Pilots must consider each of these factors, relative to both their capabilities as the pilot as well as the capabilities of the aircraft they’re flying:

• Visibility—VFR or IFR, both within the airport environment and at the altitude you’ll be flying en route.
• Ceiling—How high are the clouds; can you fly above or around them? If you’re IFR, can you fly through them without risk of icing, severe turbulence, or storm downdrafts?
• Wind—Is the direction and speed conducive to the runway alignment at both the departure and arrival airport? How will the tailwind or headwind impact your ground speed and therefore fuel planning?
• Turbulence and Wind Shear—Ironically, it’s often bumpiest when the skies are the clearest. Can you, your passengers and the aircraft handle the increased structural loads with the sky bumps?
• Thunderstorms—These can be pop-up events or contained with other weather and require a wide berth to fly around—no one should be flying through a thunderstorm.
• Temperatures—Most general aviation aircraft have limited heating and cooling capabilities while still on the ground and rely on airflow over the engine when in the air. This tends to result in extreme conditions.

While more than 80% of all aircraft accidents are put into the “human factors” category, this also includes decision-making, often related to weather and poor flight planning. With all the variables and uncertainty that comes with weather, the number of flights that go uninterrupted and as planned daily is remarkable.

Learn more about making the best decisions based on weather conditions in the Aviation Weather Handbook, available on the ASA website.

The post Aviation Decision-Making and Spring Weather first appeared on Learn To Fly.

First of all, METAR as an abbreviation is vague at best. Different sources will tell you this comes from METeorological Aerodrome Report, Meteorological Terminal Aviation Routine Weather Report, Meteorological Terminal Air Report, or Meteorological Airfield Report. Let’s stick with aviation routine weather report and get straight into the decrypting.

Here’s an example of a routine METAR report for a station location:

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

Each METAR contains specific information in sequential order. Let’s go through each bit of the standard formatted coding.

1. METARs begin with the type of report (shown in red).

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR]

You will see two types of METAR reports. The routine METAR report, transmitted at a regular time interval, or the aviation selected SPECI, a special report that can be given at any time to update the METAR for rapidly changing weather conditions, aircraft mishaps, or other critical information.

2. Next is the station identifier. A four-letter code as established by the International Civil Aviation Organization (ICAO).

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

In the 48 contiguous states, a unique three-letter identifier is preceded by the letter “K.” For example, Gregg County Airport in Longview, Texas, is identified by the letters “KGGG,” with K as the country designation and GGG as the airport identifier. In other regions of the world, including Alaska and Hawaii, the first two letters of the four-letter ICAO identifier indicate the region, country, or state. Alaska identifiers always begin with the letters “PA,” and Hawaii identifiers always begin with the letters “PH.” Station identifiers can be found on various websites, such as the Aviation Weather Center or  NOAA’s METAR Observation Station Identifiers.

3. The third grouping is the date and time of the report. Depicted in a six-digit group (161753Z). The first two digits are the date. The last four digits are the time of the METAR/SPECI, which is always given in coordinated universal time (UTC). A “Z” is appended to the end of the time to denote the time is given in Zulu time (UTC) as opposed to local time. This METAR was issued on the 16th at 1753 Zulu.

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

4. The modifier denotes whether the METAR/SPECI came from an automated source or if the report was corrected.

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

If the notation “AUTO” is listed in the METAR/SPECI, the report came from an automated source. It also lists “AO1” (for no precipitation discriminator) or “AO2” (with precipitation discriminator) in the “Remarks” section to indicate the type of precipitation sensors employed at the automated station. When the modifier “COR” is used, it identifies a corrected report sent out to replace an earlier report that contained an error. If this example had been corrected, the word AUTO would be replaced with COR.

5. The wind is reported with five digits (14021) unless the speed is greater than 99 knots, in which case the wind is reported with six digits. The first three digits indicate the direction the true wind is blowing from in tens of degrees. If the wind is variable, it is reported as “VRB.” The last two digits indicate the speed of the wind in knots (KT) unless the wind is greater than 99 knots, in which case it is indicated by three digits. If the winds are gusting, the letter “G” follows the wind speed. After the letter “G,” the peak gust recorded is provided (G26KT). If the wind direction varies more than 60° and the wind speed is greater than six knots, a separate group of numbers, separated by a “V,” will indicate the extremes of the wind directions.

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

Without gusts, this METAR would include only 14021KT.

6. The prevailing visibility is reported in statute miles as denoted by the letters “SM.” It is reported in both miles and fractions of miles (¾ SM).

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

The runway visual range (RVR) may be reported following the prevailing visibility. RVR is the distance a pilot can see down the runway in a moving aircraft. When RVR is reported, it is shown with an R, then the runway number followed by a slash (/), then the visual range in feet. For example, when the RVR is reported as R17L/1400FT, it translates to a visual range of 1,400 feet on runway 17 left.

7. Now we get to the weather. It can be broken down into two different categories: the qualifiers (+TSRA) and the weather phenomenon (BR).

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

First, the qualifiers of intensity, proximity, and the descriptor of the weather are given. The intensity may be light (–), moderate ( ), or heavy (+). Proximity only depicts weather phenomena that are in the airport vicinity. The notation “VC” indicates a specific weather phenomenon is in the vicinity of 5–10 miles from the airport. Descriptors are used to describe certain types of precipitation and obscurations. Weather phenomena may be reported as being precipitation, obscurations, or other phenomena, such as squalls or funnel clouds. Descriptions of weather phenomena, when they begin or end, and hailstone size are also listed in the “Remarks” sections of the report. The coding for qualifier and weather phenomena are shown in this chart. The weather groups are constructed by considering columns 1–5 in this table sequence: intensity, followed by descriptor, followed by weather phenomena.

The notation +TSRA BR is “heavy thunderstorms and rain with mist.” Another example, “heavy rain showers” is coded as +SHRA.

8. Next we have the sky condition. This is always reported in the sequence: amount, height, and type or indefinite ceiling/height (vertical visibility).

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

The heights of the cloud bases are reported with a three-digit number in hundreds of feet AGL. Clouds above 12,000 feet are not detected or reported by an automated station. The types of clouds, specifically towering cumulus (TCU) or cumulonimbus (CB) clouds, are reported with their height. The amount of cloud coverage and obscuring phenomena is described using fractions, then reported based on the amount of sky coverage in eighths of the sky from horizon to horizon.

Less than 1/8 is abbreviated as Sky Clear, Clear, or Few; 1/8–2/8 as Few; 3/8–4/8, Scattered; 5/8–7/8, Broken; and 8/8, Overcast. For aviation purposes, the ceiling is the lowest broken or overcast layer, or vertical visibility into an obscuration.

9. The air temperature and dew point are always given in degrees Celsius (C).

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

Temperatures below 0 °C are preceded by the letter “M” to indicate minus.

10. The altimeter setting is reported as inches of mercury (inHg) in a four-digit number group.

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

It is always preceded by the letter “A.” Rising or falling pressure may also be denoted in the “Remarks” sections as “PRESRR” (rising) or “PRESFR” (falling).

11. Lastly, we have the remarks section, which always begins with the letters “RMK.”

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

Comments may or may not appear in this section of the METAR. The information contained in this section may include wind data, variable visibility, beginning and ending times of phenomenon, pressure information, and various other information deemed necessary. An example of a remark regarding weather phenomenon that does not fit in any other category would be: OCNL LTGICCG. This translates as occasional lightning in the clouds and from cloud to ground. Automated stations also use the remarks section to indicate the equipment needs maintenance.

Our sample METAR would be read as follows:

Routine METAR for Gregg County Airport for the 16th day of the month at 1753 zulu from an automated source. Winds are 140 at 21 knots gusting to 26 knots. Visibility is ¾ statute mile. Thunderstorms with heavy rain and mist. Ceiling is broken at 800 feet, overcast at 1,200 feet with cumulonimbus clouds. Temperature 18 °C and dew point 17 °C. Barometric pressure is 29.70 inHg and falling rapidly.

A few more examples:

To find more examples and learn even more about weather events that can affect flying, check out the Aviation Weather Handbook, available at asa2fly.com.

The post METAR Deciphered first appeared on Learn To Fly.

Blog Post – Weather: Clouds

Before you jump right into the quiz let’s highlight some knowledge pertaining to clouds you should know.

• Stability determines which of two types of clouds will be formed: cumuliform or stratiform.
• Cumuliform clouds are the billowy-type clouds having considerable vertical development, which enhances the growth rate of precipitation. They are formed in unstable conditions, and they produce showery precipitation made up of large water droplets.
• Stratiform clouds are the flat, more evenly based clouds formed in stable conditions. They produce steady, continuous light rain and drizzle made up of much smaller raindrops.
• Steady precipitation (in contrast to showery) preceding a front is an indication of stratiform clouds with little or no turbulence.
• Clouds are divided into four families according to their height range: low, middle, high, and clouds with extensive vertical development.
• The first three families—low, middle, and high—are further classified according to the way they are formed. Clouds formed by vertical currents (unstable) are cumulus (heap) and are billowy in appearance. Clouds formed by the cooling of a stable layer are stratus (layered) and are flat and sheet-like in appearance. A further classification is the prefix “nimbo-” or suffix “-nimbus,” which means raincloud.
• High clouds, called cirrus, are composed mainly of ice crystals; therefore, they are least likely to contribute to structural icing (since it requires water droplets).

Ready, set, pop quiz!

Pop Quiz – Weather, Clouds

1. Clouds, fog, or dew will always form when
A—water vapor condenses.
B—water vapor is present.
C—relative humidity reaches 100 percent.

2. If an unstable air mass is forced upward, what type clouds can be expected?
A—Stratus clouds with little vertical development.
B—Stratus clouds with considerable associated turbulence.
C—Clouds with considerable vertical development and associated turbulence.

3. The suffix ‘nimbus,’ used in naming clouds, means
A—a cloud with extensive vertical development.
B—a rain cloud.
C—a middle cloud containing ice pellets.

4. Clouds are divided into four families according to their
A—outward shape.
B—height range.
C—composition.

5. What clouds have the greatest turbulence?
A—Towering cumulus.
B—Cumulonimbus.
C—Nimbostratus.

So, how do you think you did? Check out the Answers & Explanations.

Note, the question above are sample questions representative to what you might see on your FAA Private Pilot Knowledge Exam. 

The post CFI Brief: Pop Quiz—Clouds first appeared on Learn To Fly.

Clouds are visible indicators and are often indicative of future weather. For clouds to form, there must be adequate water vapor and condensation nuclei, as well as a method by which the air can be cooled. When the air cools and reaches its saturation point, the invisible water vapor changes into a visible state. Through the processes of deposition (also referred to as sublimation) and condensation, moisture condenses or sublimates onto miniscule particles of matter like dust, salt, and smoke known as condensation nuclei. The nuclei are important because they provide a means for the moisture to change from one state to another.

Cloud type is determined by its height, shape, and characteristics. They are classified according to the height of their bases as low, middle, or high clouds, as well as clouds with vertical development.

Low clouds are those that form near the Earth’s surface and extend up to about 6,500 feet AGL. They are made primarily of water droplets but can include supercooled water droplets that induce hazardous aircraft icing. Typical low clouds are stratus, stratocumulus, and nimbostratus. Fog is also classified as a type of low cloud formation. Clouds in this family create low ceilings, hamper visibility, and can change rapidly. Because of this, they influence flight planning and can make visual flight rules (VFR) flight impossible.

Middle clouds form around 6,500 feet AGL and extend up to 20,000 feet AGL. They are composed of water, ice crystals, and supercooled water droplets. Typical middle-level clouds include altostratus and altocumulus. These types of clouds may be encountered on cross-country flights at higher altitudes. Altostratus clouds can produce turbulence and may contain moderate icing. Altocumulus clouds, which usually form when altostratus clouds are breaking apart, also may contain light turbulence and icing.

High clouds form above 20,000 feet AGL and usually form only in stable air. They are made up of ice crystals and pose no real threat of turbulence or aircraft icing. Typical high level clouds are cirrus, cirrostratus, and cirrocumulus.

Clouds with extensive vertical development are cumulus clouds that build vertically into towering cumulus or cumulonimbus clouds. The bases of these clouds form in the low to middle cloud base region but can extend into high altitude cloud levels. Towering cumulus clouds indicate areas of instability in the atmosphere, and the air around and inside them is turbulent. These types of clouds often develop into cumulonimbus clouds or thunderstorms. Cumulonimbus clouds contain large amounts of moisture and unstable air and usually produce hazardous weather phenomena, such as lightning, hail, tornadoes, gusty winds, and wind shear. These extensive vertical clouds can be obscured by other cloud formations and are not always visible from the ground or while in flight. When this happens, these clouds are said to be embedded, hence the term, embedded thunderstorms.

To pilots, the cumulonimbus cloud is perhaps the most dangerous cloud type. It appears individually or in groups and is known as either an air mass or orographic thunderstorm. Heating of the air near the Earth’s surface creates an air mass thunderstorm; the upslope motion of air in the mountainous regions causes orographic thunderstorms. Cumulonimbus clouds that form in a continuous line are nonfrontal bands of thunderstorms or squall lines.

Since rising air currents cause cumulonimbus clouds, they are extremely turbulent and pose a significant hazard to flight safety. For example, if an aircraft enters a thunderstorm, the aircraft could experience updrafts and downdrafts that exceed 3,000 fpm. In addition, thunderstorms can produce large hailstones, damaging lightning, tornadoes, and large quantities of water, all of which are potentially hazardous to aircraft.

Cloud classification can be further broken down into specific cloud types according to the outward appearance and cloud composition. Knowing these terms can help a pilot identify visible clouds.

The following is a list of cloud classifications:

• Cumulus—heaped or piled clouds
• Stratus—formed in layers
• Cirrus—ringlets, fibrous clouds, also high level clouds above 20,000 feet
• Castellanus—common base with separate vertical development, castle-like
• Lenticularus—lens-shaped, formed over mountains in strong winds
• Nimbus—rain-bearing clouds
• Fracto—ragged or broken
• Alto—middle level clouds existing at 5,000 to 20,000 feet

The post Weather: Clouds first appeared on Learn To Fly.

Rime Ice

Rime ice is rough, milky, and opaque ice formed by the instantaneous freezing of small, supercooled water droplets after they strike the aircraft. It is the most frequently reported icing type. Rime ice can pose a hazard because its jagged texture can disrupt an aircraft’s aerodynamic integrity.

Rime icing formation favors colder temperatures, lower liquid water content, and small droplets. It grows when droplets rapidly freeze upon striking an aircraft. The rapid freezing traps air and forms a porous, brittle, opaque, and milky-colored ice. Rime ice grows into the air stream from the forward edges of wings and other exposed parts of the airframe.

Clear Ice

Clear ice (or glaze ice) is a glossy, clear, or translucent ice formed by therelatively slow freezing of large, supercooled water droplets. Clear icing conditions exist more often in an environment with warmer temperatures, higher liquid water contents, and larger droplets.

Clear ice forms when only a small portion of the drop freezes immediately while the remaining unfrozen portion flows or smears over the aircraft surface and gradually freezes. Few air bubbles are trapped during this gradual process. Thus, clear ice is less opaque and denser than rime ice. It can appear either as a thin smooth surface, or as rivulets, streaks, or bumps on the aircraft.

Clear icing is a more hazardous ice type for many reasons. It tends to form horns near the top and bottom of the airfoils leading edge, which greatly affects airflow. This results in an area of disrupted and turbulent airflow that is considerably larger than that caused by rime ice. Since it is clear and difficult to see, the pilot may not be able to quickly recognize that it is occurring. It can be difficult to remove since it can spread beyond the deicing or anti-icing equipment, although in most cases it is removed nearly completely by deicing devices.

Mixed Ice

Mixed ice is a mixture of clear ice and rime ice. It forms as an airplane collects both rime and clear ice due to small-scale (tens of kilometers or less) variations in liquid water content, temperature, and droplet sizes. Mixed ice appears as layers of relatively clear and opaque ice when examined from the side.

Mixed icing poses a similar hazard to an aircraft as clear ice. It may form horns or other shapes that disrupt airflow and cause handling and performance problems. It can spread over more of the airframe’s surface and is more difficult to remove than rime ice. It can also spread over a portion of airfoil not protected by anti-icing or deicing equipment. Ice forming farther aft causes flow separation and turbulence over a large area of the airfoil, which decreases the ability of the airfoil to keep the aircraft in flight.

Effects of Icing

Remember when I said a few paragraphs earlier that ice sucks? Well I didn’t really explain myself as to why.

When structural icing forms, it reduces aircraft efficiency by increasing weight, reducing lift, decreasing thrust, and increasing drag. Each effect will either slow the aircraft or force it downward.  As ice accumulates the performance characteristics of the aircraft will continually deteriorate eventually to a point where the aircraft can no longer maintain sustained flight and stalls.  The image below is a good depiction of this.

As ice forms on an airfoil, it will destroy the smooth flow of air over the surface of the wing resulting in drag and diminishing the maximum lift capable of the wing. NASA wind tunnel testing has shown that icing on the leading edge or upper surface of a wing no thicker then coarse sandpaper can reduce lift by 30 percent and increase drag by 40 percent.

In addition icing can also cause instrumentation errors, frozen or unbalanced control surfaces, engine failures and/or structural damage due to chunks of ice breaking off.

Additional Knowledge to Know

The post CFI Brief: Icing first appeared on Learn To Fly.

A weather front exists where air masses with different properties meet. The terms “warm” and “cold” are relative: 30°F air is warmer than 10°F air, but that “warm” air doesn’t call for bathing suits. Cold air is more dense than warm air, so where two dissimilar masses meet, the cold air stays near the surface. Figure 1 shows a cold front: cold air advancing from west to east and displacing warm air. Because the cold air is dense and relatively heavy, it moves rapidly across the surface, pushing the warm air up. Notice that in both cases the warm air is forced aloft and the cold air stays at the surface. When air is lifted, stuff happens. Just how bad that “stuff” might be is determined by the moisture content of the warm air and where that moisture is coming from.

Friction slows the cold air movement at the surface, so that the front is quite vertical in cross-section and the band of frontal weather is narrow. Cold fronts can move as fast as 30 knots. Your awareness of this rapid movement, together with facts you already know about temperature and dew point will allow you to make the following generalizations about cold front weather.

Visibility: Good behind the front. Warm air and pollutants rise rapidly because warm air is less dense than cold air.
Flight conditions: Bumpy as thermal currents rise.

Precipitation: Showery in the frontal area as the warm air is forced aloft and its moisture condenses. The ability of the air to hold moisture decreases as the air cools, and as the moisture contained in each column of rising air condenses into water droplets, showers result.

Cloud type: Cumulus, due to air being raised rapidly to the condensation level. Cumulus clouds are a sign of unstable air; the rising air columns are warmer than the surrounding air and continue to rise under their own power.

Icing possibility: Clear ice. Cumulus clouds develop large water droplets which freeze into clear sheets of ice when they strike an airplane.

A warm front exists when a warm air mass overtakes a slow-moving cold air mass; the lighter warm air cannot displace the heavier cold air, and the warm air is forced to rise as it moves forward (Figure 2). This slow upward movement combined with the slow forward movement characteristic of warm fronts allows the warm air to cool slowly. As it reaches the condensation level, stratiform clouds develop. While cold frontal conditions exist over a very short distance, warm fronts slope upward for many miles, and warm frontal weather may be extensive.

You may encounter warm front clouds 50 to 100 miles from where the front is depicted on the surface analysis chart. The following are the characteristics of warm frontal weather:

Visibility: Poor; pollutants trapped by warm air aloft. Air warmed at the surface can only rise until it reaches air at its own temperature.

Flight conditions: Smooth, no thermal activity.

Precipitation: Drizzle or continuous rain as moist air is slowly raised to the condensation level.

Cloud type: Stratus or layered, the result of slow cooling.

Icing possibility: Rime ice; small water droplets freeze instantly upon contact with an airplane and form a rough, milky coating.

Occasionally, a fast-moving cold front will overtake a warm front (Figure 3) and lift the warm air away from the surface. This is called an occlusion, and occluded frontal weather contains the worst features of both warm and cold fronts: turbulent flying conditions, showers and/or continuous precipitation, poor visibility in precipitation, and broad geographic extent of frontal weather conditions.

Air masses can maintain their warm/cold identity and yet not exert any displacement force. When this happens, the front becomes stationary, and the associated weather covers a large geographic area. In your planning, what you see is probably what you will get during the flight.

When you look at a weather map which shows frontal positions, cold fronts will be marked in blue, warm fronts in red, occluded fronts will be purple, and stationary fronts will alternate red and blue. You can identify fronts on black-and-white charts because the cold front symbols look like icicles and warm front symbols appear as blisters (Figure 4). Visualize the lifting process, and you will be on your way to being your own weather forecaster.

In flight, when you fly through a front you will notice a change in outside air temperature and wind direction; you will change heading to the right in order to stay on course.

Occluded fronts show both icicles and blisters on the same side of the front in the direction of movement, and stationary fronts show the symbols on opposite sides of the frontal line, indicating opposing forces.

The post Weather: Fronts first appeared on Learn To Fly.

Atmospheric pressure historically was measured in inches of mercury (“Hg) by a mercurial barometer. The barometer measures the height of a column of mercury inside a glass tube. A section of the mercury is exposed to the pressure of the atmosphere, which exerts a force on the mercury. An increase in pressure forces the mercury to rise inside the tube. When the pressure drops, mercury drains out of the tube decreasing the height of the column. This type of barometer is typically used in a laboratory or weather observation station, is not easily transported, and difficult to read.

An aneroid barometer is the standard instrument used to measure pressure; it is easier to read and transport. The aneroid barometer contains a closed vessel called an aneroid cell that contracts or expands with changes in pressure. The aneroid cell attaches to a pressure indicator with a mechanical linkage to provide pressure readings. The pressure sensing part of an aircraft altimeter is essentially an aneroid barometer. It is important to note that due to the linkage mechanism of an aneroid barometer, it is not as accurate as a mercurial barometer.

To provide a common reference, the International Standard Atmosphere (ISA) has been established. These standard conditions are the basis for certain flight instruments and most aircraft performance data. Standard sea level pressure is defined as 29.92 “Hg and a standard temperature of 59 °F (15 °C). Atmospheric pressure is also reported in millibars (mb), with 1 “Hg equal to approximately 34 mb. Standard sea level pressure is 1,013.2 mb. Typical mb pressure readings range from 950.0 to 1,040.0 mb. Surface charts, high and low pressure centers, and hurricane data are reported using mb.

Since weather stations are located around the globe, all local barometric pressure readings are converted to a sea level pressure to provide a standard for records and reports. To achieve this, each station converts its barometric pressure by adding approximately 1 “Hg for every 1,000 feet of elevation. For example, a station at 5,000 feet above sea level, with a reading of 24.92 “Hg, reports a sea level pressure reading of 29.92″Hg. Using common sea level pressure readings helps ensure aircraft altimeters are set correctly, based on the current pressure readings.

By tracking barometric pressure trends across a large area, weather forecasters can more accurately predict movement of pressure systems and the associated weather. For example, tracking a pattern of rising pressure at a single weather station generally indicates the approach of fair weather. Conversely, decreasing or rapidly falling pressure usually indicates approaching bad weather and, possibly, severe storms.

The post Weather: Measurement of Atmospheric Pressure first appeared on Learn To Fly.

We’re seeing rain for the first time in over two months in the Seattle area right now, so how about a refresher on precipitation today on the Learn to Fly Blog? Today’s post is excerpted from Aviation Weather (AC 00-6B).

Precipitation is any of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. The precipitation types are: drizzle, rain, snow, snow grains, ice crystals, ice pellets, hail, and small hail and/or snow pellets.

Precipitation formation requires three ingredients: water vapor, sufficient lift to condense the water vapor into clouds, and a growth process that allows cloud droplets to grow large and heavy enough to fall as precipitation. Significant precipitation usually requires clouds to be at least 4,000 feet thick. The heavier the precipitation, the thicker the clouds are likely to be. When arriving or departing from an airport reporting precipitation of light or greater intensity, expect clouds to be more than 4,000 feet thick.

All clouds contain water, but only some produce precipitation. This is because cloud droplets and/or ice crystals are too small and light to fall to the ground as precipitation. Because of their microscopic size, the rate at which cloud droplets fall is incredibly slow. An average cloud droplet falling from a cloud base at 3,300 feet (1,000 meters) would require about 48 hours to reach the ground. It would never complete this journey because it would evaporate within minutes after falling below the cloud base. Two growth processes exist which allow cloud droplets (or ice crystals) to grow large enough to reach the ground as precipitation before they evaporate (or sublimate). One process is called the collision-coalescence, or warm rain process (see Figure 14-1). In this process, collisions occur between cloud droplets of varying size and different fall speeds, sticking together or coalescing to form larger drops. Finally, the drops become too large to be suspended in the air, and they fall to the ground as rain. This is thought to be the primary growth process in warm, tropical air masses where the freezing level is very high.

Figure 14-1. The collision-coalescence or warm rain process.

Most cloud droplets are too small and light to fall to the ground as precipitation. However, the larger cloud droplets fall more rapidly and are able to sweep up the smaller ones in their path and grow.

The other process is the ice crystal process. This occurs in colder clouds when both ice crystals and water droplets are present. In this situation, it is easier for water vapor to deposit directly onto the ice crystals so the ice crystals grow at the expense of the water droplets. The crystals eventually become heavy enough to fall. If it is cold near the surface, it may snow; otherwise, the snowflakes may melt to rain. This is thought to be the primary growth process in mid- and high-latitudes.

The vertical distribution of temperature will often determine the type of precipitation that occurs at the surface. Snow occurs when the temperature remains below freezing throughout the entire depth of the atmosphere (see Figure 14-2).

Figure 14-2. Snow temperature environment.

Ice pellets (sleet) occur when there is a shallow layer aloft with above freezing temperatures and with a deep layer of below freezing air based at the surface. As snow falls into the shallow warm layer, the snowflakes partially melt. As the precipitation reenters air that is below freezing, it refreezes into ice pellets (see Figure 14-3).

Figure 14-3. Ice pellets temperature environment.

Freezing rain occurs when there is a deep layer aloft with above freezing temperatures and with a shallow layer of below freezing air at the surface. It can begin as either rain and/or snow, but becomes all rain in the warm layer. The rain falls back into below freezing air, but since the depth is shallow, the rain does not have time to freeze into ice pellets (see Figure 14-4). The drops freeze on contact with the ground or exposed objects.

Figure 14-4. Freezing rain temperature environment.

Rain occurs when there is a deep layer of above freezing air based at the surface (see Figure 14-5).

Figure 14-5. Rain temperature environment.

The post Weather: Precipitation first appeared on Learn To Fly.

SunsetWX.com has come up with an algorithm to forecast the sunrise and sunset quality throughout the United States and all over the world! Take a look below at the sample sunset forecast for the United States.

Areas of better sunset quality are denoted by warmer colors like the yellows, oranges and reds. It appears that the highest quality sunset will be visible throughout Central California according to this forecast. So if you happen to live in say Sacramento, CA it would be an excellent evening for that sunset cruise.

For the latest forecast visits www.SunsetWX.com and follow them on twitter @sunset_wx .

Now remember, since you will potentialy be flying prior to civil twilight, it is important to make sure your aircraft has the minimum required equipment under 14 CFR 91.205 for night flight. This is in addition to required equipment for day flight.

14 CFR 91.205

…(c) Visual flight rules (night). For VFR flight at night, the following instruments and equipment are required:

(1) Instruments and equipment specified in paragraph (b) of this section.

(2) Approved position lights.

(3) An approved aviation red or aviation white anticollision light system on all U.S.-registered civil aircraft. Anticollision light systems initially installed after August 11, 1971, on aircraft for which a type certificate was issued or applied for before August 11, 1971, must at least meet the anticollision light standards of part 23, 25, 27, or 29 of this chapter, as applicable, that were in effect on August 10, 1971, except that the color may be either aviation red or aviation white. In the event of failure of any light of the anticollision light system, operations with the aircraft may be continued to a stop where repairs or replacement can be made.

(4) If the aircraft is operated for hire, one electric landing light.

(5) An adequate source of electrical energy for all installed electrical and radio equipment.

(6) One spare set of fuses, or three spare fuses of each kind required, that are accessible to the pilot in flight.

To help you remember you can use this simple mnemonic ‘FLAPS’.

F uses (spare) or circuit breakers

L anding light (if for hire)

A nticollision lights

P osition lights

S ource of electricity

The post CFI Brief: Sunset Weather first appeared on Learn To Fly.

Atmospheric stability is defined as the resistance of the atmosphere to vertical motion. A stable atmosphere resists an upward or downward movement. An unstable atmosphere allows an upward or downward disturbance to grow into a vertical (convective) current.

Determining the stability of the atmosphere requires measuring the difference between the actual existing (ambient) temperature lapse rate of a given parcel of air and the dry adiabatic rate (a constant 3°C per 1,000 feet lapse rate).

A stable layer of air would be associated with a temperature inversion. Warming from below, on the other hand, would decrease the stability of an air mass.

The conditions and characteristic of stable or unstable air masses are shown in the figure below.

Like most things in life their are benefits and drawbacks to both atmospheric conditions. While the visibility may be excellent in an unstable air mass you are likely to encounter turbulent conditions and possibly wind shear. In a stable air mass because the air is stagnant (or lack of vertical motion) you will have a much smoother ride however visibility could be decreased due to hazy conditions.

Take a look at some of these sample FAA knowledge test questions and see if you can answer them.

1. What type weather can one expect from moist, unstable air, and very warm surface temperatures?
A—Fog and low stratus clouds.
B—Continuous heavy precipitation.
C—Strong updrafts and cumulonimbus clouds.

2. What are the characteristics of stable air?
A—Good visibility; steady precipitation; stratus clouds.
B—Poor visibility; steady precipitation; stratus clouds.
C—Poor visibility; intermittent precipitation; cumulus clouds.

3. A moist, unstable air mass is characterized by
A—poor visibility and smooth air.
B—cumuliform clouds and showery precipitation.
C—stratiform clouds and continuous precipitation.

4. Which is a characteristic typical of a stable air mass?
A—Cumuliform clouds.
B—Showery precipitation.
C—Continuous precipitation.

Answers will be posted in the comments section.

The post CFI Brief: Atmospheric Stability first appeared on Learn To Fly.