Bay Ridges Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bay Ridges Observatory clear sky chart from 2005-11-06 to 2025-10-04.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	14% 8% 3 8% 5% 5% 5% 4 6% 8% 34%	126738
	Night	15% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	122637
Transparency	Day	46% 6% 14% 21% 12% 1	130329
	Night	48% 6% 12% 21% 12% 1	127132
Seeing	Day	17% 6% 21% 31% 20% 5%	15600
	Night	19% 7% 26% 31% 15% 3	24927

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 10% 3 9% 6% 5% 5% 4 5% 7% 33%	3597
	11th to 20th	Day	10% 16% 6% 17% 9% 6% 5% 4 4 5% 17%	3857
	21st to end	Day	16% 16% 6% 13% 5% 6% 5% 3 5% 6% 19%	4466
February	1st to 10th	Day	15% 14% 5% 11% 7% 6% 6% 4 5% 6% 23%	1800
	11th to 20th	Day	13% 13% 5% 12% 7% 5% 5% 4 5% 7% 26%	1753
	21st to end	Day	17% 12% 3 10% 7% 5% 4 4 4 6% 27%	1535
March	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 4 5% 6% 31%	4932
	11th to 20th	Day	10% 10% 4 9% 6% 5% 6% 4 6% 8% 31%	4980
	21st to end	Day	17% 9% 3 7% 5% 4 5% 3 5% 6% 35%	5465
April	1st to 10th	Day	17% 10% 4 9% 5% 5% 4 4 6% 6% 30%	2379
	11th to 20th	Day	18% 9% 2 7% 4 4 5% 4 6% 6% 35%	2427
	21st to end	Day	19% 7% 3 8% 5% 4 4 4 5% 6% 34%	2600
May	1st to 10th	Day	18% 8% 3 7% 5% 5% 5% 4 6% 7% 32%	5924
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	6318
	21st to end	Day	17% 6% 2 6% 4 4 5% 4 6% 8% 39%	6949
June	1st to 10th	Day	13% 6% 3 6% 5% 5% 5% 4 7% 9% 37%	2800
	11th to 20th	Day	10% 5% 2 7% 5% 5% 5% 4 7% 10% 40%	2800
	21st to end	Day	12% 6% 2 5% 5% 5% 5% 4 7% 9% 40%	2800
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	6360
	11th to 20th	Day	7% 4 2 6% 4 4 5% 4 7% 9% 48%	6360
	21st to end	Day	6% 4 2 5% 4 5% 5% 4 7% 9% 47%	6562
August	1st to 10th	Day	14% 7% 3 6% 4 4 4 5% 5% 8% 40%	4234
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 10% 40%	4174
	21st to end	Day	8% 6% 2 6% 4 4 6% 4 7% 9% 45%	4461
September	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 5% 6% 8% 44%	2400
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 5% 8% 49%	2308
	21st to end	Day	13% 5% 2 7% 5% 5% 5% 4 6% 9% 39%	2143
October	1st to 10th	Day	16% 7% 2 8% 4 4 5% 4 5% 7% 38%	1930
	11th to 20th	Day	18% 9% 3 7% 5% 4 5% 4 4 7% 33%	1900
	21st to end	Day	21% 9% 4 9% 6% 5% 4 4 5% 7% 26%	2090
November	1st to 10th	Day	13% 8% 3 8% 6% 5% 5% 3 7% 8% 34%	1688
	11th to 20th	Day	15% 10% 4 9% 7% 6% 6% 5% 7% 7% 26%	1600
	21st to end	Day	19% 10% 4 10% 6% 6% 4 4 6% 8% 23%	1600
December	1st to 10th	Day	18% 13% 5% 10% 7% 7% 7% 5% 6% 7% 15%	2600
	11th to 20th	Day	20% 14% 5% 8% 5% 5% 5% 4 5% 7% 24%	2986
	21st to end	Day	27% 15% 6% 12% 6% 5% 4 3 5% 5% 13%	3960
January	1st to 10th	Night	15% 10% 4 9% 6% 5% 5% 4 6% 7% 29%	6349
	11th to 20th	Night	13% 12% 5% 13% 8% 7% 6% 4 6% 6% 19%	6025
	21st to end	Night	16% 13% 4 10% 6% 6% 5% 4 5% 7% 24%	6490
February	1st to 10th	Night	13% 15% 4 9% 6% 6% 5% 4 6% 6% 25%	2600
	11th to 20th	Night	15% 12% 4 10% 6% 5% 5% 5% 5% 7% 26%	2533
	21st to end	Night	14% 11% 4 9% 7% 5% 5% 4 6% 6% 29%	2089
March	1st to 10th	Night	16% 9% 4 8% 6% 5% 5% 3 5% 6% 33%	4908
	11th to 20th	Night	10% 8% 3 7% 5% 5% 5% 4 6% 8% 37%	4979
	21st to end	Night	20% 7% 2 7% 4 4 4 3 5% 5% 39%	5467
April	1st to 10th	Night	18% 10% 3 8% 5% 4 4 3 5% 7% 32%	2020
	11th to 20th	Night	15% 8% 3 6% 6% 5% 5% 3 6% 5% 39%	1938
	21st to end	Night	16% 7% 3 7% 4 5% 4 5% 7% 7% 35%	1800
May	1st to 10th	Night	23% 8% 2 6% 4 4 4 3 5% 5% 34%	4017
	11th to 20th	Night	14% 7% 3 8% 5% 6% 6% 5% 5% 7% 34%	3575
	21st to end	Night	17% 5% 2 6% 4 4 4 4 5% 7% 42%	3926
June	1st to 10th	Night	12% 5% 3 6% 4 5% 6% 4 6% 8% 43%	1600
	11th to 20th	Night	8% 6% 2 6% 6% 5% 4 5% 6% 8% 44%	1600
	21st to end	Night	12% 5% 2 5% 5% 4 4 4 6% 9% 43%	1600
July	1st to 10th	Night	6% 5% 3 7% 5% 4 5% 4 6% 9% 46%	3600
	11th to 20th	Night	9% 4 2 6% 4 4 4 4 5% 8% 49%	3600
	21st to end	Night	9% 4 2 5% 3 3 4 4 6% 8% 52%	4286
August	1st to 10th	Night	10% 5% 2 5% 5% 4 4 4 6% 8% 47%	2915
	11th to 20th	Night	11% 5% 2 6% 3 3 3 3 5% 7% 51%	2875
	21st to end	Night	9% 5% 2 6% 4 5% 4 4 6% 8% 46%	3403
September	1st to 10th	Night	9% 5% 2 7% 4 4 5% 5% 6% 7% 48%	2000
	11th to 20th	Night	10% 4 2 4 3 3 3 3 5% 7% 55%	2089
	21st to end	Night	15% 6% 3 7% 4 4 4 4 5% 7% 42%	2256
October	1st to 10th	Night	15% 6% 3 7% 4 4 4 4 6% 7% 40%	2317
	11th to 20th	Night	18% 8% 2 7% 5% 4 5% 4 6% 6% 33%	2280
	21st to end	Night	21% 9% 3 7% 4 4 5% 4 5% 7% 31%	2508
November	1st to 10th	Night	16% 7% 2 6% 5% 5% 4 3 6% 7% 38%	2608
	11th to 20th	Night	17% 9% 3 8% 6% 5% 5% 5% 6% 8% 31%	2800
	21st to end	Night	20% 10% 3 8% 5% 5% 6% 4 7% 8% 25%	2800
December	1st to 10th	Night	17% 13% 4 10% 6% 5% 5% 4 6% 6% 21%	4580
	11th to 20th	Night	18% 15% 4 8% 5% 5% 5% 4 6% 6% 24%	5208
	21st to end	Night	26% 13% 6% 11% 6% 5% 5% 4 4 5% 15%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	50% 8% 16% 15% 11% 1	3598
	11th to 20th	Day	70% 7% 10% 9% 4	3891
	21st to end	Day	65% 6% 11% 10% 7%	4466
February	1st to 10th	Day	62% 6% 11% 16% 4 1	1800
	11th to 20th	Day	57% 7% 12% 17% 7% 1	1762
	21st to end	Day	57% 5% 13% 17% 6%	1535
March	1st to 10th	Day	52% 6% 13% 15% 13% 1	4935
	11th to 20th	Day	49% 5% 18% 18% 8% 2	4980
	21st to end	Day	48% 4 11% 22% 14% 1	5465
April	1st to 10th	Day	51% 6% 12% 18% 11% 1	2379
	11th to 20th	Day	48% 6% 10% 22% 14% 1	2427
	21st to end	Day	49% 6% 11% 19% 12% 3	2600
May	1st to 10th	Day	49% 5% 13% 16% 12% 4	5933
	11th to 20th	Day	49% 9% 10% 15% 15% 1	6322
	21st to end	Day	43% 5% 13% 24% 13% 3	6949
June	1st to 10th	Day	41% 7% 14% 22% 15% 1	2800
	11th to 20th	Day	37% 7% 14% 25% 16% 1	2800
	21st to end	Day	37% 6% 17% 25% 14% 1	2800
July	1st to 10th	Day	35% 5% 16% 27% 16% 2	6290
	11th to 20th	Day	31% 5% 14% 33% 16% 1	6285
	21st to end	Day	32% 5% 15% 31% 16% 1	6374
August	1st to 10th	Day	41% 5% 15% 23% 15% 1	4221
	11th to 20th	Day	37% 5% 18% 28% 11% 1	4213
	21st to end	Day	33% 5% 15% 36% 10% 1	4461
September	1st to 10th	Day	35% 8% 14% 30% 12% 1	2400
	11th to 20th	Day	31% 7% 16% 31% 14% 1	2308
	21st to end	Day	41% 6% 16% 27% 10% 1	2143
October	1st to 10th	Day	35% 5% 16% 28% 15% 1	3190
	11th to 20th	Day	37% 5% 18% 25% 14% 1	3140
	21st to end	Day	49% 4 18% 23% 6%	3428
November	1st to 10th	Day	47% 6% 16% 21% 9% 1	1688
	11th to 20th	Day	54% 8% 14% 15% 8% 1	1600
	21st to end	Day	58% 7% 15% 17% 3	1600
December	1st to 10th	Day	67% 4 17% 8% 3	2600
	11th to 20th	Day	61% 7% 13% 10% 9%	2986
	21st to end	Day	72% 8% 7% 9% 4	3960
January	1st to 10th	Night	53% 7% 14% 20% 6%	6349
	11th to 20th	Night	65% 10% 9% 11% 4	6066
	21st to end	Night	60% 9% 11% 11% 9% 1	6490
February	1st to 10th	Night	59% 7% 12% 17% 5% 1	2600
	11th to 20th	Night	58% 6% 11% 16% 8% 1	2535
	21st to end	Night	55% 5% 13% 19% 8%	2089
March	1st to 10th	Night	52% 5% 10% 18% 12% 2	4918
	11th to 20th	Night	44% 4 18% 22% 10% 1	4980
	21st to end	Night	46% 4 9% 21% 18% 2	5467
April	1st to 10th	Night	51% 4 10% 19% 14% 1	2020
	11th to 20th	Night	47% 5% 10% 20% 16% 2	1938
	21st to end	Night	45% 7% 11% 19% 16% 2	1800
May	1st to 10th	Night	52% 5% 11% 14% 13% 6%	4024
	11th to 20th	Night	46% 9% 11% 19% 13% 3	3576
	21st to end	Night	42% 4 13% 24% 15% 2	3927
June	1st to 10th	Night	39% 5% 12% 25% 16% 3	1600
	11th to 20th	Night	37% 5% 12% 24% 20% 2	1600
	21st to end	Night	38% 6% 14% 24% 17% 1	1600
July	1st to 10th	Night	35% 4 15% 24% 20% 2	3560
	11th to 20th	Night	33% 5% 13% 32% 17% 1	3552
	21st to end	Night	30% 5% 12% 28% 23% 1	4159
August	1st to 10th	Night	35% 5% 15% 28% 15% 2	2904
	11th to 20th	Night	34% 5% 15% 32% 13% 1	2902
	21st to end	Night	35% 6% 12% 32% 14% 1	3403
September	1st to 10th	Night	34% 7% 14% 30% 14% 1	2000
	11th to 20th	Night	28% 6% 12% 35% 18%	2089
	21st to end	Night	42% 6% 12% 27% 12% 1	2256
October	1st to 10th	Night	35% 5% 10% 26% 24% 1	3837
	11th to 20th	Night	43% 5% 15% 28% 9%	3779
	21st to end	Night	41% 5% 16% 28% 10% 1	4120
November	1st to 10th	Night	44% 6% 13% 23% 13% 1	2608
	11th to 20th	Night	51% 7% 15% 17% 9% 1	2800
	21st to end	Night	56% 8% 17% 15% 4	2800
December	1st to 10th	Night	62% 5% 15% 15% 4	4580
	11th to 20th	Night	60% 7% 11% 13% 9%	5208
	21st to end	Night	70% 7% 11% 8% 4	6996

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	21% 15% 29% 27% 9% 1	200
	11th to 20th	Day	22% 15% 35% 18% 10% 1	262
	21st to end	Day	24% 10% 34% 27% 5% 1	330
February	1st to 10th	Day	20% 13% 39% 22% 6%	300
	11th to 20th	Day	19% 15% 38% 23% 5%	300
	21st to end	Day	22% 18% 28% 27% 5%	284
March	1st to 10th	Day	21% 7% 34% 29% 8%	500
	11th to 20th	Day	17% 12% 30% 30% 9% 1	500
	21st to end	Day	25% 7% 30% 29% 9% 1	550
April	1st to 10th	Day	19% 9% 28% 36% 7% 1	500
	11th to 20th	Day	25% 6% 28% 27% 12% 3	492
	21st to end	Day	20% 6% 27% 33% 12% 1	500
May	1st to 10th	Day	20% 4 19% 34% 18% 5%	503
	11th to 20th	Day	14% 2 17% 38% 24% 5%	527
	21st to end	Day	13% 2 14% 36% 28% 7%	573
June	1st to 10th	Day	14% 4 16% 41% 22% 3	530
	11th to 20th	Day	12% 3 14% 37% 31% 4	530
	21st to end	Day	15% 3 11% 40% 26% 5%	530
July	1st to 10th	Day	8% 1 10% 35% 38% 8%	525
	11th to 20th	Day	13% 2 9% 32% 35% 9%	524
	21st to end	Day	8% 3 10% 39% 33% 7%	576
August	1st to 10th	Day	11% 2 9% 32% 34% 12%	526
	11th to 20th	Day	10% 2 10% 33% 32% 13%	519
	21st to end	Day	11% 2 13% 30% 33% 10%	583
September	1st to 10th	Day	11% 3 15% 31% 28% 12%	530
	11th to 20th	Day	8% 2 11% 29% 28% 22%	530
	21st to end	Day	15% 4 18% 31% 22% 9%	530
October	1st to 10th	Day	19% 5% 22% 26% 20% 7%	502
	11th to 20th	Day	19% 8% 31% 29% 11% 3	490
	21st to end	Day	26% 8% 26% 27% 12% 1	539
November	1st to 10th	Day	21% 9% 26% 28% 15% 2	295
	11th to 20th	Day	20% 13% 27% 31% 10% 1	200
	21st to end	Day	24% 9% 36% 24% 7% 1	200
December	1st to 10th	Day	23% 9% 34% 24% 8% 3	200
	11th to 20th	Day	25% 15% 37% 17% 7% 1	200
	21st to end	Day	28% 11% 37% 18% 5% 1	220
January	1st to 10th	Night	24% 11% 39% 19% 7%	800
	11th to 20th	Night	27% 12% 32% 24% 5%	800
	21st to end	Night	19% 10% 36% 27% 7% 1	880
February	1st to 10th	Night	21% 13% 39% 23% 4	800
	11th to 20th	Night	21% 12% 36% 25% 5%	783
	21st to end	Night	20% 10% 40% 25% 5%	660
March	1st to 10th	Night	19% 7% 34% 33% 6% 1	800
	11th to 20th	Night	19% 8% 36% 28% 8% 1	800
	21st to end	Night	20% 7% 28% 32% 12% 1	880
April	1st to 10th	Night	23% 6% 28% 34% 8% 1	800
	11th to 20th	Night	18% 6% 27% 35% 12% 2	754
	21st to end	Night	21% 5% 22% 38% 13% 1	600
May	1st to 10th	Night	20% 3 17% 38% 20% 2	590
	11th to 20th	Night	16% 1 19% 39% 23% 2	394
	21st to end	Night	10% 3 12% 41% 29% 4	433
June	1st to 10th	Night	13% 2 15% 43% 24% 3	400
	11th to 20th	Night	10% 4 15% 41% 29% 3	400
	21st to end	Night	12% 2 14% 41% 29% 3	400
July	1st to 10th	Night	7% 3 8% 44% 33% 6%	390
	11th to 20th	Night	12% 3 10% 36% 34% 5%	388
	21st to end	Night	9% 3 8% 42% 33% 4	593
August	1st to 10th	Night	10% 2 10% 34% 38% 7%	597
	11th to 20th	Night	9% 2 10% 36% 35% 8%	594
	21st to end	Night	11% 3 15% 35% 31% 5%	762
September	1st to 10th	Night	11% 3 17% 34% 27% 8%	800
	11th to 20th	Night	12% 4 13% 28% 32% 12%	800
	21st to end	Night	18% 5% 16% 30% 26% 6%	800
October	1st to 10th	Night	18% 4 24% 27% 21% 5%	773
	11th to 20th	Night	23% 7% 26% 31% 12% 2	760
	21st to end	Night	26% 7% 26% 31% 9% 1	836
November	1st to 10th	Night	21% 6% 25% 34% 12% 2	780
	11th to 20th	Night	21% 9% 32% 27% 8% 2	800
	21st to end	Night	25% 9% 31% 28% 7% 1	800
December	1st to 10th	Night	28% 10% 34% 23% 5% 1	800
	11th to 20th	Night	21% 11% 40% 23% 5% 1	800
	21st to end	Night	30% 9% 33% 23% 4	880

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