WX Therory PHAK Flashcards
What percentage of the atmosphere is nitrogen?
78%
What percentage of the atmosphere is oxygen?
21%
What are the main layers of the Earth’s atmosphere based on thermal characteristics?
Troposphere, Stratosphere, Mesosphere, Thermosphere
In which atmospheric layer do most weather phenomena occur?
Troposphere
How does temperature change with altitude in the troposphere?
Decreases by about 2°C per 1,000 feet
What happens to air pressure as altitude increases in the troposphere?
Decreases by about 1 inch per 1,000 feet
What is the name of the boundary between the troposphere and stratosphere?
Tropopause
Which atmospheric layer extends from the tropopause to about 160,000 feet?
Stratosphere
Why does warm air rise in the atmosphere?
Warm air is less dense than cooler air
What causes the Coriolis force?
The rotation of the Earth
How does the Coriolis force affect air movement in the Northern Hemisphere?
Deflects air to the right
What is the standard atmospheric pressure at sea level?
29.92 inches of mercury (Hg) or 1013.2 millibars (mb)
How does air pressure change with altitude?
Decreases by 1 inch of mercury for every 1,000 feet
What happens to atmospheric pressure when altitude increases?
Atmospheric pressure decreases
What instrument is used to measure atmospheric pressure in most aircraft?
Aneroid barometer
How does altitude affect takeoff and landing distances?
Higher altitudes increase takeoff and landing distances.
Why do aircraft engines and propellers become less efficient at higher altitudes?
Air density decreases at higher altitudes, reducing engine and propeller efficiency.
What is the effect of altitude on climb rates in aircraft?
Climb rates decrease at higher altitudes due to reduced air density.
At what altitude does oxygen deprivation (hypoxia) start to affect human performance?
Around 10,000 feet, though some may experience effects as low as 5,000 feet.
How does wind flow around high-pressure systems in the Northern Hemisphere?
Clockwise (anticyclonic) circulation.
How does wind flow around low-pressure systems in the Northern Hemisphere?
Counterclockwise (cyclonic) circulation.
What type of weather is typically associated with high-pressure systems?
Good weather, due to descending dry air.
What type of weather is typically associated with low-pressure systems?
Cloudy, bad weather with precipitation, due to rising air.
What are convective currents and how do they form?
Small, localized circulations caused by uneven heating of the Earth’s surface.
What is a sea breeze and when does it occur?
An onshore wind that occurs during the day when land heats faster than water.
What is a land breeze and when does it occur?
An offshore wind that occurs at night when land cools faster than water.
How do convective currents affect an aircraft during final approach?
Rising air from warm terrain can cause overshooting, while sinking air over water or vegetation can cause undershooting.
How can ground obstructions like buildings or mountains affect wind?
They can create gusts and turbulence that change wind direction and speed rapidly.
What is low-level wind shear and why is it hazardous to aircraft?
A sudden change in wind speed and/or direction close to the ground, potentially causing violent turbulence.
What atmospheric conditions are commonly associated with low-level wind shear?
Passing frontal systems, thunderstorms, temperature inversions, and strong upper-level winds.
What is a microburst and how does it affect aircraft?
A sudden, intense downdraft that can produce severe turbulence and wind direction changes, dangerous during takeoff or landing.
How do isobars on a weather map help pilots understand wind conditions?
Closely spaced isobars indicate strong winds, while widely spaced isobars indicate light winds.
How does wind at 2,000 feet above ground level compare to surface winds?
It is faster and blows 20-40° to the right of surface winds.
What determines the stability of the atmosphere?
Its ability to resist vertical motion.
What happens in an unstable atmosphere?
Small vertical movements become larger, leading to turbulence and convective activity.
How does rising air cool in the atmosphere?
By expansion due to decreasing air pressure with altitude.
What is the dry adiabatic lapse rate for unsaturated air?
3°C (5.4°F) per 1,000 feet.
How does moisture affect air density and stability?
Moist air is less dense and rises, making it generally less stable than dry air.
What is a temperature inversion?
A situation where air temperature increases with altitude, trapping weather and pollutants below.
How do surface-based temperature inversions typically form?
On clear, cool nights when the ground cools the air near the surface.
How does humidity affect the air’s moisture-holding capacity?
Every 20°F increase in temperature doubles the air’s moisture-holding capacity.
What is relative humidity?
The actual amount of moisture in the air compared to the maximum it can hold at that temperature.
What is the dew point?
The temperature at which air becomes fully saturated and can hold no more moisture.
What happens when air temperature and dew point converge?
Clouds often form as air reaches saturation.
How do you estimate cloud base altitude using temperature and dew point?
Subtract dew point from temperature, divide by 4.4°F, then multiply by 1,000 feet.
What are the four methods by which air reaches the saturation point?
Cooling by surface contact, mixing of warm and cold air, cooling at night, and lifting in the atmosphere.
How does frost affect aircraft performance?
Frost disrupts airflow over wings, reducing lift and increasing drag.
How does radiation fog form?
On clear nights, when the ground cools rapidly and the air near the surface reaches its dew point.
How does advection fog form?
When warm, moist air moves over a cooler surface, typically requiring light winds.
What is steam fog?
Fog that forms when cold air moves over warm water, resembling smoke.
What are the main types of clouds based on height?
Low, middle, high, and clouds with vertical development.
What type of clouds typically pose a significant hazard to flight due to turbulence?
Cumulonimbus clouds.
What cloud type is associated with thunderstorms?
Cumulonimbus clouds.
What is virga?
Rain that evaporates before reaching the ground.
What is an air mass?
A large body of air with uniform temperature and moisture characteristics.
What type of air mass brings cool, dry air from polar regions?
Continental polar air mass.
What type of air mass brings warm, moist air from tropical waters?
Maritime tropical air mass.
What is the boundary between two different air masses called?
A front.
What are the four main types of fronts?
Warm, cold, stationary, and occluded.
What type of air mass replaces colder air in a warm front?
A warm air mass.
What types of clouds are typically seen before a warm front passes?
Cirriform or stratiform clouds, sometimes cumulonimbus in summer.
What happens to visibility after a warm front passes?
Visibility improves, but hazy conditions may persist for a short period.
What weather is associated with the passage of a warm front?
Light to moderate precipitation, fog, poor visibility, and stratiform clouds.
What type of air mass replaces warmer air in a cold front?
A cold, dense air mass.
What clouds are commonly associated with a passing cold front?
Towering cumulus or cumulonimbus clouds.
What type of severe weather can be produced by strong cold fronts?
Thunderstorms, lightning, hail, and possibly tornadoes.
What happens to temperature and dew point during a cold front passage?
Both drop rapidly.
How does the weather change after a cold front passes?
Skies clear, visibility improves, and temperatures remain cooler.
What is a squall line, and why is it hazardous to pilots?
A continuous line of thunderstorms that forms ahead of a cold front, causing intense, fast-moving storms.
How do cold fronts differ from warm fronts in terms of speed and weather?
Cold fronts move faster (20-35 mph) and bring sudden storms, while warm fronts move slower (10-25 mph) with steady precipitation.
What is a stationary front?
A boundary between two air masses that are relatively equal, causing weather to linger for days.
What is an occluded front?
A front formed when a fast-moving cold front overtakes a slow-moving warm front.
What weather is typical before the passage of an occluded front?
Cirriform and stratiform clouds with light to heavy precipitation.
How does a cold front occlusion differ from a warm front occlusion?
In a cold front occlusion, the cold air behind the cold front is colder than the air ahead of the warm front; in a warm front occlusion, the cold air ahead of the warm front is colder.
What types of clouds and weather can occur during the passage of an occluded front?
Nimbostratus, cumulonimbus, and towering cumulus clouds with light to heavy precipitation and poor visibility.
How do winds change with the passage of a front?
Winds shift direction, especially with cold fronts.
What are the three stages of a thunderstorm’s life cycle?
Cumulus, mature, and dissipating stages.
What characterizes the mature stage of a thunderstorm?
Strong updrafts and downdrafts, rain or hail, and violent turbulence.
What happens during the dissipating stage of a thunderstorm?
Downdrafts replace updrafts, and the storm loses its energy.
How high can severe thunderstorms reach in altitude?
50,000 to 60,000 feet, depending on latitude.
What are the three main conditions needed for a thunderstorm to form?
Sufficient water vapor, unstable lapse rate, and an initial lifting action.
What is an air mass thunderstorm?
A thunderstorm caused by surface heating, usually lasting an hour or two.
What is a squall line, and why is it dangerous?
A narrow band of intense thunderstorms, often associated with cold fronts, presenting significant hazards to aircraft.
What is a tornado, and how does it form in a thunderstorm?
A concentrated vortex of rapidly rotating air extending from the cloud base, formed when strong updrafts create intense rotation.
What type of turbulence is most dangerous in thunderstorms?
Shear turbulence between updrafts and downdrafts, both inside and outside the storm.
Why is hail dangerous to aircraft in thunderstorms?
Hailstones can grow large and damage aircraft, even several miles from the storm.
How can thunderstorms affect altimeters?
Rapid pressure changes can lead to altimeter errors of more than 100 feet.
How can lightning affect an aircraft?
It can puncture the skin, damage electronics, and cause temporary blindness.
What can happen to turbine engines in thunderstorms with high water concentration?
Engine flameout or structural failure due to excessive water ingestion.
How far should you circumnavigate a thunderstorm to avoid hail and turbulence?
At least 20 nautical miles.
What type of icing occurs in thunderstorms, and where is it most hazardous?
Clear icing from supercooled water droplets, especially between 0°C and -15°C.
