# Presentation: 658675

## Temperature and Humidity Paul M. Fransioli, CCM

- AMS Short Course on
- Introduction to Measurements and
- Observation Techniques
- January 14, 2001 - Albuquerque, New Mexico

- Temperature and Humidity

- 01/14/2001

## Temperature and Humidity

- Topics to be covered
- Instrumentation
- Field Measurements
- Field Checks and Calibrations
- Calculations

- 01/14/2001

## Introduction

- First Step: What is the question?
- General weather conditions
- Support another activity, e.g. environmental monitoring
- Representative of specific area
- Vertical gradient (e.g. atmospheric stability or flux)
- Time average - mean and/or extreme

- Temperature and Humidity

- 01/14/2001

## Temperature Instruments

- Electrical Thermometers
- Thermocouple
- More commonly used in industrial applications
- Resistance temperature detector (RTD)
- Good features, but can be sensitive to installation wiring
- Thermistor
- Good answer for many applications

- Temperature and Humidity

- 01/14/2001

## Temperature Instruments

- Manual Thermometers and Thermographs
- Liquid in glass thermometers
- Basic standard for many years
- Bi-metallic sensor
- Mechanical link to pen for graphs
- Slower response times, less resolution, etc.
- Understand historical data limitations

- Temperature and Humidity

- 01/14/2001

## Humidity Instruments

- Electrical Hygrometers
- Hygroscopic films detect atmospheric moisture
- Dew point impedance from partial pressure of water vapor
- Chilled mirror
- Optically sense presence of dew or frost on mirror
- Measure temperature of mirror for dew point or frost point
- Recent optical sensing improvements

- Temperature and Humidity

- 01/14/2001

## Humidity Instruments

- Manual hygrometers
- Psychrometer
- Paired dry-bulb and wet-bulb thermometers
- Manual and semi-automated
- Hygrographs
- Hygroscopic material sensor (e.g. hair)
- Mechanical link to pen

- Temperature and Humidity

- 01/14/2001

## Field Measurement

- Siting and Instrument Exposure
- Base on purpose of measurement and local logistics
- See monitoring guidance in text References
- Aspirated or naturally ventilated shields (important)
- Avoid local sources of heat and moisture

- Temperature and Humidity

- 01/14/2001

## Field Measurement

- Delta-temperature (vertical gradient)
- Choose levels, such as 2 to 10 meters agl for EPA stability
- Match sensors to optimize accuracy
- Documentation
- Pictures worth many megabytes of words
- Geographic coordinates and reference system

- Temperature and Humidity

- 01/14/2001

## Field Measurement

- Measurement system and data collection
- Merge into total system requirements
- On-site processing
- Time averages (fractional seconds to hour or daily)
- Glass thermometer: approximately 1-min average
- ASOS 5-min; ASTM 10-min; typical hourly
- Mean (true or median of range), extremes
- Other calculations

- Temperature and Humidity

- 01/14/2001

## Field Checks and Calibrations

- Routine checks
- External changes to sensor or shield (material, coating)
- Component checks (electronic)
- Calibrations
- Determine accuracy and resolution requirements
- Place sensor in known environment
- Total system check - use normal readout in operating mode

- Temperature and Humidity

- 01/14/2001

## Calculations

- Measure temperature and relative humidity
- Calculate dew point directly, or
- Calculate dew point indirectly use vapor pressure
- Dew point is based on vapor pressure over water,             by convention
- Calculation is sensitive to RH below about 40 percent

- Temperature and Humidity

- 01/14/2001

## Calculations

- Measure dry-bulb and wet-bulb temperatures
- To convert to dew point:
- Calculate vapor pressure, then dew point
- Calculate saturation vapor pressure from temperature
- Now you can worry about frozen bulbs

- Temperature and Humidity

- 01/14/2001

## Calculate vapor pressure

- Calculate saturation vapor pressure from temperature
- 	Temperature: *t*, degrees Celsius
- 	Saturation vapor pressure: *e**s*, millibars or hecto-Pascals
- Reasonable option: Campbell Scientific P56 statement
- Also to calculate vapor pressure from dew-point temperature

- Temperature and Humidity

- 01/14/2001

## Calculate dew point from vapor pressure

- Dew point: *T**d*, degrees Celsius
- Vapor pressure: *e*, millibars or hecto-Pascals
- (hint: inverse of previous equation)

- Temperature and Humidity

- 01/14/2001

## Calculate dew point from temperature and relative humidity (1)

- Intermediate calculated term *A* from:
- 	Temperature: *t*, degrees Celsius
- 	Relative humidity: *RH*, percent

- Temperature and Humidity

- 01/14/2001

## Calculate dew point from temperature and relative humidity (2)

- Dew point: *T**d*, degrees Celsius
- Intermediate term: *A* (see previous slide)
- 	note: *T**d **= 0 C *for* RH = 100% *and* t = 0 C*

- Temperature and Humidity

- 01/14/2001

## Calculate vapor pressure from wet-bulb and dry-bulb temperatures

- Vapor pressure: *e*, millibars or hecto-Pascals
- Dry-bulb temperature: *t*, degrees Celsius
- Wet-bulb temperature: *tw*, degrees Celsius
- Barometric pressure: *p*, millibars or hecto-Pascals
- Then calculate dew point from equation on slide 15

- Temperature and Humidity

- 01/14/2001

## Equipment

- Relative humidity probe with built-in temperature sensor
- Motor aspirated shield for temperature sensor
- (courtesy Met One, Inc.)

- Temperature and Humidity

- 01/14/2001

## Equipment

- Naturally ventilated shield
- Wooden shelter for thermometers or hygrothermograph
- (courtesy Nova Lynx, Inc.)

- Temperature and Humidity

- 01/14/2001

## Equipment

- Motor aspirated shield
- Naturally ventilated shield
- (courtesy R. M. Young, Inc.)

- Temperature and Humidity

- 01/14/2001

## Equipment

- Motor aspirated shield
- (courtesy Climatronics, Inc.)

- Temperature and Humidity

- 01/14/2001