Aviation Weather Theory Flashcards

The Importance of Weather Theory in Aviation

• Understanding weather theory is a fundamental requirement for pilot certification (Private, Commercial, and Flight Instructor).

• Pilots must understand the weather to:     - Make better decisions to avoid dangerous situations.     - Comprehend the factors that directly affect aircraft performance and limitations.

• Key primary factors affecting weather and aircraft performance include pressure and temperature.

Atmospheric Structure and Temperatures

• The majority of weather phenomena occur within the first layer of the atmosphere.

• Layers of the Atmosphere:     - Troposphere: Extends from the surface to approximately $20,000\,\text{feet}$. This is where most weather occurs.     - Stratosphere: Extends up to approximately $160,000\,\text{feet}$. Commercial jets fly here to avoid weather and take advantage of faster winds for better performance.     - Mesosphere: The layer above the stratosphere.     - Thermosphere: The highest layer, extending toward space.

• Standard Temperature Lapse Rate: Within the troposphere, the average temperature decrease is $2\,^{\circ}\text{C}$ for every $1,000\,\text{feet}$ of climb.

• Temperature Measurement: Degrees Celsius ($^{\circ}\text{C}$) is the international standard for aviation reporting, though Fahrenheit ($^{\circ}\text{F}$) is occasionally used in the U.S., specifically for calculating cloud bases.

Large-Scale Atmospheric Circulation

• Cause of Air Movement: Circulation is primarily driven by the uneven heating of the Earth’s surface by the sun.

• Basic Principles of Air Density:     - Warm air rises because it is less dense.     - Cold air sinks because it is more dense.

• General Global Circulation: Hot air rises at the Equator and moves toward the colder Poles, while colder air from the Poles sinks and moves toward the Equator.

The Coriolis Force

• Definition: A force resulting from the Earth's rotation that deflects moving air masses.

• Deflection Direction: In the Northern Hemisphere, air is deflected to the right.

• Merry-Go-Round Analogy:     - Imagine two people on opposite sides of a spinning merry-go-round throwing a ball to each other.     - If the merry-go-round is stationary, the ball travels in a straight line to the recipient.     - If the merry-go-round is rotating (e.g., to the left), the ball still travels in a straight line through space, but relative to the rotating observers, it appears to curve to the right because the recipient has moved.

• Characteristics on Earth:     - Affects large air masses and bodies of water over long distances.     - For short distances (e.g., throwing a baseball), the deflection is negligible (roughly the thickness of a fingernail or a tenth of a millimeter).     - The force is greatest at the Poles and reaches zero at the Equator.

Atmospheric Pressure and Standards

• Weight of Air: Air has weight, which can be measured in pounds per square inch ($\text{psi}$).

• Standard Sea Level Pressure:     - $14.7\,\text{psi}$     - $1.0\,\text{Atmosphere}$     - $29.92\,\text{inches of mercury (inHg)}$ (Standard unit in the U.S. and Canada).     - $1013.2\,\text{hectopascals (hPa)}$ or millibars (Standard unit in Europe and internationally).     - $760\,\text{millimeters of mercury (mmHg)}$.

• Pressure Lapse Rate: Pressure decreases at a rate of approximately $1\,\text{inHg}$ per $1,000\,\text{feet}$ of altitude gain.

• The Standard Day Concept: A reference model for calculations ($\text{Sea Level}$, $15\,^{\circ}\text{C}$, and $29.92\,\text{inHg}$).

Pressure Altitude and Density Altitude

• Reported Pressure: Atmospheric pressure in weather reports (such as ATIS) is always corrected to sea level. Even at an airport like Flagstaff (elevation $5,000\,\text{feet}$), the reported pressure on a standard day will be $29.92\,\text{inHg}$, even though the actual local pressure would be approximately $24.92\,\text{inHg}$.

• Pressure Altitude: The altitude corrected for non-standard pressure. It is the height above the standard datum plane ($29.92\,\text{inHg}$).

• Density Altitude: The pressure altitude corrected for non-standard temperature. This is the "perceived" altitude at which the aircraft feels it is flying.

• Performance Impact: Higher density altitude (caused by low pressure or high temperature) results in decreased aircraft performance. Lower density altitude improves performance.

Wind and Wind Patterns

• Lateral vs. Vertical Movement:     - Wind: Lateral movement of air parallel to the Earth's surface.     - Convective Current: Vertical movement of air.

• Pressure Gradient: Air moves from areas of high pressure to low pressure.

• Isobars: Lines on a weather map connecting areas of equal pressure (typically reported in millibars).     - Steep Pressure Gradient: Closely spaced isobars indicate a rapid change in pressure and result in strong winds.     - Shallow Pressure Gradient: Widely spaced isobars indicate calm or light winds.

• High-Pressure Systems:     - Air moves outward, downward, and in a clockwise motion (Northern Hemisphere).     - Generally associated with fair weather.

• Low-Pressure Systems:     - Air moves inward, upward, and in a counter-clockwise motion (Northern Hemisphere).     - Sucks air toward the center and lifts it, often resulting in bad weather.

Convective Currents and Localized Winds

• Surface Heating Effects:     - Barren surfaces (rocks, sand, concrete, parking lots) reflect heat, creating updrafts.     - Vegetation and water absorb heat, creating descending currents (downdrafts).

• Real-World Example (Melbourne International Airport, FL): A pilot flying final to Runway 5 encounters updrafts over large parking lots (Harris Corporation) followed by downdrafts over a line of trees right before the runway threshold.

• Turbulence: Strongest close to the ground due to rising pockets of hot air; flying at higher altitudes can often provide smoother air.

• Coastal Wind Patterns:     - Sea Breeze (Day): Land heats faster than water; warm air over land rises, and cooler air from the sea moves inland.     - Land Breeze (Night): Land cools faster than water; the water is now warmer than the land, causing air to move from the land toward the sea.

• Topographical Obstructions: Mountains can cause dangerous downdrafts on the leeward side. Pilots should fly sufficiently high above mountain peaks to avoid these currents.

Air Mass Stability

• Definition: The ability of an air mass to resist vertical motion.

• Stable Air: Resists vertical movement; typically associated with cool, dry air.

• Unstable Air: Prone to vertical movement; typically associated with warm, humid air (e.g., Florida in the summer).

• Lapse Rate and Stability: Stability is determined by how the temperature changes with altitude. Heating from below decreases stability; cooling from below increases stability.

• Types of Stability:     - Absolute Stability: The ambient lapse rate is slower than the parcel lapse rate (e.g., $1\,^{\circ}\text{C}$ per $1,000\,\text{feet}$).     - Neutral Stability: The ambient lapse rate matches the parcel lapse rate.     - Absolute Instability: The ambient lapse rate is much faster than the parcel rate (Super-adiabatic), preventing the surrounding air from containing the rising air mass.     - Conditional Stability: Exists when the lapse rate is between the dry and moist adiabatic rates; stability depends on whether the air becomes saturated.

Thermodynamic Processes

• Adiabatic Lapse Rate: The rate at which an air mass cools as it is raised.

• Sublimation: Transition from solid directly to gas (no liquid phase).

• Evaporation: Transition from liquid to vapor (adds moisture to the air).

• Condensation: Transition from vapor to liquid (removes moisture from the air).

• Specific Lapse Rates (Fahrenheit approximations):     - Dry Adiabatic Lapse Rate: Temperature decreases at $3\,^{\circ}\text{F}$ per $1,000\,\text{feet}$; dew point decreases at $0.5\,^{\circ}\text{F}$ per $1,000\,\text{feet}$.     - Moist Adiabatic Lapse Rate: Ranges from $1.2\,^{\circ}\text{C}$ to $3\,^{\circ}\text{C}$ per $1,000\,\text{feet}$ (average $2\,^{\circ}\text{C}$). Once saturated, the dew point decreases at the same rate as the air temperature.

Low-Level Wind Shear (LLWS)

• Definition: A sudden, drastic change in wind speed and/or direction.

• Occurrence: Can happen at any altitude but is most dangerous near the ground during takeoff or landing.

• Causes: Passing fronts and thunderstorms.

• Microbursts: A severe type of LLWS.     - Duration: $5$ to $15\,\text{minutes}$.     - Intensity: Downdrafts up to $6,000\,\text{feet per minute (fpm)}$.     - Effect: Initial encounter with a headwind (increased performance), followed by a massive downdraft, and finally a tailwind (decreased performance).

• Survival Strategy: Use full power and a nose-high attitude, but be wary of the increased stall speed during banked turns.

Thunderstorms

• Formation Requirements:     1. Sufficient water vapor (moisture).     2. An unstable lapse rate.     3. An initial lifting action (e.g., surface heating or terrain).

• Three Phases of a Thunderstorm:     - Cumulus Stage: Characterized by continuous updrafts.     - Mature Stage: The most violent stage. Rain begins at the surface. Features both updrafts and downdrafts. An anvil top forms.     - Dissipating Stage: Characterized primarily by downdrafts as the storm dies out.

• Hazards: Lightning, severe turbulence, and hail (which can be ejected miles away from the main cloud through the anvil).

• Avoidance: Pilots should stay at least $20$ to $30\,\text{miles}$ away from thunderstorm clouds.

Fronts and Air Masses

• Front: The boundary layer between two air masses with different characteristics.

• Standard Frontal Characteristics: Every frontal passage involves a change in wind direction and a change in temperature.

• Types of Fronts:     - Warm Front: Warm air replaces cold air; often brings red code symbols, gentle slopes, and steady precipitation.     - Cold Front: Cold air replaces warm air; acts like a snowplow, lifting warm air rapidly; often associated with violent weather.     - Stationary Front: Two air masses are in conflict but neither is moving.     - Occluded Front: A fast-moving cold front overtakes a warm front.

• Squall Line: A narrow band of active thunderstorms often found ahead of a cold front. Produce the most intense thunderstorms

Temperature Inversions

• Normal Condition: Temperature decreases with altitude.

• Inversion Condition: Temperature increases with altitude for a specific layer.

• Characteristics: Usually occurs in stable air. Often creates a layer that traps smoke, dust, and moisture, leading to poor visibility.

• Ground-Based Inversion: Occurs at night due to terrestrial radiation cooling the air near the surface while the air above remains warmer. Can lead to radiation fog if humidity is high.

Relative Humidity and Dew Point

• Relative Humidity: The ratio of the amount of water in the air compared to the maximum amount the air can hold at that temperature.

• Capacity vs. Temperature: Warm air can hold much more water than cold air.     - $10\,^{\circ}\text{C}$ cubic meter: Can hold $9\,\text{grams}$ of water.     - $20\,^{\circ}\text{C}$ cubic meter: Can hold $17\,\text{grams}$ of water.     - $30\,^{\circ}\text{C}$ cubic meter: Can hold $30\,\text{grams}$ of water.

• Saturation: Reaching $100\%$ relative humidity. Cooling an air mass reduces its capacity; if cooled below its dew point, visible moisture forms (condensation).

• Dew Point: The temperature to which air must be cooled to become fully saturated.

• Cloud Base Formula:     - $\text{Cloud Base (ft AGL)} = \left( \frac{\text{Temperature (}^{\circ}\text{F}) - \text{Dew Point (}^{\circ}\text{F})}{4.4} \right) \times 1,000$

• Frost: Forms when the dew point is below freezing and the surface temperature is also below freezing.

Fog Types

• Radiation Fog: Forms in valleys on calm, clear nights when the ground cools rapidly. Burns off with the sun.

• Ice Fog: Forms in extremely cold temperatures (below $-25\,^{\circ}\text{F}$) when water vapor turns directly into ice crystals.

• Advection Fog: Forms when warm, moist air moves over a cold surface (common in coastal areas like San Francisco). Requires wind to move the air mass.

• Upslope Fog: Forms when moist air is forced up a mountain slope and cools adiabatically. May persist even under sunlight.

• Steam Fog: Forms when cold, dry air moves over warm water. Water evaporates into the cold air and immediately saturates it.

Clouds and Visibility

• Low Clouds:     - Stratus: Layer-like, associated with stable air.     - Cumulus: Puffy, associated with unstable air and vertical development.     - Nimbostratus: Dark, rain-bearing clouds.

• Middle/High Clouds:     - Altocumulus: Often indicates strong turbulence.     - Cirrus: High altitude, composed of ice crystals; indicates potential icing.

• Lenticular Clouds (Autocumulus Standing Lenticularis): Lens-shaped clouds found near mountain peaks indicating extremely high winds and heavy turbulence.

• Ceiling: Defined as the lowest layer of clouds reported as Broken (BKN) or Overcast (OVC).

• Cloud Coverage (Octas):     - Clear (CLR/SKC): $0/8$ coverage.     - Few (FEW): $1/8$ to $2/8$ coverage.     - Scattered (SCT): $3/8$ to $4/8$ coverage.     - Broken (BKN): $5/8$ to $7/8$ coverage (constitutes a ceiling).     - Overcast (OVC): $8/8$ coverage (constitutes a ceiling).

• Visibility: The greatest horizontal distance at which prominent objects can be seen. Reported in statute miles (SM). Maximum reported is $10\,\text{SM}$.

Aircraft Icing

• Requirements: Visible moisture (clouds) and temperature at or below $0\,^{\circ}\text{C}$.

• Hazards: Impacts lift, weight, and instrument accuracy.

• Mitigation: Use Pitot heat and Carburetor heat. Know the freezing level. Descending is not always the solution; sometimes climbing gets you out of moisture.

Precipitation and High-Altitude Phenomena

• Types of Precipitation: Rain, Snow, Drizzle, Ice Pellets (indicate temperature inversion/freezing rain aloft), Freezing Rain, Hail, and Virga (rain that evaporates before hitting the ground).

• Jet Stream: A high-altitude band of strong winds ($50$ to $240\,\text{knots}$). Moves North and weakens in summer; moves South and strengthens in winter.

• Clear Air Turbulence (CAT): Sudden, severe turbulence occurring above $15,000\,\text{feet AGL}$ in cloudless regions. Often associated with the jet stream or cirrus clouds.

Summary of Air Mass Characteristics

• Unstable Air:     - Cumuliform clouds.     - Showery precipitation.     - Rough/turbulent air.     - Good visibility (due to vertical mixing).

• Stable Air:     - Stratiform clouds or fog.     - Continuous precipitation.     - Smooth air.     - Poor visibility (haze/smoke trapped near surface).

Questions & Discussion

• Q: Why does the wind direction change between 5,000 feet and the surface?     - A: Surface friction slows the wind near the ground, which reduces the Coriolis effect, causing the wind to flow more directly toward lower pressure.

• Q: What indicates the presence of freezing rain at higher altitudes?     - A: The presence of ice pellets at the surface.

• Q: What is a squall line?     - A: A non-frontal narrow band of active thunderstorms that often develops ahead of a cold front.

• Q: How do you calculate the base of cumulus clouds given a surface temp of 70F and dewpoint of 48F at an airport of 1,000ft MSL?     - A: $\frac{70 - 48}{4.4} = 5$. $5 \times 1,000 = 5,000\,\text{feet}$ above the surface. Since the surface is at $1,000\,\text{feet MSL}$, the base is at $6,000\,\text{feet MSL}$.