_Parhelion._--A mock sun, or sun dog; a form of halo consisting of a more or less distinctly colored image of the sun at the same altitude as the latter above the horizon, and hence lying on the parhelic circle, if present. The ordinary parhelia are 22 degrees from the sun in azimuth, or a little more, according to the altitude of the luminary. Parhelia have occasionally been seen about 46 degrees from the sun. Analogous phenomena seen in connection with the moon are called _paraselenæ_, _mock moons_, _or moon dogs_. _Penetrating Radiation._--A form of radiation that has the property of passing through a great extent of air without being absorbed and of ionizing the air inside hermetically sealed metal vessels. It is supposed to consist of a special kind of Gamma rays and to come from the higher levels of the atmosphere. _Phenology._--The study of the periodic phenomena of animal and plant life and their relations to weather and climate. _Photochemical Climate._--The chemical activity of sunlight characteristic of any place or region. The variations of this feature of solar radiation are more or less strikingly different from those of solar heat and the brightness of solar light. Certain types of “chemical actinometer” are used in its measurement. _Pilot Balloon._--A small free balloon the drift of which, as observed from the ground, indicates the movements of the air aloft. _Pocky Cloud._--Mammato-cumulus. _Pogonip._--A fog, composed of fine needles of ice, which occurs in mountainous regions of the western United States and is reputed to be very injurious to the lungs. _Pollution Gauge._--A gauge for measuring soot and other impurities found in the atmosphere. _Pontias._--A wind that blows by night from a narrow valley at Nyons, France. _Potential Gradient._--(See _gradient_.) _Precipitation._--The collective name for deposits of atmospheric moisture in liquid and solid form, including rain, snow, hail, dew, hoarfrost, etc. _Pressure._--An elliptical expression, current in meteorological literature, for _atmospheric pressure_, or _barometric pressure_. _Prevailing Westerlies._--The belts of winds lying on the poleward sides of the subtropical high-pressure belts. _Psuchrainometer._--An instrument devised by J. R. Milne for measuring the cooling power of the atmosphere with reference to physiological effects. _Psychrometer._--An instrument for measuring atmospheric humidity, consisting usually of a _dry-bulb thermometer_ and a _wet-bulb thermometer_. The former is an ordinary mercurial thermometer. The latter has its bulb covered with muslin or other fabric, which is either permanently wet or is wetted before use. In some psychrometers there is only one thermometer, readings being taken both before and after moistening the bulb.