XC-day weather: forecast and strategy

The three-day forecast: you choose the day, not the route

In the basic module we covered how to tell from a forecast whether today is a flying day. On a route the question is a different one. Not “am I flying” but “where to, how far, and until what hour”. The answer to that is assembled not in the morning but three days ahead.

Three days out, a model does not yet know the details. It already guesses reasonably well which pressure system will sit over the area and where a front will go. But the exact hour the front passes, the height of the base and the strength of the lift are beyond it at that range. To demand such figures from it is to deceive yourself.

So at three days out you ask only the large questions:

• Which system is coming — a high, a trough, a front?
• Where will the air flow from: the sea, the plains, the north across the range?
• Should you expect precipitation and thunderstorms in the area of the route?
• Which of the next few days looks better than the rest?

Look separately at the spread of the ensemble. An ensemble is when the same model is run several dozen times, changing the initial data slightly each time. Usually 20 to 50 runs are used. If they all draw a similar picture, the model is confident. If the runs diverge, there is no confidence, and it is too early to plan a long route for that day.

The synoptic chart: what the pressure system promises a route

A synoptic chart is a chart of surface pressure with fronts on it. The lines of equal pressure on it are called isobars. For a pilot it answers a question that no app addresses: why today will be the way it will be.

You read it in four steps.

1. Find the centres: H for high pressure, L for low. In the northern hemisphere the air goes clockwise around a high and anticlockwise around a low.
2. Look at how tightly packed the isobars are. Closer lines mean stronger wind. The figures on the isobars are secondary; what matters is the spacing between them.
3. Find the fronts. A blue line with triangles is cold, a red one with semicircles is warm, a purple one with both is an occlusion — that is, the cold front has caught up with the warm one.
4. Find the trough — an elongated tongue of low pressure. On the chart it is a black line with no symbols. A trough means unstable, turbulent air and showers.

What each system gives a route.

For an XC pilot the second day after a cold front has passed matters most. On the first day the air is too fresh and ragged, the lift punches, and the cumulus grows into towers. On the second or third day high pressure builds over the area, the air subsides and dries out. The lift becomes smoother, and the inversion above stops the clouds overdeveloping. That is the classic XC day.

The air mass and its history: where today’s air came from

Two reports with identical figures can produce completely different days. The difference is set not by today’s temperature but by the history of the air: where it was born and what happened to it on the way.

Masses are classified by two features. The first is humidity: maritime or continental. The second is the temperature of the source: arctic, polar or tropical. A maritime polar mass arrives humid and cool; a continental tropical one dry and warm.

Then the mass changes on its way. The rule is short and it works:

• Cold air travelling over a warm surface is heated from below and becomes unstable. Cumulus, showers, good visibility between them. For a route that is a strong but ragged day.
• Warm air travelling over a cold surface is cooled from below and becomes stable. Layer cloud, haze, fog. There will be no route.

Five things work on changing a mass: the temperature difference between the air and the surface, moisture added by evaporation and lost as precipitation, the terrain along the way, the shape of the track and its age — the longer a mass has been travelling, the less is left in it of its source.

How to look at the history of the air in practice. Take a wind chart for the 700 hPa level, which is about 3,000 m, and wind it back a day. That is where the air came from. Passed over the sea — expect moisture and a low base. Came down off the northern ranges — dry and cold aloft, and the lift will be strong. Crossed the mountains with precipitation — on our side it will arrive dry and warm, and that is already a conversation about the föhn.

The emagram of an XC day: working out not “will we fly” but “how far”

In the basic module we covered how to find the top of the thermals and the cloudbase on an emagram. On a route more is asked of the same graph: it has to tell you the thickness of the working layer and the quality of the sky along the whole line.

Four things to look for on the emagram of an XC day.

1. The thickness of the convective layer. That is the distance from the ground to the top of the thermals. In the practice of soaring forecast services a layer of 1,500 m or more is considered favourable. The real working ceiling is usually a little below the calculated one: near the upper limit the lift is already weak.
2. The spread at the level of the ceiling. The difference between temperature and dew point up there decides what the sky will be like. A benchmark from practice: 2–4 °C gives handsome high cumulus. Less than 2 °C and the clouds spread into an unbroken sheet, and the day switches itself off. More, and the sky will be blue, with no clouds to read.
3. The dryness of the upper level. If the temperature and dew-point curves diverge by 10–15 °C or more up there, expect no high cloud and no storm towers.
4. Stable layers inside. Every bend of the curve to the right is a step at which the day will stop and wait.

Working it out step by step. The example is for teaching; the figures are taken to demonstrate the calculation, not for a particular day.

• The environmental curve gives −2 °C at 2,800 m.
• Launch is at 2,200 m. The difference is 600 m.
• The dry adiabat is 1 °C per 100 m, that is, 6 °C over 600 m.
• Trigger temperature for the 2,800 m level: −2 + 6 = +4 °C at launch.
• The forecast gives a maximum of +7 °C. There is a margin, and the lift will reach above 2,800 m.
• Temperature at launch +7 °C, dew point −3 °C. Spread 10 °C. Base ≈ 10 × 125 = 1,250 m above launch, that is, around 3,450 m.

Then compare the base with the ceiling of the area. In Gudauri the ceiling of the UGA8 paragliding area is 10,500 ft (3,200 m). So in this example it is not the cloud that limits you but the rule.