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 2024-03-18.

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% 10% 4 7% 5% 3 4 4 4 6% 30%	79102
	Night	21% 11% 4 8% 6% 3 4 4 5% 6% 28%	80588
Transparency	Day	54% 6% 11% 18% 10% 1	78810
	Night	55% 7% 11% 18% 9% 1	80415
Seeing	Day	30% 19% 30% 15% 5% 1	16122
	Night	28% 16% 31% 20% 5%	23879

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% 14% 6% 10% 8% 5% 5% 4 4 6% 19%	2422
	11th to 20th	Day	20% 15% 6% 16% 6% 3 5% 4 5% 5% 15%	2840
	21st to end	Day	21% 12% 6% 12% 7% 3 4 3 5% 4 21%	3124
February	1st to 10th	Day	20% 17% 7% 9% 9% 4 4 4 4 5% 17%	1710
	11th to 20th	Day	19% 13% 6% 12% 9% 4 5% 4 3 6% 19%	1663
	21st to end	Day	15% 14% 7% 9% 7% 3 4 3 5% 6% 26%	1499
March	1st to 10th	Day	20% 14% 5% 9% 7% 4 5% 4 3 4 26%	2057
	11th to 20th	Day	20% 12% 5% 7% 6% 3 3 4 3 6% 30%	2059
	21st to end	Day	20% 13% 5% 8% 5% 3 3 3 4 5% 31%	2178
April	1st to 10th	Day	26% 15% 4 6% 4 3 4 4 4 5% 26%	2088
	11th to 20th	Day	22% 9% 3 5% 5% 2 4 5% 4 6% 36%	2262
	21st to end	Day	26% 10% 2 5% 4 2 4 4 4 5% 33%	2340
May	1st to 10th	Day	27% 11% 4 6% 4 3 4 3 4 5% 29%	2340
	11th to 20th	Day	23% 8% 3 5% 4 3 3 4 4 6% 36%	2407
	21st to end	Day	25% 8% 3 5% 4 2 5% 4 5% 7% 32%	2724
June	1st to 10th	Day	28% 7% 3 5% 5% 2 4 3 5% 7% 31%	2520
	11th to 20th	Day	24% 6% 3 6% 5% 3 4 3 4 6% 37%	2696
	21st to end	Day	26% 8% 3 5% 5% 3 4 5% 5% 7% 29%	2700
July	1st to 10th	Day	22% 6% 3 5% 4 3 3 4 5% 8% 36%	2700
	11th to 20th	Day	21% 6% 4 6% 5% 2 5% 6% 6% 9% 31%	2506
	21st to end	Day	22% 8% 3 5% 5% 3 5% 5% 5% 7% 31%	2548
August	1st to 10th	Day	18% 7% 3 4 5% 3 4 6% 7% 7% 36%	2327
	11th to 20th	Day	19% 8% 3 6% 5% 3 5% 5% 6% 6% 36%	2304
	21st to end	Day	19% 6% 2 4 4 3 4 4 4 7% 42%	2520
September	1st to 10th	Day	18% 6% 3 4 4 2 4 3 4 7% 45%	2160
	11th to 20th	Day	19% 8% 3 5% 4 3 4 3 4 6% 43%	1996
	21st to end	Day	21% 7% 3 4 5% 3 3 3 3 6% 42%	1980
October	1st to 10th	Day	20% 7% 3 5% 5% 2 4 3 5% 7% 39%	1800
	11th to 20th	Day	21% 7% 3 6% 5% 4 5% 5% 4 6% 34%	1800
	21st to end	Day	23% 8% 4 7% 6% 3 5% 4 5% 6% 30%	1970
November	1st to 10th	Day	23% 9% 3 6% 6% 3 3 4 4 6% 34%	1613
	11th to 20th	Day	21% 14% 5% 7% 5% 2 5% 4 5% 8% 24%	1520
	21st to end	Day	22% 12% 6% 9% 7% 3 5% 4 5% 5% 23%	1520
December	1st to 10th	Day	26% 15% 5% 8% 6% 3 5% 4 4 5% 19%	1520
	11th to 20th	Day	29% 14% 5% 7% 6% 3 5% 4 4 5% 17%	1906
	21st to end	Day	23% 13% 8% 8% 6% 3 4 4 4 6% 21%	2783
January	1st to 10th	Night	17% 13% 6% 16% 8% 4 5% 5% 5% 5% 18%	4425
	11th to 20th	Night	19% 16% 6% 13% 8% 4 4 4 4 5% 15%	4120
	21st to end	Night	19% 14% 5% 12% 7% 5% 6% 4 4 6% 18%	4532
February	1st to 10th	Night	20% 13% 6% 9% 7% 3 6% 5% 5% 5% 20%	2470
	11th to 20th	Night	17% 13% 5% 9% 9% 4 6% 5% 5% 6% 19%	2403
	21st to end	Night	13% 11% 5% 9% 8% 4 5% 5% 5% 7% 27%	1951
March	1st to 10th	Night	19% 11% 4 7% 8% 4 5% 4 4 6% 27%	2108
	11th to 20th	Night	20% 11% 4 7% 8% 4 5% 4 5% 5% 27%	2066
	21st to end	Night	18% 9% 4 6% 5% 3 4 4 5% 6% 35%	2178
April	1st to 10th	Night	25% 11% 3 7% 5% 3 4 4 4 6% 28%	1872
	11th to 20th	Night	25% 9% 3 5% 4 3 4 4 4 7% 33%	1645
	21st to end	Night	26% 9% 3 4 5% 3 3 4 5% 6% 31%	1620
May	1st to 10th	Night	24% 11% 4 5% 5% 3 5% 4 5% 5% 30%	1620
	11th to 20th	Night	25% 8% 2 7% 5% 2 4 4 4 6% 33%	1484
	21st to end	Night	21% 9% 3 6% 5% 2 4 4 6% 6% 32%	1550
June	1st to 10th	Night	28% 8% 3 5% 5% 2 5% 4 6% 6% 30%	1440
	11th to 20th	Night	21% 8% 4 7% 4 3 4 4 5% 6% 35%	1277
	21st to end	Night	27% 8% 4 5% 4 3 4 4 4 7% 29%	1260
July	1st to 10th	Night	24% 7% 2 5% 4 3 3 5% 5% 10% 31%	1260
	11th to 20th	Night	19% 9% 4 6% 5% 3 5% 5% 6% 9% 27%	1428
	21st to end	Night	24% 8% 3 5% 5% 3 4 4 5% 8% 32%	1770
August	1st to 10th	Night	17% 9% 3 7% 4 3 3 4 5% 8% 38%	1612
	11th to 20th	Night	20% 10% 3 6% 5% 2 3 4 4 7% 35%	1596
	21st to end	Night	19% 6% 3 4 4 3 3 5% 6% 7% 41%	1834
September	1st to 10th	Night	18% 6% 3 4 4 2 4 4 5% 7% 42%	1800
	11th to 20th	Night	21% 6% 2 4 3 2 4 4 4 6% 44%	1962
	21st to end	Night	22% 7% 3 6% 4 2 4 4 4 6% 38%	1980
October	1st to 10th	Night	22% 7% 2 5% 4 3 4 4 5% 6% 38%	2160
	11th to 20th	Night	20% 9% 2 5% 4 3 3 4 4 7% 39%	2160
	21st to end	Night	25% 8% 3 5% 4 2 4 4 5% 7% 32%	2384
November	1st to 10th	Night	21% 7% 3 6% 5% 3 4 4 5% 7% 35%	2464
	11th to 20th	Night	21% 11% 3 7% 6% 3 6% 4 6% 7% 26%	2660
	21st to end	Night	20% 12% 5% 6% 5% 3 5% 5% 5% 7% 27%	2660
December	1st to 10th	Night	25% 15% 4 8% 6% 3 5% 4 4 7% 20%	2660
	11th to 20th	Night	28% 13% 4 8% 6% 3 4 5% 5% 6% 19%	3293
	21st to end	Night	25% 13% 5% 9% 5% 3 4 4 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	60% 5% 9% 16% 8% 1	2061
	11th to 20th	Day	56% 5% 10% 16% 12% 2	2059
	21st to end	Day	56% 5% 8% 17% 11% 2	2178
April	1st to 10th	Day	61% 4 8% 14% 12% 1	2088
	11th to 20th	Day	50% 4 11% 21% 12% 1	2262
	21st to end	Day	52% 5% 10% 18% 12% 2	2340
May	1st to 10th	Day	59% 5% 8% 17% 10% 1	2340
	11th to 20th	Day	49% 6% 9% 21% 14% 2	2411
	21st to end	Day	52% 7% 11% 19% 11% 1	2724
June	1st to 10th	Day	54% 5% 10% 20% 11% 1	2520
	11th to 20th	Day	50% 5% 10% 19% 14% 3	2696
	21st to end	Day	54% 8% 12% 16% 9% 1	2700
July	1st to 10th	Day	45% 7% 14% 19% 12% 2	2625
	11th to 20th	Day	49% 8% 13% 21% 9% 1	2428
	21st to end	Day	50% 9% 12% 21% 7% 1	2406
August	1st to 10th	Day	44% 10% 13% 20% 13% 1	2314
	11th to 20th	Day	47% 7% 14% 21% 11%	2304
	21st to end	Day	41% 6% 11% 25% 16% 1	2520
September	1st to 10th	Day	40% 5% 12% 23% 18% 2	2160
	11th to 20th	Day	43% 6% 9% 23% 17% 3	1996
	21st to end	Day	45% 5% 7% 23% 19% 2	1980
October	1st to 10th	Day	46% 4 11% 25% 12% 2	1800
	11th to 20th	Day	49% 6% 12% 22% 10%	1800
	21st to end	Day	54% 6% 10% 17% 11% 1	1970
November	1st to 10th	Day	51% 6% 11% 21% 10% 1	1613
	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	57% 6% 11% 15% 9% 2	2117
	11th to 20th	Night	56% 6% 12% 13% 12% 1	2067
	21st to end	Night	49% 5% 10% 20% 13% 2	2178
April	1st to 10th	Night	57% 4 10% 14% 11% 3	1872
	11th to 20th	Night	51% 5% 10% 21% 12% 1	1645
	21st to end	Night	53% 5% 11% 19% 11% 2	1620
May	1st to 10th	Night	55% 6% 9% 19% 10% 1	1620
	11th to 20th	Night	52% 5% 8% 23% 11% 1	1485
	21st to end	Night	53% 7% 12% 19% 9% 1	1551
June	1st to 10th	Night	54% 6% 9% 21% 9%	1440
	11th to 20th	Night	51% 6% 11% 19% 12% 1	1277
	21st to end	Night	56% 7% 11% 14% 10% 1	1260
July	1st to 10th	Night	48% 9% 14% 19% 9%	1225
	11th to 20th	Night	52% 8% 13% 18% 9% 1	1383
	21st to end	Night	50% 9% 12% 21% 7% 1	1672
August	1st to 10th	Night	45% 7% 13% 23% 11%	1600
	11th to 20th	Night	49% 8% 11% 20% 11%	1596
	21st to end	Night	42% 8% 12% 25% 13%	1834
September	1st to 10th	Night	42% 6% 9% 27% 14% 2	1800
	11th to 20th	Night	41% 5% 8% 27% 17% 2	1962
	21st to end	Night	47% 5% 7% 22% 17% 2	1980
October	1st to 10th	Night	47% 6% 11% 21% 14% 2	2160
	11th to 20th	Night	44% 6% 13% 26% 11%	2160
	21st to end	Night	51% 5% 10% 21% 11% 1	2384
November	1st to 10th	Night	47% 6% 15% 21% 11%	2464
	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	37% 23% 25% 10% 4 1	449
	21st to end	Day	29% 27% 29% 9% 6%	462
April	1st to 10th	Day	42% 22% 27% 7% 2	420
	11th to 20th	Day	32% 23% 32% 10% 2	412
	21st to end	Day	35% 20% 29% 11% 5% 1	420
May	1st to 10th	Day	37% 23% 27% 10% 2 1	423
	11th to 20th	Day	27% 24% 29% 15% 4 1	553
	21st to end	Day	32% 16% 31% 17% 4 1	680
June	1st to 10th	Day	33% 16% 31% 17% 4	630
	11th to 20th	Day	24% 19% 32% 17% 7% 1	630
	21st to end	Day	33% 13% 30% 19% 5% 1	630
July	1st to 10th	Day	26% 12% 32% 22% 8% 1	620
	11th to 20th	Day	22% 11% 37% 21% 9% 1	583
	21st to end	Day	23% 15% 36% 19% 7%	488
August	1st to 10th	Day	19% 11% 38% 23% 8% 1	446
	11th to 20th	Day	24% 13% 33% 19% 9% 2	439
	21st to end	Day	23% 15% 31% 20% 10% 2	495
September	1st to 10th	Day	18% 15% 34% 25% 7% 1	450
	11th to 20th	Day	25% 17% 33% 14% 7% 4	450
	21st to end	Day	24% 18% 33% 17% 6% 3	450
October	1st to 10th	Day	26% 13% 31% 18% 9% 3	360
	11th to 20th	Day	24% 18% 31% 20% 6% 2	360
	21st to end	Day	29% 20% 28% 14% 7% 1	396
November	1st to 10th	Day	28% 17% 32% 15% 8%	365
	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	27% 19% 30% 18% 5%	752
	21st to end	Night	25% 20% 33% 16% 6%	792
April	1st to 10th	Night	34% 17% 32% 15% 2	612
	11th to 20th	Night	28% 21% 34% 14% 3	534
	21st to end	Night	30% 15% 29% 20% 5% 1	540
May	1st to 10th	Night	29% 17% 35% 16% 4	540
	11th to 20th	Night	29% 12% 36% 18% 4	532
	21st to end	Night	25% 14% 37% 19% 4 1	583
June	1st to 10th	Night	30% 11% 35% 21% 3	540
	11th to 20th	Night	23% 11% 37% 24% 6%	377
	21st to end	Night	31% 5% 35% 23% 7%	360
July	1st to 10th	Night	25% 7% 34% 29% 5%	350
	11th to 20th	Night	21% 7% 40% 24% 7%	482
	21st to end	Night	24% 7% 36% 29% 4	590
August	1st to 10th	Night	19% 9% 33% 32% 7%	538
	11th to 20th	Night	23% 9% 30% 30% 9%	532
	21st to end	Night	21% 8% 32% 28% 10% 1	594
September	1st to 10th	Night	19% 9% 33% 28% 9% 1	540
	11th to 20th	Night	22% 11% 29% 23% 13% 2	702
	21st to end	Night	25% 11% 28% 23% 12% 2	720
October	1st to 10th	Night	26% 10% 30% 24% 10% 1	720
	11th to 20th	Night	24% 17% 26% 23% 9% 2	720
	21st to end	Night	30% 17% 27% 21% 5%	792
November	1st to 10th	Night	26% 14% 27% 25% 7% 1	767
	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 2024-03-15 15:01:44. Version ac07

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