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Upper-air Data

Upper-air information is not as bountiful as surface data in midlatitudes, and it is even less prevalent in the tropics. Maximum use must be made of the reports you do receive.

UPPER WINDS. Pilot balloon and Rawin data generally are representative except when wind speeds are less than 5 knots. The main thing to ascertain is the quality of the upper-wind sound-ings. At times, upper winds plotted on time sections at certain stations may yield an unrealistic sequence. For example, wind directions and speeds change violently with height and time, though the weather remains the same and no other station shows such remarkable fluctuations. When this happens, you, as the analyst, must separate the good from the bad. Never discard a whole report, because part of it is usually good. You just have to find it. Here again, with experience you will learn which stations can be relied upon to transmit accurate wind data.

SOUNDING ACCURACY. Certain in-formation is provided with considerable accuracy by radiosonde data. Temperature inversions, especially the large trade-wind inversions, are recorded very satisfactorily. The same holds true for the low-level moisture distribution. You will note that temperature and moisture distribution varies considerably when a sonde ascends just outside a thunderstorm or shower and that the precipitation is of a local nature and unrepresenta-tive of the surrounding area.

Comparisons between careful measurements made by specially equipped aircraft and raob data show that a dry adiabatic lapse rate is normal for the subcloud layer but that the radiosonde often shows a more stable structure. In the high troposphere observations, observers frequently encode only the required standard levels and do not transmit enough significant levels. As a result, the lapse rate shows changes at each standard level, which is obviously not true. Tropopause pressures are also affected by this practice, even though many observers are apt to enter one significant level for very well pronounced tropopauses rather than put them at one of the standard levels.

Upper-air soundings can be most unreliable in the tropics at certain times; therefore, care should be taken before a sounding is accepted as valid.

AIRCRAFT REPORTS. Since weather data is not overly abundant in the tropics, pilot reports from both military and civilian aircraft are used to supplement surface and upper-air reports received from land stations and ships. The aircraft reports that receive the most attention are those containing reconnaissance flight information on tropical cyclones. Satellites now supply us with real-time pictures of the tropics, but recon-naissance flights by USAF weather aircraft provide on-scene data. We will discuss the information received from such flights and its usefulness later in this unit.

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