Nature of the Atmosphere Flashcards
Describe the nature of air flow of high and low pressure systems
Low Pressure - inward, upward, and counterclockwise
High pressure - outward, downward, and clockwise
If your route of flight takes you towards a LOW pressure system, what kind of Wx can you expect?
A low-pressure system is characterized by rising air, which is conducive to cloudiness, precipitation, and bad wx.
If your route of flight takes you towards a HIGH pressure system, what kind of Wx can you expect?
A high-pressure system is characterized by descending air, which means dissipation of clouds and good weather.
Describe a cold front
Occurs when a mass of cold, dense, and stable air advances and replaces a body of warmer air.
Describe an occluded front
A frontal occlusion occurs when a fast-moving cold front catches up with a slow moving warm front.
Describe a warm front
Its the boundary area formed when a warm air mass contacts and flows over a colder air mass.
Describe a stationary front
When the forces of two air masses are relatively equal, the boundary or front that separates them remains stationary and influences the wx for days. The wx is typically a mixture of warm and cold fronts.
What are some general characteristics of the wx a pilot would encounter when operating near a cold front?
As the front passes, expected wx can included towering cumulus or cumulonimbus, heavy rain, accompanied by lightning, thunder, and/or hail. Poor visibility, winds variable and gusting, temp and dew point and barometric pressure drop rapidly
What are some general characteristics of the wx a pilot would encounter when operating near a warm front?
As the front passes, expected wx can included stratiform clouds, drizzle, low ceilings, and poor visibility, variable winds, rise in temp
What is a “trough”?
A trough is an elongated area of relatively low atmospheric pressure. At the surface when the air converges into a low, it cannot go outward against the pressure gradient, not can it go downward into the ground, it must go upward.
Therefore, a low or trough is an area of rising air. Rising air is conducive to cloudiness and precipitation. Low pressure, bad wx
What is a “ridge”?
A ridge line is an elongated area of relatively high atmospheric pressure. Air moving out of a high or ridge depletes the quantity of air. Therefore, these are areas of descending air. Descending air favors dissipation of cloudiness, hence the association of high pressure and good wx
What are the standard temp and pressure values for sea level?
15 degrees C and 29.92 Hg
What are “isobars”?
An isobar is a line on a wx chart which connects areas of equal or constant barometric pressure.
If the isobars are close together on a surface wx chart or a constant pressure chart, what info will this provide?
The spacing of isobars on these charts defines how steep or shallow a pressure gradient is. When isobars are spaced close together, a steep pressure gradient exists which indicated higher wind speeds.
A shallow pressure gradient (isobars not close together) usually means winds speeds will be less.
What causes the winds aloft to flow parallel to the isobars?
The Coriolis force
At what rate does atmospheric pressure decrease with an increase in altitude?
1” Hg per 1,000 ft
What does dew point mean?
Dew point is the temperature to which a sample of air must be cooled to attain a state of saturation
When temp and dew point are close together (5 degrees) what type of wx is likely?
Visible moisture in the form of clouds, dew, or fog. Also, these are ideal conditions for carb icing
Explain the difference b/w a stable atmosphere and unstable atmosphere. Why is the stability of the atmosphere important?
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 wx.
During your pre-flight planning, what type of meteorological info should you be aware of with respect with icing?
Location of fronts - A fronts location, type, speed, and direction of movement.
Cloud layers - The location of cloud bases and tops, which is valuable when determining if you’ll be able to climb above icing layers or descend beneath those layers into warmer air. Reference PIREPS and area forecasts.
Freezing levels - important when dtermining how to avoid icing and how to exit icing conditions if accidentally encountered.
Air temp/pressure - Icing tends to be found in low pressure areas and at temps at or around freezing.
What is the definition of a freezing level and how can you determine where that level is?
The freezing level is the lowest altitude in the atmosphere over a given location at which the air temperature reaches 0 degrees C. Its possible to have multiple freezing layers. You can use a freezing level graphics charts, area forecasts, PIREPS, AIRMET’s, SIGMET’s, and surface analysis charts, low-level significant wx charts, and temperatures aloft.
What conditions are necessary for structural icing to occur?
Visible moisture and below freezing temps at the point moisture strikes the aircraft.
Name the three types of icing an aircraft may encounter during flight.
Structural, induction system, and instrument icing
Name the three types of structural icing that may occur in flight.
Clear ice - forms after initial impact when the remaining portion of the drop flows out over the aircraft surface gradually freezing as smooth sheet of ice.
Rime ice - liquid portion of the drop freezes rapidly due to its small size
Mixed ice - forms when drops mix with snow or ice particles. The ice particles become embedded in clear ice, building a very rough accumulation.
What action is recommended if you encounter icing conditions?
Leave the area of of visible moisture. This means fly at a lower altitude of fly above the cloud layer. Or turn course.
Is frost considered to be hazardous to flight?
Yes because the roughness of the frosts surface spoils the flow of smooth air over the wing. This slowing of air causes early airflow separation, resulting in a loss of lift. This frost could produce an insufficient margin of airspeed above a stall.
What factors must be present for a thunderstorm to form?
Sufficient water vapor, an unstable lapse rate, an initial upward boost of lifting air
What are the three stages of a thunderstorm?
Cumulus stage - updrafts cause raindrops to increase in size
Mature stage - rain at the earths surface. It falls through the updrafts; lightning; roll clouds
Dissipating stage - downdrafts and rain begin to dissipate. Anvil cloud
What is a temperature inversion?
An inversion is an increase in temp with height, a reversal of the normal decrease with height. An inversion aloft permits warm rain to fall through cool air below. Temperature in the cold air can be critical too icing. A ground based inversion favors poor visibility by trapping fog, smoke. The air is stable, with little or not turbulence.
State two ways fog can form.
Cooling air to the dew point, adding moisture to the air near the ground.
Name some types of fog.
Radiation, advection, upslope, frontal, steam
What causes radiation fog to form?
The ground cools the adjacent air to the dew point on calm clear nights.
What is advection fog and where is it most likely to form?
Advection fog results from the transport or warm humid air over a cold surface.A pilot can expect advection fog to form primarily along coastal areas during the winter. Unlike radiation fog, it may occur with winds, cloudy skies, over a wide geographical area and at any time of the day or night.
What is upslope fog?
Upslope fog forms as a result of moist, stale air, being cooled as it moves up sloping terrain. Once upslope wind ceases, the fog dissipates. Upslope fog is quite dense and extends to high altitudes.
Define the term wind shear and state the areas in which it is likely to occur.
Wind shear is defined as the rate of change of wind velocity (direction/speed) per unit distance. Expressed as vertical or horizontal wind shear. It may occur at any level in the atmosphere but three areas are of special concern:
a) Wind shear with a low level temperature inversion b) wind shear in a frontal zone or thunderstorm c) clear air turbulence at high levels associated with a jet stream
Why is wind shear an operational concern to pilots?
Because unexpected changes in wind speed and direction can be potentially hazardous to aircraft operations at low altitudes on approach to and departing from airports.
What types of wx information will you examine to determine if wind shear conditions might affect your flight?
Terminal forecasts, METAR’s, SIGMET’s, PIREP’s, LLWAS (low level windshear alert system)
