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 2024-11-02.

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	23% 11% 3 8% 5% 4 4 3 5% 6% 30%	102043
	Night	22% 12% 4 8% 5% 4 4 3 5% 6% 28%	96881
Transparency	Day	54% 6% 11% 18% 10% 1	101804
	Night	55% 6% 11% 18% 9% 1	96740
Seeing	Day	30% 19% 30% 15% 5% 1	17072
	Night	28% 16% 31% 20% 5%	24603

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	20% 15% 5% 11% 6% 6% 4 3 4 5% 19%	2422
	11th to 20th	Day	20% 15% 6% 18% 5% 5% 4 3 5% 5% 15%	2840
	21st to end	Day	21% 13% 5% 14% 6% 4 4 3 5% 4 21%	3124
February	1st to 10th	Day	20% 19% 6% 10% 8% 5% 4 4 4 5% 17%	1710
	11th to 20th	Day	18% 14% 5% 13% 8% 4 5% 3 4 6% 19%	1663
	21st to end	Day	15% 15% 6% 10% 7% 4 4 3 5% 6% 26%	1499
March	1st to 10th	Day	27% 15% 4 8% 5% 4 4 3 4 3 24%	3390
	11th to 20th	Day	20% 19% 4 8% 5% 4 4 3 4 5% 25%	3480
	21st to end	Day	32% 14% 5% 10% 4 3 2 2 3 4 21%	3816
April	1st to 10th	Day	25% 16% 3 6% 4 3 4 3 4 4 27%	2204
	11th to 20th	Day	21% 10% 2 6% 4 3 4 3 4 6% 36%	2390
	21st to end	Day	25% 10% 2 5% 3 3 3 4 5% 5% 34%	2470
May	1st to 10th	Day	24% 12% 4 7% 4 4 4 3 5% 6% 27%	4106
	11th to 20th	Day	23% 11% 3 7% 4 4 4 3 5% 6% 29%	4285
	21st to end	Day	21% 7% 3 6% 4 3 4 3 6% 8% 35%	4836
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 8% 5% 4 4 3 4 6% 31%	2079
November	1st to 10th	Day	23% 9% 3 7% 4 3 4 3 5% 6% 34%	1623
	11th to 20th	Day	21% 15% 4 8% 4 3 4 4 6% 7% 24%	1520
	21st to end	Day	21% 13% 5% 10% 6% 4 4 4 5% 5% 22%	1520
December	1st to 10th	Day	25% 16% 5% 9% 6% 4 5% 3 4 5% 19%	1520
	11th to 20th	Day	29% 15% 4 8% 5% 5% 5% 3 5% 5% 17%	1906
	21st to end	Day	23% 13% 7% 9% 5% 5% 4 3 4 6% 21%	2783
January	1st to 10th	Night	17% 13% 5% 17% 7% 5% 4 4 5% 5% 18%	4425
	11th to 20th	Night	19% 17% 6% 15% 7% 5% 4 3 4 5% 15%	4120
	21st to end	Night	19% 15% 5% 13% 6% 6% 5% 4 4 6% 18%	4532
February	1st to 10th	Night	20% 15% 5% 10% 7% 5% 6% 4 5% 5% 19%	2470
	11th to 20th	Night	17% 14% 4 11% 8% 5% 6% 3 6% 6% 19%	2403
	21st to end	Night	13% 12% 4 10% 8% 5% 4 4 5% 7% 27%	1951
March	1st to 10th	Night	25% 13% 3 8% 8% 5% 3 3 3 4 26%	3448
	11th to 20th	Night	18% 16% 5% 9% 5% 4 4 3 5% 6% 23%	3479
	21st to end	Night	33% 10% 5% 9% 3 2 3 2 3 4 27%	3816
April	1st to 10th	Night	24% 12% 3 8% 4 3 3 3 4 6% 29%	1976
	11th to 20th	Night	25% 10% 2 5% 3 3 4 3 5% 7% 33%	1737
	21st to end	Night	26% 10% 2 6% 4 3 4 4 5% 6% 32%	1710
May	1st to 10th	Night	21% 15% 4 7% 5% 4 4 3 5% 5% 28%	2834
	11th to 20th	Night	23% 12% 3 8% 5% 3 3 3 4 5% 29%	2613
	21st to end	Night	20% 11% 3 8% 4 4 3 3 6% 5% 33%	2749
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% 9% 3 6% 4 3 3 3 5% 7% 34%	2517
November	1st to 10th	Night	20% 8% 2 7% 4 4 3 3 6% 7% 35%	2488
	11th to 20th	Night	21% 11% 3 7% 5% 5% 4 4 6% 7% 26%	2660
	21st to end	Night	20% 12% 5% 7% 5% 4 5% 4 5% 7% 27%	2660
December	1st to 10th	Night	25% 16% 4 8% 5% 4 4 3 5% 7% 19%	2660
	11th to 20th	Night	28% 13% 4 9% 5% 4 4 4 5% 6% 19%	3293
	21st to end	Night	24% 14% 5% 10% 4 4 4 3 4 4 24%	4884

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	62% 7% 16% 13% 3	2423
	11th to 20th	Day	68% 7% 11% 11% 3	2840
	21st to end	Day	63% 6% 11% 13% 7%	3124
February	1st to 10th	Day	67% 6% 11% 9% 6%	1710
	11th to 20th	Day	64% 7% 10% 14% 5%	1671
	21st to end	Day	57% 5% 12% 15% 8% 2	1499
March	1st to 10th	Day	63% 5% 9% 15% 6% 2	3394
	11th to 20th	Day	57% 7% 12% 15% 8% 1	3480
	21st to end	Day	69% 3 6% 12% 9% 1	3816
April	1st to 10th	Day	60% 4 8% 14% 12% 1	2204
	11th to 20th	Day	50% 5% 11% 21% 12% 1	2390
	21st to end	Day	50% 5% 10% 18% 14% 2	2470
May	1st to 10th	Day	56% 7% 11% 19% 7%	4120
	11th to 20th	Day	52% 9% 12% 18% 8% 1	4289
	21st to end	Day	46% 7% 16% 16% 13% 1	4836
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	46% 4 11% 24% 13% 2	1900
	11th to 20th	Day	48% 6% 12% 22% 12%	1900
	21st to end	Day	53% 6% 11% 18% 11% 1	2079
November	1st to 10th	Day	51% 6% 11% 21% 10% 1	1623
	11th to 20th	Day	57% 6% 13% 17% 7% 1	1520
	21st to end	Day	61% 6% 13% 16% 3	1520
December	1st to 10th	Day	65% 6% 11% 13% 4	1520
	11th to 20th	Day	68% 6% 9% 12% 5%	1905
	21st to end	Day	62% 7% 15% 11% 5%	2783
January	1st to 10th	Night	62% 8% 12% 14% 3	4425
	11th to 20th	Night	66% 8% 11% 11% 4	4120
	21st to end	Night	64% 8% 11% 12% 5%	4532
February	1st to 10th	Night	64% 7% 11% 13% 6% 1	2470
	11th to 20th	Night	62% 6% 11% 14% 7%	2406
	21st to end	Night	53% 7% 13% 18% 9% 1	1951
March	1st to 10th	Night	63% 5% 9% 13% 10% 1	3457
	11th to 20th	Night	57% 7% 14% 13% 8% 1	3480
	21st to end	Night	63% 3 8% 15% 10% 2	3816
April	1st to 10th	Night	57% 4 10% 15% 12% 3	1976
	11th to 20th	Night	51% 5% 10% 21% 12% 1	1737
	21st to end	Night	53% 5% 10% 18% 12% 2	1710
May	1st to 10th	Night	55% 8% 11% 18% 6% 1	2840
	11th to 20th	Night	55% 7% 9% 21% 6% 1	2614
	21st to end	Night	52% 8% 11% 22% 7%	2750
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	47% 6% 11% 21% 14% 2	2280
	11th to 20th	Night	42% 6% 13% 26% 13%	2280
	21st to end	Night	50% 5% 10% 22% 12% 1	2517
November	1st to 10th	Night	47% 6% 15% 22% 11%	2488
	11th to 20th	Night	53% 7% 13% 18% 8%	2660
	21st to end	Night	55% 9% 15% 16% 5%	2660
December	1st to 10th	Night	63% 6% 13% 14% 4	2660
	11th to 20th	Night	63% 7% 10% 14% 5%	3295
	21st to end	Night	61% 5% 13% 17% 4	4884

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	36% 23% 28% 11% 3	302
	11th to 20th	Day	35% 19% 30% 14% 1	370
	21st to end	Day	24% 27% 33% 13% 3	407
February	1st to 10th	Day	35% 19% 29% 14% 2	370
	11th to 20th	Day	36% 26% 25% 11% 2	366
	21st to end	Day	32% 21% 32% 13% 2	349
March	1st to 10th	Day	35% 25% 28% 10% 2	460
	11th to 20th	Day	38% 22% 25% 9% 4 1	460
	21st to end	Day	32% 26% 27% 9% 5%	506
April	1st to 10th	Day	40% 24% 26% 7% 2	460
	11th to 20th	Day	31% 23% 33% 10% 3	452
	21st to end	Day	32% 22% 29% 11% 5% 1	460
May	1st to 10th	Day	36% 23% 28% 10% 2 1	463
	11th to 20th	Day	27% 24% 28% 17% 4 1	600
	21st to end	Day	31% 16% 31% 17% 4 1	735
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% 31% 15% 8%	369
	11th to 20th	Day	36% 21% 24% 13% 5%	370
	21st to end	Day	34% 20% 30% 13% 2 1	370
December	1st to 10th	Day	36% 22% 29% 12% 1	370
	11th to 20th	Day	43% 22% 22% 11% 2	370
	21st to end	Day	36% 21% 26% 12% 3 1	407
January	1st to 10th	Night	32% 20% 28% 17% 2	848
	11th to 20th	Night	32% 18% 29% 19% 3	760
	21st to end	Night	26% 21% 31% 18% 3	836
February	1st to 10th	Night	32% 18% 30% 18% 2	760
	11th to 20th	Night	31% 22% 30% 15% 2	743
	21st to end	Night	22% 20% 35% 18% 4	628
March	1st to 10th	Night	29% 19% 33% 18% 2	760
	11th to 20th	Night	26% 20% 31% 18% 5%	760
	21st to end	Night	27% 20% 32% 15% 6%	836
April	1st to 10th	Night	33% 16% 33% 15% 2	646
	11th to 20th	Night	28% 20% 34% 15% 3	564
	21st to end	Night	29% 16% 30% 20% 5% 1	570
May	1st to 10th	Night	29% 17% 35% 16% 3	570
	11th to 20th	Night	28% 13% 36% 19% 4	562
	21st to end	Night	24% 14% 37% 19% 5% 1	616
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% 14% 27% 25% 7% 1	775
	11th to 20th	Night	29% 17% 29% 19% 6%	850
	21st to end	Night	29% 22% 26% 18% 5%	850
December	1st to 10th	Night	34% 16% 28% 17% 4	850
	11th to 20th	Night	35% 21% 26% 15% 2	850
	21st to end	Night	33% 21% 25% 18% 3	935

Page generated at 2024-10-30 14:19:22. Version ac07

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