Meteorology

The weather decides more about a flight than almost anything else: which runway is in use, how much fuel you need, how smooth the ride will be and sometimes whether you should go at all. Meteorology teaches you what the atmosphere is doing and how to read the reports and forecasts that describe it.

1. The atmosphere and the ISA

We measure everything against the International Standard Atmosphere (ISA), an average atmosphere:

• At sea level: 15 °C and 1013.25 hPa.
• Temperature falls about 2 °C per 1,000 ft up to the tropopause at about 36,000 ft, where it stays at −56.5 °C.
• Near the ground, pressure falls about 1 hPa every 30 ft.

Real days are rarely standard. A summer afternoon in Amman might be ISA +25 °C – the air is thinner than standard, so engines and wings perform as if the airport were much higher (high density altitude).

2. Pressure and wind

• High pressure (anticyclone): sinking air, usually settled weather, but can trap haze and fog.
• Low pressure (depression): rising air, cloud and rain.
• In the northern hemisphere wind blows clockwise around a high and anticlockwise around a low, roughly along the isobars. Buys Ballot's law: stand with your back to the wind and the low is on your left.
• Close isobars mean strong wind. Near the ground, friction slows the wind and turns it towards the low pressure: the wind at 2,000 ft is usually stronger and veered (turned clockwise) compared with the surface wind.
• Wind direction is always where it blows from, in degrees true in forecasts and METARs (magnetic when ATC gives it for the runway).

3. Humidity, clouds and stability

Air holds more water vapour when it is warm. As air cools to its dew point, the vapour condenses into cloud or fog. A small gap between temperature and dew point (the spread) warns of fog or low cloud.

• Stable air resists rising: layer clouds (stratus), poor visibility, smooth air.
• Unstable air keeps rising once it starts: heaped clouds (cumulus), showers, good visibility outside them, bumpy air.

Level	Base (mid-latitudes)	Cloud types
Low	Surface to 6,500 ft	Stratus (ST), stratocumulus (SC), nimbostratus (NS)
Medium	6,500–23,000 ft	Altostratus (AS), altocumulus (AC)
High	Above 16,500–23,000 ft	Cirrus (CI), cirrostratus (CS), cirrocumulus (CC)
Vertical development	Low base, very high tops	Cumulus (CU), towering cumulus (TCU), cumulonimbus (CB)

4. Air masses and fronts

• A warm front: warm air slides up over cold air. Cloud lowers gradually from cirrus to stratus over hundreds of kilometres, with long periods of light to moderate rain and poor visibility.
• A cold front: cold air pushes under warm air. A narrow band of cumulonimbus, heavy showers, gusts and thunderstorms, then clearing skies and good visibility.
• An occluded front: a cold front catches a warm front; a mix of both.

In the Levant, winter depressions from the Mediterranean bring rain, low cloud and sometimes snow to Amman and Damascus; summers are dry and hot.

5. Hazards

Thunderstorms

A cumulonimbus goes through three stages: cumulus (strong updraughts), mature (updraughts and downdraughts, heavy rain, hail, lightning) and dissipating. Inside and near it: severe turbulence, icing, hail, lightning and microbursts – violent downdraughts that can push an aircraft into the ground on approach. Never fly into or under a CB; stay at least 10–20 NM away.

Icing

Airframe ice forms when you fly through visible moisture at temperatures between about 0 °C and −20 °C. Clear ice (large drops, near 0 °C) is heavy and hard to see; rime ice (small drops, colder) is white and rough. Carburettor icing can happen even on a warm day in humid air – up to about +30 °C.

Fog and visibility

• Radiation fog: clear night, light wind, moist air – the ground cools and the air above it reaches its dew point. Burns off after sunrise.
• Advection fog: warm, moist air moving over a cold surface (for example sea fog). Can last all day.
• Dust and sand: desert winds such as the spring khamsin can lift dust and cut visibility to a few hundred metres across Jordan, Iraq and Saudi Arabia.

Wind shear and turbulence

Sudden changes in wind speed or direction – near thunderstorms, temperature inversions, mountains (mountain waves) and on approach in gusty conditions. Add a margin to your approach speed in gusts.

6. Reading a METAR

OJAI 252300Z 30006KT 4000 BR NSC 19/16 Q1016 NOSIG

Group	Meaning
OJAI	Amman Queen Alia
252300Z	25th of the month, 23:00 UTC
30006KT	Wind from 300° at 6 knots (G = gusts, e.g. 27015G28KT; VRB = variable)
4000 BR	Visibility 4,000 m in mist (FG fog, HZ haze, DU dust, SA sand, RA rain, TS thunderstorm)
NSC	No significant cloud. Otherwise FEW (1–2 oktas), SCT (3–4), BKN (5–7), OVC (8) with the base in hundreds of feet, e.g. BKN025 = broken at 2,500 ft
19/16	Temperature 19 °C, dew point 16 °C (M = minus)
Q1016	QNH 1016 hPa
NOSIG	No significant change expected in the next 2 hours

CAVOK ("ceiling and visibility OK") means visibility 10 km or more, no cloud below 5,000 ft (or the minimum sector altitude if higher), no cumulonimbus or towering cumulus and no significant weather.

The ceiling is the lowest BKN or OVC layer.

7. Forecasts: the TAF

A TAF forecasts an airport's weather for 9 to 30 hours. Change groups:

• BECMG 2512/2514: conditions becoming different gradually between 12:00 and 14:00 UTC.
• TEMPO: temporary changes lasting less than an hour each, in total less than half the period.
• PROB30 / PROB40: a 30% or 40% chance.
• FM: a lasting change from a given time.

Other products: SIGMETs (severe hazards en route), significant weather charts (SWC) and wind and temperature charts for cruise levels.

Flying it at Levant

With live weather in the simulator, read the departure and arrival METARs before you plan – SimBrief shows them on the OFP – and look at the TAF for your arrival time. The wind decides the runway; the QNH goes on your altimeter. Watch for CB and TS in summer afternoons over Turkey and Iraq, and for low visibility in winter mornings.

Check yourself

1. What are the ISA sea-level temperature and pressure? 15 °C and 1013.25 hPa.
2. Back to the wind in the northern hemisphere – where is the low pressure? On your left.
3. Decode 27012G25KT 9999 BKN015. Wind 270° at 12 kt gusting 25, visibility 10 km or more, broken cloud at 1,500 ft (the ceiling).
4. Temperature 12 °C, dew point 11 °C, clear night, light wind. What do you expect? Radiation fog.
5. What does CAVOK mean? Visibility ≥ 10 km, no cloud below 5,000 ft (or MSA), no CB/TCU, no significant weather.

Simulation training material based on standard PPL meteorology. Always use current official weather reports and forecasts.