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Using two temperature sensors at one location

Using two temperature sensors at one location by Victor Reijs is licensed under CC BY-NC-SA 4.0

Introduction

Definitions of abbreviations

Measuring temperatures at different heights

• Experiences due to influence of solar radiation (the top curves are temperature, the bottom curves are air pressure):
  
  Blue line: is at the front in an open balcony (the balcony is inside the house: M5/B1). The senor is at ~4.8m from ground level
  Purple line:  is at the back beside a shed in the open (against the shed wall: M3/B2). The senor is at ~1m from ground level.
  Orange line: is at the front in an open capony at the garage (M3/B3). The sensor is at ~1m from ground level.
  
1. The air pressure (at the heigth it is measured) for the location higher up (blue) is lower (purple and orange) around 1.5mbar/3.8m. The general increase is expected as there is less air on top. In general 1mbar change per 8m, so we measure a little higher: around 3mbar/8m.
   
2. During the night; the temperature at higher location (blue) is in many instances higher (negative temperature gradient: night inversion)
3. During the day: the temperature at lower location (purple and orange) is in many instances higher, due to Sun's activity say an hour after Sun rise (positive temperature gradient) and an hour before Sunset. Ths effect is depending of cloud cover.
4. The peak of the temperature is a little later for the high location than for the low location. This is expected as the ground temperature will increased faster than the air temperature [Watts, 2007, figure 8.1].
   
• Exposure of temperature sensors to Sun influences the values considerable. Good protection is essential. The purple sensors (M3/B2) is in the shade always.
  
• One sensor (M3/B2) stands against a PVC North facing wall and opposite a pond. It is not expected that this pond will have influence on the measurements.
  
• The influence of a low pressure region and its fronts
• Looking at temperatures (blue: at 4.8m height and purple: at 1m height) and air pressure (again, as expected the air pressure at 1m height is a little higher than at 4.8m) during the passage of two deep low pressure regions (Bert and Conall). The weather maps are from KNMI.
  
1. When going through a warm front to a warm sector; around 18:00 23-11-2024, the temperatures rise and air pressure is constant.
2. Between 10:00 and 16:00 24-11-2024, the low height temperature is higher than the high height temperature (positive temperature gradient during the day).
3. When being in the warm sector (or perhaps the convergence line, although that is more to the south); around 12:00 24-11-2024, the temperatures stay constant and air pressure is constant.
4. When going through a cold front; around 12:00 25-11-2024, the temperature at the low height drops with regard to the temeprature at the high height, and both drop and air pressure rises.
5. The air pressure behavior in period between bullets 1 en 4, can also be seen in literature [Karnetzki, 2001, page 45]. WF=Warm front; KF=Cold front.
   
   This type of air pressure behavior can indicate the miller for an upcoming or present low pressure region. So not really a warning!
   
6. When going through a warm front to a warm sector; around 12:00 27-11-2024, the temperatures rise and air pressure drops.
7. When going through a cold front; around 18:00 27-11-2024, the temperature at the low height drops with regard to the temeprature at the high height, and both drop and air pressure rises.
• Looking at temperatures (blue: at 4.8m height and purple: at 1m height) and air pressure (again, as expected the air pressure at 1m height is a little higher than at 4.8m) from 29-11-2024 until 5-12-2024. The weather maps are from KNMI.
  
1. One can see the day-to-day weather due to the solar radiation (yellow lines which are more or less centered around midday).
2. Around 07:00 2-12-2024 the high and low location temperatures become similar (through a cold front: part of a stationary front?) and the temperatures diverse again after an occlusion (around 14:00 2-12-2024).
3. It looks that if high and low temperatue are similar (during the night) a low pressure region is coming/passing (remember ths does not have to be a low pressue value).
   So this is an indicator for the miller that a low pressure region is coming.
   
• There a relation between low and high level temperatues with low and high pressure regions. The temperature gradient looks to become smaller in a low presssure regime (see the blue circled area)
  
  or towards a lower pressure (see below green circled area):
  
  This is because at a low pressure region the tropopause is higher than at a high pressure region.
  
  If we assume the temperature at the top and bottom of the air column stay more or less the same; the temperature gradient in low pressure region is lower than for a high pressure region.
  
• When the Sun is above the horizon, the air pressure looks to be influenced: first a very small rise around 10:00 and than a larger lowering around 13:00: see above for the red circled areas.
  This could happen because a sensor's air pressure measurement depends a little on temperature. The BME280 datasheet gives a temperature coefficient TCO_P of ~0.015mbar/°C.
  Three things:
• The air pressure rise/set is independent of the location of the sensor, so both the low and high level sensors have this behavior.
  
• The temperatue increase could map the slight increase of air pressure, but the end of the air pressure decrease looks to happen around the peak in temperature (around 14:00) and not later if there would a direct relation between air pressure and temperature.
  
• The air pressure decrease is around 0.2mbar while the temperature increase is around 1°C (but can be more, in above graph up to 3°C, on 16 Febr due to Sun ligth through tree leaves).
  So the air pressure decrease looks to be a factor 10 larger than expected due to the temperature coefficient TCO_P.
  So is there an atmospheric reason for this air pressure behavior?
  
• Passing of high trough
  This gives three short cold spells, with rain and hail.
  
  So dips in temperature can indicate to the miller that a high trough is present (so is not really a warning, as it is quite imenent and thus not an ahead warning).
  
• asdfsafs
  

Why monitor temperature and air pressure sensors

The following aspects can be derived when having two temperature sensors (high and low) and one air pressure sensor (more air pressure sensors are not needed):

• Sun rise/set (see here):
  The temperature strats to rise more or less one hour after Sun rise
  The temperature strats to set more or less one hour before Sun set
• The temperature gradient (lapse rate):
  ${\displaystyle \mathrm{\Gamma }}$ = -(T_high-T_low)/(H_high-H_low) [°C/km]
• Freezing point can help around slippery surfaces.
  
• Terminology of air pressure change:
  Falling (or rising) slowly: 0.1 - 1.5mb/3hours
  Falling (or rising): 1.6 - 3.5mb/3hours
  Falling (or rising) quickly: 3.6 - 6.0mb/3hours
  Falling (or rising) very rapidly: More than 6.0mb/3hours
• Wind velocity depending on rise/fall of air pressure
• At sea:
  1mb/hour: likely 6Bft
  2mb/hour: likely 7 - 8Bft
  3mb/hour: likely exceeding Force 8Bft
• At land:
  1 or 2 Bft lower
• This change in air pressure can indicate to the miller that severe wind is possibly coming.
  
• Understanding of the behavior of low pressure region and the wam and cold fronts on temperatures and air pressure.
  

Conslusions

• Air pressure at different height is behaving as expected: the height has influence due to the air column being different. No significant changes yet seen due to low/high pressure regions.
  
• More conclusions will be added when made;-)
  

References

Acknowledgements

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