 – 3: PressureWind Remember this picture to help you recall whether the wind blows clockwise or counter-clockwise around a high or low… It is a picture of a “high clock over top of a low counter”. high = clockwise low = counter-clockwise The wind blows clockwise around a high, and counter-clockwise around a low.
Chapter 1 – 3: PressureWind Here’s another trick… When outdoors, you can always tell where the low pressure area is if you stand with your back to the wind…
Chapter 1 – 3: PressureWind In this position, the low will be to your left.
Chapter 1 – 3: PressureWind There is one more element that affects the precise direction of the wind. In fact, the wind does not blow exactly parallel to the Isobars. SURFACE FRICTION between the moving air and the Earth’s surface tends to slow down its motion and retards the effect of the Coriolis Force. Therefore, the air tends to move across the isobars at an angle inward toward a low, and outward from a high.
Chapter 1 – 4: Wind WindAs you can imagine, the wind is a very important factor forpilots. The wind can have a negative or positive effect for us: - on takeoff - wind affects takeoff distance - wind affects takeoff safety (gusts, crosswind) - in cruise - wind affects groundspeed (time, fuel, money) - on landing - wind affects landing distance - wind affects landing safety (gusts, crosswind)We will now look at different types of wind…
Chapter 1 – 4: WindSea Breeze A SEA BREEZE is a wind that blows from the sea (or a large body of water) to the land. Note: When referring to wind direction, we always refer to the direction from which it is blowing (e.g. a north wind blows from the north).
Chapter 1 – 4: WindSea Breeze A Sea Breeze blows during the day. The Earth’s surface is a better conductor of heat than water. During the day, the sun heats the Earth (more-so than the water), which in turn heats the air above it. This warmed air (over the land) rises. Note: Warm air, which is less dense, tends to rise. Cool are, which is more dense, tends to sink.
Chapter 1 – 4: WindSea Breeze This rising air (over the land) creates a low pressure area over the land. (Because the air is rising, there is less downward pressure created by the atmosphere, resulting in a lower pressure). In contrast, the air over the water (sea) will be of a higher pressure.
Chapter 1 – 4: WindSea Breeze We know that the air tends to move from a high pressure area to a low pressure area. So, during the day, the wind will blow from the sea to the land, creating a Sea Breeze.
Chapter 1 – 4: WindLand Breeze A LAND BREEZE is a wind that blows from the land to the sea (or a large body of water).
Chapter 1 – 4: WindLand Breeze A Land Breeze works opposite to a Sea Breeze, and blows at night. At night, all the sun’s warmth radiates from the Earth’s surface into the upper atmosphere, and the air over the land becomes cool. Water retains heat better, so the air over the water remains warmer. The warmer air over the water will rise, creating a low pressure area over the water. In contrast, the air over the land will be of a higher pressure.
Chapter 1 – 4: WindLand Breeze With the high pressure over the land at night, the wind will blow from the land to the sea, creating a Land Breeze.
Chapter 1 – 4: WindMountain Wind Wind in the vicinity of mountains can be extremely challenging for a pilot. In fact, it is recommended that you seek the advise (or perhaps even training) of a pilot with mountain flying experience before flying in the mountains. When the wind blows through a mountain valley, the valley creates a “funnel effect”, whereby the wind velocity increases substantially. This strong wind can also lead to pronounced turbulence. We’ll now look at some specific types of mountain winds…
Chapter 1 – 4: WindMountain Wind An ANABATIC WIND blows up a mountain slope during the day. As the sun heats the dark surface of the mountain slope, the warmed surface radiates its heat to warm the air above it. This warm air rises, creating a wind that blows up the mountain slope.
Chapter 1 – 4: WindMountain Wind A KATABATIC WIND blows down a mountain slope. If the mountain tops are snow covered, the air at the caps will be cooled. This cold dense air will sink, causing the wind to blow down the slope. An Anabatic Wind can turn into a Katabatic Wind at night. The removal of the sun’s heat causes the mountain slope to cool, thereby cooling the air above it. Again, this cool dense air will flow down the mountain slope.
Chapter 1 – 4: WindMountain Wind A MOUNTAIN WAVE forms when the wind blows over the top of a mountain peak. Just like the airflow over the top of a wing, the wind blowing over a mountain top will have: -increased speed -decreased pressure -decreased temperature In the chapter on Flight Instruments, we learned how this effect can cause the Altimeter to read in error by as much as 3,000 feet! The decreased temperature can lead to airframe icing (ice accumulation on the airplane).
Chapter 1 – 4: WindMountain Wind Turbulence associated with a mountain wave is most frequent and most severe just beneath the wave crest at or below mountaintop level.
Chapter 1 – 4: WindMountain Wind On the leeward side of the mountain there can be strong downdrafts (as much as 2000 to 5000 feet per minute) and very turbulent eddies.
Chapter 1 – 4: WindWind Gust A wind GUST is a rapid change of wind speed or direction, that is of brief duration (seconds). Gusts are usually caused by obstacles being in the way of the wind’s path (e.g. hangars, buildings, irregular terrain, etc.)
Chapter 1 – 4: WindWind Squall A wind SQUALL is a rapid change of wind speed or direction, that is of prolonged duration. A squall is usually caused by the passage of a fast moving cold front.
Chapter 1 – 4: WindEddies/Mechanical Turbulence EDDIES, also known as MECHANICAL TURBULENCE, is disturbed airflow (similar to eddies of water in a river or stream). They are caused by irregular surfaces in the wind’s path (like rocks in a shallow river) such as hills, buildings, etc. Mechanical Turbulence only occurs in the lower levels of the atmosphere (usually below 3,000 feet), and depends on the strength of the wind being disturbed.
Chapter 1 – 4: WindWind Shear A wind SHEAR is a sudden or violent change in wind speed or direction. Wind shears are most commonly associated with thunderstorms. They can be extremely dangerous because the wind can change much faster than an airplane’s ability to accelerate or decelerate. They are especially dangerous near the ground during takeoff and landing.
Chapter 1 – 4: WindJet Stream A JET STREAM is a tube-like band of high speed wind at high altitudes (20,000 to 40,000 feet). This band can be from 3,000 to 7,000 feet thick, with a core wind of 100 to 150 knots. This wind flows from west to east. There are two Jet Streams across North America: one lies approximately across Canada and the other across the USA. The Jet Streams migrate south in the summer, and move back north in the winter.
Chapter 1 – 4: WindClear Air Turbulence CLEAR AIR TURBULENCE (CAT) is a very turbulent condition that occurs in a cloudless sky, usually associated with a Jet Stream or Mountain Wave. Because it occurs in a clear sky, CAT is almost impossible to forecast.
Chapter 1 – 4: WindWind Speed and Direction In aviation, wind speed is expressed in knots (nautical miles per hour). Wind direction is the direction from which it is blowing. Using the compass rose to express precise direction, a wind blowing from the south would be a wind of 180o.
Chapter 1 – 4: WindWind Speed and Direction A wind of 040o would be blowing from the north-east (NE).
Chapter 1 – 4: WindWind Speed and Direction A VEER is a clockwise change in wind direction. For example, if the wind changed from 270o to 300o, we would say that the wind veered.
Chapter 1 – 4: WindWind Speed and Direction A BACK is a counter-clockwise change in wind direction. For example, if the wind changed from 270o to 240o, we would say that the wind backed.
Chapter 1 – 4: WindDiurnal (Daily) Wind Variations We all know that the wind tends to increase during a hot afternoon, and then calms at night. This is due to Diurnal Variation…
Chapter 1 – 4: WindDiurnal (Daily) Wind Variations During the hot afternoon, the sun heats the Earth’s surface. The Earth then heats the air above it by radiation. This warming air rises. As it rises, it expands, cools, and begins to fall again. As it falls, it transfers the higher level wind (from about 3,000 feet) to the surface. The higher level wind in unaffected by surface friction and is therefore stronger, and flows more parallel to the Isobars.
Chapter 1 – 4: WindDiurnal (Daily) Wind Variations As a result, during the daytime, the wind veers and increases in strength. At night, the wind resumes its normal direction and speed: it backs and decreases.
Chapter 1 – 5: HumidityHumidity HUMIDITY is the amount of moisture in the air. This moisture can be one of 2 forms: - invisible form (which is water vapour) - visible form (which is water droplets or ice crystals, making up clouds or fog)
Chapter 1 – 5: HumidityCondensation CONDENSATION is when water vapour changes into water droplets. In other words, the moisture changes from a gas to a liquid, or from its invisible form to its visible form. Condensation can be seen as moisture on the inside of a window on a cold winter day.
Chapter 1 – 5: HumiditySublimation SUBLIMATION is when water vapour changes into ice crystals. In other words, the moisture changes from a gas to a solid. Again, it changes from its invisible form to its visible form, but in this case, the liquid stage is bypassed. Sublimation can be seen as frost on a car window on a cold winter morning.
Chapter 1 – 5: HumiditySublimation Here is an important point to remember about humidity: Warm air can hold more moisture than cold air A parcel of warm air has the ability to hold more water molecules than a similar parcel of cold air.
Chapter 1 – 5: HumiditySaturated Air SATURATED AIR is when a parcel of air contains the maximum amount of water vapour that it can hold at a given temperature.
Chapter 1 – 5: HumiditySaturated Air If the air is saturated (i.e. it contains all the moisture it can hold), and then the temperature drops, that parcel of air will have more moisture than it can hold. (Remember: warm air can hold more moisture than cold air). This excess moisture (or vapour) will be forced into condensation or sublimation. The excess moisture will change from its invisible form to its visible form, creating either cloud, fog, dew, or frost. You’ve noticed that fog, dew and frost tend to form at night, when the temperature drops. Clouds form in the higher altitudes. Remember… the temperature decreases with increasing altitude.
Chapter 1 – 5: HumiditySaturated Air So, saturated air can be forced into condensation or sublimation be decreasing the temperature. Another way for this to happen is to increase the moisture content of the air. If the air is already saturated, then adding more moisture will also force the excess vapour into condensation or sublimation. An example of this is when you see your breath on a cold day (since the air you breathe out has a lot of moisture in it from your lings)
Chapter 1 – 5: HumiditySuper-Cooled Water Droplets SUPER-COOLED WATER DROPLETS are liquid water droplets that exist in the liquid form at temperatures well below 0oC. This is a condition that does not normally happen, and requires specific atmospheric conditions to exist. They are sometimes associated with thunderstorms cells. They can exist at temperatures as low as -40oC. Super-cooled water droplets are a hazard because, when they are disturbed (e.g. by a wing), they turn into ice instantaneously. They create a rapid accumulation of airframe icing.
Chapter 1 – 5: HumidityDewpoint The DEWPOINT is the temperature to which unsaturated air must be cooled to become saturated. The dewpoint is the temperature at which invisible moisture changes into visible moisture. It is the temperature at which fog, dew, frost, or clouds form.
Chapter 1 – 5: HumidityRelative Humidity RELATIVE HUMIDITY is the ratio of the amount of water vapour present in the air to the amount it would hold if it were saturated (at the same pressure and temperature). For example, if the air is holding 80% of the moisture that it can hold, then we say that the Relative Humidity is 80%. Saturated air has a Relative Humidity of 100%.
Chapter 1 – 5: HumidityRelative Humidity If a parcel of air is heated, then its Relative Humidity decreases. (Remember: warm air can hold more moisture than cold air). If a parcel of air is cooled, then its Relative Humidity increases.
Chapter 1 – 5: HumidityRelative Humidity The smaller the spread between the temperature and the dewpoint, the higher the Relative Humidity.
Chapter 1 – 6: TemperatureTemperature As we’ve already stated, the sun heats the Earth, and the Earth heats the atmosphere above it by radiation. This is an important point to remember. The atmosphere is heated from below, not from above.
Chapter 1 – 6: TemperatureSeasonal Variation So, why is the atmosphere’s temperature different at different places? One reason is due to SEASONAL VARIATION. The Earth’s axis of rotation is not perpendicular to the Earth’s path of travel, but is at a “tilt”. Hence, during North America’s summer months, the sun’s rays are more perpendicular to the continent’s surface (shine from overhead). But in the winter, the sun is lower on the horizon, so the sun’s rays are at more of an angle to the continent’s surface.
Chapter 1 – 6: TemperatureSeasonal Variation Like a beam of light shining directly onto a surface, the light’s rays are concentrated in a small area. But if we shine the light at an angle to the surface, then that same beam covers a larger surface area. If both beams are producing the same amount of energy (heat), then the beam of light shining from directly above will concentrate its heat over a smaller are. Therefore, this surface will be warmer. The sun has the same effect on the Earth’s surface in summer vs. winter.
Chapter 1 – 6: TemperatureLatitudinal Variation This same principle explains LATITUDINAL VARIATION of the Earth’s temperature. Locations near the Equator have the sun more directly overhead than locations further north or south of the Equator. Hence, near the Equator the temperatures are warmer.
Chapter 1 – 6: TemperatureTopography TOPOGRAPHY (the makeup of the Earth’s surface) also has an effect on temperature. Since dark colours absorb more light than light colours do (this is Physics!), dark colours get warmer when the sun shines on them. The same holds true for the Earth’s surface. Dark coloured terrain (dark soil, asphalt, etc.) gets hotter than does light coloured terrain (water, snow, etc.). Therefore, the atmosphere above a dark surface will be warmer.
Chapter 1 – 6: TemperatureCloud Cover CLOUD COVER can have an effect on temperature. During the day, the absence of cloud cover allows for maximum heat from the sun to heat the Earth’s surface, creating warmer air (by radiation). However, at night, a clear sky allows all the Earth’s heat (gained during the daytime) to radiate into the upper atmosphere, creating cool temperatures at the surface. A cloudy night produces a sort of blanket, keeping the heat near the surface, creating a warmer night.
Chapter 1 – 6: TemperatureHow the Atmosphere is Heated The atmosphere can be heated by any one of 4 methods: -Convection -Advection -Turbulence -Compression We will look at each of these…
Chapter 1 – 6: TemperatureHow the Atmosphere isHeated/Convection CONVECTION works much like bubbles that form in a pot of boiling water. The air nearest the Earth’s surface is warmed. Because warm air is less dense than cold air, it begins to rise. As it rises, it cools, by expansion. (Remember… at higher altitudes the pressure decreases, allowing the air to expand. When it expands, it cools). As the air cools, it becomes more dense (heavier), and begins to fall again, replacing the rising warm air below. It is the rising air that warms the air aloft.
Chapter 1 – 6: TemperatureHow the Atmosphere isHeated/Advection ADVECTION refers to the horizontal movement of air from one place to another. Advection heating occurs when cool air moves over a warm surface. The warm surface warms the air above it.
Chapter 1 – 6: TemperatureHow the Atmosphere isHeated/Turbulence When an obstruction in the path of the air’s movement (such as a hill or irregular terrain) disturbs it, TURBULENCE is created. This turbulence can push the warm air aloft.
Chapter 1 – 6: TemperatureHow the Atmosphere isHeated/Compression When a parcel of air is COMPRESSED, it warms. This can occur on the leeward side of a mountain range. As the air flows down the mountain, it is compressed at the mountain’s base. The compressed air becomes warmer.
Chapter 1 – 6: TemperatureHow the Atmosphere is Cooled The atmosphere can be cooled by any one of 3 methods: -Radiation -Advection -Expansion We will look at each of these…
Chapter 1 – 6: TemperatureHow the Atmosphere isCooled/Radiation At night, solar RADIATION ceases. All the heat absorbed by the Earth’s surface from the previous day radiates, or transfers, into the upper atmosphere. As a result, the lower levels of the atmosphere cool.
Chapter 1 – 6: TemperatureHow the Atmosphere isCooled/Advection Remember, ADVECTION refers to the horizontal movement of air from one place to another. Advection cooling occurs when warm air moves over a cool surface. The cool surface cools the air above it.
Chapter 1 – 6: TemperatureHow the Atmosphere isCooled/Expansion When a parcel of air EXPANDS, it cools. (This is the opposite of compression). If you’ve ever used a can of spray paint, then you’ve experienced this. The paint inside the can is under compression. As it comes out of the nozzle, it expands, and cools. You may have noticed that the tip of your finger on the nozzle gets cold!
Chapter 1 – 6: TemperatureHow the Atmosphere isCooled/Expansion The same thing happens when air is forced to rise… the pressure decreases, so the air expands. When it expands, the temperature decreases.
Chapter 1 – 6: TemperatureIsotherms ISOTHERMS are lines drawn on a Weather Map that join places of equal temperature.
Chapter 1 – 6: TemperatureTemperature Scales The international aeronautical unit used to express temperature is Degrees Celsius. In Degrees Celsius: - the freezing point of water = 0oC - the boiling point of water = 100oC However, you may come across some airplane manuals (especially for airplanes built in the USA) that express temperature in Degrees Fahrenheit. In Degrees Fahrenheit: - the freezing point of water = 32oF - the boiling point of water = 212oF
Chapter 1 – 6: TemperatureTemperature Scales To convert from oC to oF: oF = 9/5 oC + 32 oC = 5/9 (oF – 32) Or, simply use your E6B Flight Computer!
Chapter 1 – 6: TemperatureDensity vs. Temperature Cold air is more dense than warm air. Therefore, it is heavier and tends to sink. Warm air is less dense, or lighter, and tends to rise.
Chapter 1 – 6: TemperatureLapse Rate LAPSE RATE is the rate of decrease in temperature with height. There are 3 different Lapse Rates: - ICAO Standard Lapse Rate - Dry Adiabatic Lapse Rate - Saturated Adiabatic Lapse Rate We will look at each of these…
Chapter 1 – 6: TemperatureICAO Standard Lapse Rate The ICAO STANDARD LAPSE RATE is an average lapse rate, as derived by ICAO. (Remember, ICAO = International Civil Aviation Organization). This standard lapse rate is 1.98oC/1000 feet For simplicity, we commonly say that it is 2oC/1000 feet. This ICAO Standard Lapse Rate is an assumption used for the calibration of aircraft Altimeters.
Chapter 1 – 6: TemperatureDry Adiabatic Lapse Rate The DRY ADIABATIC LAPSE RATE is the actual lapse rate in air that is not saturated. This is the lapse rate when the temperature is greater that the dewpoint. By definition, this lapse rate is 3oC/1000 feet.
Chapter 1 – 6: TemperatureStandard Adiabatic Lapse Rate The SATURATED ADIABATIC LAPSE RATE (or sometimes called the WET ADIABATIC LAPSE RATE) is the lapse rate in air that is saturated. This is the lapse rate when the temperature meets the dewpoint. (Remember that at the dewpoint, moisture changes into its visible form. Clouds form at the dewpoint. The base of clouds, then, represents the altitude at which the temperature meets the dewpoint). By definition, the Saturated Adiabatic Lapse Rate is 1.5oC/1000 feet. So, at or above the base of clouds, the lapse rate becomes 1.5oC/1000 feet.
Chapter 1 – 6: TemperatureStandard Adiabatic Lapse Rate Here is a sample problem: Question: If the surface temperature is 10oC and the dewpoint is 1oC, what is the altitude of the base of the clouds? Solution: We know that below the cloud base, the air is unsaturated, so the lapse rate is 3oC/1000 feet (we use the Dry Adiabatic Lapse Rate below the cloud base). So, at: -1000 feet above ground, the temperature = 7oC -2000 feet above ground, the temperature = 4oC -3000 feet above ground, the temperature = 1oC Therefore, at 3000 feet we’ve reached the dewpoint of 1oC, and cloud will begin to form.
Chapter 1 – 6: TemperatureInversion An INVERSION is when the temperature increases with height. Inversions are not the norm. They are usually associated with a frontal surface. (We will talk more about fronts soon).
Chapter 1 – 6: TemperatureIsothermal Layer An ISOTHERMAL LAYER is when the temperature remains constant (neither decreases nor increases) throughout a layer for some depth.
Chapter 1 – 7: StabilityStability STABLE air is air that resists upward of downward displacement. UNSTABLE air is air that tends to move further away when displaced.
Chapter 1 – 7: StabilityUnstable Air If a parcel of air is warmer than the surrounding air, it will tend to rise. (Remember: warm air is less dense, or lighter, then cool air) This parcel of air is therefore unstable.
Chapter 1 – 7: StabilityStable Air Air that is cooler than the surrounding air will resist upward motion. (Remember: cool air is more dense and therefore will not want to rise) This parcel of air is therefore stable.
Chapter 1 – 7: StabilityLapse Rate vs. Stability The steeper the lapse rate, the more unstable the air.
Chapter 1 – 7: StabilityFlight Characteristics in Stable Air Flight through STABLE air will provide the following flight characteristics: - poor low level visibility (because stable air tends not to rise, so the pollutants get trapped near the surface) - stratus type cloud (layer cloud) - steady precipitation (e.g. that “all day” type of rain, which is characteristic of stratus type cloud) - steady (constant) winds - smooth flying conditions
Chapter 1 – 7: StabilityFlight Characteristics in Unstable AirFlight through UNSTABLE air will provide the followingflight characteristics: - good visibility (except in precipitation) - cumulus type cloud (heap type cloud of vertical development, built from unstable, rising air) - showery precipitation (e.g. “bursts” of rain, which are characteristic of cumulus type cloud) - gusty winds - turbulent flying conditions (produced by rising columns of unstable air) - smooth flying conditions
Chapter 1 – 7: StabilityLifting Agents LIFTING AGENTS are the forces or conditions that provide the lift to initiate rising currents of air. If a lifting agent provides a force onto a parcel of unstable air, then this air will experience significant lift. Stable air will have substantially less lift. When air is lifted to a higher altitude, it expands and cools. If it expands and cools sufficiently, then cloud formation occurs, and hence weather is produced.
Chapter 1 – 7: StabilityLifting Agents There are 5 different Lifting Agents: -Convection -Orographic Lift -Frontal Lift -Mechanical Lift -Convergence We will look at each of these…
Chapter 1 – 7: StabilityLifting Agents/Convection CONVECTION works much like the bubbles that form in a pot of boiling water. (We talked about this earlier when we looked at how the atmosphere is heated). The warm surface heats the air above it. This warm, less dense air rises, expands, cools, and falls again, replacing the rising warm air below.
Chapter 1 – 7: StabilityLifting Agents/Orographic Lift OROGRAPHIC LIFT occurs when air that is moving horizontally meets uneven terrain. It gets disturbed and is pushed upward.
Chapter 1 – 7: StabilityLifting Agents/Frontal Lift FRONTAL LIFT occurs when a wedge of cold dense air moves horizontally and pushes under a mass of warm air (much like a snow plow). The warm air will be forced aloft. This is an example of a “Cold Front” (hence the term “Frontal Lift”).
Chapter 1 – 7: StabilityLifting Agents/Mechanical Turbulence MECHANICAL TURBULENCE occurs when air that is moving horizontally meets an obstruction (e.g. a building or hangar). It is disturbed and becomes turbulent. This turbulence can cause the air to be pushed upward.
Chapter 1 – 7: StabilityLifting Agents/Convergence When two horizontally opposing air masses meet, they will be forced to rise by CONVERGENCE.