Meteorological Measurements and Instrumentation

Giles Harrison is Professor of Atmospheric Physics at the Department of Meteorology at the University of Reading, UK. His research focuses on one of the oldest experimental topics in meteorology, atmospheric electricity and the development of new surface and balloon-carried instruments for environmental measurements.

Series Foreword xi

Preface xiii

Acknowledgements xv

Disclaimer xvii

1 Introduction 1

1.1 The instrumental age 2

1.2 Measurements and the climate record 2

1.3 Clouds and rainfall 3

1.4 Standardisation of air temperature measurements 4

1.5 Upper air measurements 5

1.5.1 Manned balloon ascents 6

1.5.2 Self-reporting upper air instruments 7

1.6 Scope and structure 8

2 Principles of Measurement and Instrumentation 9

2.1 Instruments and measurement systems 9

2.1.1 Instrument response characterisation 10

2.1.2 Measurement quality 12

2.2 Instrument response time 14

2.2.1 Response to a step-change 14

2.2.2 Response to an oscillation 15

2.3 Deriving the standard error 18

2.3.1 Sample mean 18

2.3.2 Standard error 20

2.3.3 Quoting results 20

2.4 Calculations combining uncertainties 21

2.4.1 Sums and differences 21

2.4.2 Products and quotients 22

2.4.3 Uncertainties from functions 23

2.5 Calibration experiments 23

3 Electronics and Analogue Signal Processing 27

3.1 Voltage measurements 28

3.2 Signal conditioning 28

3.2.1 Operational amplifiers 29

3.2.2 Operational amplifier fundamentals 30

3.2.3 Signal amplification 31

3.2.4 Buffer amplifiers 33

3.2.5 Inverting amplifier 33

3.2.6 Line driving 35

3.2.7 Power supplies 36

3.3 Voltage signals 38

3.3.1 Electrometers 38

3.3.2 Microvolt amplifier 40

3.4 Current measurement 41

3.4.1 Current to voltage conversion 42

3.4.2 Photocurrent amplifier 43

3.4.3 Logarithmic measurements 44

3.4.4 Calibration currents 45

3.5 Resistance measurement 46

3.5.1 Thermistor resistance measurement 46

3.5.2 Resistance bridge methods 47

3.6 Oscillatory signals 50

3.6.1 Oscillators 50

3.6.2 Phase-locked loops 53

3.6.3 Frequency to voltage conversion 54

3.7 Physical implementation 54

4 Data Acquisition Systems and Initial Data Analysis 57

4.1 Data acquisition 57

4.1.1 Count data 59

4.1.2 Frequency data 60

4.1.3 Interval data 60

4.1.4 Voltage data 61

4.1.5 Sampling 63

4.1.6 Time synchronisation 66

4.2 Custom data logging systems 66

4.2.1 Data acquisition cards 67

4.2.2 Microcontroller systems 67

4.2.3 Automatic Weather Stations 68

4.3 Management of data files 69

4.3.1 Data logger programming 69

4.3.2 Data transfer 70

4.3.3 Data file considerations 71

4.4 Preliminary data examination 72

4.4.1 In situ calibration 72

4.4.2 Time series 73

4.4.3 Irregular and intermittent time series 75

4.4.4 Further data analysis 75

5 Temperature 77

5.1 The Celsius temperature scale 77

5.2 Liquid in glass thermometry 78

5.2.1 Fixed interval temperature scales 78

5.2.2 Liquid-in-glass thermometers 79

5.3 Electrical temperature sensors 80

5.3.1 Thermocouple 81

5.3.2 Semiconductor 81

5.3.3 Thermistor 82

5.3.4 Metal resistance thermometry 83

5.4 Resistance thermometry considerations 86

5.4.1 Thermistor measurement 87

5.4.2 Platinum resistance measurement 89

5.5 Thermometer exposure 90

5.5.1 Radiation error of air temperature sensors 90

5.5.2 Thermometer radiation screens 91

5.5.3 Radiation errors on screen temperatures 93

5.5.4 Lag times in screen temperatures 95

5.5.5 Screen condition 98

5.5.6 Modern developments in screens 99

5.6 Surface and below-surface temperature measurements 99

5.6.1 Surface temperatures 99

5.6.2 Soil temperatures 100

5.6.3 Ground heat flux density 100

6 Humidity 103

6.1 Water vapour as a gas 103

6.2 Physical measures of humidity 105

6.2.1 Absolute humidity 106

6.2.2 Specific humidity 106

6.2.3 Relative humidity 107

6.2.4 Dew point and wet bulb temperature 107

6.3 Hygrometers and their operating principles 109

6.3.1 Mechanical 109

6.3.2 Chemical 111

6.3.3 Electronic 111

6.3.4 Spectroscopic 112

6.3.5 Radio refractive index 113

6.3.6 Dew point meter 114

6.3.7 Psychrometer 114

6.4 Practical psychrometers 116

6.4.1 Effect of temperature uncertainties 118

6.4.2 Ventilation effects 118

6.4.3 Freezing of the wet bulb 120

6.5 Hygrometer calibration using salt solutions 121

6.6 Comparison of hygrometry techniques 122

7 Atmospheric Pressure 123

7.1 Introduction 123

7.2 Barometers 123

7.2.1 Liquid barometers 124

7.2.2 Mercury barometers 125

7.2.3 Hypsometer 127

7.2.4 Aneroid barometers 127

7.2.5 Precision aneroid barometers 128

7.2.6 Flexible diaphragm sensors 129

7.2.7 Vibrating cylinder barometer 129

7.3 Corrections to barometers 129

7.3.1 Sea level correction 130

7.3.2 Wind speed corrections 131

8 Wind Speed and Direction 133

8.1 Introduction 133

8.2 Types of anemometer 133

8.2.1 Pressure plate anemometers 133

8.2.2 Pressure tube anemometer 134

8.2.3 Cup anemometers 134

8.2.4 Propeller anemometer 136

8.2.5 Hot sensor anemometer 137

8.2.6 Sonic anemometer 139

8.3 Wind direction 141

8.3.1 Wind vanes 142

8.3.2 Horizontal wind components 144

8.3.3 Multi-component research anemometers 146

8.4 Anemometer exposure 146

8.4.1 Anemometer deficiencies 146

8.5 Wind speed from kite tether tension 148

9 Radiation 151

9.1 Introduction 151

9.2 Solar geometry 154

9.2.1 Orbital variations 154

9.2.2 Diurnal variation 155

9.2.3 Solar time corrections 155

9.2.4 Day length calculation 156

9.2.5 Irradiance calculation 157

9.3 Shortwave radiation instruments 158

9.3.1 Thermopile pyranometer 158

9.3.2 Pyranometer theory 159

9.3.3 Silicon pyranometers 162

9.4 Pyrheliometers 162

9.5 Diffuse solar radiation measurement 164

9.5.1 Occulting disk method 164

9.5.2 Shade ring method 165

9.5.3 Reflected shortwave radiation 168

9.5.4 Fluctuations in measured radiation 169

9.6 Reference solar radiation instruments 171

9.6.1 Cavity radiometer 172

9.6.2 Secondary pyrheliometers 172

9.7 Longwave instruments 173

9.7.1 Pyrradiometer theory 173

9.7.2 Pyrradiometer calibration 174

9.7.3 Pyrgeometer measurements 175

9.7.4 Commercial pyrradiometers 175

9.7.5 Radiation thermometry 177

9.8 Sunshine duration 178

9.8.1 Campbell–Stokes sunshine recorder 180

9.8.2 Electronic sensors 181

10 Clouds, Precipitation and Atmospheric Electricity 183

10.1 Introduction 183

10.2 Visual range 183

10.2.1 Point visibility meters 184

10.2.2 Transmissometers 185

10.2.3 Present weather sensors 185

10.3 Cloud base measurements 186

10.4 Rain gauges 187

10.4.1 Tilting siphon 188

10.4.2 Tipping bucket 188

10.4.3 Disdrometers 191

10.5 Atmospheric electricity 191

10.5.1 Potential Gradient instrumentation 191

10.5.2 Variability in the Potential Gradient 192

10.5.3 Lightning detection 193

11 Upper Air Instruments 195

11.1 Radiosondes 195

11.1.1 Sounding balloons 196

11.2 Radiosonde technology 197

11.2.1 Pressure sensor 199

11.2.2 Temperature and humidity sensors 200

11.2.3 Wind measurements from position information 201

11.2.4 Data telemetry 202

11.2.5 Radio transmitter 203

11.3 Uncertainties in radiosonde measurements 204

11.3.1 Response time 204

11.3.2 Radiation errors 204

11.3.3 Wet-bulbing 206

11.3.4 Location error 207

11.3.5 Telemetry errors 208

11.4 Specialist radiosondes 209

11.4.1 Cloud electrification 209

11.4.2 Ozone 209

11.4.3 Radioactivity and cosmic rays 210

11.4.4 Radiation 210

11.4.5 Turbulence 211

11.4.6 Supercooled liquid water 211

11.4.7 Atmospheric aerosol 212

11.5 Aircraft measurements 212

11.5.1 Air temperature 212

11.5.2 Wind 212

11.5.3 Pressure 213

11.5.4 Altitude 213

11.6 Small robotic aircraft 213

12 Further Methods for Environmental Data Analysis 215

12.1 Physical models 215

12.1.1 Surface energy balance 215

12.1.2 Turbulent quantities and eddy covariance 217

12.1.3 Soil temperature model 218

12.1.4 Vertical wind profile 220

12.2 Solar radiation models 222

12.2.1 Langley’s solar radiation method 222

12.2.2 Surface solar radiation: Holland’s model 224

12.3 Statistical models 225

12.3.1 Histograms and distributions 226

12.3.2 Statistical tests 226

12.3.3 Wind gusts 229

12.4 Ensemble averaging 229

12.4.1 Solar radiation variation 230

12.4.2 Pressure tides 231

12.4.3 Carnegie curve 231

12.5 Spectral methods 233

12.5.1 Power spectra 233

12.5.2 Micrometeorological power spectra 235

12.6 Conclusion 237

Appendix A Writing a Brief Instrumentation Paper 239

A.1 Scope of an instrument paper 239

A.2 Structure of an instrument paper 239

A.2.1 Paper title 239

A.2.2 Abstract 240

A.2.3 Keywords 240

A.2.4 Motivation 240

A.2.5 Description 240

A.2.6 Comparison 241

A.2.7 Figures 241

A.2.8 Summary 242

A.2.9 Acknowledgements 242

A.3 Submission and revisions 242

Appendix B Anemometer Coordinate Rotations 243

References 247

Index 253