Weather Theory Objective To develop basic understanding of weather theory. Outline The atmosphere Pressure Wind and Currents Moisture and Temperature Clouds Fronts Weather hazards Schedule Discussion 2:00 Equipment Whiteboard and color markers Content 1. The atmosphere Blanket of air made up of a mixture of gases that reaches almost 350 miles from the surface Constant motion - if the atmosphere were visible it might look like an ocean with swirls and eddies, rising and falling air and waves that travel for great distances 1.1. Composition 78% Nitrogen, 21% Oxygen, 1% - Argon, carbon dioxide, trace gases; also 1-5% water vapor 4 layers - troposphere (SL - 20K/48K ft), stratosphere (20K/48K - 160K ft), mesosphere (160K - 280 K ft), thermosphere (280K - 350 K ft); tropopause at the top of the troposphere The majority of weather, clouds, storms and temp. variances occur within troposphere, temp. decreases about 2 C/1000 ft, pressure decreases about 1 in/1000 ft 1.2. Circulation The primary source of all weather changes is the Sun - heats the surface at varying rates Heated air becomes less dense, rises and creates low pressure areas; cooled air descends and creates high pressure areas 1
Air heated at the Equator rises, cools and moves towards the North and South Poles - this creates circulation patterns (cells) 1.3. Coriolis Force 2. Pressure Because of the Earth s rotation, a phenomenon known as Coriolis force deflects the moving air (to the right in Northern Hemisphere) causing it to follow a curved path instead of a straight line In mid latitudes (US, Europe) the prevailing upper winds are westerly As altitude increases, atmospheric pressure decreases, on average 1 in per 1000 ft; air becomes less dense or thinner Differences in air density caused by changes in temp. result in a change in pressure; this creates motion in the atmosphere both vertically and horizontally in the form of currents and wind 3. Wind and Currents 3.1. Wind Patterns Air flows from areas of high pressure into areas of low pressure (air always seeks out lower pressure) In the Northern Hemisphere, the flow of air from areas of high to low pressure is deflected to the right and produces a clockwise circulation around an area of high pressure (anticyclonic); the opposite is true of low pressure areas - the air flows toward a low and is deflected to create counterclockwise (cyclonic) circulation Coriolis force deflects the wind so that the direction of air motion actually crosses the isobars at an angle High pressure are generally areas of dry, stable, descending air - good weather; in a low air rises, tends to be unstable and usually brings cloudiness and precipitation - bad weather Pilot can take advantage of tailwinds created by high and low patterns On the surface the wind patterns do not seem to agree with any rule - because of local conditions and surface friction (up to 2000 ft AGL) 3.2. Convective Currents Different surfaces radiate heat in varying amounts (plowed ground, rocks, sand give off heat; water, trees absorb and retain heat) - the resulting uneven heating creates small areas of local circulation called convective currents 2
Currents cause the bumpy, turbulent air at low altitude Convective currents are mostly noticeable in areas with a land mass adjacent to a large body of water; during the day land heats faster than water, the air over the land becomes warmer and less dense, it rises and is replaced by cooler, denser air flowing in from over the water - this causes an wind called a sea breeze (at night the pattern reverses) 3
Effect of Obstructions on Wind 3.3. Obstructions on the ground affect the flow of wind - ground topography and large buildings can create wind gusts that change rapidly in speed Isobars are lines drawn on the chart to depict area of equal pressure (connect stations with equal pressure); closely spaced isobars indicate a steep wind gradient and strong winds Isobars help identify low and high pressure systems, as well as location of ridges (elongated area of high pressure), troughs (elongated area of low pressure) and cols (between 2 highs and lows) Close to the ground wind direction and speed is modified by the surface friction; generally the wind at 2000 ft AGL is 20-40 to the right (clockwise) of surface winds and the speed is greater Moisture and Temperature 4.1. 4 The same condition is even more noticeable when flying in mountainous regions - the wind on the leeward side follows the contour of the terrain and is increasingly turbulent Wind and Pressure on Weather Maps 3.4. 4. Stability The stability of the atmosphere depends on its ability to resist vertical motion; a stable atm. makes vertical movement difficult and small disturbances dampen out; in un unstable atm. small vertical air movements tend to become larger, resulting in turbulence and convective activity Rising air expands and cools, descending air compresses and heats - adiabatic cooling/heating Standard lapse rate is 2 C per 1000 ft
Dry air cools 3 C per 1000 ft - stable Moist air cools at a slower rate 1.1 C - 2.8 C per 1000 ft - less stable Cold air over warm air is unstable - warm air will tend to rise (and cool slower, so rise even more) 4.2. Inversion Inversion exists when air temperature rises with altitude (to a certain point - top of the inversion) The air at the top acts as a lid keeping weather and pollutants trapped below; if humidity is high it can contribute to the formation of clouds, fog, haze, smoke, resulting in diminished visibility Surface based inversion occur on clear, cool nights when the air close to the ground in cooled by lowering temperature of the ground (radiation); frontal inversion occur when warm air spreads over a layer of cooler air, or cooler air is forced under a layer of warmer air 4.3. Moisture The amount of moisture (water vapor) in the atm. depends upon the temp. of the air - warmer air can hold more moisture (2x for every 11 C) Relative humidity is the amount of moisture in the air compared to the max. amount the air can hold invisibly at present temperature When the air contains 100% of the moisture it can hold, the moisture becomes visible The temp. at which the air (when cooled) becomes saturated is called the dew point The measure of relative humidity is the spread between temp. and dew point - 2 C or less could be an indication of possible fog forming at night Dew point decreases with altitude slower than the air temp; temp. / dew point spread can be used to estimate the height of cloud bases (1000 ft per each 2.5 C / 4.4 F of the spread) 4.4. Visible Moisture 5. Clouds When the temp. of the ground and surface objects causes the surrounding air temp. to drop below dew point, moisture condenses and deposits itself on the ground and other objects (grass/cars in the morning, soda can taken out of the dispenser) If the temp. is below freezing, the moisture is deposited in the form of frost (safety hazard) Other types of visible moisture: clouds, fog, precipitation 4 cloud families: high, middle, low and clouds with vertical development Low clouds form near the surface and extend up to 6,500 ft, made of water droplets (can include supercooled water); typical clouds are stratus, stratocumulus, nimbostratus, also fog; they create low ceiling, low visibility and change rapidly 5
Middle clouds form around 6,500 ft and extend up to 20,000 ft; composed of water, ice crystals and supercooled droplets; typical clouds include altostratus and altocumulus (forms usually when altostratus is breaking apart), can contain turbulence and icing High clouds form above 20,000 ft, usually in stable air; made of ice crystals and pose no threat; typical clouds are cirrus, cirrostratus, cirrocumulus Clouds with extensive vertical development are cumulus clouds that build vertically into towering cumulus or cumulonimbus; bases are in low to middle range but can extend into high altitudes; towering cumulus indicate areas of instability and turbulent air, they often develop into CBs Cumulonimbus clouds contain large amounts of moisture and unstable air and usually produce hazardous weather (lightning, hail, tornadoes, gusty winds, turbulence, wind shear); can be embedded or isolated, can form lines; most dangerous cloud type to pilots " # # $ " " % % " " 6 CB begins in cumulus stage, warm moist air rises, the instability allows this process to feed on itself as the air column, even though cooling with altitude, is warmer than the surrounding air. The
moisture in the air condenses into water, adding more heat. Droplets rise with air and grow colliding. When big enough to overcome upward current they drop causing downdrafts (mature stage). This updraft/downdraft combination is hazardous for any aircraft. Eventually downdrafts overpower updrafts and the storm cell dissipates into heavy rain. Severe turbulence should be expected up to 20 miles from a severe thunderstorm. 6. Cumulus - heaped or piled; stratus - formed in layers, cirrus - fibrous, castellanus - castle-like (common base with separate vertical develop.), lenticularis - lens shaped (formed over mountains in strong winds), nimbus - rain-bearing, fracto - broken, alto - high (middle level) Ceiling is the lowest layer of clouds reported as being broken (5-7/8) or overcast, or the vertical visibility into an obscuration like fog or haze Fronts 6.1. Air Masses Air masses are large bodies of that take on the temperature and moisture characteristics of the surrounding area or source region. A source region is usually an area where air remains for a period of day or longer. 6.2. Fronts 6.3. An air mass eventually comes in contact with another air mass with different characteristics. The boundary layer is a front. There are 4 types of fronts, named according to the temp. of advancing air relative to the temp. of the air it is replacing. Warm Front Move slowly, 10-25 mph, the slope slides over the top of the cooler air and gradually pushes it out of the area; contain warm, often very humid air. As warm air is lifted, the temp. drops and condensation occurs. Prior to passage cirriform or stratiform clouds, fog. In summer CBs are likely to develop. Light to or drizzle, poor vis., SSE winds. As the front moderate precip., usually rain, sleet, snow approaches, pressure continues to fall until the front passes completely, dew point increases. During passage, stratiform clouds, drizzle, poor visibility (improves with var. winds), temp. rises steadily, dew point steady and pressure levels off. 7
After passage, stratocumulus clouds and rain showers possible, improving vis., SSW winds. Dew point rises and levels off, slight rise in pressure followed by a decrease in pressure. 6.4. Cold Front Moves more rapidly than warm fronds, 25-30 mph (extreme: 60 mph), several hundred miles long. Air is dense and stays close to the ground, acts like a snowplow, sliding under the warmer air and forcing the less dense air aloft. Rapidly ascending air causes the temp. to drop suddenly, forcing the creation of clouds. Type of clouds depends on the stability of the warmer air mass. Prior to passage, cirriform or towering cumulus clouds (CB possible). SSW winds. Rain showers and haze possible. High dew point and falling pressure indicate an imminent cold front passage. As the front passes, towering cumulus or CB clouds. Possible heavy rain showers, lightning, thunder, hail (extreme: tornadoes). Vis. poor, winds variable and gusty, temp. and dew point drop rapidly. Quickly falling pressure bottoms out during passage, then begins a gradual increase. After passage, towering cumulus and CB dissipate to cumulus clouds, prep. decreases. Good vis., WNW winds, temp. remain cooler and pressure continues to rise. Fast-moving cold fronts are pushed by intense pressure systems far behind the front. The friction between the ground and cold front retards the movement, creating a steeper frontal surface. Very narrow band of weather. If the warmer air is unstable, scattered thunderstorms and rain showers may form. A squall line (continuos line of thunderstorms) may form along or ahead. Fast-moving cold front usually leaves clear sky, gusty, turbulent winds and colder temp. 8
!!!!! Occluded Front 6.5. When a cold front catches a slower moving warm front, an occluded front occurs. If the air of the cold front is colder that the air ahead of the warm front, the occlusion is cold; the cold air replaces the cool air and forces the warm front aloft; mixture of weather found in warm and cold fronts. 9
If the air of the cold front is warmer that the air ahead of the warm front, the occlusion is warm; the cold front rides up and over the warm front. If the air forces aloft is unstable, the weather is more severe: embedded thunderstorms, rain, fog. 6.6. Stationary Front When the forces of two air masses are relatively equal, the boundary that separates them remains stationary and influences the weather for days. The weather associated with a stationary front is typically a mixture of warm and cold fronts. 7. Weather hazards 7.1. Thunderstorms 7.2. Icing Anytime the temp. approaches 0 C and visible moisture (clouds, precip.) is present, icing can occur. Rain droplets that fall through a cold air mass can become supercooled and freeze on impact with an aircraft. The presence of ice pellets at the surface indicates a freezing rain at a higher altitude. Freezing rain will very quickly coat the entire airplane with ice. 7.3. Fog Fog can form and dissipate almost instantaneously. Radiation fog forms when clear skies allow the land to radiate heat, cooling the air near the surface below dew point; it s a night and early morning phenomenon and a few hours of sunlight will raise the temp. above dew point again, burning off the fog. Fog may seem harmless from an altitude (a pilot can see through) but on landing fog obscures everything because the pilot is looking at the fog layer horizontally. Advection fog results when moist air moves, typically from over water to over colder land Upslope fog forms when moist, stable air is moved towards higher terrain (cools and condenses) Precipitation induced fog is caused by rain falling into a cool, almost saturated air, increasing the moisture content (fronts, mostly warm and occluded) In industrial areas (high level of pollutants in the air) the fog will form more readily and dissipate more slowly - water vapor has something to condense on (condensation nuclei) Completion Standards Lesson is complete when the students can demonstrate basic understanding of weather theory during oral quizzing. References 1. FAR/AIM 2. FAA-H-8083-25A Pilot s Handbook of Aeronautical Knowledge - Chapter 11 3. FAA-H-8083-3A Airplane Flying Handbook 4. ASA The Complete Private Pilot - Lesson 6 10