Chilled Mirror Hygrometer (CMH)
Instrument Categories: Surface Meteorology
General Overview
Important Note to CM Data Users: Chilled mirror (CM) moisture and temperature measurements may not be representative of the true atmospheric state because the observations are made inside a large mailbox aspirator. The aspirator is used to equilibrate radiosondes with the atmosphere before launch, and the CM observations are designed to be used for radiosonde scaling. Due to the physical size of the mailbox aspirator, the temperature and relative humidity inside the aspirator box will be different from the free atmosphere. During the day, the temperature inside the box may be too high owing to solar heating, which will result in an anomalously low relative humidity. The opposite may be true at night. Calculating a dew point from the temperature and relative humidity should result in an accurate moisture measurement, but this assumes several things that may or may not be true at any given time. Therefore, it is recommended that measurements from the CM system be used with caution. Please contact the instrument mentor with any questions as to the validity and applicability of the CM data.
The CM systems have been developed for the ARM Program to act as a moisture standard traceable to National Institute of Standards and Technology (NIST). There are three CM systems that are each fully portable, self-contained, and require only 110 V AC power. The systems include a CM sensor, air sampling and filtration system, a secondary reference (Rotronic HP043 temperature and relative humidity sensor) to detect system malfunctions, a data acquisition system, and data storage for more than one month of 1-minute data. The CM sensor directly measures dew point temperature at 1 m, air temperature at 2 m, and relative humidity at 2 m. These measurements are intended to represent self-standing data streams that can be used independently or in combinations.
One CM was installed at the Southern Great Plains (SGP) Central Facility (CF) Balloon-Borne Sounding System (SONDE) location for continuous operation. There are a total of three CM systems available for use in field campaigns and special projects. For example, CMs were located on the CF 60-m tower at 25 and 60 m during field campaigns.
The CM system uses a General Eastern Instruments D2/E4 combination system with a dew point inaccuracy of approximately +/- 0.1 to 0.2°C. The response time of the system is roughly 5 minutes due to the sampling system and it has a maximum dew point depression of about 65°C.
For more detailed information, see the Chilled Mirror Dew Point Hygrometer Handbook.
IOPs Where the Instrument was Used
ARM-FIRE Water Vapor Experiment
Southern Great Plains, November - December 2000
Fall 1997 Water Vapor IOP
Southern Great Plains, September 15 - October 5, 1997
Primary Measurements Taken
The following measurements are those considered scientifically relevant. Refer to the datastream (netcdf) file headers for the list of all available measurements, including those recorded for diagnostic or quality assurance purposes.
Mentor(s)
Michael Ritsche
(630) 252-1554
mtritsche@anl.gov
David Cook
associate
(630) 252-5840
drcook@anl.gov
