Dew Point Mirror

Instrumentation

The BCO is equipped with a “Ventilated Thermohygrometer VTP6 Thygan” manufactured by Meteolabor AG. The instrument combines a ventilated and heated thermometer with a mirror-type dew point hygrometer and has been measuring since 2021-07-08.

Fig. 1 The Thermohygrometer VTP 6 with the the air intake visible at the bottom. The air exits the housing through a duct below the lid.

The instrument is installed \(29\,\mathrm{m}\) above mean sea level.

The measurable temperature range is -50 to +50 °C, while the dew point range is -65 to +50 °C. Every ten minutes, the device takes about ten measurements of air temperature and dew point temperature within a 40-second interval. The device provides the mean values along with their standard deviations, allowing data quality to be assessed. The temperature resolution is 0.1 K, and the relative humidity resolution is 0.1%.

The program evaluates three types of device messages: measured values, status message, and raw data. These messages contain the following 17 variables:

Abbreviation	Variable	Device message	Unit
ST	Measurement status	Measured values	-
TT	Air temperature	Measured values	degree_Celsius
DT	Dew point temperature	Measured values	degree_Celsius
RH	Relative humidity	Measured values	%
RTT	Air temperature	Raw data	degree_Celsius
RSTT	Standard deviation of air temperature	Raw data	degree_Celsius
RDT	Dew point temperature	Raw data	degree_Celsius
RSDT	Standard deviation of dew point temperature	Raw data	degree_Celsius
RRH	Relative humidity	Raw data	%
TG	Housing temperature	Raw data	degree_Celsius
VU	Fan voltage	Raw data	V
VI	Fan current	Raw data	mA
K	Value	Raw data	-
RST	Measurement status	Raw data	-
OA	Option 0	Status message	-
OB	Option 1	Status message	-
SW	Status byte W3	Status message	-

The measurement principle

The air temperature is measured by a copper-constantan thermocouple. To measure the dew point temperature, moist air is passed over a mirror. The mirror is cooled until the dew point temperature is reached and condensation occurs. To detect the optical change, a light beam is directed onto the mirror and reflected by it. The intensity of the reflected light beam decreases when condensation forms on the mirror. This reduction can be detected by an optical system.

Data Availability

import intake

cat = intake.open_catalog("https://tcodata.mpimet.mpg.de/catalog.yaml")
ds_dew = cat.BCO.dewpoint.to_dask()
ds_dew

/builds/tco/bco/docs/.venv/lib/python3.12/site-packages/intake_xarray/base.py:21: FutureWarning: The return type of `Dataset.dims` will be changed to return a set of dimension names in future, in order to be more consistent with `DataArray.dims`. To access a mapping from dimension names to lengths, please use `Dataset.sizes`.
  'dims': dict(self._ds.dims),

<xarray.Dataset> Size: 35MB
Dimensions:  (time: 242696)
Coordinates:
    alt      int64 8B ...
    lat      float64 8B ...
    lon      float64 8B ...
  * time     (time) datetime64[ns] 2MB 2021-07-08T20:20:00 ... 2026-07-31T23:...
Data variables: (12/17)
    DT       (time) float64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
    K        (time) float64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
    OA       (time) int64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
    OB       (time) int64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
    RDT      (time) float64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
    RH       (time) float64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
    ...       ...
    ST       (time) int64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
    SW       (time) int64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
    TG       (time) float64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
    TT       (time) float64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
    VI       (time) float64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
    VU       (time) float64 2MB dask.array<chunksize=(242696,), meta=np.ndarray>
Attributes:
    Conventions:           CF-1.12
    _logical_cutoff_date:  2026-08-01T00:00:00Z
    bcoproc_version:       0.0.0.post1660.dev0+9b77b3c
    featureType:           timeSeries
    institution:           Max Planck Institute for Meteorology, Hamburg
    license:               CC0-1.0
    location:              The Barbados Cloud Observatory (BCO), Deebles Poin...
    platform:              BCO
    source:                Ventilated Thermohygrometer VTP 6 (Thygan), Meteol...
    summary:               VTP 6 dew point, air temperature, and relative hum...
    title:                 VTP 6 data from BCO (Level 1)
    tool_versions:         {"Python": "3.11.2 (main, Apr 28 2025, 14:11:48) [...

xarray.Dataset

• Dimensions:
  • time: 242696
• Coordinates: (4)
  • alt
    ()
    int64
    ...

    long_name :

    height above mean sea level

    standard_name :

    altitude

    units :

    m

    [1 values with dtype=int64]

  • lat
    ()
    float64
    ...

    long_name :

    north latitude

    units :

    degree_north

    valid_max :

    90.0

    valid_min :

    -90.0

    [1 values with dtype=float64]

  • lon
    ()
    float64
    ...

    long_name :

    east longitude

    units :

    degree_east

    valid_max :

    180.0

    valid_min :

    -180.0

    [1 values with dtype=float64]

  • time
    (time)
    datetime64[ns]
    2021-07-08T20:20:00 ... 2026-07-...

    axis :

    T

    standard_name :

    time

    units_metadata :

    leap_seconds: utc

    array(['2021-07-08T20:20:00.000000000', '2021-07-08T20:40:00.000000000',
           '2021-07-08T20:50:00.000000000', ..., '2026-07-31T23:30:00.000000000',
           '2026-07-31T23:40:00.000000000', '2026-07-31T23:50:00.000000000'],
          shape=(242696,), dtype='datetime64[ns]')

• Data variables: (17)
  • DT
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    dew point temperature (measured values)

    units :

    degree_Celsius

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

  • K
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    value (raw data)

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

  • OA
    (time)
    int64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    option 0 (status message)

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type int64 numpy.ndarray

  • OB
    (time)
    int64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    option 1 (status message)

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type int64 numpy.ndarray

  • RDT
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    dew point temperature (raw data)

    units :

    degree_Celsius

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

  • RH
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    relative humidity (measured values)

    units :

    %

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

  • RRH
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    relative humidity (raw data)

    units :

    %

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

  • RSDT
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    standard deviation of dew point temperature (raw data)

    units :

    degree_Celsius

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

  • RST
    (time)
    int64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    measurement status (raw data)

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type int64 numpy.ndarray

  • RSTT
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    standard deviation of air temperature (raw data)

    units :

    degree_Celsius

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

  • RTT
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    air temperature (raw data)

    units :

    degree_Celsius

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

  • ST
    (time)
    int64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    measurement status (measured values)

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type int64 numpy.ndarray

  • SW
    (time)
    int64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    status byte W3 (status message)

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type int64 numpy.ndarray

  • TG
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    housing temperature (raw data)

    units :

    degree_Celsius

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

  • TT
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    air temperature (measured values)

    units :

    degree_Celsius

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

  • VI
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    fan current (raw data)

    units :

    mA

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

  • VU
    (time)
    float64
    dask.array<chunksize=(242696,), meta=np.ndarray>

    long_name :

    fan voltage (raw data)

    units :

    V

    Array Chunk Bytes 1.85 MiB 1.85 MiB Shape (242696,) (242696,) Dask graph 1 chunks in 2 graph layers Data type float64 numpy.ndarray

• Indexes: (1)
  • time
    PandasIndex

    PandasIndex(DatetimeIndex(['2021-07-08 20:20:00', '2021-07-08 20:40:00',
                   '2021-07-08 20:50:00', '2021-07-08 21:00:00',
                   '2021-07-08 21:10:00', '2021-07-08 21:20:00',
                   '2021-07-08 21:30:00', '2021-07-08 21:40:00',
                   '2021-07-08 21:50:00', '2021-07-08 22:00:00',
                   ...
                   '2026-07-31 22:20:00', '2026-07-31 22:30:00',
                   '2026-07-31 22:40:00', '2026-07-31 22:50:00',
                   '2026-07-31 23:00:00', '2026-07-31 23:10:00',
                   '2026-07-31 23:20:00', '2026-07-31 23:30:00',
                   '2026-07-31 23:40:00', '2026-07-31 23:50:00'],
                  dtype='datetime64[ns]', name='time', length=242696, freq=None))

• Attributes: (12)

  Conventions :

  CF-1.12

  _logical_cutoff_date :

  2026-08-01T00:00:00Z

  bcoproc_version :

  0.0.0.post1660.dev0+9b77b3c

  featureType :

  timeSeries

  institution :

  Max Planck Institute for Meteorology, Hamburg

  license :

  CC0-1.0

  location :

  The Barbados Cloud Observatory (BCO), Deebles Point, Barbados, West Indies

  platform :

  BCO

  source :

  Ventilated Thermohygrometer VTP 6 (Thygan), Meteolabor AG

  summary :

  VTP 6 dew point, air temperature, and relative humidity measurements at the BCO

  title :

  VTP 6 data from BCO (Level 1)

  tool_versions :

  {"Python": "3.11.2 (main, Apr 28 2025, 14:11:48) [GCC 12.2.0]", "aiohttp": "3.11.18", "propcache": "0.3.1", "ruff": "0.13.0", "jinja2": "3.1.6", "six": "1.17.0", "aiohappyeyeballs": "2.6.1", "pendulum": "3.1.0", "pluggy": "1.5.0", "toolz": "1.0.0", "pytz": "2025.2", "zipp": "3.21.0", "packaging": "25.0", "iniconfig": "2.1.0", "bcologs": "0.0.0.post1660.dev0+9b77b3c", "attrs": "25.3.0", "locket": "1.0.0", "aiosignal": "1.3.2", "netcdf4": "1.7.2", "frozenlist": "1.6.0", "tzdata": "2025.2", "cftime": "1.6.4.post1", "partd": "1.4.2", "xarray": "2025.4.0", "numpy": "2.2.5", "idna": "3.10", "dask": "2025.4.1", "certifi": "2025.4.26", "yarl": "1.20.0", "bcoproc": "0.0.0.post1660.dev0+9b77b3c", "click": "8.1.8", "datarecords": "0.0.0.post1660.dev0+9b77b3c", "multidict": "6.4.3", "cloudpickle": "3.1.1", "importlib_metadata": "8.7.0", "pyyaml": "6.0.2", "fsspec": "2025.3.2", "pandas": "2.2.3", "MarkupSafe": "3.0.3", "python-dateutil": "2.9.0.post0", "pytest": "8.3.5"}

Sample Plot

ds_dew.DT.sel(time="2025").resample(time="1D").mean().plot(label="Dew point")
ds_dew.TT.sel(time="2025").resample(time="1D").mean().plot(label="Air temperature")

[<matplotlib.lines.Line2D at 0x7f49ca051e50>]