Weather Information Flashcards
State the general characteristics in regard to the flow of air around high and lower pressure systems in the Northern Hemisphere.
Low pressure - inward, upward, counterclockwise
High pressure - outward, downward, clockwise
If your route of flight takes you towards a low-pressure system, in general, what kind of weather can you expect?
What if you were flying toward a high-pressure system?
Low pressure - characterized by rising air, which is conducive to cloudiness, precipitation, and bad weather.
High pressure - an area of descending air, which tends to favor dissipation of cloudiness and good weather
Describe the different types of fronts.
Cold front - occurs when a mass of cold, dense, stable air advances and replaces a body of warmer air
Occluded front - A frontal occlusion occurs when a fast-moving cold front catches up with a slow-moving warm front. Cold air moves under warm air, pushing it upward (which can create a thunderstorm). Two types: cold front and warm front occlusions.
Warm front - The boundary area formed when a warm air mass contacts and flows over a colder air mass.
Stationary front - When the forces of two air masses are relatively equal, the boundary or front that separates them remains stationary and influences the local weather for days. The weather is typically a mixture of both warm and cold fronts. A hot and cold front meet and don’t move.
What are the general characteristics of the weather a pilot would encounter when operating near a cold front? A warm front?
Cold front - As the front passes, expected weather can include towering cumulus or cumulonimbus, heavy rain accompanied by lightning, thunder and/or hail, tornadoes possible, during passage, poor visibility, winds variable and gusting, temperature/dew point and barometric pressure drop rapidly.
Warm front - As the front passes, expected weather can include stratiform clouds, drizzle, low ceilings, poor visibility, winds variable, rise in temperature.
The weather associated with a front depends on the amount of moisture available, the degree of stability of the air that is forced upward, the slope of the front, the speed of frontal movement, and the upper wind flow.
What is a trough?
An elongated area of lowest atmospheric pressure. At the surface when air converges into a low, it can’t go outward against the pressure gradient, nor 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, hence the general association of low pressure and bad weather.
What is a ridge?
An elongated area of highest 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 weather.
What are the standard temperature and pressure values for sea level?
59 degrees F/15 degrees C
29.92 inches mercury/1013.2 millibars
What are isobars?
A line on a weather chart which connects areas of equal or constant barometric pressure.
If the isobars are relatively close together on a surface weather chart or constant pressure chart, what information will this provide?
The spacing of isobars defines how steep or shallow a pressure gradient is. When isobars are spaced very close together, a steep pressure gradient exists which indicates higher wind speeds. A shallow pressure gradient (isobars not close together) usually means wind speeds will be less.
What is the name of the force that deflects winds to the right in the northern hemisphere and left in the southern hemisphere?
The Coriolis force. It is at a right angle to wind direction and is directly proportional to wind speed.
Why do surface winds generally flow across the isobars at an angle?
Surface friction
At what rate does atmospheric pressure decrease with an increase in altitude?
1” Hg per 1,000 feet
What does dew point mean?
The temperature to which a sample of air must be cooled to attain the state of saturation.
When temperature and dew point are close together, what type of weather is likely?
Visible moisture in the form of clouds, dew, or fog. Also, these are ideal conditions for carburetor icing.
What factor primarily determines the type and vertical extend of clouds?
The stability of the atmosphere
Explain the difference between a stable 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 weather.
List the effects of stable and unstable air on clouds, turbulence, precipitation, and visibility.
Stable air
1) Stratiform clouds
2) Smooth air
3) Steady precipitation
4) Fair to poor visibility
Unstable air
1) Cumuliform clouds
2) Rough air
3) Showery precipitation
4) Good visibility
During your preflight planning, what type of meteorological information should you be aware of with respect to icing?
Location of fronts - A front’s location, type, speed, and direction of movement.
Cloud layers - The location of cloud bases and tops, which is valuable when determining if you will be able to climb above icing layers or descend beneath those layers into warmer air, reference PIREPs and area forecasts.
Freezing level - Important when determining how to avoid icing and how to exit icing conditions if accidentally encountered.
Air temperature and pressure - Icing tends to be found in low-pressure areas and at temperatures at or around freezing.
What is the definition of the term 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. It’s possible to have multiple freezing layers when a temperature inversion occurs above the defined freezing level.
What conditions are necessary for structural icing to occur?
Visible moisture and below freezing temperatures at the point moisture strikes the aircraft
What are the two main categories of aircraft icing?
Structural and induction icing.
Structural - ice that forms on aircraft surfaces and components.
Induction - ice that forms in the engine’s induction system.
Name the three types of structural icing that may occur in flight.
Clear ice - forms after initial impact when the remaining liquid portion of the drop flows out over the aircraft surface, gradually freezing as a smooth sheet of solid ice.
Rime ice - forms when drops are small, such as those in stratified clouds or light drizzle. The liquid portion remaining after initial impact freezes rapidly before the drop has time to spread out over aircraft surface.
Mixed ice - forms when drops vary in size or when liquid drops are intermingled with snow or ice particles. The ice particles become imbedded in clear ice, building a very rough accumulation.
What action is recommended if you inadvertently encounter icing conditions?
The first course of action should be to leave the area of visible moisture. This might mean descending to an altitude below the cloud bases, climbing to an altitude above the cloud tops, or turning to a different course.
Why is icing hazardous to flight?
Ice changes the shape of the wing, which changes the chord line, which changes the AOA, which increases the stall speed.
The plane can stall in cruise with ice on the wing.
Is frost considered to be hazardous to flight? Why?
Yes, because while frost does not change the basic aerodynamic shape of the wing, the roughness of its surface spoils the smooth flow of air, thus causing a slowing of airflow. This slowing of the air causes early airflow separation, resulting in a loss of lift. Even a small amount of frost on airfoils may prevent an aircraft from becoming airborne at normal takeoff speed. It’s also possible that, once airborne, an aircraft could have insufficient margin of airspeed above stall so that moderate gusts or turning flight could produce incipient or complete stalling.
Increased ground roll by around 10-15%
Increased AOA on climb out, which could cause a power on stall
Could possibly have an AOA that’s too shallow on climb out, to build airspeed, which could result in hitting an obstacle
What factors must be present for a thunderstorm to form?
1) Sufficient water vapor
2) Unstable lapse rate
3) Initial upward boost (lifting) to start the storm process in motion
What are the hazards of thunderstorms?
1) Wind shear
2) Icing/Hail
3) Lightning
Up and down drafts can be dangerous
Why are microbursts dangerous?
Don’t need a thunderstorm to make a microburst.
On one side, the plane will have a headwind. While flying through, the plane will be pushed down by downdrafts. To compensate for this, the pilot will pitch up and go full power. But, when on the other side of the microburst, the plane will have a tailwind, which will cause the plane to stall, and since it was being pushed down by downdrafts, there won’t be enough room to recover.
What are the three stages of a thunderstorm?
Building/Cumulus stage - Updrafts cause raindrops to increase in size. Cumulus clouds form.
Mature stage - Rain at earth’s surface; it falls through or immediately beside the updrafts, lightning, hail, gusting wind, perhaps roll clouds (up and down drafts)
Dissipating stage - Downdrafts and rain begin to dissipate
What is a temperature inversion?
An increase in temperature with height - a reversal of the normal decrease with height. An inversion aloft permits warm rain to fall through cold air below. Temperature in the cold air can be critical to icing. A ground-based inversion favors poor visibility by trapping fog, smoke, and other restrictions into low levels of the atmosphere. The air is stable, with little or now turbulence.
