Weather for Fearful Flyers

Is simply the weight of the air (atmosphere) above the measuring point. As you go up the pressure goes down because there’s less air above you. That’s why we pump air into the cabin when we climb…so that the pressure is increased artificially. It’s as if you had that amount of air pressing down on you. Most plane cabins ‘pretend’ to be at about 8000 feet where there’s more than enough pressure to give you all the oxygen you need.
The temperature can change the pressure too because warm air is thinner than cold air. That’s why a hot air balloon goes up. It keeps going up until it cools and finds the same pressure around it …then it’ll start to come down again and as it cools, because the air in the balloon gets heavier. Not a technically perfect answer but near enough to explain pressure!
We all know from the weather broadcasts that we see that high pressure brings steady weather and low pressure brings changeable weather. So in summer, a high-pressure system will bring a long period of good weather but in the winter the weather will be gloomy for a period.

Although you can’t normally see any water in the air it is, in fact, present all the time. It’s in vapour form until…there’s so much of it that it can’t ‘hide’ in the air without becoming visible. That’s when we see mist and fog. Of course, if there is a great deal of moisture it often happens that the air can’t hold the vapour and all the tiny bits of water cling together and it falls as rain. When the temperature is very low it falls as hail, or snow if when the right amount of water vapour and the temperature appropriate
When the air is holding as much air as it can it’s called saturated and has a humidity of 100%. So when you see or hear that the humidity is so many % you know that’s how much water is in the air. Or do you? Surprisingly when the air is hot, it can hold more water than when the air is cool. There’s a great scientific explanation for this but …
Anyway, we know from our own experience that on a hot day when the humidity is high that it feels muggy … damp and clammy. 
 The nearer humidity is to 100% the more likely it is to condense out and appear as mist fog or rain. When it condenses it turns from vapour to water droplets like the condensation on bathroom tiles when you run a bath or take a shower. 

1. When the temperature increases the air holds more water
2. If the pressure is generally low then the weather is changeable
3. If the pressure is high the weather will be steady.
4. If the pressure is different in one place from another the air will move to equalise or ‘balance’ the pressure ( I know I haven’t said this so far but it’s true nevertheless and fits in well here)

Let’s start with the wind. As I have just said when air masses have different temperatures they probably have different pressures too. I explained that when a balloon goes up or down its pressure inside is different from the outside and it will ‘float’ or change height. This happens sideways too across the planet. If you think of the equator and how hot it is in that region, it’s easy to imagine that the air gets heated up too. This means its pressure will go down and like a hot air balloon a great bubble of air will eventually break away and float off up into the sky. Air will rush in from somewhere to fill the gap left by the departing bubble. Very roughly this air is drawn from the polar regions so there’s an enormous flow of air upwards from the equator being constantly replaced by the polar air which in turn is replaced by the air from the equator. These great masses of air don’t collide head-on because the heavy air from the poles stays near the ground (because it’s heavy) and the hot air from the equator stays high in the sky. So it’s an almost circular flow of air…almost.
But of course that’s not all the heating that happens on planet earth, landmasses like America heat up more than the Atlantic ocean so the same thing happens there…another high and Low-level exchange of air masses. And between Europe and the Atlantic. In fact, this is happening all over the world.
I want to take you shopping now. Imagine it’s a hot day, you’ve parked your car and taken a picnic in a park in town. When you sit on the grass you don’t feel that the ground is hot you just sit down and get on with your picnic. When you go back to your car you know that unless you’ve left something over the seats they will feel hot…you know instinctively that you wouldn’t put your frozen food on the roof of the car while you pack the car because the heat from the hot roof would start to warm the food. Not only is your car hot, and that’s noticeable, so is the car park and the buildings in town, in fact, the whole town has been heating up all the time the sun has been out. Away from the town, the fields have been warmed by the sun as well…but not as much as the solid objects in town…we know that’s true because the grass wasn’t hot when we sat on it for our picnic. I think you can see a bubble of hot air above the town! Sure enough, when it gets hot enough…compared with the surrounding fields up it goes off into the sky. Do you think the air replacing it is the cause of those sudden breezes you feel on a hot day?

As that air goes up some other air must go down. And that’s often the cause of those bumps you feel during turbulence as you climb away from or descend towards an airport.
Now we know enough to move on to thunderstorms. I said that the air cools down as it goes up…that’s true, but not always true. I said that the atmosphere cools down as you go up (remember the top of a mountain) that’s also true…but not always. So with these variables, it’s possible that as the air from a bubble goes up it might actually get warmer (relatively) to the air surrounding it. If it gets warmer it’ll keep on going up and then when all the moisture is released it gets cooler again but might still be warmer than the surrounding air. So on it goes upwards and upwards. And that will make it into a thundercloud. But I haven’t told you everything because as it develops its upward movement it sucks in the air underneath which will push the cloud up from underneath. That’s why thunderstorms around the equator can be so big. Lots of hot moist air being lifted into the sky.

Thunderstorms have a strict life cycle, in terms of developing and decaying. One of the things we pilots look out for when flying around a thunderstorm is the direction of the upper winds. We can tell by the shape of the cloud at its top which way the wind is blowing and we then steer around the storm from the windward side. In other words, if as we look at the storm we work out that the wind is blowing from our right to the left we’d steer to the right of the storm.
The rules say that we mustn’t fly closer than 20 nautical miles to the centre of a storm. Our weather radar shows the position of storms in relation to our route so it’s very easy to steer around them and still know where we are exactly in relation to our route (unlike the old days when we’d have to work it out all the time). In case you’re worrying Air Traffic Control allows aircraft to deviate to avoid ‘weather’ so there’s no problem in that respect. And aircraft flying the route earlier will have notified ATC so it doesn’t come as a surprise to them which means they can co-ordinate aircraft movements.

There seems to be a terrible fear of lightning striking an aircraft and wiping out all its instruments and systems or causing a fire on board. This will not happen. As I’m sure you have read elsewhere on this site an aircraft acts as a Faraday’s cage. Michael Faraday was a British scientist who, among other things, discovered that static electricity will not travel inside an object if the outside of that object is continuous, that is to say, something like an aircraft fuselage.
All that happens when a plane is struck by lightning is that there is a fairly loud bang and that’s all. I have been struck by lightning 13 times during my career and nothing at all has happened to the plane I was in. The instruments still work and to me the pilot, it’s a non-event.
But of course, the press has a field day. Their idea of a near disaster is very simplistic. Find a plane that has been struck by lightning ask a passenger if they were terrified that the plane would crash, then ask them how many people screamed and panicked then ask if they are glad to be back on the ground safely.

Ask the pilots the same question and they’d say “What? What are you talking about?”
But there’s always the photographic evidence of the near disaster …the dented nose cone and possibly leading to de-pressurization and catastrophe. In fact, the nose cone is not part of the main structure of the plane, it’s not pressurized and it’s made from thinner metal than the rest of the plane. The reason is that the weather radar is located here because it gets a clear and unobstructed view ahead. It’s also easier to send a radar beam thru’ a thin piece of metal than a thick piece. So that’s why the nose cone gets dented. It’s neither rocket science nor newsworthy and damage to it is unimportant.