Exploring Earth's Atmosphere

L'Atmosphère (The Atmosphere)

• It is a gaseous envelope that surrounds the earth for hundreds of kilometers.
• Gas becomes rarefied with altitude.
• 99% of the atmosphere's total mass resides from 0-30 km in altitude.
• It is divided into troposphere, stratosphere, mesosphere, and thermosphere layers.
• The separation surface between the troposphere/stratosphere is called the tropopause, around 11 km altitude.
• The air's composition is 99.97% dry air of:
  • 78% nitrogen
  • 21% oxygen
  • 1% argon
  • ozone (between 15-45 km)
• Includes water vapor and dust.
• Like any gas, it is described by parameters:
  • Temperature
  • Pressure
  • Humidity
  • Wind
• The standard atmosphere is an average used for aeronautical needs.
  • It describes aircraft performance and location vertically.
  • At sea level, standard temperature is 15°C and standard pressure is 1013.25 hPa.
  • 1 hPa measures every 28 feet, or 2 degrees every 1000 feet, at 330m.

La Pression (Pressure)

• Pressure is the weight of the air column from ground to the upper limit of the atmosphere over 1 m².
• The unit of measure is the Pascal, meteorologically using the hectopascal (1 hPa = 100 Pa).
• When ascending, airborne molecules decrease, as does pressure with altitude.
• In the atmosphere's lower layers, pressure decreases.
• High pressure is an anticyclone (symbol A or H).
• Low pressure is a depression (symbol D or L).
• Isobars connect points of equal pressure.
• QFE: atmospheric pressure at the aerodrome level; indicates height between aircraft and aerodrome.
• QNH: atmospheric pressure at sea level; indicates aircraft altitude in relation to sea level.
• 1013 is standard atmospheric pressure at sea level.
  • A setting of 1013 is used for flight level (FL).

La Temperature (Temperature)

• Definition: the degree of hotness or coldness that can be measured in Celsius (°C) within France
• Temperature decreases in the troposphere with increasing altitude, reaching -56.5°C at its upper limit.
• The tropopause, i.e. the transition between troposphere and stratosphere, marks the beginning of temperature inversion or increase with altitude.
• In the troposphere, temperature decreases by 2°C/1000 ft or 6.5°C/1000 m.
• Solar radiation warms the earth, which diffuses heat by radiation to lower air layers.
• Air "bubbles" rise by convection and are replaced by cooler air.
• Vertical ascending/descending convection currents are established, developing cumulus clouds at the top.
• Pilots use these 'pumps' by circling in a spiral, also known as soaring.

L'eau dans l'Atmosphere (Water in the Atmosphere)

• Water exists in the atmosphere in solid, liquid, and gas form.

Vapeur d'eau (Water vapour)

• It is water in a gaseous state contained in the air and increases with temperature.
• Humidity is the amount of water vapor in the air.
• Relative humidity is the ratio of actual to maximum water vapor.
• 100% relative humidity indicates saturation.
• Dew point refers to the temperature air must be cooled to for relative humidity to reach 100%.
• Condensation/evaporation occurs when the air temperature drops below the dew point.
  • Tiny water droplets form around various dust particles suspended in the air.
• The reverse phenomenon is evaporation.
• Solidification/fusion occurs if cooled below 0°C to become ice or snow, reversing back again during fusion.
• Superfusion describes atmospheric water droplets remaining in a liquid state below 0°C, commonly found in fog and clouds down to -40°C.
• Condensation trails are created by condensation of water vapor emitted by aircraft engines at high altitudes.
• Water droplets in suspension become ice crystals, creating white trails behind airplanes.

Les Nuages (Clouds)

• Clouds consist of visible sets of liquid water and/or ice crystal particles.
• They form via cooling of ascending air, where maximum water vapor amount increases with relative humidity, condensing around small particles when relative humidity hits 100%.
• Cloud base above 6 km consists of ice crystals with the prefix "cirro".
• Cloud base between 2 and 6 km consists of ice crystals and liquid water droplets with the prefix "alto".
• Cloud base between ground and 2 km consist of liquid water without a prefix.
• Some clouds show greater vertical extension, indicating rain or bad weather with the prefix/suffix "nimbus."

Les Precipitations (Precipitation)

• Precipitation consists of a set of liquid and/or solid particles that fall from a cloud.
• Within the cloud, negative temperatures permit coexistence of ice crystals and supercooled water droplets.
• Through water vapor transfer and collision, cloud components grow to precipitate.
• Precipitation nearly always initiates as a snowflake, transforming into raindrops if it passes a layer above 0°C.
• Drizzle refers to small water droplets of less than 0.5 mm in diameter, close together and stemming from low-lying clouds (Stratus) and fog.
• Rain represents larger droplets originating from thicker, expansive clouds (Altostratus, Nimbostratus, Cumulonimbus, Stratocumulus, Altocumulus).
• Snow comprises ramified ice crystals, sometimes star-like, formed between 0° and -10°, clustered into flakes spanning 0.5 to 2.5 cm, with the same origin as rain.
• Hail are large ice globules ranging from millimeters to centimeters in diameter, originating from unstable clouds with strong vertical development (Cumulonimbus).
• Showers are sudden, intense, localized and short-lived with unstable clouds of significant vertical development.
  • This includes rain, snow, and hail showers.

Le Vent (Wind)

• Wind is air in horizontal motion.
• If the earth was still, wind would flow from high to low pressure, but the earth's rotation deflects air movement to the right in the northern hemisphere.
• Wind direction aligns almost parallel to isobars.
• In the northern hemisphere, wind travels clockwise around anticyclones and counter-clockwise around depressions.
• In the southern hemisphere, these circulations are reversed.
• Wind force directly relates to the spacing of isobar lines, where closed lines mean stronger winds.
• Jet streams feature rapid air currents of hundreds of kilometers across and several kilometers thick, located around 10 km altitude.
• Jet streams circle the earth westward, with wind speeds reaching 200-300 km/h.

Les Brises (Breezes)

• Local, consistent wind by lakes, sea, mountains, and valleys result from temperature variations in the lower troposphere in conjunction with daily cycles.

Les brises en regions cotieres (Coastal breezes)

• Land temperature varies more and faster than water.
• Sea breezes occur during the day because land heats up faster than the sea surface; the upward air creates a depression that pulls in colder air above the sea.
• Land breezes occur because the night air in contact with soil cools faster than air contacting the sea, reversing air.

Les brises en regions montagnueses (Mountain breezes)

• Mountain and valley breezes occur during the day because the sun-facing slopes warm and rise, leading air at the valley bottom to push up the mountain.
• At night, the phenomenon reverses.
• Slope breezes combine with overall wind; and climbers use the updraft along the slopes.

Circulation Generale (General Circulation)

• Definition: a large expanse of air where temperature and humidity vary little.
• Certain global regions have uniform temperature and humidity, which air masses inherit.
• Air masses move according to general atmospheric circulation principles: cold air flows to the equator while hot air heads to the poles.
• They change when passing over regions with other characteristics.
• Air masses are classified by temperature and humidity.
• Temperature
  • Arctic (Very cold)
  • Polar (cold)
  • Tropical (warm)
  • Equatorial (very warm)
• Humidity
  • Continental (dry)
  • Maritime (humid)
• Several masses can reach France with characteristic weather:
  • Air polaire maritime: cumuliform clouds and showers.
  • Air polaire continental: clear and dry with stratus or stratocumulus.
  • Air tropical maritime: fog or low stratiform clouds.
  • Air tropical continental: storms over high grounds.

Frontologie (Frontology)

• Fronts lie on a separation surface between cold and warm air masses that deforms under air pressure.
• Front undulation produces two limits:
  • Warm front: warm air pushes cold air in front and above it.
  • Cold front: cold air pushes warm air in front and above it.
• After disruption, the cold front moves faster than the warm front.
• During occlusion, the cold front catches the warm front, pushing it upwards.

Brume Brouillard (Haze and Fog)

• Haze is suspended microscopic water droplets, reducing visibility between 1 and 5 km.
• Dry haze originates from solid particles in the air (sand, dust) not saturated humidity.
  • This disruption occurs from regional aspects (desert-based sandstorms) or effects tied to society, tech, and econ output.
• Fog consists of smaller droplets or ice crystals, reducing visibility to under 1 km.
• Fog forms primarily through cooling of a humid air mass, where refrigerating causes saturation, then condensation.

Le Givrage (Icing)

• Icing is an opaque or clear ice deposit.
• Moderate icing and heavy icing can occur.
• Icing is due to the presence of liquid-form water in clouds at sub-zero temperatures.
• Upon impact with plane surfaces exposed to relative wind (nose, wing leading edges, engine air intakes, propellers), liquid turns to ice, accumulating there.
• Icing poses an aviation hazard.
• It can cause:
  • Aircraft overload
  • Airfoil deformation (reducing lift and increasing drag)
  • Sensor obstruction
  • Instrument failure
  • Control seizing
• Prevention is best achieved on the ground by de-icing before departure and applying protective liquid.
• In flight, anticipate/manage via anti-icing systems like windshield defrosters, propeller blades, pitot tubes, and inflatable leading-edge boots.
• De-ice boots are a flexible surface on the edges.

Carte TEMSI (TEMSI Chart)

• A TEMSI is a schematic chart of significant, expected weather, produced every three hours.
• The chart incorporates significant phenomena and cloud masses, explained via symbols:
  • Fronts: cold, warm and occluded
  • isotherm levels for 0° and -10°
  • Types of precipitation
  • Cloud cover (FEW, SCT, BKN or OVC)

Carte des Vents et des Temperatures Prevus (Forecasted Wind and Temperature Chart)

• Charts indicate wind and temperature at FL020 over four periods: 00h TU, 06h TU, 12h TU, 18h TU.
• Temperature indicates a + if positive and no sign when negative.
• Wind is shown by arrows, barbs, and pennants, indicating wind direction and speed.

METAR TAF

• METAR is an airport observation report.
• TAF provides airport weather forecast.
  • Includes information on time, wind, visibility, clouds, and precipitation.