4 ORM climatology based on CAMC database

Previous research concerning meteorological parameters at ORM measured in different locations of the Observatory used data gathered from the telescopes as part as its routine operation. The wider and more systematic meteorological record comes however from the Carlsberg Automatic Meridian Circle Telescope (CAMC) . This telescope has been in continuous operation since 1984 in order to determine accurate star positions, as well as provide standard surface meteorological data to the astronomical community. From the monitoring of stars during the night extinction coefficients in the broad Johnson V band are computed for every single night.

The CAMC data were taken with sensors attached to a mast installed near the telescope building and include measurements of air temperature, atmospheric pressure, wind speed and direction and relative humidity. The interior air temperature of the telescope building is also taken. The data set is completed with the values of nightly atmospheric extinction in addition to quality flags for each meteorological parameter.

The meteorological sensors were read automatically by an Olivetti desk-top computer and stored on disk. Readings were made at 5 minute intervals throughout the day and night, even during unfavourable astronomical conditions. No readings were available when the computer was down or a sensor became faulty. The atmospheric extinction (A_V) is referred to the zenith (Carlsberg 1993).

The database includes more than ten years of measurements, ranging from 1984 to 1995 from which we excluded the volcanic eruption periods, before 1987 January and after 1991 April (Guerrero et al. 1998).

4.1 Air temperature

The statistical results of the air temperature at CAMC site are presented in Figs. 2-4. In Fig. 2, we present the monthly variation of the mean, minimum and maximum temperature at ORM. The daily mean temperature is $8.37~^{\circ}$C ( $4.95~^{\circ}$C in winter, $11.42~^{\circ}$C in summer). The absolute minimum and maximum are $-9.4~^{\circ}$C and $27~^{\circ}$C respectively. The night-time and daytime mean temperature are $% 7.27~^{\circ}$C and $9.8~^{\circ}$C, respectively. In Fig. 3 we noticed a small variation of temperature during the day, and remarkable stability during the night-time.

Figure 4 shows the difference in temperature between two consecutive nights at ORM. We noticed a very small variation (in the mean curve), which confirms the high stability of the night-to-night temperature at ORM. The mean difference value is $1.6~^{\circ}$C which gives a dome and mirror seeing about 0.17 and 0.70 arcsec, respectively. The maximum difference value is $8.6~^{\circ}$C recorded in November which might give rise to a dome and mirror seeing contribution about 1.32 and 5.29 arcsec, respectively.

4.2 Atmospheric pressure

Figure 13 shows the monthly variation of the atmospheric pressure. We observed that the pressure has low seasonal change and the mean pressure is 774.12 mbar higher than the theoretical pressure 88% of time. The theoretical value, calculated using a standard model, is equal to 760 mbar (Triplet & Roche 1986). Therefore, ORM is dominated by high pressure, the signature of stable good weather. The absolute maximum is 784.47 mbar recorded in January and the absolute minimum is 751.4 mbar recorded in December.

We also noticed that during the summer, there is little prevailing high pressure and extreme values are close, as is to be expected from anticyclonic conditions.

4.3 Relative humidity

The variation of the monthly relative humidity show a seasonal effect (Fig. 5). The daily mean humidity is 40%. The daytime and night-time mean humidity is 38% and 41%, respectively. We noticed that the humid months are from October to March ($RH > 40\%$), whereas the dry months is from April to September ($RH < 40\%$). The percentage of time with humidity lower than 90% is 89% (Fig. 6), (45% occurring in winter, 55% occurring in summer).

Figure 7 shows the daily relative humidity at ORM. We noticed very little variation.

4.4 Wind speed and direction

Wind behaviour at various sites within the ORM has been studied by McInnes & Walker (1974), Brandt & Righini (1985) and Mahoney et al. (1998).

Our results for the CAMC site are shown in Tables 4 and 5 for night- and daytime, respectively, and in Figs. 8-11, which, respectively, show for the OUK and CAMC sites the night-time wind roses, the daytime wind roses, the monthly variation in wind speed and cumulative frequency plots of wind speed. There is a dramatic difference between the two sites.

The wind roses for the CAMC site are compiled from data taken from 1987 January to 1995 July. At night (Fig. 8), all directions are indicated, but with N and NNW being prevalent and with less conspicuous components from the NE and ENE. During the daytime, however, there is a significant change in the wind-direction pattern; the prevailing tendency is still from the N and NNE but there is more scatter in all directions with a pronounced increase in southerly (especially SSE) components. These results are in broad agreement with the findings of McInnes (1976) for the Fuente Nueva site, close to where the CAMC is situated (see his wind rose reproduced in Brandt & Righini 1985). Note, however, that daytime measurements carried out by Brandt & Wohl (1982, see their Fig. 9, bottom right) on the neighbouring slope of the Roque de los Muchachos show a slightly different wind pattern with a much more pronounced easterly component, which is absent in our Fig. 9 (right) for the CAMC site.

Our night-time results differ from those of Mahoney et al. (1998, see their Fig. 1), based on data gathered at the site selected for the spanish 10 m (GTC) telescope (Muñoz-Tuñón et al. 1998). Mahoney et al. find a NE-ENE prevailing wind direction for GTC site (located below the Roque de los Muchachos on the same slope). They attribute this difference in wind pattern to local orographical influences. The NE-SE axis that is so prominent in their day- and night-time wind roses runs parallel to the local orographical contours of the GTC site; in other words, the prevailing free-air wind (coming from the NW as it reaches the Canaries) is diverted by the obstacle presented by the local orography. A plausible explanation for the discrepant behaviour of the wind at the Fuente Nueva and GTC sites could be that the contours at the former site run E-W while the site itself is situated immediately above a deep gorge running N-S, an orographical combination conducive to diverting the prevailing free-air NW wind into the N direction.

Figure 10 shows that mean wind speed varies very little. In Fig. 11, we observe that the wind speed at the site is generally low, with strong winds blowing for 0.01% of the time. As we see for April, the mean wind velocity is 2.8 m s^-1, the absolute maximum is 23 m s^-1.

4.5 Photometric data: Atmospheric extinction

The atmospheric extinction in the Canary Islands has been studied by Murdin (1985), Stickland et al. (1987), Whittet et al. (1987) Jimenez, González Jorge (1998) and Guerrero et al. (1998, 2000). To characterize ORM's site properties, Guerrero et al. ignored the external effect of eruptions of El Chichón and Mt. Pinatubo their extinction estimates. They found a modal value of 0.11 mag/airmass and their database included over 1269 extinction values. The extinction increases in the summer months (especially July and August) due to occasional irruptions of dust in suspension brought to the observatories by southerly winds from the Sahara Desert. For a more complete study of dust properties on the atmospheric extinction see Jimenez et al. (1998).

We have also studied the photometric conditions at the ORM site. CAMC data from 1984 May to 1998 May (2931 extinction values) show that the extinction varies between 0.08 to 0.98 mag/airmass, the modal value being 0.12 mag/airmass. Figure 14 records the frequency of photometric nights (for a definition of photometric nights see Murdin 1985). These occur with a mean annual frequency of 51% (40% in the winter and 60% in the summer). The maximum frequency of occurrence of photometric nights is 82% in June and the minimum is 25% in December.

The percentage of photometric nights is higher in summer, than in winter although during the months of July, August and September the sky transparency over the ORM site is occasionally affected by irruptions of airborne dust from the Sahara desert which is carried up to 3 km in altitude by southern winds which blow over the Atlantic Ocean.

 

 

Table 6: Local characteristics of the Oukaimeden and CAMC site

		ORM	OUKAIMEDEN
GEOGRAPHY	UNIT
Longitude	deg, min, sec	17 52 57 W	7 52 52 W
Latitude	deg, min, sec	28 45 36 N	31 12 32 N
distance to nearest city (time)	km(h)	(2h)	80(1h)
Altitude m above sea level	(m)	2326	2700
Shape		ridge of Caldera	peak
Distance to coast	km	10-12	100
LOCAL CLIMATOLOGY
Temperature	mean value ( oC)	8.37	6.47
	winter/summer	4.95/11.42	1.96/12.36
	min, max	-9.4 , 27	-12.9 , 25.6
	< |Tj-Tj+1|>moy	1.6	1.72
	<|Tj-Tj+1|>max	8.6	9
Relative humidity	mean value (%)	40	45
	winter/summer	47.99/31.56	47.70/41.25
	prob. $( RH < 90\% )$ (%)	89	92
Pressure	mean value (hPa)	774.12
	min, max	751.4, 784.47
	theoretical value Pt	760	724
	% of time with P > Pt	88
	winter/summer	773.26/774.91
Wind	mean value velocity (m s-1)	2.8	2.63
	max	23	50
	velocity lower than 5 m s-1 (%)	82	89.22
	velocity lower than 15 m s-1 (%)	99.99	99.84
	night-time predominant wind (%)	N(11.63)	SE(21.80)
	daytime predominant wind (%)	N(10.45)	WNW(13.16)
Precipitation	(mm)		362.5
Photometry	atmos. ext. coef. (mag/airmass)	0.11 (night-time data)	0.12 (daytime data)
	frequency of photometric nights (%)	51
	fraction of clear weather (%)		65