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 2025-05-26.

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% 12% 4 8% 5% 4 4 3 4 6% 28%	114850
	Night	21% 12% 4 9% 5% 4 4 3 5% 6% 27%	114354
Transparency	Day	55% 6% 11% 18% 10% 1	118470
	Night	56% 6% 11% 18% 9% 1	118898
Seeing	Day	30% 19% 30% 15% 5% 1	17785
	Night	28% 16% 31% 20% 5%	25434

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	17% 21% 6% 18% 8% 5% 3 2 3 4 13%	3483
	11th to 20th	Day	16% 18% 8% 21% 6% 5% 4 3 4 4 12%	4027
	21st to end	Day	17% 16% 5% 15% 8% 5% 4 2 4 4 19%	4466
February	1st to 10th	Day	19% 19% 6% 11% 8% 5% 3 3 4 5% 17%	1800
	11th to 20th	Day	18% 13% 5% 15% 8% 4 4 3 4 6% 19%	1753
	21st to end	Day	15% 16% 6% 10% 7% 4 4 3 5% 6% 26%	1576
March	1st to 10th	Day	25% 15% 5% 11% 7% 5% 4 3 3 3 20%	4876
	11th to 20th	Day	20% 17% 3 8% 4 4 4 3 5% 5% 27%	4980
	21st to end	Day	29% 14% 5% 10% 5% 4 3 2 4 4 20%	5466
April	1st to 10th	Day	25% 16% 3 6% 4 3 4 3 4 4 27%	2320
	11th to 20th	Day	21% 10% 2 6% 4 3 4 3 4 6% 35%	2517
	21st to end	Day	24% 10% 2 5% 3 3 3 4 4 5% 35%	2600
May	1st to 10th	Day	24% 12% 4 7% 4 4 4 3 5% 6% 27%	4236
	11th to 20th	Day	23% 11% 3 7% 4 4 4 3 5% 5% 29%	4421
	21st to end	Day	21% 7% 3 6% 4 3 4 3 5% 8% 35%	4906
June	1st to 10th	Day	27% 9% 2 6% 5% 3 3 3 5% 7% 30%	2660
	11th to 20th	Day	22% 7% 3 6% 4 4 3 2 4 6% 38%	2846
	21st to end	Day	27% 8% 3 6% 4 4 4 4 5% 6% 28%	2850
July	1st to 10th	Day	25% 8% 3 6% 4 4 4 3 5% 7% 31%	4760
	11th to 20th	Day	31% 7% 3 5% 3 3 3 3 5% 7% 30%	4412
	21st to end	Day	21% 9% 3 6% 5% 4 4 4 5% 7% 33%	4467
August	1st to 10th	Day	29% 10% 3 6% 4 4 3 3 5% 6% 28%	4105
	11th to 20th	Day	17% 8% 3 8% 5% 5% 5% 4 5% 6% 35%	4045
	21st to end	Day	13% 6% 2 5% 3 4 4 3 5% 8% 48%	4436
September	1st to 10th	Day	18% 6% 2 5% 4 3 3 2 4 7% 44%	2280
	11th to 20th	Day	19% 8% 2 5% 3 4 3 3 4 6% 44%	2106
	21st to end	Day	21% 7% 3 5% 4 3 3 3 3 6% 41%	2090
October	1st to 10th	Day	20% 7% 3 6% 4 3 4 3 5% 7% 38%	1900
	11th to 20th	Day	20% 7% 3 7% 4 5% 5% 4 5% 6% 34%	1900
	21st to end	Day	21% 9% 4 9% 4 4 4 3 5% 6% 31%	2079
November	1st to 10th	Day	22% 9% 3 7% 5% 3 4 4 5% 6% 33%	1699
	11th to 20th	Day	22% 15% 4 8% 4 3 4 4 5% 7% 23%	1600
	21st to end	Day	22% 14% 5% 10% 6% 5% 4 4 4 5% 22%	1600
December	1st to 10th	Day	24% 14% 6% 12% 5% 4 4 3 4 6% 18%	2610
	11th to 20th	Day	28% 13% 4 9% 5% 5% 5% 3 4 5% 20%	2996
	21st to end	Day	21% 15% 6% 9% 5% 4 3 3 4 6% 25%	3982
January	1st to 10th	Night	15% 19% 7% 20% 6% 4 4 3 4 4 14%	6331
	11th to 20th	Night	17% 16% 7% 18% 7% 6% 4 3 4 5% 13%	5858
	21st to end	Night	14% 14% 5% 15% 7% 6% 5% 4 5% 6% 19%	6490
February	1st to 10th	Night	20% 15% 5% 10% 7% 5% 6% 4 5% 5% 19%	2600
	11th to 20th	Night	17% 14% 4 13% 8% 5% 5% 3 6% 6% 19%	2533
	21st to end	Night	13% 12% 4 10% 7% 5% 4 4 6% 7% 27%	2050
March	1st to 10th	Night	25% 15% 5% 9% 8% 4 3 3 3 4 21%	4962
	11th to 20th	Night	20% 15% 6% 8% 5% 4 4 3 5% 5% 25%	4979
	21st to end	Night	31% 12% 4 7% 3 2 2 2 3 5% 27%	5466
April	1st to 10th	Night	25% 12% 3 7% 4 3 3 3 4 6% 29%	2080
	11th to 20th	Night	25% 9% 2 5% 3 3 4 3 5% 7% 32%	1829
	21st to end	Night	25% 10% 2 6% 4 3 4 4 5% 5% 32%	1800
May	1st to 10th	Night	21% 15% 4 6% 5% 4 4 3 5% 5% 28%	2924
	11th to 20th	Night	23% 12% 3 8% 5% 3 3 3 4 5% 30%	2697
	21st to end	Night	20% 11% 3 8% 4 4 3 3 6% 5% 32%	2796
June	1st to 10th	Night	28% 9% 2 5% 4 3 4 3 6% 6% 30%	1520
	11th to 20th	Night	20% 9% 3 7% 3 4 3 3 5% 6% 36%	1347
	21st to end	Night	28% 8% 3 6% 4 5% 4 4 5% 6% 28%	1330
July	1st to 10th	Night	30% 12% 3 7% 4 4 3 4 4 7% 24%	2210
	11th to 20th	Night	32% 9% 2 6% 4 3 4 3 5% 7% 25%	2516
	21st to end	Night	20% 8% 3 7% 4 3 4 3 5% 8% 34%	3087
August	1st to 10th	Night	32% 9% 2 6% 3 3 2 3 4 6% 30%	2825
	11th to 20th	Night	21% 11% 3 7% 4 3 3 3 4 6% 35%	2782
	21st to end	Night	13% 8% 2 4 3 3 3 3 5% 6% 48%	3218
September	1st to 10th	Night	18% 6% 3 5% 4 3 4 3 5% 7% 42%	1900
	11th to 20th	Night	20% 6% 2 5% 3 3 4 3 4 6% 45%	2071
	21st to end	Night	22% 8% 2 6% 3 4 3 3 4 6% 38%	2090
October	1st to 10th	Night	21% 8% 2 6% 3 3 4 3 5% 6% 38%	2280
	11th to 20th	Night	19% 9% 2 6% 3 3 4 3 4 7% 41%	2280
	21st to end	Night	24% 8% 3 6% 4 3 3 3 4 7% 34%	2517
November	1st to 10th	Night	20% 8% 2 7% 4 4 3 3 5% 7% 36%	2597
	11th to 20th	Night	21% 13% 3 7% 5% 5% 4 4 6% 7% 26%	2800
	21st to end	Night	21% 13% 5% 7% 5% 4 5% 3 5% 7% 26%	2800
December	1st to 10th	Night	17% 15% 4 13% 6% 4 5% 4 5% 6% 20%	4580
	11th to 20th	Night	24% 13% 4 7% 5% 4 4 4 5% 6% 24%	5213
	21st to end	Night	24% 15% 4 9% 6% 3 3 2 3 4 27%	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	71% 5% 11% 10% 2	3484
	11th to 20th	Day	73% 6% 9% 9% 3	4060
	21st to end	Day	65% 5% 9% 10% 10%	4466
February	1st to 10th	Day	67% 6% 10% 10% 6%	1800
	11th to 20th	Day	64% 7% 10% 13% 5%	1761
	21st to end	Day	57% 5% 12% 15% 8% 2	1576
March	1st to 10th	Day	68% 5% 8% 12% 7% 1	4880
	11th to 20th	Day	56% 6% 13% 17% 7% 1	4980
	21st to end	Day	69% 3 5% 13% 9% 1	5466
April	1st to 10th	Day	60% 4 8% 14% 12% 1	2320
	11th to 20th	Day	51% 5% 11% 21% 12% 1	2517
	21st to end	Day	50% 5% 10% 19% 13% 2	2600
May	1st to 10th	Day	57% 7% 11% 19% 7%	4250
	11th to 20th	Day	52% 8% 12% 18% 8% 1	4425
	21st to end	Day	47% 7% 16% 16% 13% 1	4906
June	1st to 10th	Day	54% 5% 10% 20% 11% 1	2660
	11th to 20th	Day	49% 5% 11% 20% 14% 3	2846
	21st to end	Day	55% 8% 12% 16% 9% 1	2850
July	1st to 10th	Day	52% 6% 11% 17% 12% 2	4685
	11th to 20th	Day	54% 6% 10% 21% 9%	4334
	21st to end	Day	49% 6% 12% 22% 10% 1	4325
August	1st to 10th	Day	57% 7% 9% 17% 10% 1	4092
	11th to 20th	Day	46% 6% 13% 26% 9%	4084
	21st to end	Day	34% 6% 12% 27% 20% 1	4436
September	1st to 10th	Day	41% 5% 11% 23% 18% 2	2280
	11th to 20th	Day	42% 6% 10% 23% 17% 3	2106
	21st to end	Day	45% 5% 7% 23% 18% 2	2090
October	1st to 10th	Day	45% 6% 11% 25% 12% 1	3160
	11th to 20th	Day	42% 5% 10% 22% 21%	3140
	21st to end	Day	44% 4 12% 24% 12% 4	3405
November	1st to 10th	Day	51% 6% 11% 21% 9% 1	1699
	11th to 20th	Day	58% 6% 12% 16% 7% 1	1600
	21st to end	Day	63% 6% 13% 15% 3	1600
December	1st to 10th	Day	64% 6% 13% 15% 2	2610
	11th to 20th	Day	64% 6% 11% 14% 6%	2995
	21st to end	Day	60% 6% 13% 14% 7%	3982
January	1st to 10th	Night	70% 7% 9% 11% 2	6331
	11th to 20th	Night	67% 8% 11% 11% 4	5900
	21st to end	Night	61% 7% 11% 12% 8%	6490
February	1st to 10th	Night	64% 7% 10% 12% 6% 1	2600
	11th to 20th	Night	62% 6% 10% 14% 6%	2536
	21st to end	Night	53% 6% 12% 18% 9% 1	2050
March	1st to 10th	Night	67% 5% 7% 10% 10% 1	4971
	11th to 20th	Night	58% 6% 13% 16% 6% 1	4980
	21st to end	Night	61% 3 8% 18% 10% 1	5466
April	1st to 10th	Night	57% 4 10% 15% 12% 3	2080
	11th to 20th	Night	51% 5% 10% 21% 11% 1	1829
	21st to end	Night	53% 5% 10% 19% 12% 2	1800
May	1st to 10th	Night	56% 8% 11% 18% 6% 1	2930
	11th to 20th	Night	55% 7% 9% 21% 7% 1	2698
	21st to end	Night	52% 8% 11% 22% 7%	2797
June	1st to 10th	Night	53% 6% 9% 22% 10%	1520
	11th to 20th	Night	49% 6% 12% 20% 12% 1	1347
	21st to end	Night	57% 7% 11% 14% 10% 1	1330
July	1st to 10th	Night	60% 6% 9% 14% 10%	2175
	11th to 20th	Night	58% 6% 10% 15% 10% 1	2471
	21st to end	Night	49% 7% 11% 21% 11% 1	2989
August	1st to 10th	Night	57% 5% 10% 19% 9%	2813
	11th to 20th	Night	49% 7% 14% 22% 7%	2809
	21st to end	Night	35% 7% 12% 27% 18% 1	3217
September	1st to 10th	Night	41% 6% 9% 26% 16% 2	1900
	11th to 20th	Night	41% 5% 8% 26% 17% 2	2071
	21st to end	Night	47% 5% 8% 22% 17% 2	2090
October	1st to 10th	Night	48% 6% 11% 21% 13% 1	3800
	11th to 20th	Night	34% 5% 12% 26% 23% 1	3779
	21st to end	Night	42% 4 10% 27% 16% 1	4141
November	1st to 10th	Night	46% 6% 15% 22% 11%	2597
	11th to 20th	Night	55% 7% 13% 18% 8%	2800
	21st to end	Night	56% 8% 15% 16% 5%	2800
December	1st to 10th	Night	59% 7% 13% 19% 3	4580
	11th to 20th	Night	57% 6% 13% 14% 9%	5215
	21st to end	Night	60% 4 11% 14% 9% 1	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	37% 23% 26% 11% 3	329
	11th to 20th	Day	35% 18% 30% 15% 2	400
	21st to end	Day	25% 27% 33% 12% 3	440
February	1st to 10th	Day	37% 20% 29% 13% 2	400
	11th to 20th	Day	37% 25% 25% 11% 2	396
	21st to end	Day	33% 21% 32% 13% 2	378
March	1st to 10th	Day	36% 24% 28% 10% 2	500
	11th to 20th	Day	37% 22% 27% 9% 4 1	500
	21st to end	Day	33% 27% 27% 9% 5%	550
April	1st to 10th	Day	42% 24% 26% 7% 2	500
	11th to 20th	Day	33% 24% 31% 10% 3	492
	21st to end	Day	32% 23% 30% 10% 4 1	500
May	1st to 10th	Day	36% 24% 28% 10% 2 1	503
	11th to 20th	Day	28% 24% 28% 16% 4 1	646
	21st to end	Day	31% 17% 31% 16% 4 1	760
June	1st to 10th	Day	33% 17% 30% 16% 4	680
	11th to 20th	Day	23% 18% 33% 18% 6% 1	680
	21st to end	Day	35% 13% 28% 18% 4 1	680
July	1st to 10th	Day	26% 12% 33% 21% 7% 1	670
	11th to 20th	Day	24% 11% 36% 20% 8% 1	631
	21st to end	Day	23% 15% 37% 19% 6%	532
August	1st to 10th	Day	21% 11% 37% 22% 8% 1	486
	11th to 20th	Day	23% 14% 34% 19% 9% 2	479
	21st to end	Day	21% 15% 31% 20% 10% 2	539
September	1st to 10th	Day	19% 17% 33% 24% 7% 1	490
	11th to 20th	Day	24% 16% 32% 16% 8% 3	490
	21st to end	Day	23% 17% 33% 18% 7% 2	490
October	1st to 10th	Day	26% 14% 30% 18% 9% 3	390
	11th to 20th	Day	24% 18% 30% 20% 5% 2	390
	21st to end	Day	28% 21% 29% 15% 7%	429
November	1st to 10th	Day	28% 17% 32% 15% 8%	395
	11th to 20th	Day	38% 21% 24% 12% 5%	400
	21st to end	Day	35% 21% 29% 13% 2 1	400
December	1st to 10th	Day	36% 22% 29% 12% 1	400
	11th to 20th	Day	43% 21% 22% 12% 2 1	400
	21st to end	Day	35% 20% 27% 13% 4 1	440
January	1st to 10th	Night	32% 21% 28% 16% 3	897
	11th to 20th	Night	32% 17% 28% 19% 4	800
	21st to end	Night	26% 22% 31% 17% 3	880
February	1st to 10th	Night	33% 18% 30% 18% 2	800
	11th to 20th	Night	31% 22% 30% 14% 2	783
	21st to end	Night	23% 20% 36% 18% 4	660
March	1st to 10th	Night	30% 19% 32% 17% 2	800
	11th to 20th	Night	26% 19% 31% 19% 5%	800
	21st to end	Night	28% 19% 33% 15% 6%	880
April	1st to 10th	Night	34% 17% 32% 15% 2	680
	11th to 20th	Night	29% 20% 33% 14% 3	594
	21st to end	Night	29% 16% 30% 20% 4 1	600
May	1st to 10th	Night	31% 17% 34% 16% 3	600
	11th to 20th	Night	29% 13% 35% 19% 4	592
	21st to end	Night	25% 14% 37% 19% 4 1	634
June	1st to 10th	Night	30% 12% 34% 21% 3	570
	11th to 20th	Night	21% 10% 38% 25% 5%	397
	21st to end	Night	32% 4 34% 23% 6%	380
July	1st to 10th	Night	26% 7% 33% 28% 5%	370
	11th to 20th	Night	22% 7% 40% 23% 7%	510
	21st to end	Night	24% 6% 37% 29% 4	623
August	1st to 10th	Night	21% 9% 32% 32% 6%	568
	11th to 20th	Night	23% 8% 30% 30% 9% 1	562
	21st to end	Night	21% 7% 32% 28% 11% 1	627
September	1st to 10th	Night	20% 9% 33% 28% 9% 1	570
	11th to 20th	Night	22% 10% 28% 25% 13% 2	741
	21st to end	Night	25% 11% 27% 23% 11% 2	760
October	1st to 10th	Night	26% 9% 29% 24% 10% 1	760
	11th to 20th	Night	24% 18% 26% 23% 9% 1	760
	21st to end	Night	30% 17% 27% 21% 5%	836
November	1st to 10th	Night	26% 15% 28% 24% 7% 1	810
	11th to 20th	Night	30% 18% 28% 19% 5%	900
	21st to end	Night	30% 22% 26% 17% 5%	900
December	1st to 10th	Night	33% 17% 28% 18% 4	900
	11th to 20th	Night	35% 21% 26% 15% 3	900
	21st to end	Night	33% 22% 25% 18% 3	990

Page generated at 2025-05-23 10:21:00. Version ac07

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