Help Page for APEX Weather Query Form

The APEX weather station is located on a free-standing 6m high tower located about 50m West of the APEX telescope. The sensors are from Vaisala, except the temperature sensor, which is from Stego. The APEX radiometer was built by Radiometer Physics GmbH (MeckenHeim, Germany). It is located in the Cassegrain cabin of the APEX telescope and it is calibrated roughly every 2 weeks by the APEX staff using a bath of liquid nitrogen.
Between 1 January and 11 April 2013, the APEX radiometer was refurbished to install 6 instead of 3 IF bands (see figure below). As a result, there are no PWV measurements for this period.

The ESO archive database contains all available APEX weather data from January, 1st 2006 onwards. Data are recorded every minute and transferred to the database in Garching every 10-15 minutes. For each record in the results table, a link to the graphical overview of the ambient data is available for the corresponding day.

The Precipitable Water Vapour (PWV) is measured by the APEX radiometer. The PWV is determined using the ATM library, using a mean atmosphere for Chajnantor (see http://www.apex-telescope.org/sites/chajnantor/atmosphere/).

Telescope azimuth and elevation are provided to determine the direction to which the water vapour radiometer (located in the APEX Cassegrain cabin) was pointing at the time of the PWV measurement. The measurement itself has been corrected towards zenith. Note that the telescope park position is Az=180, El=15.

If the APEX shutter is closed, the APEX radiometer does not see the sky. The shutter is closed as a precaution in case of bad weather when snow or ice may build up. Hence, the PWV values are somewhat biased against the worst weather conditions.

The Control Box Temperature is measured inside the electronics box on the weather tower. This box is heated to around 20 Celsius by a special probe to ensure the proper functionality of weather station. During 2008, this sensor failed, causing a FAULTED flag to be given in the weather station state. However, the measurements are still mostly reliable as the heating probe was still functioning properly.

The APEX radiometer measures the brightness temperature in three (2006 to 2012) or six (as of 2013) double side band channels centered on the 183.31 GHz line of H2O. These parameters changed after a refurbishment of the APEX radiometer in early 2013. The sideband channels are:

2006 to 2012 (blue in figure)		from 2013 (red in figure)
Off-set frequency (GHz)	Bandwidth (GHz)	Off-set frequency (GHz)	Bandwidth (GHz)
1.3	0.5	0.6	0.2
4.2	1.0	1.5	0.2
6.5	1.0	2.5	0.2
		3.5	0.2
		5.0	0.4
		7.5	0.5

Atmospheric spectrum near 183.31 GHz as seen from an observatory at 5.1 km altitude looking at the zenith, for five different PWV columns ranging from 0.2 to 4.0 mm (curves from top to bottom). The radiometer actually measures the sky brightness temperatures at three or six offset frequencies as listed in the Table above.