Weather – Forage France

19 June 202121 February 2022

Quiet Lightning?

I was out at 3am having been woken up by the wind to see the most fantastic light show (see video) it was continuous lightning, no rain yet but i could tell it was coming! The difference this time was there were no rumbles of thunder at all. It got me thinking why, what might be the cause and does it tell us anything about the storm?

I know, from my time in music college, that sound attenuates (dissipates) over distance and travels at approximately 343 meters per second (varies with environmental factors). With that knowledge one can safely assume that i’m not hearing the rumble of thunder because I am are simply too far from the source. However light travels at 299 792 458 m / s without the same levels of attenuation so we will always see it.

People often count 1, 2, 3, 4 and so on from the flash of lightning until they hear the thunder and assume that it is that many miles or kilometers away. i’ve see people do it many times and they are always half right half wrong. With the knowledge that sound travels at 343m/s (we will round it down to 333m/s for easiness) for every 3 seconds from seeing lightning to hearing it you can safely assume it is 1 kilometer per 3 seconds.

Lightning 18/6/2021

Combine that knowledge of the distance calculation with the direction of the storm winds you can work out if it is coming towards you or away from you. you can also do this by observing the change in time from the flash to the sound. If the counting decreases the storm is coming towards you if it increases it is moving away from you.

The sound itself is also an indicator of direction and distance. A high pitched crack is going to be closer than a distant low pitched rumble. Think Doppler effect; a vehicle far away sounds low, gets higher pitched the closer it comes to you and lower pitched again as it travels away from you.

Ok, i’m not quite finished with the maths… It still leaves the question of how far do I have to be away from the lightning to see it but not hear it?

What is Distance Attenuation?

Distance attenuation is defined as the way in which a sound level reduces as a listener moves away from a sound source.

As sound travels from the source, the area over which the sound is spread increases like ripples on a pond. This means that the same amount of acoustic energy is dispersed over a greater area, and as a result the sound level at any position on this surface reduces as the distance from the source is increased.

Stay with me…

If the source is suspended in the air, as in our case, for simplicity, it has spherical distribution. (I know it is actually slightly more cylindrical in nature as the sound emanates from the length of the lightning bolt)

The area of a sphere it is A = 4πr²

The area quadruples with every doubling of distance and increases by a factor of 100 if the distance is increased by a factor of 10.

Acoustically, the sound level under these conditions reduces at a rate of 6dB (the measurement of sound pressure) for every doubling of distance from a source. However, after the original distance has been doubled, this increased distance then has to be doubled again (i.e. the original distance from the source has to be quadrupled in order to achieve a further 6dB reduction and then increased to 8 times the original distance for a further 6dB reduction). This also gives a 20dB reduction if the original separation distance is increased by a factor of 10.

If that is a head spin, just know that the sound pressure (dB) reduces by 6dB every time you double the distance, compounded.

Answer:

It’s not completely straightforward because there are many variable factors, are you at sea or in a forest? this will impact the calculation so we will base it on having no variables you are standing on flat surface with nothing between you and the lightning.

A lightning strike measures roughly 120dB close to the source and at 100m it measures 100dB. That means you are up to 25km away from the source of lightning.

In reality thunder is seldom heard beyond 16 km under ideal conditions.

So there you have it… If you can see it but can’t hear it, but can see it, it is at least 16km away!

1 March 202126 February 2022

Red Sky at Night, Fact or Fiction?

Sitting at my desk and watching the sunset It made me think of “red sky at night shepherds delight” which prompted the questions “Why do we say it? and “Is it true, can it predict the coming weather?”.

The First question “why?” is fairly straightforward to answer because throughout history memorable rhymes and sayings have been used to predict the weather based on human repeated experience. These sayings are more prevalent within occupations or communities that rely on knowing the weather to bring in the harvest or shelter from a storm at sea.

So Let us begin with question two “Is it true?”…

The History

“Red sky at night, shepherd’s delight. Red sky in the morning, shepherd’s warning” Is probably one of the most famous weather related sayings, and one most people would be familiar with, probably due to the very obvious identification of a red sky. The saying first appears in the Bible in the book of Matthew. (16 v2-3,) Jesus (apparently) said, “When in evening, ye say, it will be fair weather: For the sky is red. And in the morning, it will be foul weather today; for the sky is red and lowering”. Throughout the generations it has stuck and transformed into the various versions we know today.

In order to understand why the developed saying of “Red sky at night, (enter a profession here) delight. Red sky in morning, (enter a profession here) warning” can predict the weather, we must understand more about weather and the colors in the sky.

NB: Usually, weather moves from west to east. In the mid-latitudes, the prevailing winds are westerlies. This means storm systems generally move in from the West.

The Colour

The colors we see in the sky are due to the rays of sunlight being split into colors of the spectrum as they pass through the atmosphere and ricochet off the water vapor and particles in the atmosphere.

The amounts of water vapor and dust particles in the atmosphere are good indicators of weather conditions.

During sunrise and sunset the sun is low in the sky, and it transmits light through the thickest part of the atmosphere. A red sky suggests an atmosphere loaded with dust and moisture particles. We see the red, because red wavelengths (the longest in the color spectrum) are breaking through the atmosphere. The shorter wavelengths, such as blue, are scattered and broken up.

The Science

It would seem that the saying is most reliable when weather systems predominantly come from the west as they do in the UK and most of Europe. “Red sky at night” can often be proven true, since red sky at night means fair weather is generally headed towards you.

A red sky at sunset means high pressure is moving in from the west, so therefore the next day will usually be dry and pleasant.

“Red sky in the morning, shepherds warning” means a red sky appears due to the high-pressure weather system having already moved east meaning the good weather has passed, most likely making way for a wet and windy low-pressure system.

What is it going to be tonight?

26 May 202026 February 2022

The Scent of Rain

Petrichor is the name of that smell when the summer rains fall after a hot day. What’s it called? Why does it smell?

Petrichor is the earthy scent produced when rain falls on dry soil. The word is constructed from Greek petra, “rock”, or petros, “stone”, and īchōr, the fluid that supposedly flowed in the veins of the gods. I think “Petrichor is better sounding than “rocky god blood”.

What is it?

Well, it is slightly more complicated than might be first thought.

The smell is the aeration of a combination of an oil that is produced by plants during long periods of dry weather that then settles into clay based soils and rocks. When the rain falls it gets lifted into the air and combines with another compound that is a metabolic (breaking down of compounds) by-product of Actinobacteria (see section below) which is released as the soil becomes wet.

So in short it is a mixture of plant oils and a by-product of bacteria that build up during as dry spell give us that distinctive scent from summer rain that invokes so many positive thoughts and childhood memories.

According to research from MIT University heavy rain actually produces fewer aerosols than a light or medium rainfall, according to their findings. So that might be why petrichor is associated with the moments after a drizzle.

Actinobacteria exist as either dormant spores or actively growing mycelium with filaments called hyphae. Most of these free-living organisms look like strands of hair or spaghetti under a microscope. Some are spherical in shape, others are branched, and many have knobby or hair-like projections.

Enjoy,

Tom

How rain creates an aerosol effect

13 January 202027 February 2022

Stormy weather – how?

Whatever the weather we are not happy. Us Brits love moaning about the weather, too hot, too cold, too windy, too sunny, not sunny enough, haven’t had weather like this for ages, can’t wait for summer, this summer has been going on a bit long… you name it we bloody love moaning about it. On the Brink of storm Brian, Brin, Bertie or whatever significant storm is heading our way we ramp up the moaning and groaning, waiting, just waiting for the devastation after the storm, for it to never come, to only moan that the storm was not as powerful as predicted.

British pessimism aside, what are storms and why do we get them?

Names

Firstly, every year since 2015 the Met Office (Meteorological Office) releases the names of the storms for the coming year, the names of storms go from A-W. Leaving out Q, U, X, Y & Z because the U.S. Hurricane Centre finds it difficult to find names that start with those letters, so to keep consistency we followed suit.

This year in 2020 the names are Atiyah, Brendan, Ciara, Dennis, Ellen, Francis, Gerda, Hugh, Iris, Jan, Kitty, Liam, Maura, Noah, Olivia, Piet, Róisín, Samir, Tara, Vince & Willow

The more observant of you all will notice that the storms alternate boy and girl names, and interestingly the storms with girls names tend to cause more casualties. Scientists think that this is because female names are less threatening and people take less precautions ahead of and during the storm.

Not all storms get a name from the UK list, if the storm starts in another country like the U.S.A. then it keeps it’s name when it gets to the U.K. Not all storms get a name anyway, the only ones that do are thought to have the potential to cause significant damage. Usually when it is marked as ‘Amber’ be prepared, or ‘Red’ Take action.

You can take part in naming storms for the Met Office here.

Storms

A storm is created when low pressure forms within a system of high pressure – (low pressure spins clockwise & high pressure spins anticlockwise). Imagine this as wind. When the winds are travelling in opposite directions it can create clouds like cumulonimbus – see article on clouds – a storm cloud.

Winds

Wind is produced by the uneven temperature of the earth. Hot air rises as it rises the denser cold air rushes in to take its place. That air moving across the surface, we feel as wind. Wind is commonly measured in the Beaufort wind force scale of 0 – 12

Terminology for Wind & the Beaufort Scale

Calm – 0 – (1mph)
Light air – 1 – (1-3mph)
Breeze – 2,3,4,5 (4-24mph)
Squall – 6 (25-31mph gusts that last over 1 minute)
Gale – 7,8 & 9 (32-54mph)
Storm – 10 & 11 (55-72mph)
Hurricane – 12+ (73+mph)
Light air – 1 – (1-3mph)

Rain

Taken from – Weather predicting using clouds.

The tiny water droplets float around in the cloud and bump into each other sticking to each other, something we call cohesion. All this cohesion makes the water droplets larger and this means they are less able to fight the pull of gravity and when the pull is too much, they fall from the sky, creating rain. Rain drops can vary in mass from drizzle at 0.004mg and the largest during a storm at 300mg.

Lightning (and thunder)

Lightning is created when heavier (negatively) charged particles sink to the bottom of a cloud and the positively charged particles rise to the top. When there is a large build up of both it creates a spark, this is what we see as lightning and hear as thunder. We do not see and hear lightning at the same time because light travels much faster than sound.

Light travels at 299792458 meters per second and sound travels at 343 meters per second. If we round down sound to 333.3 meters per second that makes one second roughly 1Km away. Therefore if you count the seconds between seeing lightning and hearing it it will give you the approximate distance in Km – (seconds divided by 3). If you count multiple times and you are counting more seconds the storm is heading away from you, likewise the storm is getting closer if you count less seconds.

Mostly I hope you enjoy this years storms and have fun knowing a little more about the weather.

You can find out more from the Met Office.