Weather Balloon
Twice a day, every day of the year, weather balloons are released simultaneously from almost 900 locations worldwide! This includes
92 released by the National Weather Service in the US and its territories. The balloon flights last for around 2 hours, can drift as far as
125 miles away, and rise up to over 100,000 ft. (about 20 miles) in the atmosphere!
Weather balloons, which are made of latex or synthetic rubber (neoprene), are filled
with either hydrogen or helium. The sides are about 0.051 mm thick before release
and will be only 0.0025 mm thick at typical bursting altitudes! The balloons, which start
out measuring about 6 ft. wide before release, expand as they rise to about 20 ft. in
diameter! An instrument called a radiosonde is attached to the balloon to measure
pressure, temperature and relative humidity as it ascends up into the atmosphere.
These instruments will often endure temperatures as cold as -139�F (-95�C), relative
humidities from 0% to 100%, air pressures only a few thousandths of what is found on
the Earth's surface, ice, rain, thunderstorms, and wind speeds of almost 200 mph! A
transmitter on the radiosonde sends the data back to tracking equipment on the ground
every one to two seconds. By tracking the position of the radiosonde, we can also
calculate wind speed and wind direction. The radiosonde is powered by a small
battery.
During nighttime releases, a lightstick is attached so the balloon can be tracked by
meteorologists until the tracking equipment locks onto the radio signal. A parachute,
attached to the end of the balloon, allows the radiosonde to fall slowly to the ground at
speeds less than 22 mph after the balloon bursts. Each radiosonde contains a mailing
bag and instructions on what to do if you find one. About 20% of the 75,000
radiosondes sent up each year in the US are found and returned. These instruments
are fixed and reused, saving the government money.
Weather balloons are the
primary source of data above the ground. They provide valuable input for
computer forecast models, local data for meteorologists to make forecasts
and predict storms, and data for research. Computer forecast models which
use weather balloon data are used by all forecasters worldwide, from National
Weather Service meteorologists to your local TV weatherman! Without this
information, accurate forecasts beyond a few hours would be almost
impossible!
Thermometer
Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature.
A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury thermometer) in
which some physical change occurs with temperature, plus some means of converting this physical change into
a numerical value (e.g. the scale on a mercury thermometer).
In 1665 Christiaan Huygens suggested using the melting and boiling points of water as standards. In 1724
Daniel Gabriel Fahrenheit produced a temperature scale which now bears his name. In 1742 Anders Celsius
proposed a scale with zero at the boiling point and 100 degrees at the melting point of water, though the scale
which now bears his name has them the other way around.
Comparison of the Celsius and Fahrenheit scales
Barometer
A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure
tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used
within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries.
History
Evangelista Torricelli is universally credited with inventing the barometer in 1643. French scientist and
philosopher René Descartes described the design of an experiment to determine atmospheric pressure as early
as 1631, but there is no evidence that he built a working barometer at that time.
Types
Water-based barometers
The concept that decreasing atmospheric pressure predicts stormy weather, postulated by Lucien Vidie,
provides the theoretical basis for a weather prediction device called a "storm glass" or a "Goethe barometer"
(named for Johann Wolfgang Von Goethe, the renowned German writer and polymath who developed a simple
but effective weather ball barometer using the principles developed by Torricelli).
The weather ball barometer consists of a glass container with a sealed body, half filled with water. A narrow
spout connects to the body below the water level and rises above the water level. The narrow spout is open to
the atmosphere. When the air pressure is lower than it was at the time the body was sealed, the water level in the
spout will rise above the water level in the body; when the air pressure is higher, the water level in the spout
will drop below the water level in the body.
Mercury barometers
A mercury barometer has a glass tube with a height of at least 84 cm, closed at one end, with an open mercuryfilled reservoir at the base. The weight of the mercury creates a vacuum in the top of the tube. Mercury in the
tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir.
High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low
pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the
reservoir. Since higher temperature at the instrument will reduce the density of the mercury, the scale for
reading the height of the mercury is adjusted to compensate for this effect.
The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters or
feet (torr): the pressure is quoted as the level of the mercury's height in the vertical column. 1 atmosphere is
equivalent to about 760 millimeters of mercury.
Aneroid barometers
An aneroid barometer, invented in 1843 by French scientist Lucien Vidie uses a small, flexible metal box
called an aneroid cell (capsule), which is made from an alloy of beryllium and copper. The evacuated capsule
(or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air
pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that
the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many
models include a manually set needle which is used to mark the current measurement so a change can be seen.
In addition, the mechanism is made deliberately "stiff" so that tapping the barometer reveals whether the
pressure is rising or falling as the pointer moves.
Use in Weather Forecasting
Using barometric pressure and the pressure tendency (the change of pressure over time) has been used in
weather forecasting since the late 19th century. When used in combination with wind observations, reasonably
accurate short-term forecasts can be made. Simultaneous barometric readings from across a network of weather
stations allow maps of air pressure to be produced, which were the first form of the modern weather map when
created in the 19th century. Isobars, lines of equal pressure, when drawn on such a map, gives a contour map
showing areas of high and low pressure. Localized high atmospheric pressure acts as a barrier to approaching
weather systems, diverting their course. Atmospheric lift caused by low-level wind convergence into the surface
low brings clouds and potentially precipitation. The larger the change in pressure, especially if more than
3.5 hPa, the larger the change in weather can be expected. If the pressure drop is rapid, a low pressure system is
approaching, and there is a greater chance of rain . Rapid pressure rises, such as in the wake of a cold front, are
associated with improving weather conditions, such as clearing skies.
Windsock
A windsock is a conical textile tube designed to indicate wind
direction and relative wind speed. Windsocks typically are used at
airports and at chemical plants where there is risk of gaseous leakage.
They are sometimes located alongside highways at windy locations.
Wind direction is the opposite of the direction in which the windsock is
pointing (note that wind directions are conventionally specified as
being the compass point from which the wind originates; so a
windsock pointing due north indicates a southerly wind). Windspeed is
indicated by the windsock's angle relative to the mounting pole; in low
winds, the windsock droops; in high winds it flies horizontally.
Per FAA standards referenced below, a 15-knot (28 km/h; 17 mph)
wind will fully extend the properly functioning windsock. A 3-knot
(5.6 km/h; 3.5 mph) breeze will cause the properly functioning
windsock to orient itself according to the wind.
At many airports, windsocks are lighted at night, either by flood lights
on top surrounding it or with one mounted on the pole shining inside it.
Wind Vane
A weather vane is an instrument for showing the direction of the wind. They are typically used as an
architectural ornament to the highest point of a building.
Although partly functional, weather vanes are generally decorative, often featuring the traditional cockerel
design with letters indicating the points of the compass. Other common motifs include ships, arrows and horses.
Not all weather vanes have pointers.
Operation
The design of a wind vane is such that the weight is evenly distributed on each side of the surface, but the
surface area is unequally divided, so that the pointer can move freely on its axis. The side with the larger surface
area is blown away from the wind direction, so that the smaller side, with the pointer, is pivoted to face the wind
direction. Most wind vanes have directional markers beneath the arrow, aligned with the geographic directions.
Wind vanes, especially those with fanciful shapes, do not always show the real direction of a very gentle wind.
This is because the figures do not achieve the necessary design balance: an unequal surface area but balanced in
weight.
To obtain an accurate reading, the wind vane must be located well above the ground and away from buildings,
trees, and other objects which interfere with the true wind direction. Changing wind direction can be meaningful
when coordinated with other apparent sky conditions, enabling the user to make simple short range forecasts.
History
The Tower of the Winds on the ancient Roman agora in Athens once bore on its roof a wind vane in the form of
a bronze Triton holding a rod in his outstretched hand, rotating as the wind changed direction. Below, the frieze
was adorned with the eight wind deities. The eight metre high structure also featured sundials, and a water clock
inside dates from around 50 BC.
Modern aerovanes combine the directional vane with an anemometer (a device for measuring wind speed).
Anemometer
An anemometer is a device for measuring wind speed, and is a common weather station instrument.
The first known description of an anemometer was given by Leon Battista Alberti around 1450.
Cup anemometers
A simple type of anemometer, invented (1846) by Dr. John Thomas
Romney Robinson, of Armagh Observatory. It consisted of four
hemispherical cups each mounted on one end of four horizontal arms,
which in turn were mounted at equal angles to each other on a vertical
shaft. The air flow past the cups in any horizontal direction turned the cups
in a manner that was proportional to the wind speed. Therefore, counting
the turns of the cups over a set time period produced the average wind
speed for a wide range of speeds. On an anemometer with four cups it is
easy to see that since the cups are arranged symmetrically on the end of the
arms, the wind always has the hollow of one cup presented to it and is
blowing on the back of the cup on the opposite end of the cross.
Ping-pong ball
anemometers
A common anemometer for basic use is constructed
a ping-pong ball attached to a string. When the wind
blows horizontally, it presses on and moves the ball;
because ping-pong balls are very lightweight, they
move easily in light winds. Measuring the angle
between the string-ball apparatus and the vertical gives
estimate of the wind speed.
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Rain Gauge
A rain gauge (also known as a udometer or a pluviometer or an ombrometer) is a type of instrument used by
meteorologists to gather and measure the amount of liquid precipitation over a set period of time.
History
The first known rainfall records were kept by the Ancient Greeks, about 500 B.C. This was followed 100 years
later by people in India using bowls to record the rainfall. The readings from these were correlated against
expected growth, and used as a basis for land taxes. In the Arthashastra, used for example in Magadha, precise
standards were set as to grain production. Each of the state storehouses were equipped with a standardised rain
gauge to classify land for taxation purposes.
Principles
Most rain gauges generally measure the precipitation in millimeters.The
level of rainfall is sometimes reported as inches or centimeters.
Rain gauge amounts are read either manually or by automatic weather
station (AWS). The frequency of readings will depend on the requirements
of the collection agency.
In most cases the precipitation is not retained, however some stations do
submit rainfall (and snowfall) for testing, which is done to obtain levels of
pollutants.
Rain gauges have their limitations. Attempting to collect rain data in a
hurricane can be nearly impossible and unreliable (even if the equipment
survives) due to wind extremes. Also, rain gauges only indicate rainfall in a
localized area. For virtually any gauge, drops will stick to the sides or funnel of the collecting device, such that
amounts are very slightly underestimated, and those of .01 inches or .25 mm may be recorded as a trace.
Another problem encountered is when the temperature is close to or below freezing. Rain may fall on the funnel
and freeze or snow may collect in the gauge and not permit any subsequent rain to pass through.
Rain gauges should be placed in an open area where there are no obstructions, such as building or trees, to block
the rain. This is also to prevent the water collected on the roofs of buildings or the leaves of trees from dripping
into the rain gauge after a rain, resulting in inaccurate readings.
Types
Types of rain gauges include graduated cylinders, weighing gauges, tipping bucket gauges, and simple buried
pit collectors. Each type has its advantages and disadvantages for collecting rain data.
Standard rain gauge
The standard NOAA rain gauge, developed around the start of the 20th century, consists of a funnel attached to
a graduated cylinder (2 cm in diameter) that fits inside a larger outside container (20 cm in diameter and 50 cm
tall). If the water overflows the inside graduated cylinder, the outside larger container will catch it. When
measurements are taken, the height of the water in the small graduated cylinder is measured and the excess
overflow in the large container is carefully poured into another graduated cylinder and measured to give the
total rainfall. In locations using the metric system, the cylinder is usually marked in mm and in the picture
above will measure up to 250 millimetres (9.8 in) of rainfall. Each horizontal line on the cylinder is 5
centimetres (2.0 in) in areas using the metric system; in areas using Imperial units each horizontal line
represents 0.01 inch. The larger container collects any rainfall amounts over 25 mm that flows from a small hole
near the top of the cylinder. A metal pipe is attached to the container and can be adjusted to ensure the rain
gauge is level. This pipe then fits over a metal rod that has been placed in the ground.
Weighing precipitation gauge
A weighing-type precipitation gauge consists of a storage bin, which is weighed to record the mass. Certain
models measure the mass using a pen on a rotating drum, or by using a vibrating wire attached to a data logger.
The advantages of this type of gauge over tipping buckets are that it does not underestimate intense rain, and it
can measure other forms of precipitation, including rain, hail and snow. These gauges are, however, more
expensive and require more maintenance than tipping bucket gauges.
The weighing-type recording gauge may also contain a device to measure the quantity of chemicals contained in
the location's atmosphere. This is extremely helpful for scientists studying the effects of greenhouse gases
released into the atmosphere and their effects on the levels of the acid rain. Some Automated Surface Observing
System (ASOS) units use an automated weighing gauge called the AWPAG (All Weather Precipitation
Accumulation Gauge).
Tipping bucket rain gauge
The tipping bucket rain gauge consists of a funnel that collects and channels the
precipitation into a small seesaw-like container. After a pre-set amount of
precipitation falls, the lever tips, dumping the collected water and sending an
electrical signal. An old-style recording device may consist of a pen mounted on
an arm attached to a geared wheel that moves once with each signal sent from
the collector. In this design, the wheel turns the pen arm moves either up or
down leaving a trace on the graph and at the same time making a loud click. Each
jump of the arm is sometimes referred to as a 'click' in reference to the noise.
The chart is measured in 10 minute periods (vertical lines) and 0.4 mm (0.015 in) (horizontal lines) and rotates once
every 24 hours and is powered by a clockwork motor that must be manually wound.
The tipping bucket rain gauge is not as accurate as the standard rain gauge
because the rainfall may stop before the lever has tipped. When the next period
of rain begins it may take no more than one or two drops to tip the lever. This
would then indicate that pre-set amount has fallen when in fact only a fraction of
that amount has actually fallen. Tipping buckets also tend to underestimate the
amount of rainfall, particularly in snowfall and heavy rainfall events. The
advantage of the tipping bucket rain gauge is that the character of the rain (light,
medium, or heavy) may be easily obtained. Rainfall character is decided by the
total amount of rain that has fallen in a set period (usually 1 hour) and by counting the number of 'clicks' in a 10 minute
period the observer can decide the character of the rain.
Tipping gauges can also incorporate weighing gauges. In these gauges, a strain gauge is fixed to the collection
bucket so that the exact rainfall can be read at any moment. Each time the collector tips, the strain gauge
(weight sensor) is re-zeroed to null out any drift.
To measure the water equivalent of frozen precipitation, a tipping bucket may be heated to melt any ice and
snow that is caught in its funnel. Without a heating mechanism, the funnel often becomes clogged during a
frozen precipitation event, and thus no precipitation can be measured. Many Automated Surface Observing
System (ASOS) units use heated tipping buckets to measure precipitation
Radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of
radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail, etc.). Modern
weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to
the intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and
their potential to cause severe weather.
During World War II, radar operators discovered that weather was causing echoes on their screen, masking
potential enemy targets. Techniques were developed to filter them, but scientists began to study the
phenomenon. Soon after the war, surplus radar were used to detect precipitations. Since then, weather radar has
evolved on its own and is now used by national weather services, research departments in universities as well as
in television broadcasts. Raw images are routinely used and specialized software can take radar data to make
short term forecast of future positions and intensities of rain, snow, hail, and other weather phenomena. Their
output is even incorporated into numerical weather prediction models to improve analyses and forecasts.
Exactly how does radar work?
As the radar antenna turns, it emits extremely short bursts of radio
waves, called pulses. Each pulse lasts about 0.00000157 seconds
(1.57x10-6), with a 0.00099843-second (998.43x10-6) "listening period"
in between. The transmitted radio waves move through the atmosphere
at about the speed of light.
By recording the direction in which the antenna was pointed, the
direction of the target is known as well. Generally, the better the target
is at reflecting radio waves (i.e., more raindrops, larger hailstones, etc.),
the stronger the reflected radio waves, or echo, will be.
This information is observed within the approximately 0.001-second
listening period with the process repeated up to 1,300 times per second. By keeping track of the time it takes the radio waves to
leave the antenna, hit the target, and return to the antenna, the radar can calculate the distance to the target.
The WSR-88D's pulses have an average transmitted power of about 450,000 watts. By comparison, a typical home microwave
oven will generate about 1000 watts of energy. However, because of the very short period the radar is actually transmitting,
when the time of all pulses each hour are totaled (the time the radar is actually transmitting), the radar is "on" for a little over 7
seconds each hour. The remaining 59 minutes and 53 seconds are spent listening for any returned signals.
The Doppler Advantage
By their design, Doppler radar systems can provide information regarding the movement of targets as well their position. When
the WSR-88D transmits a pulse of radio waves, the system keeps track of the phase (shape, position, and form) of the
transmitted radio waves.
By measuring the shift in phase between a transmitted pulse and a received echo, the target's radial velocity (the movement of
the target directly toward or away from the radar) can be calculated. A positive phase shift implies motion toward the radar and a
negative shift suggests motion away from the radar.
The phase shift effect is similar to the "Doppler shift" observed with sound waves. With the "Doppler shift", the sound pitch of an
object moving toward your location is higher due to compression of sound waves. As an object moves away from your location,
sound waves are stretched resulting in a lower frequency. You have probably heard this effect from an emergency vehicle or
train. As the vehicle or train passes your location, the siren or whistle's pitch lowers as the object passes by.
For the Doppler radar, atmospheric objects moving inbound (toward the radar) produce a positive shift in frequency of the radar
signal. Objects moving away from the radar (outbound) produce a negative shift in frequency. It is this change in frequency that
allows us to "see" motion in the atmosphere. The larger the phase shift, the greater the target's radial velocity.
Satellites
The weather satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth.
Satellites can be either polar orbiting, seeing the same swath of the Earth every 12 hours, or geostationary,
hovering over the same spot on Earth by orbiting over the equator while moving at the speed of the Earth's
rotation. These meteorological satellites, however, see more than clouds and cloud systems. City lights, fires,
effects of pollution, auroras, sand and dust storms, snow cover, ice mapping, boundaries of ocean currents,
energy flows, etc., and other types of environmental information are collected using weather satellites. Weather
satellite images helped in monitoring the volcanic ash cloud from Mount St. Helens and activity from other
volcanoes such as Mount Etna.[2] Smoke from fires in the western United States such as Colorado and Utah
have also been monitored.
Other environmental satellites can detect changes in the Earth's vegetation, sea state, ocean color, and ice fields.
For example, the 2002 oil spill off the northwest coast of Spain was watched carefully by the European
ENVISAT, which, though not a weather satellite, flies an instrument (ASAR) which can see changes in the sea
surface.
El Niño and its effects on weather are monitored daily from satellite images. The Antarctic ozone hole is
mapped from weather satellite data. Collectively, weather satellites flown by the U.S., Europe, India, China,
Russia, and Japan provide nearly continuous observations for a global
weather watch.
History
The first television image of Earth from space from the TIROS-1 weather
satellite. 
The first weather satellite, Vanguard 2, was launched on February 17,
1959. It was designed to measure cloud cover and resistance, but a poor
axis of rotation kept it from collecting a notable amount of useful data.
The first weather satellite to be considered a success was TIROS-1,
launched by NASA on April 1, 1960. TIROS operated for 78 days and
proved to be much more successful than Vanguard 2. TIROS paved the
way for the Nimbus program, whose technology and findings are the
heritage of most of the Earth-observing satellites NASA and NOAA
have launched since then.
Types
There are two basic types of meteorological satellites: geostationary and polar orbiting.
Geostationary
Geostationary weather satellites orbit the Earth above the equator at altitudes of 35,880 km (22,300 miles).
Because of this orbit, they remain stationary with respect to the rotating Earth and thus can record or transmit
images of the entire hemisphere below continuously with their visible-light and infrared sensors. The news
media use the geostationary photos in their daily weather presentation as single images or made into movie
loops. These are also available on the city forecast pages of noaa.gov (example Dallas, TX).[6]
Several geostationary meteorological spacecraft are in operation. The United States has three in operation;
GOES-12, GOES-13, and GOES-15. GOES-12 is located at 60 degrees west. GOES-13 is located at 75 degrees
west. GOES-15 is over the eastern Pacific Ocean. Russia's new-generation weather satellite Elektro-L 1
operates at 76°E over the Indian Ocean. The Japanese have one in operation; MTSAT-1R over the mid Pacific
at 140°E. The Europeans have Meteosat-8 (3.5°W) and Meteosat-9 (0°) over the Atlantic Ocean and have
Meteosat-6 (63°E) and Meteosat-7 (57.5°E) over the Indian Ocean.
Polar orbiting
Computer controlled motorized parabolic dish antenna for tracking LEO weather satellites.
Polar orbiting weather satellites circle the Earth at a typical altitude of 850 km (530 miles) in a north to south
(or vice versa) path, passing over the poles in their continuous flight. Polar satellites are in sun-synchronous
orbits, which means they are able to observe any place on Earth and will view every location twice each day
with the same general lighting conditions due to the near-constant local solar time. Polar orbiting weather
satellites offer a much better resolution than their geostationary counterparts due their closeness to the Earth.
The United States has the NOAA series of polar orbiting meteorological satellites, presently NOAA 17 and
NOAA 18 as primary spacecraft, NOAA 15 and NOAA 16 as secondary spacecraft, NOAA 14 in standby, and
NOAA 12. Europe has the Metop-A satellite. Russia has the Meteor and RESURS series of satellites. China has
FY-1D and FY-3A. India has polar orbiting satellites as well.
Hygrometer
A hygrometer is an instrument used for measuring the moisture content in the environmental air, or humidity.
Most measurement devices usually rely on measurements of some other quantity such as temperature, pressure,
mass or a mechanical or electrical change in a substance as moisture is absorbed. From calculations based on
physical principles, or especially by calibration with a reference standard, these measured quantities can lead to
a measurement of humidity. Modern electronic devices use temperature of condensation, or changes in
electrical capacitance or resistance to measure humidity changes.
A dial hygrometer, in this case a hair tension style.
Types
Metal/pulp coil type
The familiar metal/paper coil hygrometer is useful for giving a dial indication of humidity changes, but it
appears most often in very inexpensive devices and their accuracy is very limited. A search through many
identical units in a display might show differences in indicated humidity of 10% or more. In these devices,
humidity is absorbed by a salt-impregnated paper strip attached to a metal coil, causing it to change shape.
These changes in length (analogous to those in a bimetallic thermometer) cause an indication on a dial.
Hair tension hygrometers
These devices use a human or animal hair under tension. The length of the hair changes with humidity and the
length change may be magnified by a mechanism and/or indicated on a dial or scale. The traditional folk art
device known as a "weather house" works on this principle.
Psychrometer
A psychrometer consists of two thermometers, one which is dry and one which is kept moist with
distilled water on a sock or wick. The two thermometers are thus called the dry-bulb and the wet-bulb. At
temperatures above the freezing point of water, evaporation of water from the wick lowers the temperature, so
that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer. When
the air temperature is below freezing, however, the wet-bulb is covered with a thin coating of ice and may be
warmer than the dry bulb. Relative humidity is computed from the ambient temperature as shown by the drybulb thermometer and the difference in temperatures as shown by the wet-bulb and dry-bulb thermometers.
Relative humidity can also be determined by locating the intersection of the wet- and dry-bulb temperatures on
a psychrometric chart. Psychrometers are commonly used in meteorology, and in the HVAC industry for proper
refrigerant charging of residential and commercial air conditioning systems.
The sling psychrometer, where the thermometers are attached to a handle or length of rope and spun around in
the air for a few minutes, is sometimes used for field measurements, but is being replaced by more convenient
electronic sensors. Alternatively a whirling psychrometer uses the same principle, however the two
thermometers are fitted into a device that resembles a Ratchet or football rattle.
A sling psychrometer for outdoor use