Weather - In-Flight Weather Advisories, Thunderstorms, & Turbulence

AIRMET (WA)

• Advise of significant weather phenomena specifically dangerous to lighter aircraft

• Valid for 6 hours

AIRMET Zulu

• Moderate Icing

• Provides freezing level heights

AIRMET Tango

• Moderate turbulence

• Surface winds of 30+ KIAS

• Non-conductive low-level wind shear

AIRMET Sierra

• IFR conditions

• Extensive mountain obscurement

SIGMET (WS)

In flight advisory concerning non-convective weather that is potentially hazardous to all aircraft

Unscheduled & Valid for 4 hours.

• Severe icing not associated with a thunderstorm.

• Severe turbulence/clear air turbulence (CAT) not associated with thunderstorms.

• Dust or sandstorms lowering the surface or in-flight visibility to below 3 miles.

• Volcanic ash

Convective SIGMET (WST)

Implies severe weather & hazardous convective activity

• Valid for up to 2 hours

Issued for: Embedded thunderstorms, line of thunderstorms, as well as:

• Severe Thunderstorms with

  • Surface winds greater than 50 knots

  • Hail at the surface ≥ ¾ inch in diameter

  • Tornadoes

Thunderstorms with heavy or greater precip affecting 40% or more of at least 3,000 square miles.

PIREPS

Pilot generated report concerning weather conditions encountered in flight

• Required Elements:

  • Type

  • Location

  • Time

  • Altitude

  • Aircraft type

  • at least 1 describing element

Urgent PIREP (UUA)

• Issued for

• tornadoes

• severe/extreme turbulence

• severe icing

• hail

• LLWS within 2,000’ of the surface

• volcanic ash

• or any other phenomena considered to be hazardous

Routine PIREP (UA)

• Issued based on pilot reports that do not contain urgent information

Convective Turbulence

Strong updrafts and downdrafts of air due to uneven heating of the earths surface

Mechanical Turbulence

Caused by obstructions to the wind flow, such as trees, buildings, and mountains

Mountain Wave Turbulence

Stable air passes over a mountain, developing waves above/downwind of mountains

• Often produce violent down drafts on the immediate leeward side of the mountain barrier

Clear Air Turbulence (CAT)

It is sudden & severe turbulence occurring in cloudless regions

• Not associated with thunderstorms

• It usually occurs along an upper level temperature inversion

Wind Shear

The rate of change in wind direction and/or speed per unit distance

• Often at any level of the atmosphere but mainly occur across

  • A low-level temperature inversion

  • In a frontal zone or thunderstorm

  • Clear air turbulence (CAT) at high levels associated with a jet stream or strong circulation

3 stages of a thunderstorm

1. Cumulus

   • Continuous updrafts

2. Mature

   • @ greatest intensity heavy rain w/thunder and lighting

3. Dissipating

   • Predominantly downdrafts

3 Conditions needed for a thunder storm to develop

1. Water Vapor

2. Unstable temperature lapse rate

3. lifting force