MIMO 1.2

• Basic considerations for site selection and instrument exposure as suggested by the WMO:

  • Surface ground cover should be representative of the location
  • Outdoor instruments should be installed on level ground
  • Sharp topographical gradients in the vicinity should be avoided
  • Sites should be well away from obstructions such as buildings, walls, and trees to avoid interference
  • For most sites, the preference is for openness and homogeneity but not for every instrument; for example, rainfall amount can be significantly affected when rain gauges are exposed to wind of even moderate speed
• Instruments used for observing sunshine, radiation, clouds, cloud coverage, sky condition, and visibility should be as open as possible and command the widest possible view of the sky and the surrounding area
• External lighting should be avoided or dimmed for nighttime observations such as clouds or visibility
• Estimation of wind speed and direction are best performed outside of an enclosure
• Site locations must be physically, politically, and economically accessible and have adequate infrastructure (e.g., roads, power, communication, staff accommodations)
• Sites that will be used for global climate monitoring must be a sufficient distance from urban environments to avoid the heat island effect
• Key Components of a Measuring System:

  • The measurand is the measured property of the atmosphere
  • Three basic components: Sensor, Data acquisition/signal conditioning, Display/recording system
• Sensor:

  • Provides an output, usually an electrical property like voltage or resistance, that has a known relationship to the atmospheric property being measured
  • Includes components like sensing element, exciter, signal processor
• A display and recording system usually consists of two components: Data display and Data recorder
• Performance Characteristics of Instruments:

  • Error in measurement is the difference between the measurement and the value of the measurand
  • "Uncertainty" characterizes the dispersion of the values reasonably attributed to a measurand based on the measurement and characteristics of the instrument
• Types of Error:

  • Systematic error is the mean error that would result from averaging a very large number of measurements of the same measurand carried out under the same conditions
  • Bias can be reduced by calibration, which may indicate a correction to be applied to the measurement
  • Random error is the scatter about the mean in a set of measurements
  • “Operator errors” can be systematic (e.g., an observer consistently but incorrectly reading the top of a meniscus)
  • illegitimate (e.g., recording an incorrect value) they are so hard to identify or characterize
• Accuracy vs. Precision:

  • Accuracy refers to how close a measurement is to its agreed value
  • Precision refers to how close two or more measurements are to each other, regardless of accuracy
• Sources of Error:

  • Errors may arise from incorrect calibration, time lag, drift, or exposure
• Calibration is the process of correlating an observation from one instrument with an observation from another instrument of known quality; these are called standards
• Dimensions, units, and symbols for quantities like mass, area, volume, time, pressure, and plane angle are provided in a table

  
• The table also gives equivalencies of each unit to its base unit, for example, 1 tonne is equivalent to 1000 kilograms

  
• The table provides dimensions, units, and symbols for various quantities, including mass, area, volume, time, pressure, and plane angle

  
• The table includes equivalencies of each unit to its base unit, like 1 tonne being equivalent to 1000 kilograms

  
• The table includes dimensions, units, and symbols for quantities such as mass, area, volume, time, pressure, and plane angle

  
• The table provides dimensions, units, and symbols for quantities such as mass, area, volume, time, pressure, and plane angle

  
• The table includes equivalencies of each unit to its base unit, for example, 1 tonne is equivalent to 1000 kilograms

  
• The table provides dimensions, units, and symbols for quantities like mass, area, volume, time, pressure, and plane angle

  
• The table also gives equivalencies of each unit to its base unit, like 1 tonne is equivalent to 1000 kilograms

  
• The table includes dimensions, units, and symbols for various quantities, including mass, area, volume, time, pressure, and plane angle