What are the primary forces that affect wind?

The three primary forces that affect wind are pressure gradient force, Coriolis force, and friction.

The atmosphere has different areas of pressure.  Wind is created as the pressure systems try to equalize.  The airflow moves from high pressure to low pressure.

The Coriolis force causes the wind to deflect to the right in the Northern hemisphere.  he Coriolis force is directly proportional to the speed of the wind.  The greater the wind speed, the greater the Coriolis force at a given latitude.  The Coriolis force is zero at the equator, and more pronounced at middle and higher latitudes.  The Coriolis force affects all moving objects.  This is the same Coriolis force that affects the rotor system, such as when coning.

Wind is slowed down near the surface due to friction with the earth.  The rougher the terrain, the greater the frictional effect.  Also, the stronger the wind speed, the greater the friction.  The frictional drag of the ground normally decreases with height and becomes insignificant above a few thousand feet.

Local winds are small-scale wind systems driven by heating or cooling of the ground.  Air temperature differences develop over adjacent surfaces.  Air in contact with the ground heats during the day and cools at night.  Local winds are a significant factor for helicopters because their flight is generally local and often close to the ground.

Reference(s):

FAA AC 00-6B Aviation Weather pg. 7-1, 9-1

Other Weather and Atmosphere Topics

What is wind shear?

Wind shear is when there is a sudden change in wind direction and/or speed.

Wind shear can be dangerous as the flight environment changes significantly.  The pilot must correct for these changes. This is most important during low-altitude phases of flight as there is less terrain clearance.  For example, there may be a headwind that abruptly changes to a tailwind.

Reference(s)

AC 00-54 Pilot Windshear Guide
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge 12-11

Other Weather and Atmosphere Topics

What are types of fog?

The primary types of fog are radiation fog, advection fog, upslope fog, frontal fog, and steam fog.

Fog is visible water droplets in the air near the ground.  Fog forms when the air temperature and dew point approach the same value.  Fog is basically a cloud with the base at the surface.  From down in a valley, a pilot may see a cloud near a mountain top.  If the pilot was at the top of the mountain, it would be considered fog.

Radiation fog is formed over land by the cooling of the surface.  Radiation fog is generally shallow and burns off rapidly after sunrise.  A strong wind also displaces the fog. Factors favoring the formation of radiation fog are: 1) a shallow surface layer of relatively moist air beneath a dry layer, 2) clear skies, and 3) light surface winds.  Radiation fog is often called ground fog.

Advection fog forms when moist air moves over a cooler surface.  Advection fog is common in coastal areas, but often moves inland.  When at sea, advection fog is referred to as sea fog.  Wind greater than 15 knots lifts the fog into a layer of low stratus or stratocumulus clouds.

Upslope fog forms as moist, stable air is adiabatically cooled as it moves up sloping terrain.  Winds speeds of 5 to 15 knots are most favorable since stronger winds tend to lift the fog into a layer of low stratus clouds.

Frontal fog is associated with frontal zones and frontal passages.  When warm, moist air, is lifted over a front, clouds and precipitation may form.  Frontal fog can become quite dense and continue for an extended period of time.  Frontal fog may extend over large areas, completely suspending air operations.  Frontal fog is most commonly associated with warm fronts but can occur with other fronts as well.

Steam fog forms when cold, dry air moves over warm water.  As the water evaporates, it rises and resembles smoke.  This type of fog is common over bodies of water during the coldest times of the year.  Low-level turbulence and icing are commonly associated with steam fog.

Reference(s):

FAA AC 00-6B Aviation Weather pg. 16-1

Other Weather and Atmosphere Topics

What are the causes of turbulence?

The primary causes of turbulence are convective currents, wind flow obstructions, and wind shear.

Convective currents are strongest on warm summer days, as warm air raises.  The area right below cumulus clouds will often be very turbulent.

Obstructions to wind can cause mechanical turbulence, such as buildings, mountains, etc.  When flying near such obstructions, a helicopter pilot must be very vigilant.

As there is a significant change in winds speed and/or direction, there is often turbulence at the boundary of wind shear.

Reference(s):

FAA AC 00-6B Aviation Weather pg. 17-1

Other Weather and Atmosphere Topics

What are types of ice that form on aircraft structure?

The types of ice that form on aircraft structure are rime ice, clear ice, and mixed ice.

Rime ice is rough, milky, opaque ice formed by the instantaneous freezing of small super-cooled water droplets.
Clear ice is glossy, clear, or translucent ice formed by the relatively slow freezing of large, super-cooled water droplets.

Mixed ice is a combination of rime and clear ice.

Reference(s):

FAA AC 00-6B Aviation Weather pg. 18-1

Other Weather and Atmosphere Topics

What are the primary hazards with ice?

The primary hazard with ice is degraded aircraft performance.

As ice accumulates on the aircraft, the aircraft’s weight increases.  As ice accumulates on rotor systems, the shape of the airfoil changes, reducing its effectiveness.  Due to centrifugal force, it is likely that some of the ice will shed from the rotor system.  When this happens, it will likely shed unevenly.  This uneven sheading of ice can cause significant imbalances in the rotor system, resulting in severe vibrations that may damage the helicopter.

In addition to structural icing, there is potential for carburetor icing for normally aspirated piston engines and visibility could be significantly reduced.

Related to icing conditions, any frost on airfoil reduces an aircraft’s lift capacity as the frost causes the airflow to separate from the airfoil.  As such, a pilot should ensure there is no frost on any airfoil prior to flight.

Reference(s):

FAA AC 00-6B Aviation Weather pg. 18-1

Other Weather and Atmosphere Topics

What is a SIGMET?

SIGMETs are inflight advisories concerning non­convective weather that is potentially hazardous to all aircraft.

SIGMETs report weather forecasts that include severe icing not associated with thunderstorms, severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms, dust storms or sandstorms that lower surface or inflight visibilities to below three miles, and volcanic ash.  SIGMETs are unscheduled forecasts that are valid for 4 hours unless the SIGMET relates to a hurricane, in which case it is valid for 6 hours.

Reference(s):

FAA AC 00-45H Aviation Weather Services pg. 5-16
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 13-12

Other Aviation Weather Services

What is a Convective SIGMET?

A Convective SIGMET is issued for thunderstorms.

Instead of SIGMETs, Convective SIGMETs are issued for thunderstorms in the conterminous U.S. (CONUS).  Any Convective SIGMET implies severe or greater turbulence, severe icing, and low-level wind shear.  A Convective SIGMET will be issued when any of the following conditions are occurring or, in the judgment of the forecaster, are expected to occur:

• A line of thunderstorms at least 60 miles long with thunderstorms affecting at least 40 percent of its length
• An area of active thunderstorms affecting at least 3,000 square miles covering at least 40 percent of the area concerned and exhibiting a very strong radar reflectivity intensity or a significant satellite or lightning signature
• Embedded or severe thunderstorm(s) expected to occur for more than 30 minutes during the valid period regardless of the size of the area

A special Convective SIGMET may be issued when any of the following criteria are occurring or, in the judgment of the forecaster, are expected to occur for more than 30 minutes of the valid period.

• Tornado, hail greater than or equal to 3⁄4 inch (at the surface), or wind gusts greater than or equal to 50 knots (at the surface) are reported
• Indications of rapidly changing conditions, if in the forecaster’s judgment, they are not sufficiently described in existing Convective SIGMETs
• Special issuance is not required for a valid Convective SIGMET

Reference(s):

FAA AC 00-45H Aviation Weather Services pg. 5-7
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 13-12

Other Aviation Weather Services

What is a convective outlook?

A convective outlook primarily indicates the forecast for severe storms or convective activity.

The NWS Storm Prediction Center (SPC) issues narrative and graphical convective outlooks to provide the contiguous U.S. NWS Weather Forecast Offices (WFOs), the public, media and emergency managers with the potential for severe (tornado, wind gusts 50 knots or greater, or hail 1-inch diameter size or greater) and non-severe (general) convection and specific severe weather threats during the following three days.  The Convective Outlook defines areas of slight risk (SLGT), moderate risk (MDT) or high risk (HIGH) of severe thunderstorms for a 24-hour period beginning at 1200 UTC.  The Day 1 and Day 2 Convective Outlooks also depict areas of general thunderstorms (GEN TSTMS), while the Day 1, Day 2, and Day 3 Convective Outlooks may use SEE TEXT for areas where convection may approach or slightly exceed severe criteria.  The outlooks are available on the Storm Prediction Center web site.

Reference(s):

https://www.spc.noaa.gov/products/outlook/
FAA AC 00-45H Aviation Weather Services pg. 5-35

Other Aviation Weather Services