MET 008
Meteorological hazards
The hazards at a glance
Each hazard on one line: its code or chart symbol, where and when it is found, and what it does. A linked name leads to its section.
| Hazard | Code or chart symbol | Where and when | What it does |
|---|---|---|---|
| Wind shear | – | Over a short distance: it is microscale. Downbursts out of a CB are particularly hazardous. | When severe, it can displace an aircraft from its flight path. |
| Turbulence | Chart symbols for moderate and severe | Weather fronts, thunderstorms, the lee side of mountains, inversion layers, clear air (CAT), and wake turbulence from aircraft. | Four levels: light, moderate, severe and extreme. It feels worse in a small aircraft. |
| Icing | Chart symbols for light, moderate and severe | Generally in CB and TCU, but it can still be bad in stratiform cloud, for example NS. When liquid water is at 0 °C or less. | Disturbed airflow, more drag, added weight, engine damage in jets and reduced visibility. |
| Thunderstorm | TS | Only a CB makes one, and a CB can hide inside layer cloud: an embedded CB. | Thunder and lightning, often with gusts and heavy rain, and sometimes hail. |
| Funnel cloud | FC | Extending from a CB. | A violent, rotating column of air. In contact with the ground it is a tornado, with water a waterspout. |
| Ice pellets | PL | – | They show a layer of freezing rain above the surface (MET 006). |
| Volcanic ashOne of the lithometeors (MET 007). | VA | During eruptions, which suspend a lot of particles in the air. | – |
Wind shear
A change in the wind over a short distance.
Wind shearA change in wind speed or direction, or both, over a short distance. It is microscale (MET 005).
When severeIt can displace an aircraft from its flight path.
Downbursts: particularly hazardous
Turbulence
Four levels of severity, and where and when turbulence is found, wake turbulence included.
Four levelsLight, moderate, severe and extreme. Their chart symbols are in the severity table, below.
Where and when
- Weather frontsSquall lines and cold fronts (MET 005).
- ThunderstormsIn and near the CB, and in micro- and macrobursts, under Wind shear, above.
- MountainsWhen the wind blows over them, vertical shear is observed on the lee side: the mountain waves of MET 005.
- Inversion layersWhere the temperature rises with height (MET 003).
- Clear air turbulence (CAT)Turbulence in clear air, so nothing shows where it is.
Seen from above: a vortex trails from each wingtip. Wake turbulenceA type of CAT. It comes from aircraft, not from the weather.- Turbulence generated by an aircraft in flight.
- The vortex strength depends on the aircraft’s weight, its speed and the shape of its wing.
Wake turbulence categories are in LAW 007, and the extra spacing behind a heavier aircraft in ATM 003.
Icing
What icing is, rime ice against clear ice, and where and when icing occurs.
IcingSupercooled droplets freezing on impact with an aircraft. Supercooled water is in MET 003.
Rime or clear ice
Rime ice
Clear ice
| Rime ice | Clear ice | |
|---|---|---|
| Temperature | Colder than about −15 °C | 0 to about −15 °C |
| Droplets | Small | Large |
| The ice | Ice with trapped air | Solid and smooth |
| Forms on | Leading edges and engine intakes | – |
Where and when
Cumuliform cloudIcing is generally in cumuliform cloud: CB and TCU (towering cumulus). They have large droplets, so ice builds fast.
Stratiform cloudStratiform cloud usually has smaller droplets, but icing in it can still be bad, for example in NS.
0 °C or lessIcing occurs when liquid water is at 0 °C or less.
Ice pellets (PL)They show a layer of freezing rain above the surface (MET 006).
Effects, severity and reports
What ice does to an aircraft, one severity scale for turbulence and icing, and how icing is forecast, reported, removed and prevented.
Severity: turbulence and icing
| Severity | Turbulence symbol | Icing symbol | In icing, a change of heading or altitude |
|---|---|---|---|
| Light | – | One bar | Not necessary |
| Moderate | One peaked line | Two bars | Desirable |
| Severe | Two peaked lines | Three bars | Immediate change necessary |
| Extreme | – | – | – |
The same symbols are on the significant weather chart. ICAO charts show moderate and severe icing only.
The aircraft type. A pilot who reports turbulence also gives the aircraft type. Turbulence feels worse in a small aircraft, so the type lets others judge how severe it will be for them.
Forecast, report, de-icing and anti-icing
ForecastOn the significant weather (SIGWX) chart (also written SWC), and in a SIGMET when it is severe.
ReportIn an air-report, AIREP (ARP), or a special air-report, AIREP SPECIAL (ARS).
De-icingRemoval of existing ice, frost and snow.
Anti-icingA chemical to prevent or delay the formation of ice.
Thunderstorms
Only a CB makes one. What a thunderstorm needs, its life cycle, and what a CB brings.
Thunderstorm (TS)Only a CB makes a thunderstorm (MET 004). A CB can also hide inside layer cloud: an embedded CB.
Three necessary ingredients
- A lifting forceOne of four. How each lifts the air is drawn in MET 004.
- A frontal surface: a frontal thunderstorm.
- Differential surface heating: an air mass thunderstorm.
- Convergence.
- Sloping terrain.
- InstabilityUnstable air makes the air continue to rise (MET 004).
- MoistureCondensation releases latent heat. Without enough water vapour, the cloud does not grow.
The life cycle
What a CB brings
| Hazard | In brief |
|---|---|
| Downbursts | Air rushes down out of a CB and spreads out at the ground. A microburst is less than 4 km in diameter, a macroburst more than 4 km. |
| Turbulence | In and near the CB, where air goes both up and down, and in micro- and macrobursts. |
| Icing | Icing is generally in CB and TCU. They have large droplets, so ice builds fast. |
| Gusts, heavy rain and hail | With the thunder and lightning often come gusts and heavy rain, and sometimes hail. |
| Lightning | A discharge of static electricity. It can damage aircraft. |
| Funnel cloud (FC) | A violent, rotating column of air extending from a CB. |
| Tornado | A funnel cloud in contact with the ground. |
| Waterspout | A funnel cloud in contact with water. |
Types of thunderstorm
Four types, in brief.
Single cell or air mass thunderstormUsually isolated: the typical summer thunderstorm. Its life cycle is drawn under Thunderstorms, above.
MulticellA group of single cells at different stages: new ones grow as old ones die.
Squall lineSimilar to a multicell, but in a line. A long, roll-shaped cloud can form along its front edge.
SupercellA special kind of single cell that can last for many hours. Its strong updraft rotates: a mesocyclone.
Reading material
Meteorology
MET 008 Meteorological Hazards PowerPoint / PDF
EPN Group Training Document Meteorology, chapter 8 Reading list, course material
Ahrens, C. Donald, Meteorology Today: An Introduction to Weather, Climate, and the Environment, 8th edition, 2007 Reference, paid