Question about temperature measurements

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Discussion Overview

The discussion revolves around the nature of temperature measurements, specifically whether they are taken in the shade or under direct sunlight, and the implications of these conditions on the accuracy of the readings. Participants explore the technical aspects of measurement standards, including the use of Stevenson Screens and the effects of ventilation and albedo on temperature readings.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that temperature measurements are taken in the shade, while others emphasize that readings under direct sunlight do not accurately reflect air temperature.
  • One participant mentions that official measurements are taken at a height of 1.5 meters in a ventilated white shelter to prevent sunlight absorption.
  • Concerns are raised about the lack of specified ventilation rates for these shelters and the potential inaccuracies in historical data due to unventilated conditions.
  • There is a discussion about the albedo of the shelter and its impact on temperature readings, with one participant questioning the ideal conditions for maintaining a high albedo.
  • Another participant introduces the idea that meteorological temperatures are influenced by both true air temperature and wind speed, suggesting a complexity in defining "true" air temperature.

Areas of Agreement / Disagreement

Participants generally agree that shade temperatures are lower than those measured in direct sunlight, but there is disagreement regarding the specifics of measurement standards, ventilation rates, and the implications for historical data accuracy.

Contextual Notes

There are unresolved questions regarding the specifications for ventilation rates in temperature measurement shelters and the historical context of data collection methods. Additionally, the discussion highlights the variability in temperature readings based on environmental conditions.

Simfish
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Used in measured highs/lows/hourly temperatures.

Are they shade temperatures or temperatures under the Sun? Also - are shade temperatures really cooler than temperatures under the Sun?
 
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Yes, they are shade temperatures, and yes, these are often much lower than a reading under direct sunlight. A temperature measurement under direct sunlight will only tell you the temperature of the thermometer itself, not the air temperature.
 
http://www.rickly.com/MI/InstrumentShelter.htm
 
Last edited by a moderator:
When I was in high school, they used to call those things Stevenson Screens.
 
All official measurements are taken about 1.5 meters above the ground, in a white shelter that is ventilated at a certain rate. The white color (ideally) gives the shelter a very high albedo, close to 100%, which means that it won't absorb sunlight and warm up... the ventilation keeps the air mixed and fresh (think greenhouse effect, or lack thereof).

Unfortunately, this can lead to problems... you can imagine that shelters don't stay perfectly white throughout their existence, for instance.
 
Weather Freak said:
All official measurements are taken about 1.5 meters above the ground, in a white shelter that is ventilated at a certain rate.

What rate is that? When was such a specification added? How are "corrections" made for "unventilated" data collected prior to that time?

For the record, there is no ventilation rate specified; there are forced ventilation shelters available, but they are not in common use.

The white color (ideally) gives the shelter a very high albedo, close to 100%, which means that it won't absorb sunlight and warm up... the ventilation keeps the air mixed and fresh (think greenhouse effect, or lack thereof).

"100%?" "Certain conditions and restrictions apply. Substantial penalties for early withdrawal, or in other random events." This is for chalk, clay, lead, or titanium whites? Oil, resin, lacquer, enamel, or latex bases? 5800 K emissivities run 0.1 - 0.3; 300 K emissivities run 0.6 - 0.8; temperature rises can get to 5 - 10 K in still air.

Unfortunately, this can lead to problems... you can imagine that shelters don't stay perfectly white throughout their existence, for instance.

Meteorological temperatures are NOT true air temperatures; they are a combination of true air temperature and wind speed.
 

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