BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY

Entered as second class matter at the Post Office, Worcester, Massachusetts, under Act of Aug. 24, 1912. Issued monthly except July and August. Annual subscription, $3.50; single copies of this issue, 35c. Address contributions to the Bulletin, and correspondence relating thereto, to ROBERT G. STONE, Editor Dept. of , New York University, University Heights, N. Y.

Vol. 23 JANUARY, 1942 No. 1

The Effect of Lake Erie on the Local Distribution of in (I)* JOHN T. REMICK, 2nd Lt., U. S. A. C. 620 East Ave., Lockport, N. Y. DESCRIPTIVE the roads are closed to traffic. HE TERM " FLURRIES" is The sharpness of the edge of the generally applied to the snow— snow-flurry area is variable—some- T light or heavy—that falls on the times quite diffuse and other times lee sides of the Great Lakes and very pronounced. A sharp boundary which is not caused by any apparent to the area is more likely to be the frontal activity. The prevailing wind case in the early fall than later in over the Great Lakes in winter is the winter—the reason for which from the west so whatever lake effect will be discussed later. there is should show up on the east- If the snow flurries are of much ern side of these lakes. consequence the fact should show up A low deck of is found usu- in the normal precipitation patterns ally to prevail over the area on the for the area. October-December is lee side of the lakes. A light snow the period of maximum snow flurry is generally falling. The water con- effect and March-May is a period of tent of the snow is variable—the the least effect. While the area in ratio of snow to water ranging from the vicinity of Jamestown, N. Y., 6 to 1 to 50 to 1. The average ratio has the largest amount in both is about 18 to 1 rather than the com- cases, the area bordering the lake is monly used 10 to 1 ratio. For the in the maximum precipitation area month of January 1940, 'Watertown, in the March-May period. By smooth- New York, on the eastern end of Lake ing out the isohyets Kincer1 shows Ontario had a total snowfall of 79 vividly the effect of the lake on the inches and a total precipitation normal precipitation. amounting to 2.84" or an average Monthly normals, of course, show snow/water ratio of 27 to 1 for the the effect more precisely. The October whole month. and November maps show it best Considerable wind usually accom- while the December and January maps panies the snow flurries and conse- show the effect much diminished (figs. quently much drifting takes place. 2-4). By January the lake tempera- Visibility sometimes gets so bad that ture is 32 °F and is partly ice covered. The snow flurries occur as often then 1J. B. Kincer: Seasonal distribution of in the U. S., Monthly Review, vol. as before but are too light to affect 47, 1919, pp. 624-631. the normals except on the Alleghany •From a B. S. thesis submitted at Mass. Inst. Techn., 1941, under the direction of Plateau where the orographic effect Prof. H. C. Willett. Lt. Remick is now with the Army in the Caribbean. is important.

Unauthenticated | Downloaded 09/29/21 04:28 PM UTC FIGS. 1-4. It will be noted that there is no pronounced general rise of the land. Although the Knapp Creek, New York—Coudersport, Pa. region is about 1400r above the level of Lake Erie this means an average rise of only about 28 feet a mile. The number of stations is sufficient in all parts of the area except between Erie and Buffalo—just where the local are most important. Seven to twelve inches of snow have sometimes fallen between Fredonia and Buffalo and yet no indication of this would be reported by any of the stations. Two of the maps (2nd part) show just such cases. In these two cases the data for amount and location of heaviest precipitation were obtained from newspaper reports. The normals as given on these charts were taken from the U. S. Climatological Data. No normals were used based on less than 10 years' data. The isohyets on the lower right hand part of the maps where little data is given were drawn from the maps appearing in the Monthly Weather Review showing the distribution of normal precipitation in eastern U. S. by months.

FRICTIONAL INFLUENCE levels. Such influence is generally First the lake must be considered assumed to extend to one kilometer. as an area of considerably less fric- The increase in velocity will mean an tional influence on the wind than the increase in the Coriolis force which land surrounding it. will deflect the wind to the right. In a westerly current of polar (Left and right in this discussion origin passing out onto the lake. The refer to an observer facing down- first effect of the decrease in surface wind.) According to Rossby and 2 friction will be an increase in the Montgomery the angle between sur- velocity of the wind in the lower face and gradient winds over a land surface such as prevails in the lake 2C.-G. Rossby and R. B. Montgomery, The region is 25° while the angle becomes Layer of Frictional Influence in Wind and Ocean Currents, Pavers in Phys. Ocean and 18° over water. The total effect on Met., M. I. T. and Woods Hole Ocean Inst., Vol. Ill, 3. the wind therefore as it travels over

Unauthenticated | Downloaded 09/29/21 04:28 PM UTC the lake is to increase its velocity and None of the Great Lakes is known turn it more parallel to the isobars. in historical times to ever have been After the current crosses the lee frozen completely over, but in late shore the effect is reversed. Its veloc- winter Lake Erie is usually ice cov- ity will then decrease to its former ered except for some open stretches in value and it will become a westerly the center. And during the last fif- current again. The loss in latitude teen years no severe local was is here considered too small to be found to have occurred after Feb. 1. of any significance. There will be a (Any local giving 0.7" or more certain lag in this refraction at both of precipitation in 48 hours we have shores—the change being a gradual arbitrarily classed as severe.) one occurring over a distance of many miles. But the lag will be much less THERMAL INFLUENCES The effects of a relatively warm pronounced on the lee side because water temperature will now be con- the velocities and direction of the air sidered. The most important effect aloft and the surrounding air which is to produce pronounced instability has not gone over the lake will tend of the air by warming the lower lay- to restore the wind to its original ers. As the air moves over the warm motion. This fact helps explain why water surface it will tend to rise and the strongest effect of the lake is produce tall convective clouds3. confined close to the lee-side shore. The main consequence of all this is This same heating will also raise to produce a field of convergence on the height of the turbulence inver- the right-hand portion of the lee side sion. By examining the lapse rates of the lake. The air piles up and is of Joliet and Buffalo in cases of good banked over this area. By banking it Pc flows it was found that the inver- is meant that the fast moving air sion was raised from about 0.8 km to from the lake is forced up over the 2 or even 3 km. This would indicate slower moving air over the land and that the convective clouds would be is deflected to the left into the general permitted to build up to a maximum current. The forced ascent over the height on the lee side of the lake. surface air will cause the upper air The steepening of the lapse rate to cool adiabatically and, if sufficient, will allow more moisture to be evap- condensation and precipitation will orated into the air and therefore result. The stronger the wind the will allow the air to reach saturation more intense will be the convergence. conditions more easily. The amount The shape of the lake will exert of evaporation will depend a great considerable influence on the intensity deal on the relative vapor pressures of convergence. A funnel-shaped of the air and water and therefore lake like Erie tends to both increase on the temperature differences be- the intensity and to limit the area tween them. On Feb. 5, 1933 at Dal- of the convergent action. las, Texas, mist visibly rose from So as a result of frictional influ- small lakes such as Lake Dallas and ences alone snow flurries may occur. Lake Cliff and was converted imme- Polar air has an originally low mois- diately into low clouds and snow ture content; if it had picked up no flurries; the air temperature was additional moisture over the lake it 13 °F, while the day before it had would not be expected that any heavy been 56 °F with the lake in the upper snowfalls would occur from the fric- 3R. M. Dole: in the Lake Region, Monthly Weather Review, v. 56, 1928, tional effect alone. pp. 512-513.

Unauthenticated | Downloaded 09/29/21 04:28 PM UTC fifties on both days! in the fall and that is just what is Heating will decrease the density observed. Not one of the severe of the air and since a decrease in Lake-Erie local storms described later density is proportional to an in- in this report occurred after Dec. 16. crease in velocity (Brunt), the veloc- We can now outline the factors ity of the wind will become even upon which the occurrence and in- greater than that due to decrease in tensity of snow flurries depend. friction alone. But since the density (1) A strong flow of cold polar air at the ground varies so little it is —to intensify frictional effect. probable that this effect is negligible. (2) A large temperature contrast When the unstable stratocumulus between the air and water, the water clouds move in from the warm lake being warmer—to intensify thermal over the cold land they will cool effect. and precipitation may result. Be- (3) A long air trajectory over the cause of the irregularity of convec- lake to obtain maximum effects of tive currents, it seems that the pre- both influences. cipitation from these clouds should be OROGRAPHIC INFLUENCE of the snow- type, rather than There is still another influence uniformily continuous light snow. which in certain localities may be of These showers can be very heavy at primary importance in causing snow times because the air has picked up flurries. That is the orographic much moisture from the lake. effect. The forced ascent of the air The frictional influence alone tends up a slope may be sufficient to start to produce light continuous snow on off precipitation whereas none would the right hand portion of the lee side occur otherwise. In this way snow of the lake and the thermal influence flurries may occur anywhere between alone tends to produce showers of the Great Lakes and the Appalachian snow in light to heavy amounts all Mountains. At the eastern end of over the lee side of the lake. Lake Ontario there is a decided rise A combination of the frictional and from the lake level to the crest of the thermal influences should produce the western Adirondack Plateau and very worst conditions. The frictional in- heavy falls often occur in that region. fluence is present always—its intens- The area about which this paper is ity dependent on the strength and concerned has some upslope, the im- stability of the air flow. The thermal portance of which will be seen later. effect is at a maximum in the fall Figure 1 gives a general idea of the when the air is much colder than the topography of the region by 500-foot water. Therefore, the most severe contour lines. snow-flurry conditions should prevail (To be concluded)

On the Mean Circulation of the Atmosphere Over the Caribbean^ ROBERT G. STONE Research Fellow, Blue Hill Observatory, Harvard University SOURCE OF DATA made there lies in the fact that as a ILOT-BALLOON WORK was begun result of the unremitting interest of at the San Juan station of the Dr. Fassig an exceptionally large P U. S. Weather Bureau in the proportion of high ascents was ob- of 1920, under the direction tained over a relatively long period so of the late Dr. O. L. Fassig. The par- •Read at Int. Union Geod. and Geophysics, Sept. 1939 meeting, Washington, D.C., Ab- ticular value of the observations stract in BULLETIN, 1940, p. 111.

Unauthenticated | Downloaded 09/29/21 04:28 PM UTC