Another calm day, very beautiful and clear. Wilson and Bowers took our few dogs for a run in a sledge. Walked myself out over ice in North Bay–there are a good many cracks and pressures with varying thickness of ice, showing how tide and wind shift the thin sheets–the newest leads held young ice of 4 inches.
The temperature remains high, the lowest yesterday -13°; it should be much lower with such calm weather and clear skies. A strange fact is now very commonly noticed: in calm weather there is usually a difference of 4° or 5° between the temperature at the hut and that on Wind Vane Hill (64 feet), the latter being the higher. This shows an inverted temperature.
As I returned from my walk the southern sky seemed to grow darker, and later stratus cloud was undoubtedly spreading up from that direction–this at about 5 P.M. About 7 a moderate north wind sprang up. This seemed to indicate a southerly blow, and at about 9 the wind shifted to that quarter and blew gustily, 25 to 35 m.p.h. One cannot see the result on the Strait, but I fear it means that the ice has gone out again in places. The wind dropped as suddenly as it had arisen soon after midnight.
In the evening Simpson gave us his first meteorological lecture–the subject, ‘Coronas, Halos, Rainbows, and Auroras.’ He has a remarkable power of exposition and taught me more of these phenomena in the hour than I had learnt by all previous interested inquiries concerning them.
I note one or two points concerning each phenomenon.
Corona.–White to brown inside ring called Aureola–outside are sometimes seen two or three rings of prismatic light in addition. Caused by diffraction of light round drops of water or ice crystals; diameter of rings inversely proportionate to size of drops or crystals–mixed sizes of ditto causes aureola without rings.
Halos.–Caused by refraction and reflection through and from ice crystals. In this connection the hexagonal, tetrahedonal type of crystallisation is first to be noted; then the infinite number of forms in which this can be modified together with result of fractures: two forms predominate, the plate and the needle; these forms falling through air assume definite position–the plate falls horizontally swaying to and fro, the needle turns rapidly about its longer axis, which remains horizontal. Simpson showed excellent experiments to illustrate; consideration of these facts and refraction of light striking crystals clearly leads to explanation of various complicated halo phenomena such as recorded and such as seen by us on the Great Barrier, and draws attention to the critical refraction angles of 32° and 46°, the radius of inner and outer rings, the position of mock suns, contra suns, zenith circles, &c.
Further measurements are needed; for instance of streamers from mock suns and examination of ice crystals. (Record of ice crystals seen on Barrier Surface.)
Rainbows.–Caused by reflection and refraction from and through drops of water–colours vary with size of drops, the smaller the drop the lighter the colours and nearer to the violet end of the spectrum–hence white rainbow as seen on the Barrier, very small drops.
Double Bows–diameters must be 84° and 100°–again from laws of refraction–colours: inner, red outside; outer, red inside–i.e. reds come together.
Wanted to see more rainbows on Barrier. In this connection a good rainbow was seen to N.W. in February from winter quarters. Reports should note colours and relative width of bands of colour.
Iridescent Clouds.–Not yet understood; observations required, especially angular distance from the sun.
Auroras.–Clearly most frequent and intense in years of maximum sun spots; this argues connection with the sun.
Points noticed requiring confirmation:
Arch: centre of arch in magnetic meridian.
Shafts: take direction of dipping needle.
Bands and Curtains with convolutions–not understood.
Corona: shafts meeting to form.
Notes required on movement and direction of movement–colours seen–supposed red and possibly green rays preceding or accompanying movement. Auroras are sometimes accompanied by magnetic storms, but not always, and vice versa–in general significant signs of some connection–possible common dependents on a third factor. The phenomenon further connects itself in form with lines of magnetic force about the earth.
(Curious apparent connection between spectrum of aurora and that of a heavy gas, ‘argon.’ May be coincidence.)
Two theories enunciated:
Arrhenius.–Bombardments of minute charged particles from the sun gathered into the magnetic field of the earth.
Birkeland.–Bombardment of free negative electrons gathered into the magnetic field of the earth.
It is experimentally shown that minute drops of water are deflected by light.
It is experimentally shown that ions are given off by dried calcium, which the sun contains.
Professor Störmer has collected much material showing connection of the phenomenon with lines of magnetic force.