13 Jan Reading the Weather and Understanding Weather Fronts
Reading the Weather
Understanding Weather Fronts
After this reading the weather and understanding weather fronts workshop why not check out our upcoming online workshops and guided walks at SummitandBeyond.co.uk
Where to Find the Weather Forecast
There are various apps and forecast providers out there but in this workshop we are focusing on the weather in the UK upland areas. With that in mind we will be using the following two websites
You can download the Met Office app. Met Office weather is where the majority of the forecasts take their information from.
There are also alternative sites worth having a look at.
‘Magic Seaweed’ is dedicated to coastal weather and surf.
‘yr.no’ is a Norwegian site which shows the air pressure, air flow, temperature and precipitation in animation.
‘XC Weather’ gives an accurate localized forecast.
If you want to get an extremely good idea of what the weather will be like on a given day, it is important to keep your research within a 3 day window. By cross referencing the above sites you are taking information from the wider more advanced European weather centers (yr.no), you are considering what the weather looks like coming in off the coast (magic seaweed), you are considering how the mountains will manipulate that weather (MWIS), you are taking into consideration the localized weather predictions (XC Weather) and reading the synoptic chart will provide a wider scope of information about the fronts causing the conditions you will be experiencing.
The more you study these sites the easier it will be to condense the amount of research you need to do. Over time you will learn to trust two or three of these sites and develop a slick process which is effective when planning for weather conditions.
The Synoptic Chart
Symbols and their meanings
All our weather is determined by air flow. ‘Jet steams’ are currents of air, a lot like a rivers, flowing way above the earths surface and although they meander, the flow west to east. When you take a long flight on an air liner the plane ascends right up into the jet stream where the air flows fast helping to carry the plane along like a conveyor belt.
The jet stream also pushes warm and cold air around the globe. Think about summer time in the UK. We generally experience a lot of rain and this is because the jet stream carries warm air up from the Atlantic. The warm air is known as ‘Maritime air’ it brings low pressure and warm fronts.
In the winter of 2018 the UK experienced the ‘Beast from the East’. This was due to the ‘Polar air flow’ pushing further south and carrying cold air from the freezing north right over us. Most years we experience the polar air around November time when it becomes crisp and icy.
It feels cold in UK winter time because the earth axis is tilted meaning we see a lot less sun. Our winters are mostly wild and windy or mild and wet. This is due to the Maritime and Polar air flow fighting for dominance. The cold and warm jet streams fighting for our air space creates high winds and unpredictable weather. Welcome to the UK!
The image below symbolizes ‘low pressure’. The circular lines are called ‘isobars’ and show the air flow. If you see an ‘L’ or ‘low’ it means the air flow, or isobars, are rotating anticlockwise around the center circle or the eye of the pressure. This anti clock wise. movement occurs because of the rotation and shape of the earth, which we will cover in more depth below.
This is the part of the pressure chart (synoptic chart) that confuses most people. When people read ‘low’ most presume this means low to the earths surface. In actual fact it means low in pressure, light warm air which rises.
The contours on a map show the shape of the land, the high points, the low points and the twists and turns of the valleys. Like this image below
The isobars on the pressure chart work exactly the same but they also show the direction the air is rotating.
In this example of low pressure the center circle, with the ‘L’ or ‘Low’, resembles the peak of the air, because low pressure is created by warm light air which rises up.
Below is an example of high pressure.
High pressure means pressure is pushing down towards the earths surface. High pressure is brought along with cold air. The heavy dense cold air pushes all the moist warm air out and leaves a clear blue sky. High pressure air moves in a clockwise direction.
Why do High and Low Pressures move in opposite directions?
The short answer is that low pressure is twisting inwards on its self and high pressure pushes outward. The push of the cold air and high pressure forces the warm light air of low pressure up and away. The cold and warm air, or the high and low pressures are constantly twisting and forcing each other like fluid cogs keeping the machine (life) moving along.
The long answer is in the northern hemisphere low pressure systems move in an anticlockwise direction and high pressure systems in a clockwise direction. However, in the southern hemisphere low pressure systems move in an clockwise direction and high pressure in an anticlockwise direction.
The equator has further to travel, takes the brunt of the force of the earths rotation through space and moves quicker than the poles which have less distance to travel, move slower and experience less pressure.
As the equator is taking the full force of the earths rotation and pressures are pushed from the equator north, and from the equator south. In other words the equator acts like the line of most resistance deflecting pressures away in opposite directions.
Once the air flow has been deflected away from the equator the pressure systems form and begin twisting respective to the hemisphere they move through.
Our perception of air flow around the globe is described as the Coriolis effect. The Coriolis effect is where the movement of one object makes the movement of another appear to bend or twist. The most common example is if you are standing on a playground roundabout spinning to the left and you throw a ball, the ball appears to curve away to the right. Although, anyone watching will see the ball travel in a straight line off the roundabout.
Take a rainbow or the sun set for example. These are refractions, what we are seeing is a combination of light, air pressure and moisture forming a sight which is not actually there. Nevertheless, we do not question seeing the rainbow or sun set, we ask how it was formed and why it looks the way it does? These questions lead us to an answer which is consistent with every single rainbow or sun set we see.
The same goes for air flow and pressure systems around the globe. With todays technology we can observe air flow and recognize the consistent trends which allows us to accept, and take into consideration the Coriolis effect, or our perception vs actual forces and pressures.
This short video by met office explains the Coriolis effect using a practical example click here to watch the video
Here are some of the more common weather fronts you will see in the synoptic chart and experience here in the UK. There is also a description of the clouds you will see during each weather front however to keep it easy to understand I have only labelled the clouds you are most likely to recognize as the front builds.
The image below symbolizes a ‘Warm Front’
Imagine boiling a kettle. As the air above the spout of the kettle begins to warm up, the air pressure lowers and it rises up then reacts with the cooler air above creating a cloud. If you hold a cold plate, or flat cool surface, one foot above the cloud rising from the kettle, the warm air rising reacts with the cool surface and creates precipitation.
The steam rising from the kettle resembles how a warm front works. The air becomes warm, light and lifts up high into the earths atmosphere where it meets cool air. The cold icy airway above the earth acts like the plate or cold surface we mentioned before. As the warm light air mixes with the cold air above it begins to rain.
The clouds forming a warm front
First we will see the high cirrus clouds
Cirrus clouds are formed by ice high up in the sky.
…….then we will see Cumulous clouds
Cumulous clouds are formed by convection. This is the warm moist air beginning to accumulate in the surrounding cooler air. Thinking about a warm front, this process continues until we have a stratus cloud as in the image below.
Stratus are those dark grey clouds that blanket the sky. As the warm air continues to build and the clouds grow high and heavy nimbostratus clouds form and the rain begins to fall.
An image of a nimbostratus cloud
The image below shows a diagram of how a warm front builds. You will also see that their are different stratus clouds named in the diagram, Cirrostratus and Altostratus. Cirrostratus meaning a high stratus cloud and altostratus meaning lower stratus clouds. I haven’t provided images of cirrostratus or altostratus because it is often difficult to see the difference as stratus clouds blanket the sky.
The image below resembles a ‘Cold Front’
Remember this saying ‘cold air is Queen’.
Cold air is heavy and dominates the sky above the earth. If you have been on a long flight you will have seen the sky above 30,000ft where the cold air pushes any warm air down toward the earth surface. You are above the earth in clear blue sky and any clouds are formed below. This is because the cold air above the earth is too dense for warm air to penetrate it.
When the cold air reaches down to the earths surface it pushes the warm air out of the way. Cold air moves faster and dominates over warm air. As the cold air catches and pushes against the back end of a warm front this can generate a huge amount of energy and we experience thunder and lightening. After thunder and lightning the sky will clear as the high pressure of the cold air surrounds us.
Cold Front Clouds
As seen in the diagram above, the cold air drives into the warm air mixing ice particles with the warm wet particles from the nimbostratus cloud making up the rear of the warm front. This mixture of cold and warm forms a Cumulonimbus cloud. As cumulonimbus clouds tend to grow quickly and look like giants in the sky.
This towering mix of cold and warm air brings thunder, as the warm and cold particles rub together. The rumble of thunder is energy bursting out as electrical surged lightning strikes rip towards the earth.
The image below symbolizes an ‘occluded front’.
An occluded front occurs when we have a cold front followed by a warm front followed by a cold front. In other words, a warm front sandwich.
The high pressure of the rear cold front pushes the warm air forward into the cold air in front. The warm air has no where to go but straight up. This traps the warm air in suspension between the cold air above and at ether side. Yes, that’s right! It will rain.
Clouds of an Occluded front
Because an occlusion is a collection of warm air trapped above the clouds appear like stratus, blanketing the sky usually with persistent rain.
The image below symbolizes a Stationary Front
A stationary front is created when neither front has the energy to dominate and push the other forward. This front will hang around until another front with a good amount of energy moves in and breaks up the stationary front.
A stationary front will bring clouds and rain. When we have a stationary front hanging over us it feels dull because of the dark sky and the lack of progression with the air flow. When you notice a stationary front occurring on the synoptic chart a good game to play is predicting which future front will push it out of the way.
Clouds of a stationary front
Like an occlusion, the static front is suspended over us. Static fronts can bring a variation of cloud formations. It could appear over cast like a covering of stratus along with rain or there could be clear sky in parts. It all depends where you are located under the front.
Dry Line Front
The picture below symbolizes a Dry Line
A dry line is warm dry air meeting warm moist air, or a warm front. As mentioned previously cold air is dense and warm air light, this means that the majority of the time cold air will dominate the earths surface pushing warm air up and out of the way. Nevertheless, there are times when warm dry air meets a warm front. The warm dry air is impenetrable and acts like a barrier to the warm moist air of the warm front. The meeting of both warm dry air and warm moist air develops a ‘dry line’.
Due to the dry air been more dense, the warm moist air can no longer flow. Unlike the development of a warm front where the light, warm moist air would be pushed up by dense cold air, the dense dry air simply drives against the light, warm moist air interrupting the process of the warm front and creating unstable and stormy weather.
Clouds of a dry line
If you look back to the cloud formation of a warm front and then imagine a force driving into those clouds and pushing them back up against one another you will have a good idea what is happening during a dry line. There is a huge crash of dense clear dry air into light warm moist air, and clouds of a warm front, and then as the warm moist air reacts with both the warm dry air and the cold air above thunder and lightning occurs.