Upper atmosphere measurements During the latter part of the 19th century and the first quarter of the 20th century, the upper air information was obtained mainly by meteorographs sent aloft on tethered kites. The average altitude reached was only about 10,000 feet although some flights reached nearly 20,000 feet. The data could not be evaluated until after the observation was completed and the kite and meteorograph were brought back to the ground. Kites could be flown only in good weather and only if winds were neither too light nor too strong. There was danger of the kite breaking away and the meteorograph endangering lives and property. Weaknesses: From 1925 to 1937, upper-air data were obtained by attaching meteorographs to airplanes, observations called APOBS. * average altitude was about 17,000 feet and the maximum about 20,000 feet. * data could not be evaluated until the plane had landed. * observations could not be taken in stormy weather.

Radiosonde Radiosonde is a small, expendable instrument package that is suspended below a large balloon filled with hydrogen or helium. The radiosonde consists of sensors used to measure several meteorological parameters coupled to a radio transmitter and assembled in a lightweight box. The meteorological sensors sample the ambient temperature, relative humidity, and pressure of the air through which it rises. By tracking the position of the radiosonde, wind speed and direction aloft are also obtained.

Tracking ballown using theodolite GPS

The transmitter operates on a frequency ranging from to MHz The instrument package is attached to a parachute, and together, they are suspended from a balloon. The balloon is a spherically-shaped film of natural or synthetic rubber and is inflated with hydrogen or helium. Even though various sizes are available, 600-gram and 1200-gram balloons are most widely used. The balloon, parachute and radiosonde, known collectively as a flight train, ascend at an approximate rate of 1000 feet/minute depending on weather conditions, amount of gas, and balloon size. Tracking System The ground-based Radiosonde Tracking System which is housed in a fiberglass dome above the inflation shelter. Wind speed and direction are determined for each minute of the flight, generally 90 minutes. The combined dynamic/thermodynamic observation is termed a rawinsonde observation NWS Upper Air Inflation Building at Alabaster, AL

The altitude reached by rawinsonde varies for several reasons:  bursting height of the balloon;  faulty receiving equipment;  atmospheric interference. When the balloon reaches its elastic limit and bursts, the parachute slows the descent of the radiosonde, minimizing the danger to lives and poperties. 600-gram balloon can rise approximately 90,000 feet. The bursting altitude for larger 1,200-gram balloon exceeds 100,000 feet.

A flight is considered a failure and a second radiosonde is released if the balloon bursts before reaching the 400 mb level or if more than 6 minutes of data between surface and 400 mb are missing. Worldwide, there are more than 900 upper-air observation stations using 15 major types of radiosondes. Most stations are located in the Northern Hemisphere and all observations are taken at the same times each day at 00:00 and 12:00 UTC (Greenwich Mean Time), 365 days per year. Observations are made by the NWS at 93 stations - 72 in the conterminous United States, 13 in Alaska, 10 in the Pacific, and 1 in Puerto Rico.

NWS Operational Use of Radiosonde Observations The current operational numerical weather prediction (NWP) models for weather and hydrologic prediction are run on computers at the National Center for Environmental Prediction in Camp Springs, Maryland. Radiosonde data are fed into the models and, with other observations, assimilated to provide initial conditions for the model predictions. The highest resolution operational weather prediction model computes weather parameters on a mathematical grid 48 by 48 km horizontally, 38 levels vertically, and in 200 second time steps out to 48 hours. Experimentally, a 29 by 29 km, 50 vertical level model is being run regularly in addition to the 48 km resolution model. Radiosonde sites in the United States are about 375 km (235 miles) apart. Since the NWP model grid resolution is much smaller than the spacing of radiosonde sites, the loss of data from only one or two radiosonde sites can have significant impacts on model forecasts. Studies have shown that when missing radiosonde observations occur, small scale weather features above the surface can be lost or inaccurately positioned in the data analysis, causing significant errors in the model predictions. This especially holds true during severe weather episodes.

Importance of Radiosonde Data to Local Weather Prediction Individual soundings help forecasters determine many local weather parameters including, atmospheric instability, freezing levels, wind shear, precipitabie water, and icing potential. The following are examples of local weather phenomena that are predicted with the aid of sounding data: Severe thunderstorms Tornadoes Microbursts Flash floods Ice storms Aircraft icing conditions and turbulence Cloud heights Maximum temperature

GPS Dropsondes Dropsonde is a weather reconnaissance device created by the National Center for Atmospheric Research (NCAR), designed to be dropped from an aircraft at altitude to accurately measure tropical storm conditions as the device falls to the ground. The dropsonde contains a GPS receiver, along with pressure, temperature, and humidity sensors to capture atmospheric profiles and thermodynamic data and winds.

Launch dropsonde

Dropsonde Sensor Specifications Range Accuracy Resolution Pressure hPa ± 1.0 hPa 0.1 hPa Temperature -90 to +60 C ± 0.2 C 0.1 C Humidity 0-100% ± 5% 1.0% Horiz Wind m/s ± 0.5 m/s 0.1 m/s

Driftsondes It is a new type of observing system to track weather above hard-to-reach parts of the globe, as well as make soundings that will fill critical gaps in data coverage over oceanic and remote arctic and continental regions. These areas include (1) relatively void of in-situ measurements from radiosondes and commercial aircraft, such as the remote Pacific and Atlantic oceans, (2) covered with extensive cloud shields so that satellite measurements are limited. The across the ocean driftsonde flights will provide high resolution atmospheric profiles made by GPS dropsondes that would be difficult or impossible to obtain by deployment of aircraft alone.

Zero-pressure balloon The balloon is filled only partially on the ground. As the balloon rises, the gas bubble expands to fill out the balloon. When the gas bubble expands to completely fill the balloon, excess gas exhausts through the vent at the bottom; at this point, the ascent stops, and the balloon will hover at a constant altitude. At night, after the sun goes down, the gas bubble cools and shrinks. This causes the balloon to begin to descend. If the balloon mission must go for several days, some ballast material, such as sand, can be dropped to maintain altitude. But during the next day, the gas will heat and expand again, but since the system is now lighter, it will ascend higher. To maintain its altitude, the system has to dump some gas. And so on until the supply of ballast is used up. Eventually, it will hit the ground, but this may take several days.

Launching a driftsonde during AMMA

The straightforward westerly tracks depicted by numerical models The actual tracks taken by 2008's eight driftsondes included many loops and deviations (bottom). Dots along the tracks show where dropsondes were deployed.