Aviation Weather Hazards and Atmospheric Stability for UAS Operations

Introduction to Aviation Weather for UAS

• The course is transitioning from aerospace topics to weather, which will be the focus for the next seven or eight classes.

• The curriculum will cover weather conditions, weather formation processes, environmental factors, the effects of weather on aircraft performance, and specific weather reports used by aviators and UAS (Unmanned Aircraft Systems) operators.

• Understanding weather is a legal requirement under 14 CFR Part 107.49, which mandates that a remote pilot in command (RPIC) must check weather conditions before the first flight of the day as part of preflight items.

Atmospheric Environments: Stable vs. Unstable

• There are two primary types of atmospheric environments focused on in aviation: stable atmospheres and unstable atmospheres.

• Lapse Rate: Defined as the cooling rate of the atmosphere. It is the change in temperature as an aircraft climbs higher through the atmosphere.

• Standard Temperature: In aviation, temperatures are internationally standardized using the Celsius scale.

  • The standard temperature at sea level is defined as $15\,^{\circ}\text{C}$ (approximately $59\,^{\circ}\text{F}$ to $60\,^{\circ}\text{F}$).

• International Standards for Atmosphere/Pressure (ISA): The standard lapse rate is a decrease of $2\,^{\circ}\text{C}$ for every $1,000\,\text{ft}$ of altitude gained.

  • Example: If the temperature at sea level is $15\,^{\circ}\text{C}$, at $1,000\,\text{ft}$ in a stable atmosphere, the temperature should be $13\,^{\circ}\text{C}$.

• Stable Atmosphere: Characterized by a small, consistent cooling rate as altitude increases. It features small, predictable changes.

• Unstable Atmosphere: Characterized by random, sporadic, or unpredictable lapse rates. The temperature change may be inconsistent (e.g., cooling $2\,^{\circ}\text{C}$ one thousand feet, then $3\,^{\circ}\text{C}$ the next, then $4\,^{\circ}\text{C}$).

Cloud Formation and Air Masses

• Thermodynamics: Heat rises. This principle is visualized through the metaphor of a hot air balloon or a pocket/mass of air. If the air inside the mass is warmer than the surrounding environment, the mass will rise.

• Dew Point ($T_d$): The temperature at which the air becomes saturated and water vapor condenses into visible moisture (fog, dew, mist, or clouds).

  • As the temperature gets closer to the dew point, relative humidity increases. When they match, condensation occurs.

• Stratiform Clouds: These are thin, layered, wispy clouds.

  • They form in stable air that resists vertical development.

  • A stable environment may limit a cloud's thickness (e.g., a cloud only $1,000\,\text{ft}$ thick) because the rising air parcel eventually matches the environmental temperature and loses its lifting force.

• Cumuliform Clouds: These are clouds characterized by "lifting action" and significant vertical development.

  • They form in unstable atmospheres where the air parcel remains consistently warmer than the surrounding environment as it rises.

  • These clouds promote vertical development and can build into massive structures such as thunderstorms, which can be tens of thousands of feet thick.

Comparison of Atmospheric Characteristics

• Stable Air ("Stagnant Air"): Resists vertical development. Smoke from fires or pollutants (like clearing fields near Counts Prairie) will hit an "invisible ceiling" and spread out horizontally rather than dissipating upwards.

• Unstable Air: Promotes vertical development. While it features better horizontal visibility, it often includes high winds and turbulence.

Thunderstorms: Definitions and Ingredients

• Three Ingredients for a Thunderstorm:

  1. Lift (e.g., a cold front forcing air up).

  2. Unstable atmosphere.

  3. Moisture (carrying water vapor upwards to build the storm).

• Definition: By FAA definition, a thunderstorm always contains lightning. Thunder is the sound caused by lightning.

• Three Stages of a Thunderstorm:

  1. Cumulus Stage: The initial lifting and building stage where the storm gains energy.

  2. Mature Stage: The most dangerous stage. It features the full cycle of the storm with simultaneous updrafts and downdrafts. This is when rain begins and wind shear is most prevalent.

  3. Dissipation Stage: The final stage, predominantly characterized by downdrafts as the storm releases its energy and "fizzles out."

Severe Weather Hazards: Squall Lines and Microbursts

• Cold Fronts: As a cold front pushes into an area, it rapidly forces warm, moist air upward, creating thunderstorms.

• Squall Lines: Long lines of thunderstorms that form along or ahead of the leading edge of a cold front.

  • They can form at any altitude and are considered the most dangerous types of storms for pilots because they are difficult to fly around or over.

• Microbursts: A severe type of downdraft associated with a rapid release of energy.

  • In Salina, KS (2024), a microburst caused damage similar to a tornado with winds near $100\,\text{mph}$.

  • Duration: An individual microburst will seldom last longer than $15\,\text{minutes}$. This is a critical figure for FAA testing.

• Wind Shear: Occurs in downdrafts where a pilot may experience a sudden headwind (increasing lift) followed by a strong downdraft and then a sudden tailwind (decreasing lift). This is most dangerous during takeoff and landing when the aircraft is low to the ground and near its stall speed.

Aircraft Icing

• Icing is hazardous to UAS because it increases weight and disrupts the lift-producing shape of airfoils (wings and propellers).

• Requirements for Icing:

  1. Visible moisture (clouds, rain, mist).

  2. Freezing temperatures (at or below $0\,^{\circ}\text{C}$).

• Types of Icing:

  • Clear Ice: Forms when water strikes the airfoil and flows back before freezing. It builds "clear down" the airfoil from the point of contact.

  • Rime Ice: Forms when water droplets freeze immediately upon contact. It builds "right up" (rime up) on the leading edge of the airfoil.

  • Mixed Ice: A combination of the two; it is heavier on the leading edge but also extends down the wing.

Types of Fog

• Radiation Fog: Forms on clear nights with little to no wind and a small temperature/dew point spread. As the ground radiates away the day's heat, the air cools to the dew point. It is often thin and can be "burned off" by the sun as the temperature rises.

• Advection Fog: Requires the horizontal movement of air. It occurs when warm, moist air moves over a colder surface (e.g., air moving inland from an ocean). Movement/Wind is the keyword.

• Upslope Fog: Occurs when wind pushes warm, moist air up a slope (mountain or hill), cooling it as it rises. Like advection fog, it requires wind.

• Steam Fog: Occurs when cold air moves over warm water, causing evaporation. Often seen during cold snaps in the fall over lakes or pools.

• Valley Fog: Similar to radiation fog but settles in low-lying valleys (e.g., driving through the Flint Hills).

Temperature Inversions

• A temperature inversion occurs when the temperature actually increases with altitude instead of decreasing.

• This indicates a very stable environment.

• Hazards associated with temperature inversions include poor visibility, fog, and stagnant air, as there is no vertical movement to disperse pollutants or mist.

Questions & Discussion

• Q: What is the defining characteristic of a thunderstorm?

• A: Lightning.

• Q: Which stage of a thunderstorm is the most dangerous?

• A: The Mature Stage.

• Q: Which stage is predominantly characterized by downdrafts?

• A: The Dissipation Stage.

• Q: How long does a microburst typically last?

• A: 15 minutes or less.

• Q: Which type of fog requires horizontal movement or wind?

• A: Advection fog.