McGill's Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McGill's Island clear sky chart from 2005-11-06 to 2026-09-29.

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	22% 11% 4 8% 5% 4 4 3 5% 6% 29%	150507
	Night	21% 12% 4 9% 5% 4 4 3 5% 6% 27%	140861
Transparency	Day	55% 5% 11% 18% 10% 1	154085
	Night	56% 6% 10% 18% 9% 1	145372
Seeing	Day	30% 19% 30% 15% 5% 1	19719
	Night	28% 16% 31% 20% 5%	27129

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	16% 20% 8% 19% 7% 5% 3 2 3 4 13%	4517
	11th to 20th	Day	18% 19% 9% 21% 6% 5% 4 2 3 3 9%	5247
	21st to end	Day	15% 15% 5% 16% 12% 6% 5% 3 4 3 15%	5781
February	1st to 10th	Day	18% 18% 6% 11% 8% 5% 4 3 4 5% 17%	1890
	11th to 20th	Day	18% 13% 5% 14% 8% 4 5% 3 4 6% 20%	1843
	21st to end	Day	15% 15% 6% 10% 6% 4 4 3 5% 6% 27%	1653
March	1st to 10th	Day	25% 15% 5% 10% 6% 5% 4 3 4 3 20%	4985
	11th to 20th	Day	20% 17% 3 8% 4 4 4 3 5% 5% 27%	5090
	21st to end	Day	29% 14% 5% 10% 5% 4 3 2 4 4 20%	5587
April	1st to 10th	Day	25% 16% 3 6% 4 3 4 3 4 4 28%	2436
	11th to 20th	Day	21% 11% 3 6% 4 3 4 3 4 6% 35%	2644
	21st to end	Day	24% 10% 2 5% 3 3 3 4 5% 6% 35%	2730
May	1st to 10th	Day	30% 12% 4 7% 4 3 3 2 4 5% 27%	7639
	11th to 20th	Day	21% 11% 3 7% 4 4 4 3 5% 6% 32%	7997
	21st to end	Day	21% 10% 3 8% 5% 4 4 3 5% 6% 30%	9059
June	1st to 10th	Day	26% 9% 2 6% 5% 3 3 3 5% 7% 31%	2940
	11th to 20th	Day	21% 8% 3 6% 4 4 3 3 4 6% 38%	3146
	21st to end	Day	26% 8% 3 6% 4 4 4 4 5% 6% 29%	3150
July	1st to 10th	Day	25% 8% 3 6% 4 4 4 3 5% 7% 31%	8880
	11th to 20th	Day	23% 7% 2 5% 4 3 4 3 5% 7% 36%	8224
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 8% 37%	8342
August	1st to 10th	Day	23% 8% 3 6% 4 4 4 4 5% 7% 33%	6015
	11th to 20th	Day	17% 8% 2 7% 4 4 5% 3 5% 7% 37%	5953
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 8% 42%	6453
September	1st to 10th	Day	19% 7% 3 5% 4 3 3 2 4 7% 43%	2520
	11th to 20th	Day	18% 7% 2 5% 3 4 3 3 4 6% 46%	2328
	21st to end	Day	21% 7% 3 6% 4 4 3 3 3 6% 41%	2288
October	1st to 10th	Day	19% 6% 3 6% 4 3 4 3 5% 7% 40%	2000
	11th to 20th	Day	20% 8% 3 7% 4 5% 5% 4 5% 6% 33%	2000
	21st to end	Day	21% 9% 4 9% 4 4 4 3 5% 7% 30%	2188
November	1st to 10th	Day	23% 9% 3 8% 5% 3 3 4 4 6% 33%	1786
	11th to 20th	Day	21% 16% 5% 9% 4 3 4 4 5% 7% 22%	1680
	21st to end	Day	21% 14% 5% 11% 6% 4 4 4 4 5% 22%	1680
December	1st to 10th	Day	24% 14% 6% 12% 5% 4 4 3 4 6% 18%	2690
	11th to 20th	Day	28% 13% 4 9% 5% 5% 5% 3 4 5% 20%	3076
	21st to end	Day	21% 15% 6% 10% 5% 4 3 3 4 6% 24%	4070
January	1st to 10th	Night	15% 18% 8% 22% 7% 4 4 3 4 4 12%	8216
	11th to 20th	Night	22% 17% 6% 17% 6% 5% 4 3 4 4 12%	7638
	21st to end	Night	13% 14% 5% 16% 9% 8% 5% 3 4 5% 16%	8400
February	1st to 10th	Night	19% 14% 5% 10% 7% 5% 6% 4 5% 5% 19%	2730
	11th to 20th	Night	17% 13% 4 12% 8% 5% 5% 3 5% 6% 21%	2663
	21st to end	Night	13% 12% 4 10% 7% 5% 4 4 5% 7% 28%	2149
March	1st to 10th	Night	24% 15% 5% 9% 8% 4 3 3 3 4 21%	5073
	11th to 20th	Night	20% 15% 6% 8% 5% 4 4 3 5% 5% 25%	5089
	21st to end	Night	30% 12% 4 7% 3 2 2 2 3 5% 27%	5587
April	1st to 10th	Night	25% 12% 3 7% 4 3 3 3 4 5% 29%	2184
	11th to 20th	Night	25% 10% 2 5% 3 4 4 3 5% 7% 32%	1922
	21st to end	Night	25% 10% 2 6% 4 3 3 4 5% 5% 33%	1890
May	1st to 10th	Night	30% 14% 3 6% 4 3 3 3 4 4 26%	5259
	11th to 20th	Night	24% 13% 3 6% 4 3 3 3 4 5% 32%	4879
	21st to end	Night	18% 14% 3 8% 4 4 3 3 5% 5% 32%	5147
June	1st to 10th	Night	26% 8% 2 5% 4 3 4 3 6% 6% 31%	1680
	11th to 20th	Night	20% 9% 3 7% 3 4 3 3 5% 6% 36%	1489
	21st to end	Night	27% 8% 3 6% 4 5% 4 4 5% 6% 29%	1470
July	1st to 10th	Night	26% 9% 3 7% 4 4 4 4 6% 9% 24%	4110
	11th to 20th	Night	25% 8% 2 6% 4 3 4 3 5% 6% 34%	4692
	21st to end	Night	21% 7% 2 6% 4 4 4 3 5% 8% 37%	5736
August	1st to 10th	Night	27% 9% 3 7% 4 4 3 3 5% 6% 30%	4135
	11th to 20th	Night	20% 11% 3 7% 4 3 3 3 4 6% 36%	4092
	21st to end	Night	18% 8% 3 5% 4 4 3 3 5% 6% 41%	4670
September	1st to 10th	Night	19% 7% 3 5% 4 3 4 3 5% 7% 41%	2100
	11th to 20th	Night	20% 6% 2 5% 3 3 4 3 4 6% 46%	2289
	21st to end	Night	22% 7% 2 6% 4 4 3 3 5% 6% 38%	2289
October	1st to 10th	Night	20% 7% 2 6% 3 4 4 3 5% 6% 40%	2400
	11th to 20th	Night	19% 10% 2 6% 3 3 4 3 4 7% 39%	2400
	21st to end	Night	24% 8% 3 6% 4 3 4 3 5% 7% 33%	2650
November	1st to 10th	Night	20% 8% 2 7% 4 4 4 3 5% 7% 36%	2730
	11th to 20th	Night	20% 14% 3 8% 5% 5% 4 3 6% 7% 25%	2940
	21st to end	Night	20% 13% 4 7% 5% 4 5% 3 5% 7% 26%	2940
December	1st to 10th	Night	17% 15% 4 13% 6% 4 5% 4 5% 6% 20%	4720
	11th to 20th	Night	24% 13% 4 8% 5% 4 4 4 5% 6% 24%	5353
	21st to end	Night	23% 14% 4 10% 6% 3 3 2 4 4 27%	7150

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	72% 5% 10% 10% 2	4518
	11th to 20th	Day	78% 5% 8% 7% 2	5280
	21st to end	Day	68% 5% 9% 9% 9%	5781
February	1st to 10th	Day	67% 6% 10% 11% 6%	1890
	11th to 20th	Day	63% 7% 10% 14% 5%	1851
	21st to end	Day	56% 5% 12% 16% 9% 2	1653
March	1st to 10th	Day	67% 5% 8% 12% 7% 1	4989
	11th to 20th	Day	56% 6% 13% 17% 7% 1	5090
	21st to end	Day	69% 3 5% 13% 9% 1	5587
April	1st to 10th	Day	60% 4 8% 14% 13% 1	2436
	11th to 20th	Day	51% 4 10% 21% 12% 1	2644
	21st to end	Day	50% 5% 10% 19% 13% 2	2730
May	1st to 10th	Day	60% 5% 9% 17% 8%	7651
	11th to 20th	Day	51% 6% 12% 19% 9% 2	8001
	21st to end	Day	54% 6% 11% 14% 13% 1	9060
June	1st to 10th	Day	53% 5% 10% 20% 12% 1	2940
	11th to 20th	Day	48% 5% 11% 20% 14% 2	3146
	21st to end	Day	54% 7% 12% 16% 9% 1	3150
July	1st to 10th	Day	53% 5% 13% 18% 10% 1	8805
	11th to 20th	Day	47% 5% 10% 26% 12%	8146
	21st to end	Day	44% 5% 13% 24% 13% 1	8159
August	1st to 10th	Day	50% 7% 11% 21% 10% 1	6002
	11th to 20th	Day	46% 5% 13% 26% 9%	5992
	21st to end	Day	39% 6% 12% 26% 16% 1	6453
September	1st to 10th	Day	42% 5% 11% 23% 17% 2	2520
	11th to 20th	Day	40% 5% 10% 23% 19% 3	2328
	21st to end	Day	45% 5% 7% 22% 18% 3	2288
October	1st to 10th	Day	44% 6% 11% 26% 12% 1	3260
	11th to 20th	Day	43% 5% 10% 22% 20%	3240
	21st to end	Day	44% 4 12% 25% 11% 4	3514
November	1st to 10th	Day	52% 6% 12% 21% 9% 1	1786
	11th to 20th	Day	60% 5% 12% 15% 7% 1	1680
	21st to end	Day	63% 6% 13% 15% 3	1680
December	1st to 10th	Day	64% 6% 12% 15% 3	2690
	11th to 20th	Day	64% 6% 11% 14% 6%	3075
	21st to end	Day	60% 6% 13% 14% 7%	4070
January	1st to 10th	Night	73% 6% 9% 10% 3	8216
	11th to 20th	Night	71% 7% 9% 10% 3	7680
	21st to end	Night	65% 7% 10% 11% 7%	8400
February	1st to 10th	Night	64% 7% 10% 13% 6% 1	2730
	11th to 20th	Night	61% 6% 10% 16% 6%	2666
	21st to end	Night	53% 6% 12% 19% 9% 1	2149
March	1st to 10th	Night	66% 5% 8% 11% 10% 1	5082
	11th to 20th	Night	58% 6% 13% 15% 6% 1	5090
	21st to end	Night	61% 3 8% 18% 10% 1	5587
April	1st to 10th	Night	57% 4 9% 14% 12% 3	2184
	11th to 20th	Night	52% 5% 10% 21% 11% 1	1922
	21st to end	Night	52% 5% 10% 19% 12% 2	1890
May	1st to 10th	Night	62% 5% 9% 16% 8%	5265
	11th to 20th	Night	55% 5% 9% 20% 9% 2	4880
	21st to end	Night	54% 6% 9% 21% 10%	5148
June	1st to 10th	Night	52% 6% 9% 23% 10%	1680
	11th to 20th	Night	49% 6% 11% 21% 12% 1	1489
	21st to end	Night	55% 7% 12% 15% 10% 1	1470
July	1st to 10th	Night	57% 6% 12% 18% 7%	4075
	11th to 20th	Night	50% 5% 10% 20% 14% 1	4647
	21st to end	Night	47% 6% 10% 22% 13% 2	5605
August	1st to 10th	Night	54% 5% 12% 19% 10%	4123
	11th to 20th	Night	49% 6% 15% 23% 7%	4119
	21st to end	Night	43% 6% 12% 24% 14% 1	4669
September	1st to 10th	Night	42% 6% 9% 26% 16% 2	2100
	11th to 20th	Night	40% 5% 7% 26% 19% 2	2289
	21st to end	Night	47% 5% 7% 22% 17% 2	2289
October	1st to 10th	Night	47% 6% 11% 22% 13% 1	3920
	11th to 20th	Night	35% 5% 11% 26% 22% 1	3899
	21st to end	Night	43% 4 10% 27% 15% 1	4274
November	1st to 10th	Night	46% 6% 14% 22% 11%	2730
	11th to 20th	Night	55% 7% 13% 17% 8%	2940
	21st to end	Night	56% 8% 15% 16% 5%	2940
December	1st to 10th	Night	59% 7% 13% 19% 3	4720
	11th to 20th	Night	58% 6% 13% 14% 9%	5355
	21st to end	Night	60% 4 11% 15% 9% 1	7150

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	37% 24% 26% 10% 3	356
	11th to 20th	Day	37% 18% 29% 14% 2	430
	21st to end	Day	26% 27% 32% 12% 3	473
February	1st to 10th	Day	35% 20% 29% 14% 3	430
	11th to 20th	Day	36% 24% 26% 12% 2	426
	21st to end	Day	32% 23% 31% 12% 2	407
March	1st to 10th	Day	35% 25% 29% 9% 2	540
	11th to 20th	Day	38% 22% 26% 9% 4 1	540
	21st to end	Day	35% 27% 26% 8% 4	594
April	1st to 10th	Day	42% 25% 25% 6% 2	540
	11th to 20th	Day	34% 23% 31% 9% 2	532
	21st to end	Day	32% 23% 29% 10% 4 1	540
May	1st to 10th	Day	37% 24% 28% 9% 2 1	543
	11th to 20th	Day	28% 25% 28% 15% 3 1	692
	21st to end	Day	31% 17% 31% 16% 4	845
June	1st to 10th	Day	31% 17% 31% 17% 4	780
	11th to 20th	Day	24% 20% 31% 18% 6% 1	780
	21st to end	Day	33% 14% 30% 18% 4 1	780
July	1st to 10th	Day	26% 13% 34% 19% 6% 1	770
	11th to 20th	Day	24% 13% 35% 19% 8% 1	727
	21st to end	Day	22% 15% 38% 19% 6% 1	620
August	1st to 10th	Day	19% 10% 38% 23% 8% 1	566
	11th to 20th	Day	22% 15% 34% 19% 9% 1	559
	21st to end	Day	21% 17% 32% 19% 9% 2	627
September	1st to 10th	Day	21% 16% 33% 23% 6% 1	570
	11th to 20th	Day	22% 17% 32% 16% 8% 4	570
	21st to end	Day	24% 18% 33% 17% 6% 2	562
October	1st to 10th	Day	25% 14% 31% 18% 10% 2	420
	11th to 20th	Day	25% 17% 30% 20% 5% 2	420
	21st to end	Day	29% 20% 29% 15% 7%	462
November	1st to 10th	Day	30% 17% 31% 15% 7%	425
	11th to 20th	Day	37% 22% 24% 12% 5%	430
	21st to end	Day	34% 21% 28% 13% 3	430
December	1st to 10th	Day	37% 21% 29% 12% 1	430
	11th to 20th	Day	43% 22% 22% 11% 2	430
	21st to end	Day	36% 21% 26% 12% 4 1	473
January	1st to 10th	Night	32% 22% 28% 16% 2	947
	11th to 20th	Night	33% 17% 28% 19% 3	840
	21st to end	Night	26% 23% 31% 17% 3	924
February	1st to 10th	Night	32% 18% 30% 19% 2	840
	11th to 20th	Night	30% 22% 31% 15% 2	823
	21st to end	Night	23% 20% 36% 18% 4	692
March	1st to 10th	Night	30% 19% 33% 17% 2	840
	11th to 20th	Night	27% 19% 30% 18% 5%	840
	21st to end	Night	27% 20% 33% 14% 5%	924
April	1st to 10th	Night	35% 17% 32% 14% 2	714
	11th to 20th	Night	30% 20% 33% 14% 3	624
	21st to end	Night	29% 15% 30% 21% 4	630
May	1st to 10th	Night	31% 17% 34% 15% 3	630
	11th to 20th	Night	29% 14% 35% 18% 4	622
	21st to end	Night	24% 14% 38% 19% 4 1	682
June	1st to 10th	Night	29% 12% 34% 22% 3	630
	11th to 20th	Night	23% 11% 36% 25% 5%	439
	21st to end	Night	31% 5% 34% 24% 6%	420
July	1st to 10th	Night	26% 8% 33% 27% 5%	410
	11th to 20th	Night	22% 8% 40% 22% 7%	566
	21st to end	Night	24% 7% 36% 28% 4	689
August	1st to 10th	Night	20% 8% 32% 33% 7%	628
	11th to 20th	Night	22% 8% 30% 31% 9% 1	622
	21st to end	Night	20% 8% 32% 28% 10% 1	693
September	1st to 10th	Night	21% 9% 33% 27% 9% 1	630
	11th to 20th	Night	21% 11% 28% 25% 13% 2	819
	21st to end	Night	25% 11% 28% 23% 11% 2	833
October	1st to 10th	Night	25% 10% 30% 24% 11% 1	800
	11th to 20th	Night	25% 17% 25% 23% 9% 1	800
	21st to end	Night	29% 17% 27% 21% 6%	880
November	1st to 10th	Night	27% 14% 28% 23% 7% 1	853
	11th to 20th	Night	30% 18% 28% 19% 5%	950
	21st to end	Night	30% 22% 26% 16% 5%	950
December	1st to 10th	Night	34% 17% 28% 18% 4	950
	11th to 20th	Night	36% 21% 26% 15% 2	950
	21st to end	Night	32% 22% 26% 17% 3	1045

Page generated at 2026-09-26 05:18:40. Version ac07

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