Understanding Aviation Weather Observations and Turbulence

Automated Systems

Most airports utilize automated systems to provide continuous observations:

• ASOS (Automated Surface Observing System): Provides reports up to 25 nm and 10,000 ft.
• AWOS (Automated Weather Observing System): Provides observations every 20 minutes, but notably does not issue "special" reports (SPECI) or report thunderstorms.

These systems use various technologies to measure elements. Wind is measured via sonic anemometers, and precipitation is often measured by weighing a bucket that fills with rain or snow (AWPAG). Sky conditions are determined using LIDAR (a vertically pointing laser), which provides a 30-minute average for sky condition and a 10-minute average for surface visibility.

Special Observations (SPECI)

A "SPECI" is a special observation issued when there is a significant change in weather. This includes changes in ceiling, visibility, wind shifts, or the onset of phenomena like tornadoes or thunderstorms.

Exam Tip: You can distinguish between an ASOS and an AWOS report by looking for the "AO1" or "AO2" identifiers in the text. Remember that AWOS does not provide special reports.

Precipitation and Obscurations

Automated systems use LEDWI (Light Emitting Diode Water Indicator) to differentiate between rain and snow and to determine their intensity.

Obscurations (such as fog, mist, or dust) are often inferred from a combination of visibility, temperature, dew point, and present weather. For example, the distinction between fog and mist is often based on visibility, with mist typically having a visibility of 5/8.

Pilot Reports (PIREPs)

While automated systems are consistent, they are "point sampling" and cannot capture the experience of an aircraft moving through a volume of air. This is where Pilot Reports (PIREPs) become invaluable.

PIREPs provide real-time data on:

• Turbulence Intensities: How the air is actually being felt inside the cockpit.
• Icing Types: The type and severity of ice accumulation.
• Wind Shear: Including low-level wind shear (LLWS), defined as airspeed fluctuations of 10 knots within 2,000 ft of the surface.

ATC will actively solicit PIREPs when certain thresholds are met, such as ceilings $\leq 5,000$ ft, visibility $\leq 5$ sm, or when pilots encounter moderate or greater turbulence or light icing or greater.

Urgent Pilot Reports (UUA)

When a pilot encounters life-threatening conditions, they issue an Urgent Pilot Report (UUA). Criteria for a UUA include:

• Hail or tornadoes/funnel clouds.
• Severe icing or severe/extreme turbulence.
• Volcanic ash.

Understanding Turbulence and Shear

Turbulence is a major factor in aviation safety, categorized by its source.

Convective Turbulence

Caused by solar heating (common in summer afternoons) or cold air advection (common in spring/fall near cold fronts). Thermals rise at rates of 200–2,000 fpm and can be identified by cloud tops in PIREPs or GFA.

Shear-Induced Turbulence

Wind shear can occur due to several factors:

1. Surface Friction: Occurs in the boundary layer (500–5,000 ft AGL). It is most severe in summer afternoons over rough terrain.
2. Temperature Inversions: Nocturnal radiation inversions can cause light to moderate turbulence from the surface to 2,000 ft AGL.
3. Frontal Boundaries: Moving fronts (especially those $>30$ kts) can create moderate to severe turbulence.
4. Jet Streams (CAT): Clear Air Turbulence (CAT) is common when the jet core exceeds 110 kts.

Safety Note: When encountering turbulence during approach or landing, pilots should use more power, less flaps, and add $1/2$ of the gust factor to their approach speed.

Mechanical Turbulence

This is caused by wind interacting with physical obstacles. Small obstacles disturb airflow above $2 \times$ their height and up to $10 \times$ their height downwind. Rough terrain (features $>1,000$ ft) can produce moderate turbulence with winds $>25$ kts and severe turbulence with winds $>40$ kts.

Summary Checklist for Turbulence Scenarios

When analyzing a weather scenario to predict turbulence, use the following correlation method:

1. Large Scale: Check progression charts and satellite imagery for pressure gradients and cloud types.
2. Advisories: Look for existing SIGMETs or PIREPs in the area.
3. Vertical Profile: Examine the temperature and wind profiles. Is there a strong temperature inversion? Is there a significant wind shift with altitude?.
4. Surface/Terrain: Is the terrain rough or flat? Are there significant obstacles near the departure or arrival paths?.

Remember: The absence of an advisory or a PIREP does not guarantee the absence of turbulence.