Meteorological Measurement Systems

Sensor physics are emphasized in order to explain how sensors work and to explore the strengths and weaknesses of each design type. The book is organized according to sensor type and function (temperature, humidity, and wind sensors, for example), though several unifying themes are developed for each sensor. Functional diagrams are used to portray sensors as a set of logical functions, and static sensitivity is derived from a sensor's transfer equation, focusing attention on sensor physics and on ways in which particular designs might be improved. Sensor performance specifications are explored, helping to compare various instruments and to tell users what to expect as a reasonable level of performance. Finally, the text examines the critical area of environmental exposure of instruments. In a well-designed, properly installed, and well-maintained meteorological measurement system, exposure problems are usually the largest source of error, making this chapter one of the most useful sections of the book.

Contents

1 Overview 1

• 1.1 Instrument Design and Selection 1
• 1.1.1 Performance Characteristics 2
• 1.1.2 Functional Model 2
• 1.1.3 Sources of Error 4
• 1.2 Standards 6
• 1.2.1 Calibration 6
• 1.2.2 Performance 6
• 1.2.3 Exposure 6
• 1.2.4 Procedural 7
• 1.3 System Integration 8
• 1.3.1 Instrument Platforms 8
• 1.3.2 Communication Systems 8
• 1.3.3 Power Source 10
• 1.4 Human Aspects of Measurement 11
• 1.4.1 Human Perception versus Sensor Measurements 11
• 1.4.2 Reasons for Automation 11
• 1.4.3 Design, Implementation, and Maintenance of Measurement Systems 12
• 1.4.4 Interpretation of Sensor Specifications 12
• 1.4.5 Interpretation of Results 13
• 1.4.6 Human Judgment 14
• 1.5 Quality Assurance 15
• 1.5.1 Laboratory Calibrations 16
• 1.5.2 Field Intercomparisons 17
• 1.5.3 Data Monitoring 17
• 1.5.4 Documentation 18
• 1.5.5 Independent Review 18
• 1.5.6 Publication of Data Quality Assessment 19
• 1.6 Scope of this Text 19
• Questions 19
• Bibliography 20
• General Instrumentation References 21

• 2.1 Atmospheric Pressure 22
• 2.2 Direct Measurement of Pressure 23
• 2.2.1 Mercury Barometers 24
• 2.2.2 Aneroid Barometers 29
• 2.3 Indirect Measurement of Pressure 33
• 2.3.1 Boiling Point of a Liquid 34
• 2.4 Comparison of Barometer Types 37
• 2.4.1 Mercury Barometers 38
• 2.4.2 Aneroid Barometers 38
• 2.4.3 Hypsometer 38
• 2.5 Exposure Error 39
• 2.6 Laboratory Experiment 40
• 2.7 Calibration of Barometers 42
• Questions 43
• Bibliography 45

• 3.1 Some Definitions 47
• 3.2 Static Calibration 49
• 3.2.1 Definition of Terms Related to the Transfer Plot 50
• 3.2.2 Calibration Procedure 52
• 3.3 Example of a Static Calibration 56
• 3.4 Multiple Sources of Error 57
• 3.5 Significant Figures 58
• Questions and Problems 60
• Bibliography 61

• 4.1 Thermal Expansion 63
• 4.1.1 Bimetallic Strip 63
• 4.1.2 Liquid-in-Glass Thermometer 65
• 4.2 Thermoelectric Sensors 67
• 4.3 Electrical Resistance Sensors 70
• 4.3.1 Resistance Temperature Detectors 70
• 4.3.2 Thermistors 74
• 4.4 Comparison of Temperature Sensors 76
• 4.5 Exposure of Temperature Sensors 77
• Questions 82
• Bibliography 84
• Notes 85

• 5.1 Water Vapor Pressure 86
• 5.2 Definitions 89
• 5.3 Methods for Measuring Humidity 93
• 5.3.1 Removal of Water Vapor from Moist Air 93
• 5.3.2 Addition of Water Vapor to Air 93
• 5.3.3 Equilibrium Sorption of Water Vapor 96
• 5.3.4 Measurement of Physical Properties of Moist Air 100
• 5.3.5 Attainment of Vapor-Liquid or Vapor-Solid Equilibrium 104
• 5.3.6 Chemical Reactions 107
• 5.4 Choice of Humidity Sensor 107
• 5.5 Calibration of Humidity Sensors 108
• 5.6 Exposure of Humidity Sensors 110
• Questions 111
• Laboratory Exercises 114
• Bibliography 114
• Notes 116

• 6.1 First-Order Systems 118
• 6.1.1 Step-Function Input 119
• 6.1.2 Ramp Input 122
• 6.1.3 Sinusoidal Input 123
• 6.2 Experimental Determination of Dynamic Performance Parameters 125
• 6.3 Application to Temperature Sensors 126
• Questions 127
• Bibliography 128

• 7.1 Methods of Measurement 129
• 7.1.1 Wind Force 130
• 7.1.2 Heat Dissipation 140
• 7.1.3 Speed of Sound 141
• 7.2 Calibration 144
• 7.3 Exposure 144
• 7.4 Wind Data Processing 144
• Questions 145
• Bibliography 147

• 8.1 Generalized Dynamic Performance Models 151
• 8.2 Energy Storage Reservoirs 152
• 8.3 Second-Order Systems 154
• 8.3.1 Step Function Input 154
• 8.3.2 Ramp Input 157
• 8.3.3 Sinusoidal Input 158
• 8.4 Application to Sensors 159
• 8.5 Experimental Determination of Dynamic Performance Parameters 162
• Questions 164
• Bibliography 165

• 9.1 Definitions 166
• 9.2 Methods of Measurement 167
• 9.2.1 Point Precipitation Measurement 167
• 9.2.2 Radar Rain Measurement 182
• Questions 185
• Bibliography 187

• 10.1 Definitions 189
• 10.2 Methods of Measurement 192
• 10.2.1 Pyrheliometers 193
• 10.2.2 Pyranometers 194
• 10.2.3 Pyrgeometers 197
• 10.2.4 Pyrradiometers 197
• 10.3 Measurement Errors 198
• 10.4 Exposure 199
• Questions 200
• Bibliography 201

• 11.1 Definitions 202
• 11.2 Measurement of Visibility 204
• 11.2.1 Transmissometer 205
• 11.2.2 Forward Scatter Meters 206
• 11.3 Measurement of Cloud Height 207
• 11.3.1 Rotating Beam Ceilometer 207
• 11.3.2 Laser Ceilometer 208
• Questions 210
• Bibliography 211

• 12.1 Methods for Making Upper Air Measurements 213
• 12.1.1 Remote Sensing 214
• 12.1.2 In-Situ Platforms 214
• 12.2 Balloons 215
• 12.3 Wind Measurement 219
• 12.3.1 Theodolites 220
• 12.3.2 Radar 223
• 12.3.3 Navigation Aids 223
• 12.4 Radiosondes 225
• 12.5 Exposure Error 228
• Questions 228
• Bibliography 229

• 13.1 Signal Path 232
• 13.2 Drift 233
• 13.3 Sampling 233
• 13.4 Analog-to-Digital Conversion 235
• 13.5 Information Content of a Signal 239
• Questions 241
• Bibliography 243

• A Units and Constants 245
• International System of Units (SI) 245
• Numerical Values 246
• Bibliography 247
• B Thermistor Circuit Analysis 248
• B.1 A Thermistor 248
• B.2 A Circuit 250
• B.3 An Alternative Calibration Equation 252
• C A Data Logger 253
• C.1 The Data Logger 253
• C.2 Application in a Measurement System 255
• D Circuits 258
• D.1 Fundamentals 258
• D.2 Simple Circuits 261
• Questions 263
• E Geophysical Coordinate System 266
• E.1 Geophysical versus Mathematical Coordinate System 266
• E.2 Mathematical Coordinates 266
• E.3 Geophysical Coordinates 267
• F Instrumentation Glossary 269
• Index 285