XXX METEOROLOGY OF BEN NEVIS. 



considered as fairly well represented by the daily maximum and minimum 

 temperatures. Hence, while the daily range of temperature from the mean 

 hourly observations for January is only 0°6, the mean from the daily maxima 

 and minima is 7°0. 



The disturbing influence of cyclones on the temperature is also strikingly 

 seen in the diurnal curve for January, which shows a double maximum and 

 minimum for the four years' observations, which will no doubt disappear as 

 observations accumulate. The same disturbing influence is apparent more or 

 less in the six months from October to March, particularly as regards the 

 time of the minimum temperature of the day. 



Humidity. — The hygrometry of Ben Nevis is one of the most remarkable 

 features of its climate. The usual conditions are a saturated atmosphere. In 

 the warmer months of the year it often happens that everything outside the 

 Observatory is dripping wet, and on opening the Stevenson box for the thermo- 

 meters, a drop of water is seen at each thermometer bulb ; the box itself 

 seems just to have been lifted out of water, and everything all round is 

 wrapped in dense wetting mist. In the winter months, when the temperature 

 is below freezing, a dense mist frequently envelops everything, and the wind 

 deposits ice on all objects it sweeps past, the deposition being on the windward 

 side of the object. 



This state of things adds greatly to the labour of observing, inasmuch as it 

 renders necessary a repeated change of the thermometer box, and the labour 

 is further increased by the necessity of clearing the louvres and thermometers 

 of the fine particles of snow with which they get covered by the whirling drifts, 

 so that the essential condition of temperature observations may be secured 

 for the bulbs of the thermometers, viz., that they are kept entirely in contact 

 with the free atmosphere. Hence, as regards the observations of temperature, 

 registering instruments can never take the place of observers on Ben Nevis as 

 is the case with low-level observations ; and seeing that observations of 

 temperature are absolutely indispensable in utilising and discussing barometric 

 observations, the personal services of the observer can never be dispensed with 

 at this Observatory. 



Further, more than common care and attention must be given to keep the 

 wet bulb thermometer in good order, so that it may show unfailingly those 

 changes in the humidity, which are frequently great, sudden, and of brief 

 duration, and are so important in connection with the atmospheric changes of 

 north-western Europe. Hence it is part of the observer's regular routine of 

 duty after each observation to secure so far as possible that at next hour's 

 observation the " wet bulb " will be properly wet, and if the temperature is 

 under 32 o- 0, that it will have a proper coating of ice. For this purpose a small 

 phial of water is carried in the observer's pocket. Some idea may be formed 



