Knowledge of temperature, humidity and wind at all levels of the atmosphere from the surface to the stratosphere is essential for understanding the structure of weather systems and predicting how they will develop in the hours and days ahead. Upper air observations from a large number of locations are required to provide the initial conditions for the numerical models that forecast the weather. The Met Office makes use of routine upper air observations from three different types of system:
- Radiosonde - using a helium-filled balloon to carry a small package of sensors up through the atmosphere;
- Commercial Aircraft - capturing meteorological data from the aircraft's navigation system;
- Profilers - using upward looking radars to measure wind.
A radiosonde is a small expendable package of instruments which is carried aloft into the atmosphere below a helium-filled balloon. As the radiosonde ascends, measurements of temperature and relative humidity are made at 2 second intervals. A GPS receiver in the package allows its location to be identified from which the pressure, wind speed and direction can be calculated. Data are transmitted back to a ground receiver for encoding and onward distribution. The balloon typically ascends at 5 m/s to reach a height of about 25 km. As pressure decreases with height, the balloon expands until it bursts at which point the instrument package returns to earth borne by a parachute. Where measurements are required at very high levels in the atmosphere, larger balloons are used capable of regularly reaching 35 km. The Met Office currently uses Vaisala RS92 radiosondes, typical of those used by many other national meteorological services around the world.
There are six radiosonde stations within the United Kingdom that regularly launch radiosondes. The two manned stations at Camborne (Cornwall) and Lerwick (Shetland Islands) use larger balloons that reach the higher altitudes required for climate monitoring. Balloons are launched at 1115 UTC and 2315 UTC each day from these stations. A package for measuring ozone concentrations is launched once a week from Lerwick.
Automatic radiosonde launch over land
There are four automatic stations at Albemarle (Northumberland), Watnall (Nottinghamshire), Castor Bay (Northern Ireland) and Herstmonceux (East Sussex). Radiosondes attached to a small balloon may be launched from this system without the need for a person to be present. Launches are made routinely at 2315 UTC, but can be requested at other times by forecasters: typically 0515, 1115 or 1715 UTC.
The Met Office also supports radiosonde activities on several military, third party and overseas sites.
Radiosondes over the oceans
The six stations in the United Kingdom are part of a much larger network of about 600 land stations worldwide where regular measurements of the upper atmosphere are made by radiosondes. In an attempt to fill some of the data voids over the vast expanse of the oceans, there is an international programme, known as ASAP (Automated Shipboard Aerological Programme), that uses merchant ships to extend the network. The ASAP system is essentially an automatic balloon launching system adapted for use at sea. European countries are very active supporting these systems. There are currently 18 ships carrying ASAP systems each making two to three ascents per day while at sea. They mainly operate in the North Atlantic and Mediterranean, areas where improved coverage of upper air data is particularly beneficial for forecasts in the European area.
It is complex and expensive to meet all the requirements of modern forecasting systems for upper air observations. Aircraft offer an opportunity to acquire meteorological data over a wide area at a relatively modest additional cost. Today, aircraft fly to and from a great many locations and their routes often pass through regions poorly served by radiosondes or other land based systems that make measurements of the upper atmosphere. Each aircraft needs to monitor wind speed and air temperature during flight for purposes of navigation and air safety. By tapping into this source and capturing the data while the aircraft is in flight, a vast amount of valuable information is available for weather forecasting.
For a great many years aircrew have routinely transmitted reports of wind and temperature during flights. Although very valuable for forecasting purposes, such reports have only been available in limited numbers at cruise altitudes. More recently a fully automated data collection system has been implemented. Known as Aircraft Meteorological Data Relay or AMDAR, it records measurements made by the aircraft's own instrumentation and uses the internal communication devices on board the aircraft to transmit the data back to the ground. In addition to giving the latitude, longitude and height of the aircraft, the reports provide measurements of air temperature, wind speed, wind direction and onboard some aircraft turbulence.
The height of the aircraft is related to the static pressure, measured by an electronic barometer connected to a Pitot-static probe on the fuselage of the aircraft. Temperature is measured by a platinum resistance thermometer, also located on the fuselage, shielded in a way that diverts cloud particles from the sensor. An adjustment is made to the measured air temperature to allow for adiabatic compression of the air.
Using data from the aircraft navigation system and the airspeed system, together with data from the temperature sensors, it is possible to estimate to a high degree of accuracy the velocity of the aircraft with respect to the Earth and the velocity of the air with respect to the aircraft. The difference between the two values gives the wind speed and direction with respect to earth. The calculations are complicated and need to take into account the aircraft pitch, roll and yaw.
A few aircraft have been fitted with sensors that measure relative humidity and this type of measurement is expected to become more common in the future with the expansion of the Humidity programme in the US (WVSS - Water Vapour Sensing System). In Europe a small subset of aircraft are being equipped with other Regional AMDAR Programmes considering installation.
The observations from the AMDAR systems provide high quality data at the aircraft's cruising altitude as well as during ascent and descent. The coverage is particularly good at flight levels over the North Atlantic and at all levels over Europe and North America where there are a great many busy airports.
Global coverage of AMDAR data during a typical 24-hour period. Colours indicate height of the aircraft in thousands of feet as given by the scale on the right.
Coverage of AMDAR data over Europe during a typical 24-hour period. Note the areas of purple and red where aircraft are descending to or taking off from airports.
A wind profiler is a very sensitive upward-looking Doppler radar. Measurements are made by sending short pulses of microwave energy in several near-vertical beams and detecting the return signal. Various types of wind profiler are used worldwide, those of higher power being able to measure wind at greater heights.
A radar pulse transmitted upwards into the atmosphere by the wind profiler is scattered by small inhomogeneities, or targets, that occur naturally in the air. These targets include:
- small variations in refractive index caused by irregularities in the temperature, humidity and pressure of the air;
- small variations in refractive index caused by turbulent eddies in the air; and
- particles of precipitation.
Because each target is carried by the wind, its movement tracked by the radar is a good indication of wind speed. The profiler is capable of detecting the weak return signal from the target. The delay in receiving the signal is proportional to the height of the target while the shift in frequency is proportional to the speed it is moving away from the receiver (the Doppler effect). A pulse emitted directly upwards will allow the vertical velocity of the atmosphere above the profiler to be measured at various heights. By sending pulses in a number of near-vertical directions the wind in the horizontal plane may also be measured. This is of particular value for weather forecast models.
Two types of wind profiler are used in the UK. Boundary layer profilers provide vertical profiles of horizontal wind speed and direction, and vertical wind velocity every 100 m up to a height of 4-10 km. Tropospheric profilers emit a more powerful signal capable of providing regular measurements as high as 12-15 km.
Wind profilers must be carefully located to minimise the effects of interference from nearby trees, air traffic radars or weather radars. A flat location is better than the top of a hill. The instrument operates in fully automated mode requiring only a small number of maintenance visits a year. It provides a continuous stream of data from the site, and is capable of detecting rapid changes in wind that may occur on small timescales.
There are currently boundary layer wind profilers at Camborne (Cornwall), Dunkeswell (Devon), Wattisham (Suffolk) and the Isle of Man. A tropospheric wind profiler operates at South Uist (Outer Hebrides). In addition data are made available from a second tropospheric wind profiler at Aberystwyth managed by the University of Wales. Wind profilers of different designs operate in other European countries, alongside wind data from weather radars. Much of this data is managed as part of the E-WINPROF program.
- Learn more about the European Profiler programme.