AF 202 – Chris Dimoulis Aviation Weather Theory
Objectives The Atmosphere Energy Transfer and Wind Stability Moisture Airmasses and Fronts
Atmosphere The atmosphere is a mixture of gases along with water and ice Nitrogen 78% Oxygen 21% Other 1%
Atmosphere While the proportion of those gases remains constant, water vapor is a variable gas Over tropical oceans it can be as high as 4% of the total volume of gasses and nearly absent over deserts.
Atmosphere Even though it seems small, the water vapor plays a large role in weather production It can exist in 3 forms: vapor, liquid, solid Absorbs radiant energy keeping things cool Act as particulates
Atmosphere Particulates are liquid or solid particles that are small enough to remain suspended in the air Formation of fog and clouds Intercept solar and terrestrial radiation Provide a surface for the condensation of water vapor Reduce visibility
Structure of Atmosphere The atmosphere is relatively short compared to how large the earth is
Structure of Atmosphere 50 % of the atmospheric mass is below 18,000 feet 90 % is below 53,000 feet 99.9% is below 164,000 feet
Structure of Atmosphere The different layers of the atmosphere
Structure of Atmosphere Troposphere Tropopause Stratosphere Stratopause Mesosphere Mesopause Thermosphere Exosphere What judges where one ends or beings?
Structure of Atmosphere Troposphere About 36,000 feet at middle latitudes Trope means ‘turn’ or ‘change’ talking about air with more vertical motion is common Majority of clouds and weather at this layer Tropopause Jet stream Clear air turbulence Large thunderstorms
Structure of Atmosphere Stratosphere Temperature increases with altitude ‘Strata’ refers to the air moving more or less in a horizontal motion Stratopause About 160,000 feet Temperature increased to maximum
Structure of Atmosphere Mesosphere Temperature begins to decrease again Up to about 280,000 feet Mesopause Thermosphere Temperature increases, but that doesn’t mean much Number of air molecules is extremely small
Other Layers Ozone Layer Found in lower stratosphere High concentration of ozone (O3) which is toxic. About 80,000 feet Ionosphere A deep layer of charged particles (ions, free electrons) in mesosphere and thermosphere Radio waves can be reflected or absorbed.
Temperature We think of hot or cold Temperature is also looked at as a measurement of the motion (or Kinetic Energy) of molecules
Temperature So the temperature we understand is a measurement of the average kinetic energy of the molecules in a substance Absolute zero is the point where all molecular motion ceases.
Density The total mass of molecules in a given volume If the mass increases but volume stays the same, density increases If the mass stays the same, but the volume decreases, density increases
Pressure The force exerted by the moving molecules of gas on a given area Pressure acts equally in all directions An example for us is atmospheric pressure 29.92 = 14.7 lbs. per sq. foot
Energy Transfer The atmosphere acts like a heat engine Solar energy enters the atmosphere, is transferred into heat energy and then redistributed throughout the atmosphere This is what ultimately produces the weather systems we know today
Energy Transfer Heat gets to our planet in the form of solar radiation which means that energy is transferred by electromagnetic waves. The terrain absorbs about 51% of the radiation and the atmosphere absorbs about 19% The rest is reflected or scattered
Energy Transfer While the radiation from the sun is equal in all areas, the angle at which the radiation causes some areas to get more. The rotation and tilt of the earth means that the earth is heated unevenly
Energy Transfer Since most radiation is absorbed by the earth, the rest of the heat transfer occurs other ways Conduction: The transfer of heat through molecular motion Convection/Advection: Transfer of heat through mass motion The ocean currents Warm and cold airmasses
So what? It is this uneven heating on the earth that is the starting point for all weather. The differential heating causes some air to rise and begins to create areas of high and low pressure. This is what causes WIND!
Wind Wind is caused by pressure gradient force. That is, when you have a high pressure and a low pressure, the high pressure wants to move to the low pressure.
Wind Hot air rises making a low pressure near the surface, but a higher pressure in the upper atmosphere The cooler air sinkscausing a high pressurenear the surface and alow pressure in theupper atmosphere
Wind – Coriolis Force Wind is affected because of the earth’s rotation Max affect near the poles and zero at the equator. Breaks up the flow of large scale air movement
Wind – Friction Force Causes winds closer to the surface to be slower. Winds at 2,000 feet parallel isobars Winds at the surface cross the isobars toward the low pressure
Stability A description of the behavior of a system. Stable: If displaced or distorted, tends to return to the original configuration Unstable: If displaced or distorted, tends to move away from the original configuration
Stability When speaking in terms of the atmosphere: A stable atmosphere means it is difficult for air parcels to move upward or downward An unstable atmosphere is one that promotes vertical motion. Vertical motion of air is one of the ingredients to making severe weather
Stability How do we know if the atmosphere is stable or not? Adiabatic Lapse rate is the rate of temperature change for a parcel of air as it changes altitude 2 degrees Celsius per 1,000 feet
Stability Compare the A.L.R. to the actual Lapse Rate (measured by soundings) of the atmosphere to see how stable it is. L.R. > A.L.R. Unstable L.R. = A.L.R. Neutral L.R. < A.L.R. Stable
Affects of Stability Smooth or turbulent air Stable = smooth, Unstable = turbulent Types of clouds formed Stable = stratiform, Unstable = cumuliform Quality of visibility Stable = low (haze), Unstable = high
Moisture Remember that water vapor is a variable part of the atmospheric composition Though it is a small amount comparably, it is what ‘weather’ is made out of Most weather hazards owe their existence to water vapor. WHY?
Moisture Water is such an important ingredient in weather because it is able to change from solid, to liquid, to gas within the normal temperature ranges of the atmosphere.
Moisture Melting – Ice to Water Evaporation – Water to Vapor Sublimation – Ice to Vapor Condensation – Vapor to Water Freezing – Water to Ice Deposition – Vapor to Ice
Relative Humidity Is used to determine how close the atmosphere is to saturation Relative humidity is a percentage of how much moisture is in the atmosphere The higher the temperature, the more moisture that can be held So 50% RH at 20 degrees does NOT contain the same amount of moisture as 50 % RH at 70 degrees.
Dew Point Dew point is the temperature where, assuming moisture remains constant, the air is fully saturated (100% RH). Dew point can be used to help predict clouds, fog, and low visibility
Clouds Water droplets or ice crystals suspended in the atmosphere Higher than 50 ft. Three Ingredients: Water Vapor Condensation Nuclei Cooling
Clouds Condensation Nuclei Microscopic particle, like dust or salt, that provide a surface for the water vapor to undergo condensation Cooling Cooling can occur by contact with a colder surface or adiabatically.
Cloud Terms Stratus Layer Cumulus Heap Nimbus Rain Cirrus Ringlet Alto Mid Cirro High
Low Cloud Types Stratus Stratocumulus Nimbostratus
Middle Clouds Altocumulus Altostratus
High Clouds Cirrocumulus Cirrostratus Cirrus
Vertical Clouds Cumulus Cumulonimbus
Precipitation All forms of water which fall from the atmosphere Most clouds do not have precipitation 100% humidity does NOT mean there will be precipitation