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1. What is an observation?
1.What is an observation?
Most of us have observed the weather at some time or another, even if it is only when we become soaked whilst waiting in the rain for a bus! Some of the most beautiful and spectacular shows on earth are products of the weather, such as a summer thunderstorm or a winter snowstorm. However, in spite of its beauty, the weather can occasionally turn into a frightening killer. Each year tornadoes cause great destruction in parts of the USA, whilst during the hurricane season, areas such as the Bay of Bengal and the Caribbean are particularly vulnerable. Even in the UK, occasional high winds or flooding can cause damage and loss of life.
In order to make accurate weather forecasts it is essential that we build up a picture of what is actually happening in the atmosphere. A weather observation is defined as “a record of the measurement or assessment of one or more meteorological elements - e.g. temperature, pressure, cloud type & amount - at a particular time and place” (Met. Glossary 6th ed. p.203). For observations to be of real value, it is important that, as far as possible, the type and siting of instruments, as well as methods of observing and reporting are standardised, in order to meaningfully compare data between different places and from day to day.
There are several different types of weather observations, each fulfilling a specific function and supplying different information to customers. Observations compiled at airfields for the movement and safety of aircraft are known as METARS (Meteorological Aerodrome Reports). Of crucial importance is the accurate measurement of elements such as pressure, cloud type, amount & height and wind information.
Another type of observation is the measurement of climatological elements for the UK climatological data bank. This includes maximum & minimum temperatures, rainfall, snow depth and sunshine. The figures recorded by Met. Office sites are supplemented by a network of some 500 amateur co-operating stations who take readings at 0900 UTC each morning. The data base is used for maintaining the climate records of the UK and in monitoring of long term climatic change.
The most well known use of weather observations is for the compilation of weather forecasts. There have been great technological advances in weather forecasting techniques over the past few decades, especially in the field of computer generated forecasts. Nevertheless the “bread and butter” of all forecasters and computers is still regular, accurate observations, whether manually completed by a human observer or recorded by increasing numbers of automatic weather stations.
Readers should also be aware that it is necessary for measurements of weather in the upper atmosphere to be made, as well as at the earth’s surface, because it is high level winds at around 30-40,000 feet that drive our weather systems. A network of approximately 10 radio-sonde stations around the UK take measurements of wind, temperature and pressure, 4 times a day up to heights of about 60,000 feet or more. Weather satellites are also a vital source of upper air wind and temperature data as well as providing useful cloud imagery.
The following sections will examine the main individual elements of the surface observation and describe how each one is measured or assessed. It is worth stressing at this point that you do not have to possess sophisticated equipment to make weather observations. An enjoyable hobby for many people is to make their own equipment from everyday household items, compare their readings to the “professionals” and also keep a daily weather diary.
The term wind refers to the horizontal motion of air throughout the earth’s atmosphere and is caused by differences in air pressure between places. Generally speaking, the strength of the wind depends on the size of the pressure difference over a given distance - the greater the difference, the stronger the likely wind.
The speed of wind is normally measured in knots - nautical miles per hour, where a nautical mile is one minute (one sixtieth of a degree) of latitude at the earth’s surface. Its name comes from the old method of measuring a ship’s speed through the water: A float, shaped to offer resistance to towage and attached to a long line knotted at equal intervals, was thrown into the water. The number of knots passing freely through the hand over a period of time as the line spun out, indicated the speed of the ship. 10 knots is approximately 11.5 miles per hour
The instrument most commonly used to measure wind speed is the cup anemometer. It consists of 3 hollow cups mounted on a vertical spindle which rotates at a rate proportional to the wind speed (see Figure 1).
Figure 1 The Cup Anemometer and Wind Vane for measuring wind
Whenever we refer to wind direction we always mean the direction from which the wind is blowing. For instance if the wind is described as ‘westerly’, it means that the wind is blowing from the west. The ideal way of measuring wind direction is by the movement of a wind vane. Wind vanes have been known to exist since Greek and Roman times. For instance, in the first century BC, Marcus Varro, writer on agricultural matters, had one on his roof connected to a dial on the ceiling of the room below.
A wind vane is designed so that the tail fin is downwind of the actual wind direction. In other words, the arrow head will point towards the direction from which the wind is blowing (see Figure 1).
In order to meaningfully compare wind readings from different locations the cup anemometer and wind vane should be sited at a standard 10 metres above the earth’s surface and well away from obstructions (buildings, trees etc.). This helps to overcome the worst effects of turbulence and eddying caused by friction between the moving air and underlying surfaces.
Gale A surface wind of mean speed 34 knots or more, over a period of 10 minutes.
Hurricane A tropical storm with mean surface wind speeds of 64 knots or more, over a period of 10 minutes.
Squall A strong wind that rises suddenly, lasts for at least a minute and then dies away comparatively suddenly. It is distinguished from a gust by its longer duration.
Gust A rapid increase in the strength of the wind relative to the mean strength at the time. Much shorter in duration than a squall.
Veering Clockwise change in wind direction. e.g. from SW to NW through west.
Backing Anti-clockwise change in wind direction. e.g. from NW to SW through west.
Buys Ballot’s Law In the northern hemisphere, if an observer stands with their back to the wind, the pressure is lower to their left than to their right.
The highest gust ever recorded was 200 knots (231 mph) on 12 April 1934 at the observatory, Mt. Washington summit (1910 m.), New Hampshire, USA. In the UK the highest recorded gust was 150 knots (173 mph) from the automatic weather station on the summit of Cairngorm (1245 m.), Grampian, on 20 March 1986. The most well known recent storm to hit south-east Britain occurred in the early hours of 16 October 1987. Gusts exceeding 80 knots (92 mph) were widely reported from Norfolk to Dorset, causing 18 deaths and extensive damage, with a gust of 106 knots (122 mph) at Gorleston (Suffolk).
Temperature is normally measured in degrees & tenths Celsius, where 0.0°C is the freezing point and 100.0°C the boiling point of water (at a standard pressure). However, some countries still use the Fahrenheit scale.
A common method of measuring temperature is to use mercury or alcohol-in-glass thermometers, where the liquid expands & contracts proportionally to changes in temperature. However, at an increasing number of stations, especially where observations are carried out at regular intervals throughout the day, the temperature is obtained from an electrical resistance thermometer. It consists of a coil of platinum wire encased in a ceramic insulator and contained in a stainless steel sheath. The electrical resistance of the platinum wire, being proportional to the temperature, is measured and shown on a digital display.
Thermometers are housed in a Stevenson screen. Designed by Thomas Stevenson, the father of author Robert Louis Stevenson, it consists of a white, slatted, wooden box, supported on a stand 1.1 metres above a grass surface. This protects the thermometers from the sun, precipitation and excess wind, whilst allowing the free circulation of air around the instruments (see Figure 2).
Figure 2: The Stevenson Screen for measuring temperature
Four temperatures are normally read in the Screen:
Dry bulb: This is the current air temperature in the shade.
Wet bulb: This is achieved by wrapping the base of the thermometer in muslin, which is kept continually moist by dipping the ends in a container of purified water, hence its name. As the water on the muslin evaporates into the surrounding air, the air immediately around the bulb is cooled. A comparison of the dry-bulb and wet-bulb temperatures indicates the humidity of the air - the drier the air the greater the rate of evaporation and therefore the larger the difference between the 2 temperatures.
Another temperature which is not directly measured, but derived from the dry-bulb and wet-bulb, is the dew-point. This is the temperature at which, if the air is cooled, it will become saturated. Once the air becomes saturated, the invisible water vapour begins to condense into visible water droplets. The dew-point is extremely useful to a forecaster as an indicator of different air masses and in fog, cloud base prediction etc.
Maximum & Minimum temperatures are recorded at 0900 (& often 2100) for inclusion in the UK climatological data base. Certain stations also take readings at 1700 for inclusion in the national newspapers.
The worlds highest shade temperature of 57.8°C (136°F), was recorded at Ar Aziziyah, Libya on 13 September 1922. The maximum UK record is 37.1°C (99°F) on 3 August 1990 at Cheltenham, Gloucs. 1995 was an exceptionally warm recent British summer. At the Met. Office College, Reading, Berkshire, the maximum temperature exceeded 25°C (77°F) on 38 days between May and August, with 9 of those days exceeding 30°C (86°F). (Average number of days per year in south-east England exceeding 25°C is around 12).
The coldest place on earth is Antarctica; on the 21 July 1983, the temperature registered -89.2°C (-129°F) at Vostok. The lowest recorded temperature in the British Isles is -27.2°C (-17°F), at Braemar, Grampian, on 10 January 1982. A recent very cold spell occurred in Scotland during the last week of December 1995: -20°C was recorded somewhere across the country for 4 consecutive nights, culminating in a figure of -27.0°C at Altnaharra, Highland, on the 30th, a new December low for the UK.
Atmospheric pressure is the weight of air above any given point on the Earth’s surface. There are several different units used, but the standard UK unit is the millibar [mb] (also known as the hectopascal [hpa] )
Pressure is measured using a barometer. The mercury barometer, consists of a column of mercury which rises and falls due to changes in pressure. The length of the column is measured and the pressure obtained after making an allowance for the temperature of the instrument.
The aneroid ( “without liquid”) barometer consists of a sealed metal chamber with most of the air removed. Any increase (decrease) of pressure causes contraction (expansion) of this container and this is magnified by a system of levers connected to a pointer and a scale. A more recent development is the precision aneroidbarometer (PAB); the movement of several aneroid capsules is accurately measured using a knife point electrical contact and the resulting pressure value read digitally.
Figure 3: The Precision Aneroid Barometer for measuring pressure
The mercury barometer, although very accurate, is fragile and cumbersome and has been replaced at observing stations by the PAB. At sites which use automatic observing equipment, pressure is measured using the vibrating cylinder pressure transducer. A small cylinder is caused to vibrate by an electric coil around it. The rate of oscillation (which varies according to the outside temperature and pressure) is measured by an attached micro-processor and the air pressure calculated from it.
The lowest pressure ever recorded was 870 mb in the centre of Typhoon Tip, Pacific Ocean on 12 October 1979. Around the shores of the UK pressure generally remains between 950 & 1050 mb. However, on 10 January 1993, the centre of a deep depression between Scotland and Iceland was estimated to be 912 mb, a record for the North Atlantic. By contrast, the highest pressure ever recorded was 1083.8 mb at Agata, Siberia on 31 December 1968.
Meteorological visibility is defined as the greatest distance at which an object can be seen and recognised in daylight.
We estimate visibility by choosing a number of suitable objects in as many different directions as possible, varying in distance from as little as 20 metres up to the furthest horizon. If one of your visibility objects is just recognisable for what it is known to be, then its distance, in metres or kilometres, is the visibility. If one object is clearly visible but the next furthest one cannot be seen or is very indistinct, then the actual visibility lies somewhere between the two. If the visibility is greater than the most distant object, then it can be estimated by noting the clarity with which it stands out.
Increasing numbers of automatic visibility sensors are being installed at observing sites. The most common type in the UK is called the Forward Scatter Visiometer. It is based on the principle that the more particles (water, smoke etc.) present in the atmosphere, the poorer the resulting visibility. Light is transmitted from a projector and scattered by the atmospheric particles. A receiver records the amount of scattering and then calculates the visibility (see Figure 4).
Figure 4: Forward Scatter Visiometer for measuring visibility
It should be remembered that automatic visibility sensors can only measure the visibility between the projector and receiver and this is not necessarily a representative value. Observers are required to make a manual check of the visibility in all directions and then report the lowest visibility observed.
Clouds form when the invisible water vapour in the air condenses into visible water droplets or ice crystals. Therefore, a cloud may be defined as “a visible aggregate of minute droplets of water or particles of ice or a mixture of both floating in the free air”. For each individual cloud layer we need to know 3 pieces of information; the amount, the type of cloud and its height above station level.
The amount of a cloud layer is measured or estimated in oktas (eighths) covering the sky. For instance, a cloud sheet covering three quarters of the sky would be coded as 6 oktas.
In 1803, Luke Howard (retail chemist & amateur meteorologist) proposed a system which became the basis of the present international classification. He recognised four types of cloud and gave them Latin names as follows:
• Cumulus a heap or pile • Stratus a sheet or layer • Cirrus a filament or thread • Nimbus a rain bearer
If we include another Latin word altum meaning height, the names of the 10 main cloud types are all derived from these 5 words and based upon their appearance or characteristics. The cloud types are split into 3 groups according to the height of their base above mean sea level. Note that “medium” level clouds are prefixed by the word alto and “high” clouds by the word cirro (see Table 1). All heights given are approximate for mid-latitudes; figures will vary from one climatic region to another. If observing from a hill top or mountain site, the range of bases will accordingly be lower.
Low clouds Surface - 7000 ft
Medium clouds 7000 - 17000 ft
High clouds 17000 - 35000 ft.
Table 1: The 10 main cloud types
The height of the base of each cloud layer above station level must be obtained. In the UK these heights are measured or estimated in feet. An increasing number of stations are equipped with a laser ceilometer for measuring cloud bases up to 24,000 feet. It detects clouds by transmitting pulses of near-infrared light vertically into the atmosphere. The Receiver Telescope detects, processes and analyses the returned signals scattered by clouds and precipitation. Elapsed time between transmission of a light pulse and backscattered energy received is used to establish the distance to the reflecting object.
The collected data is temporarily stored, then sent via a modem to the Primary Display Unit (see Figure 5). The resulting cloud base can then be viewed on a monitor and printed out on a colour chart printer if required.
Figure 5: The Laser Ceilometer for measuring cloud base
T he laser ceilometer can also be used for calculating cloud amounts. Some automatic stations are now equipped with a ceilometer and the derived cloud bases and amounts are included in their weather reports.
Observers are required to report the weather occurring at the time of the observation, as well as what has occurred in the recent past (between 1 and 6 hours, depending on the time of the observation). The most common types of weather experienced in the UK are listed below in 2 groups - precipitation and non-precipitation types.
Rain/Drizzle Water droplets which are defined as being rain or drizzle according to the diameter of the droplets and NOT the intensity of the precipitation (i.e. rain>0.5 mm, drizzle<0.5 mm). Rain droplets, being larger tend to fall more quickly and create rings upon impact with puddles etc. If either freeze on impact with a surface it is known as freezingrain / drizzle, or glaze, commonly called black ice.
Showers Determined by the type of cloud the precipitation falls from, NOT its intensity or duration. Showers may be in the form of rain, snow or hail.
Snow Ice crystals, often showing a six-rayed starlike structure. However, when the temperature is near to freezing, such crystals will often stick together forming large snowflakes, thus losing their individual beauty.
Sleet A mixture of rain or drizzle and snow or snow melting as it falls. However in the USA it refers to ice pellets, therefore the term is not used by meteorologists in order to avoid confusion.
Hail Balls of ice, diameter ranging from 5 mm to 50 mm or more. Although usually spherical, large stones can be quite irregular in shape, especially if several stones have agglomerated into a lump. Large hailstones cause considerable damage to property and crops. Small, soft hail is sometimes called snow pellets.
Snow grains The frozen equivalent of drizzle. Tiny particles of ice resembling grass seed or grains of rice which occur only in small quantities.
Ice pellets similar in appearance to small hail, it falls from layered cloud, whereas hail always falls from heaped cloud.
Mist/Fog A reduction in the visibility caused by tiny water droplets suspended in the air. If the visibility is reduced to less than 1000 metres, it is termed fog, if 1000 metres or more, mist. In cold weather, the fog droplets may freeze on impact with exposed surfaces, with the resultant build up of ice known as rime.
Haze A reduction in the visibility caused by tiny, invisible, dry particles suspended in the air. The term smoke may be used when the visibility is reduced by combustion particles e.g. pollution, forest fires.
Blowing/drifting snow Previously fallen snow that is lifted by strong winds and blown around. If the snow is raised to less than 2 metres high, it is termed drifting, if greater than 2 metres, blowing.
Thunderstorm One or more sudden electrical discharges, manifested by a flash of light (lightning) and a rumbling sound (thunder). For observing purposes, a thunderstorm is said to be occurring at the station if thunder is heard. If distant lightning only is seen but no thunder heard, that does not constitute a thunderstorm. It may or may not be accompanied by precipitation.
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