CCNY Weather Station
DATA Definitions and FORMATs

Weather Station data is collected at one minute intervals and presented in columnar format.      Column headings and data descriptions are presented below.  For your convenience in data utilization and manipulation, they are numbered from 1 to 45, corresponding to the 45 columns in the daily data sets presented in the archives.  The archived data directory can be found at http://earth.engr.ccny.cuny.edu/noaa/wc/DailyData/.

 

DATASET TIME STAMP:

Col Num

Column Heading

Data Definition, Description, Derivation, References and Notes

1

Measurement Date, time  MM/DD/YY 24h

Date and time of the data collection.  The date is presented in american standard format, Julian date (MM/DD/YY) followed by a comma, then the time in 24 hour military format (hh:mm).  The weather station clock is referenced to the host computer time, which in turn is referenced to the NIST-F1 Cesium Fountain Atomic Clock, verified and corrected weekly on Sunday night at 23:45 p.m..  Please see (http://www.boulder.nist.gov/timefreq/service/its.htm) for more information.

The time is NOT corrected for Daylight Savings Time to avoid collection data loss, duplication and the unwelcome requirement to change the unattended weather station programs on variable dates.  Our data time is EST (Eastern Standard Time), and always is -05:00 hrs GMT (Greenwich Mean Time.)  More information about Daylight Savings Time may be obtained at:

http://webexhibits.org/daylightsaving/b.html.

 

WIND:

2

Wind Speed m/s inst

Wind Speed in m/s (meters per second) at the end of the one-minute collection interval, the last value collected.

3

Wind Speed m/s Max

The maximum Wind Speed detected within the one-minute data collection interval.

4

Wind Speed m/s Avg

The measured raw data is:

      Si = horizontal wind speed of an individual measurement
      Θi = horizontal wind direction of an individual measurement
      N = number of samples.

This is the time-weighted average, or mean wind speed during the one-minute data collection interval.  The short, individual head-to-tail vectors of each measurement are the input sample vectors described by Si and Θi, the sample speed and polar direction.  At the end of the output interval T, the sum of the sample vectors is described by a vector U and direction .  If the input sample interval is t, the number of samples in output interval T is N = T/t.  The mean vector magnitude is .The scalar mean horizontal wind speed is described as:

     

For discussion of the performance accuracy of this class of instrument, you may refer to:

Gill, G.C., 1973:  The Helicoid Anemometer, Atmosphere, II, 145-155.
Baynton, H.W., 1976:  Errors in Wind Run Estimates from Rotational Anemometers,  Bul. Am. Met. Soc., vol. 57, No. 9, 1127-1130.

5

Wind Dir

The wind direction, or azimuth, in degrees from true North.  East would be 90 degrees, and West 270.

6

Wind Vector S wvt

The Scalar Mean Horizontal Wind Speed during the one-minute data collection interval:

     

7

Wind Vector D1 wvt

The Unit Mean Vector Wind Direction during the one-minute data collection interval, Θ1:

      ,

where

     

     

This term may be averaged over the selected time interval.  See (8) below.

8

Wind Vector Sd1 wvt

The Standard Deviation of Wind Direction using the Yamartino algorithm.  This method complies with EPA guidelines for use with straight-line Gaussian dispersion models to model plume transport.  The standard deviation, σ(Θ1), using the Yamartino algorithm:

     

where

      ,

and Ux and Uy are as defined in column 7 above.

Standard deviation is a measure of variability about a mean.  If the wind speed direction changes over the output interval, the standard deviation is erroneously high.  The longer the output interval, the greater the chance that the wind direction will change.  This is especially true under light or meandering wind conditions.  To reduce this error, the EPA recommends that hourly standard deviation of horizontal wind direction be computed.  For our one minute data collection intervals, the following equation can be used to arrive at hourly standard deviation:

     

39

Wind Speed
U

The resultant mean horizontal wind speed, :

     

where (for our polar sensor)

     

     

40

Wind Dir
DU

Resultant mean wind direction,

     

41

Wind Dir
SDU

Standard deviation of wind direction, , using the Campbell Scientific algorithm:

      ,
where S is the scalar mean horizontal wind speed (column 4, above.)

42

Wind Dir @ Max WS

Wind direction at the time of the maximum wind speed detected within the one-minute data collection interval

 

AIR:

9

Air
Temp ºC
inst

Instantaneous Air Temperature reading in °C at the end of the one-minute collection interval.

10

Air
Temp Min.

Air Temperature Minimum in °C during the one-minute collection interval.

11

Air
Temp Max.

Air Temperature Maximum in °C at the end of the one-minute collection interval.

12

Air
Temp Avg.

Air Temperature Average in °C during the one-minute collection interval.

45

Air Temp SD

Standard deviation of the air temperatures recorded during the one-minute data collection interval.

13

Dew Point

Dew Point in °C at the end of the one-minute collection interval. Dew Point is the temperature at which the water vapor in the air begins to condense, and is calculated from the following equations and scheme:

Td=(241.88*ln(Vp/0.61078))/(17.558-ln(Vp/0.61078))

where

Td = dew point (°C)

and Vp = vapor pressure (kPa)

This equation is an inverse of a version of Teten's equation, optimized for dew points in the range -35 to 50°C, and is accurate to within ±0.1°C within that range.

Vapor pressure is calculated within the datalogger using the following equation:

Vp=RH*SVp/100

where

RH = relative humidity (%), and

SVp = saturation vapor pressure (kPa), calculated by the datalogger with the following approximating polynomial (see Lowe):

SVp=6.107799961 + T*(4.436518521*10^-1 + T*(1.428945805*10^-2 + T*(2.650648471*10^-4 + T*(3.031240396*10^-6 + T*(2.034080948*10^-8 + 6.136820929*10^-11*T)))))

where

T = air temperature (dry-bulb temperature) (°C)

Please consult the following references:

http://meted.ucar.edu/awips/validate/dewpnt.htm
Campbell Scientific Technical Note 16
Lowe, P.R. 1930. J. Appl. Meteor., 16:100-103
Tetens, O. 1930. Z. Geophys., 6:297

14

Wet Bulb Temp

Wet bulb Temperature in °C at the end of the one-minute collection interval.  It is defined as the lowest temperature that can be obtained by evaporating water into the air at a given constant pressure.  The term comes from the technique of wrapping a wet cloth around a mercury bulb thermometer and blowing air over the cloth until the water evaporates.  The wet bulb temperature is always lower than the dry bulb temperature (temperature measured without a wet cloth) in the same surroundings, because of evaporative cooling.  The wet bulb and dry bulb temperatures can be used to calculate dew point and relative humidity. Our Wet-bulb temperature is derived using an iterative process. The wet-bulb temperature lies somewhere between the dry-bulb temperature (air temperature) and the previously calculated dew point temperature.

The datalogger uses the following National Weather Service algorithm to calculate vapor pressure using the dry-bulb temperature and a first approximation wet-bulb temperature guess:

          VP = VPW-A(1+B*TW)(TA-TW)SP,

where,

          VP = ambient vapor pressure, kPa
          VPW = saturation vapor pressure at the wet bulb temp, kPa
          TW = wet bulb temperature, ºC
          TA = ambient air temperature (dry-bulb temperature), ºC
          SP = standard air pressure (kPa) at the user entered elevation
          A = 0.000660
          B = 0.00115

The resulting vapor pressure is compared to the true vapor pressure (see above) and the difference determines the next wet-bulb temperature guess.  The process repeats until the difference between the current wet-bulb temperature guess and the previous wet-bulb temperature guess is only ±0.01°C. The datalogger thus derives the wet-bulb temperature.

Additional information may be obtained at:

http://meted.ucar.edu/awips/validate/wetblb.htm

15

Heat Index

Heat Index in °C in °C during the one-minute collection interval.  Heat Index values are only valid for air temperatures greater than 27 °C (80 °F), dew point temperatures greater than 12 °C (65 °F), and relative humidities higher than 40 percent.  The weather station algorithm sets the heat index temperature equal to the current air temperature if air temperature is less than 80°F (27°C) or relative humidity is less than 40% or the heat index is less than the current air temperature.  The equation and discussion of its relevance can be found at:

http://www.srh.noaa.gov/bmx/tables/heat_index.html

16

Heat Index Max

Heat Index Maximum in °C at the end of the one-minute collection interval.

17

Heat Index Min

Heat Index Minimum in °C at the end of the one-minute collection interval.

18

Heat Index Avg

Heat Index Average in °C at the end of the one-minute collection interval.

19

Wind Chill

Wind Chill in °C at the end of the one-minute collection interval.  For a discussion of the old and new calculation methods, you may  refer to:

http://www.srh.noaa.gov/bmx/tables/the_new_chill.html

20

Wind Chill Max

Wind Chill Maximum in °C during the one-minute collection interval.

21

Wind Chill Min

Wind Chill Minimum in °C during the one-minute collection interval.

22

Wind Chill Avg

Wind Chill Average in °C during the one-minute collection interval.

23

Rel Humid inst

Instantaneous Relative Humidity in percent (%) at the end of the one-minute data collection interval (last value collected.)  The interrelationship of Relative Humidity and Dew Point is approximated by the equation:

          ,

where T = Air Temperature in °C and Td = .the Dew Point Temperature in °C.  This equation is accurate within 0.6 percent within the range of -25 to 45 °C (-13 to 113 °F.)

24

Rel Humid Max

Relative Humidity Maximum during the one-minute collection interval.

25

Rel Humid Min

Relative Humidity Minimum during the one-minute collection interval.

38

Rel Humid Avg

Relative Humidity Average during the one-minute collection interval.

43

Rel Humid SD

Standard deviation of the relative humidity measurements during the one-minute data collection interval.

26

Bar hPa inst

Barometric Pressure in hPa (mbar) at the end of the one-minute collection interval.

27

Bar hPa Max

Barometric Pressure Maximum during the one-minute collection interval.

28

Bar hPa Min

Barometric Pressure Minimum during the one-minute collection interval.

29

Bar hPa Avg

Barometric Pressure Average during the one-minute collection interval.

44

Bar hPa SD

Standard deviation of the air pressure values recorded during the one-minute data collection interval.

 

PRECIPITATION: RAIN, FREEZING RAIN, SLEET, HAIL AND SNOW:

30

Rain

Rainfall is measured by a tipping bucket rain gauge on the occurrence of the accumulation of 0.01 inch of water.  The number of measurements are accumulated and totaled at the end of the one-minute collection interval.  Our rain gage is NOT equipped with a heated collection funnel, therefore snow, sleet and freezing rain will not be correctly reported unless or until the ambient temperature is above 0 °C.

 

ENVIRONMENTAL DATA:

31

Hourly ETo

Hourly ETo (reference crop evapotranspiration) is calculated by the FAO 56 Penman-Monteith equation using the measurements of wind speed, air temperature, relative humidity, and solar radiation.

References:

Richard G. Allen, Luis S. Pereira, Dirk Raes, and Martin Smith, "Crop Evapotranspiration, Guidelines for Computing Crop Water Requirements, FAO Irrigation and Drainage Paper 56", Food and Agriculture Organization of the United Nations, Rome, 1998.

32

Daily ETo

Daily ETo is the progressive summation of the hourly data derivation of Hourly ETo.

 

SOLAR DATA:

33

Solar Flux Den W/m2

Solar Flux Density in Watts/m2 measured at the end of the one-minute collection interval.

34

Solar Flux Max W/m2

Solar Maximum Flux Density observed during the one-minute collection interval.

35

Solar Flux Min W/m2

Solar Minimum Flux Density observed during the one-minute collection interval.

36

Solar Flux Avg W/m2

Average Flux Density during the one-minute collection interval.

37

Solar Total Flux kJ/m2

Solar Total Flux in kJ/m2 accumulated during the one-minute data collection interval.  A daily total solar flux can be determined by summing this column in any daily reporting.

 

This page created by Tom Legbandt.  Last updated on 19 July, 2005.

Contact weatherman@ee.ccny.cuny.edu regarding this page's content or broken links.