Weather

Almost all weather comes from one process: the sun heats the surface unevenly, the surface heats the air above it, and warm air rises. Everything else — cloud, rain, wind, storms — is a consequence of what happens to air that goes up.

Air that rises expands, because the pressure around it falls. Expanding gas cools, and it does so without exchanging heat with anything, which is why the process is called adiabatic. Unsaturated air cools at about 3°C per 1,000 feet as it rises.

Eventually it cools to its dew point and the water vapour in it condenses into droplets. That is a cloud base. Condensation releases latent heat, so from that point upward the parcel cools more slowly — commonly around 1.1 to 1.5°C per 1,000 feet, depending on temperature and pressure.

That change of rate is the engine of severe weather. A rising parcel that is still warmer than the air around it keeps rising, and the latent heat released by condensation keeps it warmer for longer. Given enough moisture, a parcel that starts rising near the ground can keep going until it reaches the tropopause.

If a parcel of air pushed upward finds itself cooler than its surroundings, it sinks back. The atmosphere is stable, and its signature is layer cloud: stratus, poor visibility, haze that will not lift, drizzle rather than showers, and smooth flying with steady winds.

If the parcel finds itself warmer than its surroundings it keeps rising. The atmosphere is unstable, and its signature is vertical development: cumulus building through the day, excellent visibility between the clouds, showers rather than continuous rain, gusty surface winds and turbulence at low level.

For a pilot the practical distinction is what the air will do to you. Stable air is comfortable and can be dangerous for what you cannot see — fog, low ceilings, ice in the cloud layer. Unstable air is uncomfortable and dangerous for what it can build into.

Both are made worse by fronts, because a front is a place where air is being forced to rise whether or not it wants to. A cold front pushes under warm air and shoves it up steeply, producing a narrow band of intense weather. A warm front slides up over cold air at a shallow angle, producing a wide band of layer cloud, poor visibility and precipitation that may extend for hundreds of miles ahead of the surface position.

A thunderstorm requires moisture, an unstable lapse rate, and something to lift the air to start the process. Remove any one and it does not form; supply all three and it will.

The cumulus stage is dominated by updraughts. The cloud builds vertically, and inside it air is moving upward throughout. It looks impressive and may already be turbulent, and it does not yet produce rain at the surface.

The mature stage begins when precipitation starts falling. This is the crucial change: falling rain drags air down with it, so the cell now contains a powerful updraught and a powerful downdraught side by side. This is the most dangerous stage, and it is where every hazard the storm has to offer is present at once — severe turbulence, hail, lightning, icing, and the downdraught striking the ground and spreading outward as a gust front.

The dissipating stage arrives when the downdraught spreads through the cell and cuts off the supply of warm moist air feeding the updraught. The storm rains itself out. It is still dangerous while it does.

The standard guidance is to stay at least 20 nautical miles from a thunderstorm. That number surprises people, and it exists because hail can be thrown out of the top of a cell and fall clear of the visible cloud, and because severe turbulence extends well beyond the storm’s edge.

A microburst is a concentrated downdraught, typically less than 2.5 miles across, that strikes the ground and spreads outward in all directions. Vertical speeds of 6,000 feet per minute have been measured, and the outflow can exceed 60 knots.

What makes it lethal is the sequence an aircraft flying through it experiences. Entering the microburst, the aircraft meets the outflow head-on: a sudden increase in headwind, which increases airspeed and makes the aircraft balloon above the glidepath. The natural response is to reduce power and lower the nose.

Then it passes into the core and meets the downdraught, which pushes it toward the ground while the airspeed advantage disappears.

Then it exits the other side into the outflow going the other way — a sudden tailwind, which strips airspeed away at exactly the moment the aircraft is low, slow and with the power already reduced.

Every instinctive response to the first phase makes the third phase worse. This is why recognition matters more than technique: the recovery is full power and a pitch attitude that trades everything available for altitude, and it must be started at the first indication rather than after the situation develops.

Airframe icing requires two conditions at the same time: visible moisture, and an airframe temperature at or below freezing. The moisture has to be liquid — flying through dry snow or ice crystals does not accrete airframe ice, because the crystals bounce off.

The substance responsible is supercooled water: liquid droplets at below-freezing temperature, held liquid because there is nothing for them to freeze onto. An aircraft flying into them supplies exactly that.

Rime ice forms from small droplets that freeze almost instantly on contact, trapping air. It is opaque, white and rough, and it builds forward from leading edges. Clear ice forms from large droplets that flow back across the surface before freezing, producing a smooth, heavy, transparent layer that is harder to see and harder to shed. Mixed ice is both.

What ice does is worse than the weight it adds. It changes the shape of the wing, and a wing’s performance depends on its shape more than on anything else. Even a thin, rough accretion on the leading edge can substantially reduce maximum lift and, critically, reduce the angle of attack at which the wing stalls. An iced wing stalls earlier and often without the usual warning.

The most dangerous form is freezing rain, where liquid drops fall through a below-freezing layer. It produces clear ice extremely rapidly and it means there is warmer air above — which is the one useful piece of information it gives you, because climbing may be the escape.

A METAR is an observation: what the weather at an airport actually is, usually issued hourly with special reports when something changes significantly. A TAF is a forecast: what the weather at an airport is expected to be, typically covering 24 or 30 hours and issued four times a day.

The distinction matters more than it sounds. A METAR is measured and a TAF is predicted, and a TAF is a forecast for a small area around the airport rather than for the region.

Both use a compressed international format, which looks impenetrable and is learned in about an hour. Wind as direction and speed, visibility, cloud as amount and base, temperature and dew point, pressure, then remarks.

What experienced pilots read out of a sequence of METARs is not the current line but the trend. A dew point that has been climbing toward the temperature all evening is a fog forecast regardless of what the TAF says, and a pressure that has been falling steadily is a system arriving whether or not anybody has forecast the time.