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Aims.  Some astronomical facilities are in operation at the Sierra Negra volcano (SNV), at ∼4.5 km over the sea level (o.s.l.) in Mexico. We asses whether it is possible to estimate the opacity for millimeter-wavelength observations based on the meteorological parameters at the site. A criterion for allowing astronomical observations at SNV depends on the atmospheric opacity at 225 GHz, which has to be  τ  225  ≤ 0.30 Nepers. The correlation of the opacity at SNV, measured with a radiometer at 225 GHz,  τ  225 , with the local dew point temperature,  T  DP , the water vapor pressure,  P  H2O  and the water vapor content (WVC) at SNV is studied with the aim to determine whether these parameters can be used to estimate the opacity at similar high-altitude locations for astronomical observations at millimeter wavelengths.   Methods.  We used radiosonde data taken in various decades in Mexico City (MX) and Veracruz City (VR) to compute the WVC in 0.5 km altitude ( h ) intervals from 0 km for VR and from 2.0 km for MX to 9.5 km o.s.l. to study the altitude profile WVC( h ) at SNV by interpolating data of MX and VR. We also fit exponential functions to observed WVC (WVC obs ( h )), obtaining a fit WVC (WVC ftd ( h )). The WVC obs ( h ) and WVC ftd ( h ) were integrated, from lower limits of  h  low  = 2.5–5.5 km to the upper limit of 9.5 km as a measure of the input of WVC obs ( h  ≥  h  low ) to the precipitable water vapor.   Results.  The largest differences between WVC obs  and WVC ftd  values occur at low altitudes. The input of WVC obs ( h ) to the precitpitable water vapor for  h  ≥ 4.5 km ranges from 15% to 29%. At 4.5–5.0 km, the input is between 4% and 8%. This means that it is about a third of the WVC ( h  ≥ 4.5 km). The input above our limit (from 9.5–30.0 km) is estimated with WVC ftd ( h ) and is found to be lower than 1%. The correlation of  τ  225  with  T  DP ,  P  H2O , and WVC SNV  takes values between 0.6 and 0.8. A functional relation is proposed based on simultaneous data taken in 2013–2015, according to which it is possible to estimate the opacity with the  T  DP ,  P  H2O , or WVC SNV  at the site.   Conclusions.  With local meteorological parameters, it is possible to know whether the opacity meets the condition  τ  225  ≤ 0.30 Nepers, with an uncertainty of ±0.16 Nepers. The uncertainty is low for low opacities and increases with increasing opacity.

Local dew-point temperature, water vapor pressure, and millimeter-wavelength opacity at the Sierra Negra volcano