Weather Theory: Weather Theory


Atmosphere Composition: Atmosphere Composition Protection from ultra violet rays Supports human animal and plant life 78% nitrogen, 21% Oxygen, 1% Argon, Carbon dioxide, and other trace gases


Layers of the Atmosphere: Layers of the Atmosphere


Troposphere: Troposphere Extends from sea level up to 20,000 feet over the northern and southern poles and up to 48,000 over equatorial regions Majority of weather Temperature decreases at a rate of about 2 degrees Celsius every 1,000 feet


Tropopause: Tropopause Boundary between the troposphere and the stratosphere which acts as a lid to confine most of the water vapor, and associated weather, to the troposphere. Altitude varies with latitude and season of the year Elliptical Shape Commonly associated with the location of jetstream and possible clear air turbulence


Stratosphere: Stratosphere Extends from the tropopause to about 160,000 feet. Little weather Stable


Mesosphere: Mesosphere Mesopause boundary to 128,000 ft. Temperature decreases rapidly with an increase in altitude -90 Degrees Celsius Mesopause


Thermosphere: Thermosphere Starts above mesosphere and fades into outer space


Atmospheric Pressure: Atmospheric Pressure At sea level, the atmosphere is exerting a pressure of 14.7 pounds per square inch over the entire surface of your skin Atmospheric Pressure will vary with temperature, altitude and density of the air. Pressure will effect performance (takeoff, rate of climb, engine efficiency, landing roll)


International Standard Atmosphere (ISA): International Standard Atmosphere (ISA) Provides common reference Standard sea level pressure 29.92 in Hg. At 59 degrees F (or 15 degrees C) 1013.2 millibars For every 1,000 ft altitude increase, pressure falls about 1 inch of mercury If pressure is increasing, it is generally a sign of good weather If pressure is decreasing, it is an indicator of poor weather and possibly severe storms


Atmospheric Circulation: Atmospheric Circulation Sun heats the Earth’s surface unequally Upsets equilibrium of atmosphere Changes in air movement and atmospheric pressure Generally, areas of low pressure exist over the equatorial regions and higher pressure exist over the polar regions due to heating differences


Atmospheric Circulation: Atmospheric Circulation Solar heating causes air to become less dense and rise in equatorial areas and results in a low. The resulting low pressure allows the high pressure air at the poles to flow along the planet’s surface toward the equator As warm air flows toward poles, it cools, becoming more dense and sinks back toward the surface


Coriolis Force: Coriolis Force Created by the rotation of the Earth Deflects air to the right in the Northern Hemisphere – causing it to follow a curved path rather than a straight line Amount of deflection depends on latitude Greatest at poles, diminishes toward equator


Coriolis Effect: Coriolis Effect The speed of the earths rotation causes the general flow to break up into three distinct cells in each hemisphere NH: warm air at equator rises upward, travels northward, but is deflected eastward by rotation of the Earth. Coriolis force bends the flow to the right, creating the northeasterly trade winds that prevail from 30 degrees latitude to the equator


Air Mass Circulation: Air Mass Circulation Friction modifies the movement of the air within 2,000 feet of the ground Air moves slower Wind is diverted from its path because the frictional force reduces the Coriolis force Wind direction at surface varies from the wind direction a few thousand feet aloft.


Wind Patterns: Wind Patterns Air flows from areas of high pressure to areas of lower pressure In the Northern Hemisphere, the flow of air from areas of high to low pressure is deflected to the right, producing a clockwise circulation (anti-cyclonic) around High pressure Around Low pressures, air flows counter – clockwise (cyclonic)


High Pressure V. Low Pressure: High Pressure V. Low Pressure High Pressure Dry, stable, descending air Good weather Low Pressure Unstable, increasing clouds and precipitation Bad Weather


Convective Currents: Convective Currents Caused by different surfaces radiating different amounts of heat Rocks, plowed fields, sand, barren land give off large amounts of heat (cause updrafts) Water, trees, vegetation retain heat (cause downdrafts) Uneven heating creates small areas of local circulation called Convective Currents Convective currents cause bumpy, turbulent air sometimes encountered at lower altitudes Avoid by flying at higher altitudes, possibly above the clouds


Sea Breeze: Sea Breeze Convective currents especially noticeable in areas where landmasses are directly adjacent to a large body of water Day: Land heats faster than the water causing the land air to become warmer and less dense Rises and is replaced by cooler, less dense air flowing in from the water


Land Breeze: Land Breeze Night: Land cools faster than water, so does the air above the land Warmer air over the water rises and is replaced by the cooler, denser air from the land


Effects of Convective Currents: Effects of Convective Currents Can affect a pilots ability to control the aircraft Rising air from terrain devoid of vegetation on final approach can cause you to balloon and overshoot intended landing spot Approach over water or dense vegetation can cause you to sink and land short


Effect of Obstructions on Wind: Effect of Obstructions on Wind Ground topography and large buildings can break up the flow of wind and create wind gusts that change rapidly in direction and speed


Mountainous Regions: Mountainous Regions As air flows down leeward side of mountain, air follows the contour of the terrain and is increasingly turbulent. Tends to push aircraft into the side of the mountain. Stronger the wind the greater the downward pressure and turbulence become


Low Level Wind Shear: Low Level Wind Shear Sudden drastic change in windspeed and/or direction over a very small area Can subject an aircraft to violent updrafts and downdrafts as well as abrupt changes in horizontal movement Can occur at any altitude, Low level is especially hazardous due to proximity to ground Directional wind changes of 180 degrees and speed changes of 50 kts associated with LLWS Commonly associated with passing frontal systems, thunderstorms and temperature inversions with strong upper level winds


Microbursts: Microbursts Most severe type of LLWS Associated with convective precipitation (rain from thunderstorms) Occurs in a space of less than 1 mile horizontally and within 1,000 feet vertically 15 minute lifespan Downdrafts up to 6,000 feet per minute, wind change of 45 knots or more


Microburst: Microburst Difficult to detect because of confined area Alert systems for LLWS installed a several airports around country LLWAS (Low Level Wind shear alert system)


Atmospheric Stability: Atmospheric Stability The stability of the atmosphere depends on its ability to resist vertical motion. A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an unstable atmosphere, small vertical air movements tend to become larger, resulting in turbulent airflow and convective activity. Instability can lead to significant turbulence, extensive vertical clouds, and severe weather.


Adiabatic Cooling and Heating: Adiabatic Cooling and Heating Adiabatic Cooling As air increases in altitude, pressure decreases and it will expands and cools Adiabatic Heating As air decreases in altitude it is compressed and heated to the increased pressure Average rate of temperature change Moist adiabatic lapse rate (saturated air) 2 degrees per 1000 feet Dry adiabatic lapse rate (unsaturated air) 3 degrees per 1000 feet


Inversions: Inversions When air warms as it rises in altitude Shallow layers of smooth, stable air near the ground Can contribute to clouds, fog, haze and smoke which can diminish visibility Clear, cool nights as the ground cools and cools the air a few hundred feet above Frontal inversions occur when a warm air mass moves over a colder air mass


Moisture: Moisture Atmosphere naturally contains moisture in the form of water vapor Amount of moisture depends on temperature A 20 degree F increase in temperature doubles the atmospheres ability to hold moisture


Moisture: Moisture Melting Solid to liquid Freezing Liquid to solid Evaporation Liquid to gas Sublimation Solid to Gas Condensation Gas to Liquid Deposition Gas to solid


Humidity: Humidity Amount of water vapor in the atmosphere at a given time Relative Humidity Actual amount of moisture in the atmosphere compared to the amount of moisture that it could hold at that temperature Ex. If today’s relative humidity is 55%, the atmosphere is holding 55% of what it could be holding


Dewpoint: Dewpoint Temperature at which the atmosphere could hold no more moisture When temperature reaches the dewpoint, the air is completely saturated and the moisture will need to begin to come out in the form of fog, dew, frost, clouds, rain, hail, or snow. Clouds will often form at the altitude where temperature and dewpoint meet


Relative Humidity, Temperature and Dewpoint: Relative Humidity, Temperature and Dewpoint


Methods by which air will meet saturation point: Methods by which air will meet saturation point When warm air moves over a cold surface, the air’s temperature drops and reaches the saturation point. When cold air and warm air mix. When air cools at night through contact with the cooler ground, air reaches its saturation point. When air is lifted or is forced upward in the atmosphere.


Dew and Frost: Dew and Frost On cool, calm nights, the temperature of the ground and objects on the surface can cause temperatures of the surrounding air to drop below the dewpoint. Moisture in the air condenses and deposits itself on the ground, buildings, and other objects like cars and aircraft. If below freezing, we get frost Disrupts flow over wing, drastically reducing lift Adds drag


FOG: FOG Cloud forming within 50 feet of surface Temp near ground cooled to dewpoint Classified according to the manner in which it forms Dependent upon the current temperature and the amount of water vapor in the air.


Radiation Fog: Radiation Fog Clear nights Little to no wind Low – lying areas such as mountain valley’s Occurs when ground cools rapidly due to terrestrial radiation Will burn or blow off


Advection Fog: Advection Fog Warm, moist air moves over a cold surface Winds up to 15 knots required Above 15 knots = low stratus Coastal areas Sea Breezes


Upslope Fog: Upslope Fog Coastal areas Moist, stable air forced up sloping surface (mountain) Needs wind for formation and continued existence Will not burn off, lasts for days


Steam Fog: Steam Fog Cold, dry air over warm water Water evaporates and rises – looks like smoke Bodies of water Cold season Low level turbulence Icing


Ice Fog: Ice Fog Temperature well below freezing Water Vapor forms directly to ice crystals Much like radiation fog, but in mostly artic regions


Clouds: Clouds Formation Must have adequate water vapor and condensation nuclei, as well as a method by which the air can be cooled. When the air cools and reaches its saturation point, the invisible water vapor changes into a visible state. Through the processes of deposition (also referred to as sublimation) and condensation, moisture condenses or sublimates onto miniscule particles of matter like dust, salt, and smoke known as condensation nuclei. The nuclei are important because they provide a means for the moisture to change from one state to another.


Clouds: Clouds Type Height Shape Behavior Low, Middle or High depending on altitude of cloud base Vertical development


Low Clouds: Low Clouds Surface – 6,500 ft AGL Primarily water droplets Can include supercooled ice droplets that induce iceing Stratus, Nimbostratus, Stratocumulus, Fog Hamper visibility, change rapidly, Low ceilings


Middle Clouds: Middle Clouds 6,500 – 20,000 ft AGL Altostratus, Altocumulus Turbulence, Moderate Icing


High Clouds: High Clouds Above 20,000 Only form in stable air Cirrus, Cirrostratus, Cirrocumulus Ice crystals – no real threat


Vertically Developing Clouds: Vertically Developing Clouds Cumulus, towering cumulus, cumulonimbus clouds Form at low to middle altitudes but extend into high Instability, turbulence Lighting, hail, gusty winds, tornadoes and wind shear Can be embedded (hidden)


Thunderstorms: Thunderstorms


Air Masses: Air Masses Large bodies of air that take on the characteristics of their source region Areas of stagnation 4 areas Tropical/Polar Continental/Maritime


Air Masses: Air Masses A continental polar air mass forms over a polar region and brings cool, dry air with it. Maritime tropical air masses form over warm tropical waters like the Caribbean Sea and bring warm, moist air. As the air mass moves from its source region and passes over land or water, the air mass is subjected to the varying conditions of the land or water, and these modify the nature of the air mass.


Air Masses: Air Masses An air mass passing over a warmer surface is warmed from below and convective currents cause the air too rise Unstable, Good Visibility Cumulus clouds, showers and turbulence Air mass passing over a colder surface is stable Poor surface visibility: Smoke dust and other particles cannot rise Low stratus clouds and fog


Fronts: Fronts As air masses move, the will eventually come in contact with another mass with different characteristics Boundary between the two masses is a front A change of weather will always be associated with a front


Warm Front: Warm Front Warm mass of air advances and replaces a colder body of air Move slowly; 10 – 25 miles per hour Slides over the top of the cooler air and gradually pushes it out of the area Warm air masses contain high humidity, as air is lifted, the temperature drops and condensation will occur


Warm Front: Warm Front Prior to Passage (along frontal boundary) Cirriform or stratiform clouds Summer: Cumulonimbus likely to form (thunderstorms) Fog Light to moderate precip – sleet, snow, drizzle Poor visible S/SE wind Cool/cold Falling pressure During Passage Stratiform Clouds Drizzle Poor visibility Temperature rises steadily Pressure and dewpoint level off After Passage Stratocumulus Possible rain Showers Visibility will eventually improve S/SW wind Dewpoint rises and levels Slight rise then decrease in pressure


Warm Front: Warm Front


Cold Front: Cold Front Mass of cold, dense and stable air advances and replaces a body of warmer air Move 25 – 30 mph (some up to 60mph) Stays close to ground, slides under warmer air forcing it aloft Temperature will decrease suddenly forcing creation of clouds


Cold Front: Cold Front Prior to Passage Cirriform, towering cumulus, cumulonimbus Rain showers/haze High dewpoint, falling pressure During Towering cumulus, CB Heavy rain, lighting, thunder, hail Possible Tornadoes Poor Visibility Variable, gusty winds Temp, DP drop rapidly After Dissipating clouds Good visibility W/NW winds Pressure rises


Cold Front: Cold Front


Stationary Fronts: Stationary Fronts Forces from two air masses relatively equal Boundary remains stationary and effects local weather for days Effects can be a mix a both warm and cold front characteristics


Occluded Front: Occluded Front Cold front catches up with a warm front Cold Front Occlusion Cold front air is colder than the air in front of the warm front Forces warm air aloft Mix of weather Warm Front Occlusion Air in front of warm front is colder than the air in the cold front Cold front will be forced on top of the warm front and aloft If air is unstable, weather will be more severe than in a cold front occlusion Embedded thunderstorms, rain, fog


Cold Front Occlusion: Cold Front Occlusion