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Understanding the Hartland ( Stoke ) AWS weather data

Barometric Pressure

Climatology

Dew Point Temperature

Humidity

Rain

Snowfall

Solar Energy

Solar Radiation

Sunshine

Temperature

Wind

Wind chill

Caveat: As explained on the "Site" page, this AWS cannot fully meet some World Meteorological Organisation ( WMO ) recommendations for placement of recording equipment. Therefore this Site's data cannot be regarded as giving an accurate representation of weather affecting the wider Hartland peninsula - just the weather at this Site. However the data may give a fair idea of what has been happening generally on the Hartland peninsula. It will give comparable site-specific information over time

Time spans :

Daily: begins / ends at midnight ... and "Today" starts and finishes at midnight as well

Monthly: begins / ends at midnight on the first of the month

Yearly: begins / ends at midnight on January 1st

Please note that data recording began at this AWS in mid October 2017, with Solar Radiation being recorded from mid April 2018


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Wind :

Wind Speed is reported here in knots ( kt ) ... as a rough guide, 1 kt is just over 1mph, 10 kt is 11 mph 20 kt is 23 mph , and 30 kt is 34 mph

This AWS measures wind speed every 2.5 seconds ; the highest "Hi Wind" value in a 5 minute period is recorded, as is the average "Current Wind Speed" over a 10 minute period.

A "gust" of wind is defined by the WMO as the maximum observed wind speed over a 3 second period over the observing cycle. Thus the "Hi Wind" recorded by this AWS is the fastest "gust" in a five minute period.


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Rain :

When comparing your experience of the day's rain with the figures you may read here, do bear in mind that some wet stuff is more wetting than other wet stuff !

One heavy shower in a day may be recorded as significantly more rainfall than a whole day of Hartland Drizzle yet, as locals will attest, drizzle, mist and low cloud can not only be grizzly but can make you much wetter than the odd shower .

So the rainfall readings may not reflect how wet you got nor how much you were able to do in the garden in comfort !

This AWS has a Tipping Bucket Rainguage (TBR) . Rain falling into a collecting funnel runs into the TBR mechanism ; in this AWS a small spoon on each end of a little see-saw tips when full generating an electronic recording of 0.2mm of rain with each event.

The software measures how many events (tippings) occur and records the time interval between each event.

The number of events gives the "Rain" figures - the total rainfall over a chosen period of time

The time interval between events enables calculation of the "Rain Rate" (or rain intensity)figures - how much rain would fall in an hour if this rate of tipping were to be maintained.

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Snowfall :

Snowfall is not measured by this AWS rain gauge. Snowfall depth may be measured manually by 'getting out there' with a ruler, though with drifting etc, finding an area of "representative average depth" can be next to impossible.

Weight and/or volume of snow falling, and snow fall rate, cannot be measured with any accuracy at all with this raingauge as, for example, snow blows across the top of the collector unpredictably, and melts erratically over perhaps many days.

In fact unless snow (or ice) blockage of the collecting funnel is cleared, subsequent rainfall measurement can be interfered with until all is melted.

The "snow to liquid equivalent" is the amount of liquid precipitation that is produced after melting snow. The "average" snow to liquid ratio is 10:1. This is saying that if 10 inches of snow fell and that snow was melted it would produce 1 inch of liquid precipitation in the rain gauge.

Wet, settled, snow has a high liquid content . A falling wet snow will often have large snowflakes and a lower number of snowflakes. The snow is sticky (due to its high partial liquid content)so making snowballs is easy. The snow to liquid equivalent ratio for wet snow will be less than 10:1. For example, a 5:1 ratio may occur in which it takes 5 inches of snow to produce 1 inch of liquid equivalent.

A dry snow has little to no liquid water content thus this snow will be less dense than average. Since dry snowflakes are less sticky they are less inclined to stick together as they fall, thus a dry snow will often be composed of a large number of small snowflakes. Dry snow is not sticky and thus it is difficult to make snowballs with it and the wind blows it around substantially even after reaching the surface. The ratio for dry snow will be greater than 10:1. In extreme cases it can be 30:1 or greater.

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Temperature :

This AWS measures "air temperature" , from a sensor within a standard non-aspirated radiation shield.

The "non-aspirated" bit means that the radiation shield around the temperature sensor relies upon wind and local convection to ventilate it ; there may be significant lag in temperature change at the sensor in light winds, and appreciable warming (perhaps a degree or two) in strong sunshine, particularly in light winds.

An aspirated shield would give a constant ventilation rate (5-10m /sec)to the temperature sensor but requires a constant power supply - all costing more !

Temperature is what makes us feel too cold, too hot, or just right. But it isn't just the temperature that determines how comfortable we are - the speed that air is flowing over our skin (Wind Chill) , and how wet that air is (Humidity) are very important as well.

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Wind Chill :

Wind chill takes into account how the speed of the wind affects our perception of the air temperature.

Our bodies warm the surrounding air molecules by transferring heat from the skin.

If there's no air movement, this insulating layer of warm air molecules stays next to the body and offers some protection from cooler air molecules.

However, wind sweeps that warm air surrounding the body away.

The faster the wind blows, the faster heat is carried away and the colder you feel.Wind has a warming effect at higher temperatures.

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Humidity :

Humidity itself simply refers to the amount of water vapour in the air. However, the total amount of water vapour that the air can contain varies with air temperature and pressure.

Relative Humidity takes into account these factors and offers a humidity reading which reflects the amount of water vapour in the air as a percentage of the amount the air is capable of holding. 100% relative humidity means that the air is totally saturated with water vapour and cannot hold any more, creating the possibility of rain.

This doesn't mean that the relative humidity must be 100% in order for it to rain -- it must be 100% where the clouds are forming, but the relative humidity near the ground could be much less.

Relative humidity is an important factor in determining the amount of evaporation from wet surfaces ; warm air with low humidity has a large capacity to absorb extra water vapour, whereas at low temperatures it requires less water vapour to attain high relative humidity.

For example, for your washing to dry on a clothes line, the moisture in your washing has to evaporate into the air, requiring the air to be to be less than 100% relative humidity (saturated). The lower the relative humidity, the faster washing will dry (though drying time is also affected by factors such as wind speed and heat energy from sunlight).

When this AWS uses the term humidity it means relative humidity.

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Dew Point Temperature is the temperature to which air must be cooled to become saturated with water vapour(100% relative humidity).

Dew point is a good indicator of the air's actual water vapour content (as opposed to relative humidity). High dew point indicates high vapour content and low dew point indicates low vapour content.

The dew point is always lower than (or equal to) the air temperature. Thus if air temperature falls to dew point temperature the airborne water vapour will condense to form liquid water (dew, fog, or cloud) . Similarly when air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface as dew or, if the Dew Point is below 0C, frost.

Dew Point Temperature as measured and reported generally refers to the air around the observer or measuring station; but dew point phenomena can apply at a much more local level - for example on a very hot day, air temperature can fall to the dew point temperature where the air contacts a very cold surface ... hence the water that condenses on the outside of a glass of iced lager!

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Relative Humidity, Dew Point Temperature, and Comfort
We are very sensitive to humidity, as the skin relies on the air to get rid of moisture. The process of sweating is your body's attempt to keep cool and maintain its current temperature.

If Relative Humidity is 100%, sweat will not evaporate into the air and as a result we feel the temperature to be much hotter than it actually is.

If the relative humidity is low, we can feel much cooler than the actual temperature because our sweat evaporates easily, cooling us off.

However this does not take into account the difference in temperature between our body and the air. Thus in cold air there is little need for our body to cool by evaporating sweat, so even fully saturated air does not feel unpleasantly humid. Contrariwise, when our body needs to cool in hot air even relatively low humidity levels (perhaps 50%) can impair sweat evaporation - thus making us feel uncomfortable.. "hot and sticky".

3 day Dew Point
Relative Humidity past 3 days
3 day Dew Point
Dew Point past 3 days
Relative Humidity on its own cannot give a good appreciation of how humid it will feel. Dew Point Temperature can give a much better indication of comfort . Although acclimatisation helps us to tolerate more humid air, generally in the UK a dew point temperature above 17C will begin to feel uncomfortable, whilst above 20C it will feel very uncomfortable for most people.

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Barometric Pressure :

We may say "as light as air" implying something has no weight - but air does have weight, as the 50km or so thickness of air - the atmosphere - between the ground and space is pulled towards the Earth by gravity. The column of air above a square metre of earths surface weighs, on average, 11 tonnes ! Barometric pressure is how one expresses this weight/pressure of the air/atmosphere on the surface - it is the "weight of the air".

The pressure reading can be expressed in several different units, including mmHg (mm of mercury, from the days of mercury barometers) and hPa (hectopascals - the preferred SI unit of pressure). For this station, it is expressed in mb (millibars - a familiar standard unit of pressure in meteorology use since 1914 , and numerically identical to hPa)

The "normal" barometric pressure at sea level would be 1013.25 mbar - but as the thickness, weight and characteristics of the atmosphere above any spot on the surface is changing all the time the measured barometric pressure varies either side of this.The highest in Britain was 1054.7mb (31.15 inHg), in Aberdeen on 31 January 1902. and the lowest was 925.4 mb (27.33 inHg) in Ochtertyre in Perthshire on 26 January 1884.

As the weight of air/atmosphere at the surface of the Earth seeks balance to be the same everywhere, air in high pressure areas moves down and outwards towards low pressure areas, changing the barometric pressure that we measure.

Generally, we experience areas of high pressure (high barometer readings) as being calm and settled, dry, and often with clear and sunny skies. In areas of low pressure (low barometer readings), we experience weather that is changeable, often wet and windy , frequently cloudy, and occasionally stormy. The higher - or lower- the reading the more extreme these features of high and low pressure will be.

Barometric pressure changes continually, although not often very rapidly. As an indicator of short-term changes in the weather, a graphical display of barometer recordings shows not only what the barometric pressure has been over time, but illustrates how rapidly that pressure has changed, and in which direction : the steeper the line and the greater the change in pressure the more rapidly the weather is changing, and whether for better (higher pressure) or worse (lower pressure).

past day Barometer
Barometric Pressure past 24 hrs
As atmospheric pressure falls rapidly with altitude, barometric pressure readings are often adjusted to a sea-level value to take account of the station's height above sea level. This station is 90 metres above mean sea level (AMSL) and all atmospheric pressure readings ("weight of the air" above the station) measured here are adjusted to show as Barometric Pressure at Mean Sea Level .

The barometric pressure shown on this website is for Hartland only. If the barometric pressure is taken from many sites at the same time, a map of surface pressures over a wide area can be drawn - such as at this Met Office page (opens in a new window).

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Solar Radiation,   Solar Energy,  and   Sunshine :

Solar Radiation

What this AWS calls "current solar radiation" is technically known as 'Global Solar Radiation', a measure of the intensity of the suns radiation reaching a horizontal surface. This irradiance includes both the direct component from the sun and the reflected component from the rest of the sky.

The solar radiation reading gives a measure of the amount of solar radiation hitting the solar radiation sensor at any given time, expressed in Watts per square meter (W/m2). The value logged by WeatherLink is the average solar radiation measured over the archive interval (currently set as 30 minutes).

'Hi Solar Radiation' is the peak solar radiation measured during the archive interval (currently set as 30 minutes). In general terms, on a 'clear blue sky' day, maximum possible solar radiation varies both through the day and through the year . It will be at its highest when the sun is highest in the sky ... in other words each day at noon (Greenwich Mean Time - GMT) , and each year at noon on Midsummer Day.

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Solar Energy

The amount of accumulated solar radiation energy over a period of time is measured in Langleys (Ly)
1 Langley (Ly) =
    11.622 Watt-hours per square meter
     3.687 BTUs per square foot
    41.84 kilojoules per square meter

The daily solar energy total will be greater on sunnier days than overcast days, greater on longer (summer) than shorter (winter) days.

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Sunshine

This solar radiation sensor is thus, roughly speaking, a "brightness recorder" and not a "sunshine recorder", where sunshine is generally taken to mean a clear enough sun to cast shadows.

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So , looking at the graphs, can I see what sort of a day it was?... was it a nice, sunshining day?
Here are solar radiation graphs from four consecutive days in mid April, to the same scale, with comments below about how it looked on the day.

 
 
largely blue, some cloud

A nice sunny day , blue sky with passing cumulus clouds
good start and finish

After a slightly hazy start, blue sky morning and evening, but significant cloud cover in rest of the day
middling day

A bright blue start, but clouding as the morning progressed. Then a brief sunny spell, and a slow thinning of the cloud which never fully cleared, and getting a little hazy in the evening
no sun

A terrible day. No sign of the sun, much rain, low cloud and mist all day
 

Climatology :

For this Automatic Weather Station's monthly Climatological reports click here

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 For some "learn about the weather" information visit the Met Office by clicking the Met Office link at the bottom of this page



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