the map is not the area

[Hans Blom, 960912]

An example that Bill Powers mentioned a few times is the wet-bulb
thermometer. Not being familiar with the instrument, I did a net
search and came up with some definitions. Here they are:

  psychrometer: Any instrument used to measure the water vapor
  content of an air parcel (atmospheric humidity) by evaporative
  cooling of the air to its wet bulb temperature. Examples include
  a sling and an aspirated psychrometer. The psychrometer consists
  of two liquid-in-glass thermometers, one of which (dry-bulb)
  measures the ambient air temperature. The bulb of the other
  thermometer (wet-bulb) is covered with a muslin sock which is
  saturated with distilled water prior to an observation. When
  ventilated, the instrument indicates the dry-bulb and wet-bulb
  temperatures. Dewpoint and relative humidity can be determined
  from the wet bulb depression using a psychrometric table. See
  also hygrometer.

  wet-bulb thermometer (or wet-bulb): The thermal sensor in a
  psychrometer that has a muslin covered bulb which is saturated
  with water and then ventilated thoroughly to attain the wet-bulb
  temperature. Contrast with dry-bulb thermometer.

  wet-bulb temperature: The temperature obtained by covering the
  bulb of a dry-bulb thermometer with a silk or cotton wick
  saturated with distilled water and drawing air over it at a
  velocity not less than 1000 ft/min. This is often accomplished
  by swinging the covered thermometer on the end of a string or
  rope. If the atmosphere is saturated with water vapor, the water
  in the wick will not evaporate and the dry and wet bulb
  temperatures will be the same. If the atmosphere is not
  completely saturated, the water will evaporate from the wick at
  a rate dependent upon the degree of saturation. The evaporation
  will cool the bulb and lower the temperature reading over that
  of the dry-bulb temperature to that of the wet-bulb temperature.

  dry-bulb temperature: The temperature of the ambient air and
  water vapor mixture measured by a thermometer where the thermal
  element is dry and shielded from radiation.

Thus, wet-bulb temperature and dry-bulb temperature are measure-
ments obtained with suitable sensors under appropriate conditions.
"Dewpoint and relative humidity can be determined from the wet
bulb [temperature] depression using a psychrometric table." So
dewpoint and relative humidity are measurement-derived abstract
concepts that are part of our scientific theories. Do they (also)
have an "independent existence in the environment"? How about
force, acceleration, mass?