Air Pressure Effect on Water Level 
Jack Holliday 
High and low air pressure areas moving over large bodies of water affect the 
“height” of the water. This change can best be observed where the shores of the 
water body (lake, river, ocean) are gradually sloped and shallow. 
My first experience of this phenomenon was at Constance Bay, Ontario. We 
had rented a cottage there for several weeks in August, 1958.1 was out on the 
river, fishing from a small boat anchored about 200 meters from shore. The 
afternoon was pleasantly warm, now and again a fish grabbed my bait, and I 
was content. 
Across the river, on the Quebec side, a small thundercell flashed lightning and 
poured rain on the escarpment beyond Eardley. It remained in the same area 
for hours. I kept a wary eye on it, because once I was caught by just such a 
thundercell which changed direction and came upon me, unprotected, in the 
middle of a lake. Not a pleasant experience. 
After three hours I thought the storm had moved imperceptibly towards me. I 
studied it even more intensely and the wary eye was rewarded with the certain 
knowledge that the storm was coming at me, and quickly. By the time I had 
pulled in the anchor, started the small outboard motor and headed for “home,” 
the rain curtain had obscured the Quebec shore and the storm was quickly ap¬ 
proaching Constance Bay. Luckily it was only five minutes to the landing where 
I beached the boat, lifted the motor, and pulled the boat ten meters up the 
beach, beyond the storm waves, I hoped. 
Gathering my equipment, I dashed into the cottage and we battened down to 
await the rain and lightning and thunder. Like many such summer storms it 
never arrived at Constance Bay; it turned and headed down the Ottawa River 
instead. 
After being all prepared for a storm, I was disappointed when it didn’t arrive, 
so I walked to the beach to see what had happened. There was the thundercell 
above the centre, and moving down river. To my amazement, the waters’ edge 
was now thirty meters from where I had pulled the boat. 
I concluded that the low air pressure under the thundercell in the centre of the 
river and the relatively high pressure at the shore, three kilometres distant, had 
forced the water out from the shore. The drop in water height, perhaps 15-20 
centimeters, was quite obvious because the shore was gently sloping and the 
water shallow. After several hours, the water gradually returned to its former 
edge. 
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