Observatoires de Cantley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoires de Cantley 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	17% 9% 3 8% 5% 5% 5% 4 5% 6% 33%	126847
	Night	17% 11% 4 9% 5% 5% 4 3 5% 6% 31%	122525
Transparency	Day	51% 6% 12% 21% 10% 1	130312
	Night	53% 6% 11% 19% 10% 1	127147
Seeing	Day	22% 15% 29% 22% 10% 3	15613
	Night	22% 11% 29% 27% 10% 1	24632

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	14% 16% 7% 16% 6% 5% 4 3 4 4 20%	3597
	11th to 20th	Day	12% 16% 5% 14% 6% 5% 5% 4 4 6% 22%	3570
	21st to end	Day	11% 17% 6% 14% 6% 6% 5% 4 5% 6% 22%	4116
February	1st to 10th	Day	13% 16% 4 13% 7% 5% 4 4 5% 6% 25%	1800
	11th to 20th	Day	13% 15% 6% 10% 6% 5% 4 3 5% 5% 27%	1758
	21st to end	Day	18% 13% 4 11% 7% 5% 4 4 5% 5% 25%	1650
March	1st to 10th	Day	20% 13% 4 10% 5% 5% 4 3 5% 5% 25%	4476
	11th to 20th	Day	13% 11% 4 9% 5% 5% 5% 3 6% 7% 31%	4971
	21st to end	Day	19% 11% 3 8% 5% 4 4 3 4 4 36%	5779
April	1st to 10th	Day	22% 11% 3 6% 5% 4 4 3 4 6% 33%	2400
	11th to 20th	Day	22% 9% 3 7% 4 4 3 3 4 5% 36%	2374
	21st to end	Day	23% 9% 4 7% 5% 5% 4 3 4 5% 32%	2552
May	1st to 10th	Day	24% 9% 3 8% 5% 4 5% 4 6% 6% 26%	6346
	11th to 20th	Day	18% 8% 3 8% 6% 5% 5% 3 5% 7% 32%	6318
	21st to end	Day	22% 8% 3 7% 5% 4 4 4 5% 6% 33%	6949
June	1st to 10th	Day	22% 7% 3 7% 5% 5% 4 4 6% 6% 31%	2800
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 7% 39%	2800
	21st to end	Day	20% 6% 2 6% 5% 5% 6% 5% 6% 6% 32%	2800
July	1st to 10th	Day	15% 5% 3 7% 5% 5% 5% 5% 7% 9% 35%	6360
	11th to 20th	Day	13% 4 2 6% 4 5% 5% 4 6% 8% 44%	6360
	21st to end	Day	10% 4 2 5% 3 4 5% 4 6% 10% 48%	6926
August	1st to 10th	Day	14% 5% 3 7% 4 5% 5% 4 6% 8% 38%	4509
	11th to 20th	Day	17% 5% 2 6% 4 4 5% 5% 6% 8% 39%	4121
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 6% 9% 41%	4312
September	1st to 10th	Day	14% 5% 2 6% 4 4 5% 3 5% 8% 44%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 49%	2400
	21st to end	Day	19% 5% 2 5% 4 4 4 3 5% 8% 40%	2135
October	1st to 10th	Day	21% 7% 2 7% 3 4 4 4 5% 7% 36%	1930
	11th to 20th	Day	24% 8% 2 7% 5% 5% 5% 4 5% 6% 29%	1900
	21st to end	Day	25% 10% 3 6% 5% 5% 5% 3 4 5% 29%	1929
November	1st to 10th	Day	15% 9% 3 7% 5% 5% 4 5% 5% 7% 35%	1761
	11th to 20th	Day	19% 12% 4 8% 4 5% 5% 4 5% 6% 28%	1602
	21st to end	Day	23% 15% 4 8% 6% 5% 5% 4 5% 6% 21%	1600
December	1st to 10th	Day	21% 20% 6% 9% 5% 4 5% 4 6% 6% 15%	2600
	11th to 20th	Day	18% 16% 4 10% 6% 5% 4 3 4 3 27%	2986
	21st to end	Day	24% 18% 4 9% 5% 4 5% 3 3 4 21%	3960
January	1st to 10th	Night	14% 22% 8% 14% 5% 4 3 2 3 5% 20%	6349
	11th to 20th	Night	11% 15% 6% 14% 8% 6% 6% 3 4 5% 22%	6315
	21st to end	Night	13% 18% 6% 12% 6% 6% 5% 3 4 5% 21%	6837
February	1st to 10th	Night	13% 14% 5% 12% 5% 5% 5% 4 5% 5% 25%	2600
	11th to 20th	Night	13% 14% 5% 11% 7% 5% 4 4 4 5% 29%	2528
	21st to end	Night	16% 13% 4 9% 5% 4 4 4 5% 5% 30%	1980
March	1st to 10th	Night	23% 14% 5% 9% 5% 4 4 3 3 4 28%	5364
	11th to 20th	Night	13% 12% 3 10% 5% 5% 5% 4 6% 8% 30%	4984
	21st to end	Night	19% 10% 4 8% 5% 4 4 3 5% 6% 34%	5152
April	1st to 10th	Night	21% 10% 2 8% 5% 3 4 3 5% 6% 33%	2000
	11th to 20th	Night	20% 8% 3 6% 5% 4 4 4 5% 5% 36%	1975
	21st to end	Night	22% 9% 3 6% 4 4 3 3 5% 6% 35%	1864
May	1st to 10th	Night	23% 8% 3 9% 5% 5% 5% 4 6% 7% 25%	3594
	11th to 20th	Night	18% 7% 2 7% 5% 4 4 4 5% 6% 38%	3575
	21st to end	Night	20% 8% 3 7% 5% 4 5% 4 5% 5% 32%	3926
June	1st to 10th	Night	17% 6% 2 7% 5% 5% 5% 5% 6% 8% 35%	1600
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 6% 7% 43%	1600
	21st to end	Night	19% 7% 2 5% 5% 6% 4 4 6% 7% 36%	1600
July	1st to 10th	Night	15% 6% 3 7% 5% 6% 5% 5% 7% 8% 35%	3600
	11th to 20th	Night	14% 6% 2 5% 4 4 4 4 6% 6% 45%	3600
	21st to end	Night	10% 4 2 6% 5% 4 4 4 6% 7% 48%	3926
August	1st to 10th	Night	13% 5% 3 7% 4 4 6% 4 7% 8% 40%	2636
	11th to 20th	Night	16% 5% 2 6% 4 4 4 4 5% 7% 44%	2894
	21st to end	Night	15% 5% 2 7% 5% 5% 4 4 6% 7% 42%	3586
September	1st to 10th	Night	12% 5% 2 6% 5% 4 4 4 5% 7% 48%	2000
	11th to 20th	Night	14% 5% 1 4 4 3 3 3 5% 6% 51%	2000
	21st to end	Night	18% 6% 2 6% 5% 3 4 3 4 6% 45%	2260
October	1st to 10th	Night	19% 7% 2 5% 4 4 5% 4 5% 6% 40%	2317
	11th to 20th	Night	22% 7% 2 7% 6% 5% 5% 3 5% 6% 33%	2280
	21st to end	Night	23% 10% 3 7% 5% 4 4 4 4 6% 30%	2667
November	1st to 10th	Night	20% 9% 3 7% 4 4 4 4 5% 6% 34%	2537
	11th to 20th	Night	19% 10% 3 9% 5% 4 5% 3 5% 6% 30%	2795
	21st to end	Night	21% 12% 4 9% 5% 5% 4 4 5% 6% 25%	2800
December	1st to 10th	Night	19% 17% 5% 10% 6% 5% 6% 4 5% 6% 18%	4580
	11th to 20th	Night	24% 14% 4 8% 5% 4 4 3 3 4 27%	5208
	21st to end	Night	27% 17% 4 9% 4 4 4 3 4 4 22%	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	69% 6% 8% 13% 4 1	3598
	11th to 20th	Day	63% 5% 11% 13% 8%	3600
	21st to end	Day	63% 6% 10% 11% 8% 1	4116
February	1st to 10th	Day	60% 5% 10% 18% 6% 1	1800
	11th to 20th	Day	59% 4 12% 15% 9% 1	1767
	21st to end	Day	60% 5% 10% 13% 10% 2	1650
March	1st to 10th	Day	61% 5% 8% 12% 11% 3	4479
	11th to 20th	Day	52% 6% 17% 15% 10% 1	4971
	21st to end	Day	52% 4 5% 19% 17% 2	5779
April	1st to 10th	Day	54% 4 8% 18% 13% 3	2400
	11th to 20th	Day	51% 4 6% 21% 15% 2	2374
	21st to end	Day	56% 4 8% 15% 13% 4	2552
May	1st to 10th	Day	57% 6% 7% 19% 8% 2	6357
	11th to 20th	Day	50% 8% 13% 18% 10% 1	6322
	21st to end	Day	51% 6% 11% 19% 12% 1	6949
June	1st to 10th	Day	52% 6% 11% 18% 12% 1	2800
	11th to 20th	Day	42% 8% 12% 24% 13% 2	2800
	21st to end	Day	48% 8% 13% 20% 10% 1	2800
July	1st to 10th	Day	44% 7% 14% 26% 9%	6290
	11th to 20th	Day	38% 7% 13% 31% 12%	6285
	21st to end	Day	32% 7% 16% 35% 11%	6728
August	1st to 10th	Day	44% 7% 16% 24% 9%	4495
	11th to 20th	Day	40% 8% 15% 29% 8% 1	4160
	21st to end	Day	39% 6% 17% 25% 12% 1	4312
September	1st to 10th	Day	38% 7% 14% 29% 12%	2400
	11th to 20th	Day	35% 6% 13% 32% 13% 1	2400
	21st to end	Day	42% 7% 14% 25% 11%	2135
October	1st to 10th	Day	46% 6% 12% 28% 7% 1	3190
	11th to 20th	Day	46% 7% 15% 24% 7%	3140
	21st to end	Day	49% 5% 19% 20% 7%	3154
November	1st to 10th	Day	48% 7% 14% 23% 7% 1	1761
	11th to 20th	Day	56% 6% 13% 16% 9% 1	1602
	21st to end	Day	64% 7% 12% 10% 6% 1	1600
December	1st to 10th	Day	69% 6% 13% 10% 2	2600
	11th to 20th	Day	63% 4 10% 16% 7%	2986
	21st to end	Day	66% 5% 8% 15% 5%	3960
January	1st to 10th	Night	69% 5% 11% 12% 4	6349
	11th to 20th	Night	64% 7% 11% 13% 5%	6360
	21st to end	Night	65% 5% 8% 10% 11% 1	6837
February	1st to 10th	Night	61% 5% 10% 16% 7% 1	2600
	11th to 20th	Night	59% 4 10% 19% 8% 1	2530
	21st to end	Night	56% 6% 9% 19% 10% 1	1980
March	1st to 10th	Night	61% 4 6% 14% 12% 3	5374
	11th to 20th	Night	52% 7% 15% 16% 8% 3	4985
	21st to end	Night	52% 4 7% 21% 13% 2	5152
April	1st to 10th	Night	53% 5% 9% 17% 14% 2	2000
	11th to 20th	Night	50% 6% 7% 20% 16% 1	1975
	21st to end	Night	51% 5% 8% 17% 16% 3	1864
May	1st to 10th	Night	56% 8% 7% 19% 8% 2	3600
	11th to 20th	Night	45% 8% 13% 19% 14% 2	3576
	21st to end	Night	51% 6% 11% 17% 14% 1	3927
June	1st to 10th	Night	46% 7% 12% 24% 10% 1	1600
	11th to 20th	Night	40% 7% 11% 24% 16% 2	1600
	21st to end	Night	46% 8% 11% 19% 14% 1	1600
July	1st to 10th	Night	45% 8% 16% 20% 10% 1	3560
	11th to 20th	Night	37% 7% 13% 32% 11%	3552
	21st to end	Night	34% 7% 12% 31% 15%	3809
August	1st to 10th	Night	41% 8% 16% 25% 10%	2626
	11th to 20th	Night	40% 7% 14% 31% 8%	2921
	21st to end	Night	41% 8% 15% 23% 12% 2	3586
September	1st to 10th	Night	36% 8% 11% 30% 14% 1	2000
	11th to 20th	Night	33% 8% 12% 35% 11% 1	2000
	21st to end	Night	42% 4 12% 29% 12%	2260
October	1st to 10th	Night	39% 8% 13% 28% 12%	3837
	11th to 20th	Night	43% 7% 14% 27% 8%	3779
	21st to end	Night	46% 6% 18% 20% 9%	4392
November	1st to 10th	Night	51% 6% 15% 20% 9%	2537
	11th to 20th	Night	55% 6% 12% 17% 10% 1	2795
	21st to end	Night	61% 6% 12% 15% 5%	2800
December	1st to 10th	Night	66% 6% 13% 11% 3	4580
	11th to 20th	Night	62% 4 7% 15% 11% 1	5208
	21st to end	Night	67% 4 7% 12% 9%	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	25% 17% 33% 23% 3 1	200
	11th to 20th	Day	25% 15% 39% 15% 6% 1	200
	21st to end	Day	21% 15% 38% 21% 5%	253
February	1st to 10th	Day	30% 23% 32% 12% 2	300
	11th to 20th	Day	27% 27% 33% 12% 1	305
	21st to end	Day	28% 19% 32% 18% 3	414
March	1st to 10th	Day	25% 17% 33% 19% 6%	500
	11th to 20th	Day	25% 19% 32% 20% 4 1	500
	21st to end	Day	29% 21% 28% 15% 7% 1	550
April	1st to 10th	Day	27% 23% 29% 14% 6%	500
	11th to 20th	Day	30% 20% 24% 16% 9% 1	492
	21st to end	Day	29% 20% 32% 13% 6% 2	500
May	1st to 10th	Day	25% 15% 31% 20% 7% 2	503
	11th to 20th	Day	19% 17% 31% 20% 10% 3	527
	21st to end	Day	20% 15% 29% 24% 10% 2	573
June	1st to 10th	Day	24% 12% 29% 24% 9% 2	530
	11th to 20th	Day	15% 15% 31% 25% 12% 2	530
	21st to end	Day	22% 12% 30% 23% 11% 2	530
July	1st to 10th	Day	13% 8% 28% 34% 13% 4	525
	11th to 20th	Day	13% 9% 32% 28% 16% 3	524
	21st to end	Day	11% 8% 31% 31% 15% 3	576
August	1st to 10th	Day	13% 8% 24% 33% 18% 4	526
	11th to 20th	Day	14% 9% 22% 29% 19% 8%	519
	21st to end	Day	14% 11% 28% 26% 15% 7%	583
September	1st to 10th	Day	16% 10% 28% 22% 16% 8%	530
	11th to 20th	Day	11% 8% 22% 24% 22% 14%	530
	21st to end	Day	20% 12% 26% 23% 14% 5%	530
October	1st to 10th	Day	23% 12% 25% 22% 13% 5%	502
	11th to 20th	Day	27% 15% 30% 18% 8% 1	490
	21st to end	Day	29% 14% 27% 24% 5%	454
November	1st to 10th	Day	25% 17% 28% 19% 9% 1	395
	11th to 20th	Day	24% 20% 37% 14% 4 1	202
	21st to end	Day	30% 19% 30% 17% 4 2	200
December	1st to 10th	Day	27% 16% 34% 20% 4 1	200
	11th to 20th	Day	26% 24% 33% 15% 2 1	200
	21st to end	Day	31% 19% 31% 14% 4 1	220
January	1st to 10th	Night	28% 13% 35% 21% 4	800
	11th to 20th	Night	27% 15% 29% 23% 6% 1	800
	21st to end	Night	20% 10% 35% 28% 6% 1	880
February	1st to 10th	Night	22% 14% 35% 26% 3	800
	11th to 20th	Night	20% 17% 36% 22% 4	783
	21st to end	Night	23% 15% 30% 28% 4	660
March	1st to 10th	Night	25% 15% 32% 24% 4	800
	11th to 20th	Night	23% 17% 33% 23% 4	800
	21st to end	Night	24% 15% 29% 24% 7%	880
April	1st to 10th	Night	28% 13% 30% 25% 5%	800
	11th to 20th	Night	23% 16% 30% 22% 8%	791
	21st to end	Night	27% 11% 28% 25% 8% 1	664
May	1st to 10th	Night	25% 8% 28% 30% 9% 2	400
	11th to 20th	Night	22% 9% 25% 33% 10% 1	394
	21st to end	Night	16% 7% 38% 28% 11%	433
June	1st to 10th	Night	18% 5% 29% 38% 10%	400
	11th to 20th	Night	16% 6% 28% 35% 14% 2	400
	21st to end	Night	20% 6% 26% 33% 15% 1	400
July	1st to 10th	Night	10% 4 27% 43% 16%	390
	11th to 20th	Night	12% 4 21% 45% 18% 1	388
	21st to end	Night	9% 3 27% 42% 17% 2	438
August	1st to 10th	Night	11% 4 20% 41% 24%	422
	11th to 20th	Night	14% 3 18% 37% 25% 3	600
	21st to end	Night	14% 7% 21% 36% 20% 2	880
September	1st to 10th	Night	13% 6% 27% 33% 20% 1	800
	11th to 20th	Night	16% 6% 19% 31% 25% 4	800
	21st to end	Night	21% 7% 23% 27% 20% 2	800
October	1st to 10th	Night	22% 8% 26% 24% 18% 2	773
	11th to 20th	Night	24% 11% 29% 25% 9% 1	760
	21st to end	Night	28% 12% 28% 27% 6%	836
November	1st to 10th	Night	26% 12% 26% 27% 8% 1	780
	11th to 20th	Night	25% 15% 33% 18% 9% 1	800
	21st to end	Night	28% 16% 28% 22% 6%	800
December	1st to 10th	Night	30% 14% 30% 22% 4	800
	11th to 20th	Night	29% 14% 31% 21% 5% 1	800
	21st to end	Night	34% 13% 29% 20% 5%	880

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