Weather is the biggest factor in making a flight probable, difficult, or impossible to execute. Understanding weather can make flight planning more safe and efficient. 

Follow along in Chapter 12 of the Pilot Handbook of Aeronautical Knowledge: highlight key points and tab out the book. (if using the paperback book)

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Tools for weather activity on Aviation Weather

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Major weather sources for seeing trends, current activity, and forecasts include:

• AviationWeather.gov

• Weather radar & charts on the application ForeFlight

• 1800WXBRIEF website or call for a briefing (1800-992-7433)

• TAF & METAR 

• NOAA general overview or forecast

• FSS or HIWAS for in-air weather advisories

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Aviation Weather is a highly recommended source that provides frontal activity, pressure systems, turbulence, radar, and pressure differentials on the prognostic chart. It also provides wind vectors at altitude, and pilot reports of specific weather activity at specific areas along with aviation specific SIGMETS and AIRMETS. (more on SIGMETS & Airmets) 

 

Charts for current SGMETS and AIRMETS
 

Significant Weather Charts ​

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Surface Level Significant Weather Charts show forecasts of frontal systems, pressure systems/differentials, and precipitation activity.

Fronts

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• Cold Front: When a mass of cold, dense, and stable air advances and replaces a body of warm air. They move more rapidly at a rate of 25-30mph and have reached speeds of 60mph. The air is dense and close to the ground. It operates like a snow plow, sliding under the warm air, lifting the warmer air aloft. This can produce storms. 

• Produces:

  • Violent weather activity

  • Storms, gusty winds, turbulence and hail or possibly tornadoes

 

• Warm Front: When a warm mass of air advantages and replaces a body of cooler air. They move slowly at 10-25mph. The slope of the warm air mass pushes the cold air out of the area as it passes through. The front contains warm humid air. Prior to passage, cirriform or stratiform clouds form. 

• Produces:

  • Low ceilings

  • Poor visibility

  • Rain

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• Stationary Front: When the forces of two air masses are relatively equal. The boundary remains stationary and will influence the local weather for days. 

• Produces:

  • Mixture of both warm and cold front weather

 

• Occluded Front: When a fast moving cold front overtakes a slow-moving warm front. 

• Produces:

  • Warm front weather prevails but is immediately followed by cold front weather

 

Pressure systems

Pressure defines circulation of wind. The solid black line is known as isobars. These are lines that represent constant pressure and show the pressure in millibars along the line. Areas where the pressure is significantly high or low are represented with an “H” or ”L”. 

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• High Pressure System:

  • Circulates air in a clockwise rotation

  • Air flows downward in the center of the system and outward at the surface

  • Associated with good weather 

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• Low Pressure System:

  • Circulates air in a counterclockwise rotation

  • Air flows upward and inward 

  • Associated with poor weather

 

Low Level Significant Weather Charts show forecasts of significant cloud coverage in blue & red, other hazards significant to flight such as freezing in light blue and turbulence in orange. 

• Blue representing ceilings 3,000’ - 1,000’ and/or visibility of 3-5 miles. If the blue “puffy-like” line surrounds the area of departure/arrival when the flight will be conducted, then exercise extreme caution due to these ceilings or visibility being potentially hazardous without proper experience. Visibility or ceiling can potentially degrade to worse flight conditions.

• Red representing ceilings less than 1,000’ and/or visibility of less than 3 miles. If the red line surrounds the area of departure/arrival when the flight will be conducted, then the flight is highly discouraged. 

 

Taking the information from the prognostic charts can help a pilot conclude what type of weather is likely for a prolonged period of time, along with what hazards from pressure or frontal activity may come in the future with seeing the foretasted predictions with 12 hour windows of predictions.

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These tools help pilots take advantage of wind directions produced by pressure systems with beneficial tailwinds. Additionally, forecasts of frontal activity along with the characteristics associated with each is helpful to flight planning in order to make a good go, no-go decision for safety or legality to fly in Visual Meteorological Conditions (VMC) However, during flight, take advantage of HIWAS. See WX Chcklist for more details

 

 

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Atmosphere

 

The atmosphere consists of 78% nitrogen, 21% oxygen and 1% other gasses. In the first 48,000’, the troposphere, where the majority of weather occurs. 

In the troposphere, the rate of temperature decrease with altitude, is -2°C per 1,000’ and is known as the lapse rate, or adiabatic cooling.

 

Pressure is a large role in weather

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Pressure differential is the causing force to winds. 

• Heat lowers pressure and density, which also rises the air.

• Cooler air increases pressure and density, which sinks the air.

• An increase in altitude will decrease pressure at a rate of -1 inch of mercury (measurement for pressure) per 1,000’

 

Air flows from an area of high pressure to an area of low pressure. High seeks low. Pressure differential can be seen on a prognostic surface analysis chart with gray lines representing areas of constant pressure called isobars. The closer he isobars the faster the wind. 

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Uneven heating and cooling causes pressure to fluctuate which causes convection from the warm rising air and wind from pressure differential which in turn, is the root cause of all weather.  

 

Plowed ground, rocks, sand absorb solar energy quickly and give off heat. 

Water, trees, and areas of vegetation slowly absorb heat and give off less heat.

The uneven heating give off convective pockets which are responsible for low level turbulence.

 

 

Water Cycle:

 

Evaporation - As the Earth’s surface is heated up by the sun, liquid on the surface evaporates and is lifted since warm air rises. 

 

Condensation - Temperature decreases as altitude increases. The evaporated moisture in the air being lifted reaches an altitude where the temperature is cool enough to condense the moisture back into a liquid state. Small foreign particles in the air, like dust or smoke, attract the water particles together, forming a cloud. 

 

Precipitation - Warm air rising from Earth’s surface faster than the cloud descending keeps the cloud in the air. As the cloud stays at a stationary altitude, the evaporation process continues to fuel the cloud of moisture, building it until the cloud cannot carry any more water droplets. 

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Clouds:

 

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3 levels of cloud height:

 

High: Cirro- above 20,000’

• Cirrus: wispy appearance composed entirely of ice crystals. Often the first sign of an approaching warm front

• Cirrostratus: form more of a widespread, veil-like layer. As a warm front approaches, cirrus clouds tend to thicken into cirrostratus.

• Cirrocumulus: layered clouds permeated with small cumuliform lumpiness. They also may line up in rows of clouds across the sky 

 

Middle: Alto- between 6,500’ and 20,000’

• Altostratus: possess a flat and uniform type texture in the mid levels. They frequently indicate the approach of a warm front and may thicken and lower into stratus, then nimbostratus resulting in rain or snow. Occasionally light showers may occur from a thick altostratus deck.

• Altocumulous: heap-like clouds with convective elements. Like cirrocumulus, altocumulus may align in rows. May denote the presence of elevated instability, especially in the morning, which could become boundary-layer based and be released into deep convection during the afternoon or evening.

 

Low: below 6,500’

• Stratus:  uniform and flat, producing a gray layer of cloud cover which may be precipitation-free or may cause periods of light precipitation or drizzle. 

• Cumulous: develop vertically as heap-like clouds with convective elements.

 

Precipitation Clouds:

• Nimbostratus: dense stratus or stratocumulus clouds producing steady rain or snow often

• Cumulonimbus: Most dangerous and relevant cloud type that affects aviation. Produces thunderstorms, severe turbulence, high winds, and heavy precipitation. Described below is the cumulonimbus. 

Three stages of a thunderstorm:

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Cumulus stage: With a lifting action, moisture, and an unstable lapse rate, the cloud will continue to build in height rapidly. The strong updrafts prevent moisture from falling. 

 

Mature stage: Most violent period of time with continuous lifting action within the cloud combined with precipitation-induced downdrafts that causes circulation. The lifting action near the top of the cloud loses energy and bows outward giving the cloud an anvil-like shape. Heaviest precipitation and lightning occurs during this stage. 

 

Dissipating stage: Downdrafts spread out and begin to replace all updrafts needed to sustain the storm.

 

Storms should be avoided by all costs, navigating when a cell is present

• It is impossible to fly over a storm cell since it can reach heights of 50,000-60,000’ 

• Flying under or through a thunderstorm is dangerous with damaging lightning, hail, and extreme turbulence

• Circumnavigation should be executed no less than 20nm (nautical miles) away from a thunderstorm that is giving an extreme radar echo since hail may fall miles outside of the clouds.

• Take note of where the anvil top is sheared off, one side will be more pronounced; the longer end indicates the direction the storm is traveling. 

 

Hazards associated with thunderstorms: 

 

- Heavy Precipitation
- Severe Turbulence
- Heavy convective action
- Excessive updrafts and downdrafts
- Temperature inversion
- Vortices
- Lightning

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With more active thunderstorms:

• Wind Shear - Drastic change in wind velocity and direction. This is especially hazardous for aircraft flying low into a headwind. A windshear can turn into a tailwind resulting in an extreme loss in performance and altitude. Associated with fronts passing, thunderstorms, and temperature inversions. 

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• Microburst - Strong downdrafts and outflow. Downdrafts up to 6,000 feet per minute (fpm) The outflow can be increasingly problematic at low altitudes on takeoff or departure. The outflow can simulate a strong headwind followed by a strong downdraft, then a downward tailwind 

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• Tornado - Violent tornadoes draw air into the base of the cloud. The air may have a rotating motion to it, or a vortex. If a vortex touches the ground, it becomes a tornado and can reach velocities of over 200 kts. 

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• Icing - Updrafts in a thunderstorm support water, that, at high altitudes and/or an extremely cold relative temperature, will bring the moisture to a super-cooled state. In this state the water will freeze instantly once it comes in contact with a surface that is below freezing. The ice is considered rime or clear ice depending on the droplet size. This is particularly hazardous due to the disruption of airflow over the airfoil and instruments. Additionally, the added weight of the ice can negatively affect the weight and balance on performance drastically. 

 

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Visibility

 

Moisture in the air is the most common reason for a reduced visibility. This is due to visible moisture becoming present when the air can no longer hold the water. 

Relative humidity refers to the amount of water present in the atmosphere at a given time. For example, if the current relative humidity is 50%, then the air is holding 50% of the total moisture it can hold. The dew point, given in degrees, is the temperature at which the air will be at 100% moisture content. When the temperature of the air is reduced to the dew point, the air is completely saturated and visible moisture is present in the following - there are 6 types of fog:

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• Radiation fog: During the day, heat is radiated from the ground from the heating from the sun, but at night, the loss of energy decreases the temperature of the surface and as the ground cools, the surrounding air is also cooled which lowers the air temperature to the dewpoint and condenses into visible moisture in the air. Usually occurs in the early morning or late evening and is generally localized. 

                     - No wind: dew or frost will layer over the ground. (Frost can be hazardous due to the disruption of a smooth surface on the wing which 

                       dissolves lift.)

                     - Light wind 5-15kts: fog

                     - Higher winds: low stratus cloud formation

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• Up-slope fog: Air flowing upward over rising terrain is cooled since the temperature lapse rate decreases as altitude increases. This adiabatic cooling results in the air cooling, condensing, and reaching saturation point. This fog can be dense and can remain stationary for as long as the air is being blown up-slope.

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• Advection fog: From a warm surface, air is lifted, then blown over to an area where the surface temperature is lower than the dew point. As the air moves over a colder surface, it is cooled and reaches the saturation point. This fog is dense and covers a large area. It can form at any time as long as the circumstances exist.  

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• Steam fog: When water moisture is added to the air over a surface, usually over water, the air is instantly condensed then visible moisture becomes present. 

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• Precipitation-induced fog: Precipitation falls into cooler air. Evaporation of the liquid increases the dew point, and the temperature meets this point which causes visible moisture. This fog can be sporadic depending on the conditions that exist. Frontal activity can make it thick, widespread, and persistent. Whereas, thunderstorms produce patchy, thin, and temporary fog.   

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• Ice fog: Cold temperatures less than -30°F. As long as moisture is added to the air, then ice fog can exist as long as the dew point and temperature reach each other. 

 

If haze exists: 

• Objects in the distance appear further than they actually are

• Visibility is degraded

• The sun can cause glare through the moisture making it very difficult to see through.

 

Smoke, volcanic ash, smog, and other atmospheric obscurations are a hazard to flying visual. Smoke can be unforecasted and pop up anywhere, giving the pilot great difficulty to make visual contact with the surface. Spotting the source out in the distance can help avoid flying into it.