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 2024-01-12.

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	18% 9% 3 7% 6% 3 5% 5% 5% 6% 33%	74724
	Night	18% 10% 4 7% 6% 3 4 4 5% 6% 33%	74974
Transparency	Day	51% 6% 11% 20% 10% 1	74429
	Night	53% 6% 11% 19% 10% 1	74803
Seeing	Day	22% 15% 29% 22% 10% 3	13344
	Night	23% 11% 29% 27% 10% 1	22546

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	18% 16% 6% 10% 6% 4 5% 4 5% 4 22%	1517
	11th to 20th	Day	20% 15% 5% 8% 6% 4 5% 5% 3 5% 24%	1448
	21st to end	Day	14% 13% 5% 11% 7% 3 6% 5% 5% 6% 26%	1642
February	1st to 10th	Day	13% 15% 5% 12% 8% 4 4 4 5% 6% 24%	1620
	11th to 20th	Day	14% 14% 6% 9% 6% 4 4 4 5% 6% 28%	1577
	21st to end	Day	19% 13% 5% 9% 8% 4 4 5% 4 5% 24%	1480
March	1st to 10th	Day	16% 10% 5% 8% 8% 4 5% 5% 5% 6% 29%	1797
	11th to 20th	Day	16% 11% 4 7% 7% 4 6% 4 5% 6% 31%	1971
	21st to end	Day	20% 8% 4 7% 6% 3 4 3 4 5% 37%	2297
April	1st to 10th	Day	23% 10% 3 6% 6% 3 4 3 4 5% 33%	2160
	11th to 20th	Day	22% 8% 3 6% 5% 2 4 4 4 5% 37%	2134
	21st to end	Day	23% 9% 4 7% 6% 3 4 4 4 5% 31%	2295
May	1st to 10th	Day	21% 8% 3 6% 6% 3 5% 4 5% 6% 34%	2520
	11th to 20th	Day	18% 6% 3 5% 6% 3 5% 4 5% 6% 39%	2478
	21st to end	Day	17% 6% 3 5% 6% 4 5% 5% 6% 7% 36%	2725
June	1st to 10th	Day	23% 7% 3 6% 6% 3 5% 5% 6% 6% 30%	2520
	11th to 20th	Day	15% 6% 2 5% 5% 4 5% 5% 7% 8% 39%	2520
	21st to end	Day	20% 5% 3 6% 5% 3 7% 6% 6% 7% 32%	2520
July	1st to 10th	Day	13% 5% 3 5% 5% 3 5% 6% 7% 8% 40%	2520
	11th to 20th	Day	13% 5% 2 6% 5% 4 5% 5% 6% 9% 39%	2520
	21st to end	Day	13% 5% 3 5% 5% 4 5% 6% 6% 10% 39%	2744
August	1st to 10th	Day	11% 5% 3 5% 7% 4 5% 5% 6% 8% 40%	2482
	11th to 20th	Day	13% 4 2 6% 6% 3 6% 6% 7% 7% 40%	2280
	21st to end	Day	14% 6% 2 5% 5% 3 4 5% 6% 9% 42%	2376
September	1st to 10th	Day	15% 4 2 5% 5% 3 5% 4 5% 8% 42%	2160
	11th to 20th	Day	14% 4 2 5% 5% 4 4 4 5% 7% 47%	2160
	21st to end	Day	19% 5% 2 5% 5% 3 4 4 5% 8% 40%	1922
October	1st to 10th	Day	23% 6% 3 7% 4 3 3 5% 5% 6% 36%	1800
	11th to 20th	Day	25% 8% 3 6% 6% 4 5% 5% 5% 6% 27%	1800
	21st to end	Day	27% 9% 3 6% 6% 4 4 5% 4 5% 28%	1828
November	1st to 10th	Day	15% 9% 3 6% 5% 3 5% 6% 5% 7% 34%	1671
	11th to 20th	Day	20% 11% 4 7% 5% 4 5% 5% 5% 6% 27%	1522
	21st to end	Day	23% 14% 4 7% 7% 4 6% 5% 5% 6% 21%	1520
December	1st to 10th	Day	23% 15% 6% 9% 6% 3 5% 5% 5% 5% 18%	1520
	11th to 20th	Day	21% 12% 4 9% 7% 3 5% 5% 4 4 26%	1906
	21st to end	Day	29% 19% 5% 7% 5% 2 4 3 3 4 19%	2772
January	1st to 10th	Night	18% 17% 6% 11% 8% 4 4 3 4 4 20%	2649
	11th to 20th	Night	19% 17% 5% 9% 7% 4 5% 4 3 4 24%	2535
	21st to end	Night	13% 13% 6% 9% 7% 3 4 4 4 6% 31%	2711
February	1st to 10th	Night	15% 13% 6% 11% 7% 4 5% 5% 5% 5% 24%	2340
	11th to 20th	Night	14% 12% 5% 9% 8% 4 5% 4 4 5% 29%	2269
	21st to end	Night	18% 12% 4 8% 6% 3 4 5% 5% 5% 30%	1776
March	1st to 10th	Night	16% 12% 6% 8% 7% 4 5% 5% 4 5% 30%	2150
	11th to 20th	Night	16% 12% 5% 8% 6% 3 4 4 4 4 33%	1984
	21st to end	Night	18% 9% 3 6% 5% 3 3 3 4 5% 40%	2058
April	1st to 10th	Night	22% 9% 4 6% 5% 3 4 4 4 6% 34%	1800
	11th to 20th	Night	19% 7% 3 5% 5% 3 5% 5% 5% 5% 37%	1775
	21st to end	Night	21% 9% 3 6% 4 4 4 4 5% 6% 34%	1679
May	1st to 10th	Night	21% 7% 2 6% 4 3 4 4 5% 8% 37%	1440
	11th to 20th	Night	20% 6% 3 5% 5% 3 3 5% 4 6% 40%	1415
	21st to end	Night	14% 5% 3 5% 5% 2 4 6% 5% 6% 44%	1550
June	1st to 10th	Night	18% 5% 2 5% 6% 4 5% 6% 6% 9% 35%	1440
	11th to 20th	Night	15% 5% 2 5% 5% 3 4 5% 6% 7% 43%	1440
	21st to end	Night	17% 6% 2 5% 5% 5% 5% 5% 6% 8% 37%	1440
July	1st to 10th	Night	12% 4 2 5% 5% 3 5% 6% 5% 8% 46%	1440
	11th to 20th	Night	12% 6% 3 4 4 3 5% 5% 6% 8% 44%	1440
	21st to end	Night	10% 4 2 5% 7% 3 5% 6% 7% 9% 43%	1574
August	1st to 10th	Night	11% 5% 2 6% 5% 3 5% 5% 6% 9% 43%	1453
	11th to 20th	Night	11% 5% 2 5% 5% 3 5% 5% 6% 7% 46%	1602
	21st to end	Night	12% 4 2 5% 5% 4 5% 4 6% 7% 44%	1980
September	1st to 10th	Night	13% 5% 1 6% 5% 3 4 5% 5% 7% 46%	1800
	11th to 20th	Night	16% 5% 2 4 4 3 3 4 5% 6% 49%	1800
	21st to end	Night	19% 5% 2 6% 6% 2 4 4 4 6% 44%	2034
October	1st to 10th	Night	20% 7% 2 5% 5% 3 5% 4 4 6% 39%	2160
	11th to 20th	Night	23% 6% 2 5% 8% 3 6% 4 5% 6% 32%	2160
	21st to end	Night	24% 9% 4 6% 6% 3 4 5% 4 6% 29%	2526
November	1st to 10th	Night	20% 8% 3 6% 5% 3 4 5% 5% 6% 35%	2407
	11th to 20th	Night	20% 9% 4 8% 6% 3 5% 5% 5% 6% 30%	2655
	21st to end	Night	22% 11% 5% 8% 6% 4 4 4 5% 6% 25%	2660
December	1st to 10th	Night	22% 16% 5% 8% 6% 4 5% 5% 5% 6% 19%	2660
	11th to 20th	Night	23% 13% 4 8% 6% 3 3 3 4 4 28%	3288
	21st to end	Night	30% 17% 4 8% 5% 2 3 3 4 4 19%	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	67% 5% 9% 13% 5%	1518
	11th to 20th	Day	64% 5% 8% 11% 11% 1	1448
	21st to end	Day	59% 7% 11% 14% 9% 1	1642
February	1st to 10th	Day	62% 5% 10% 17% 5% 2	1620
	11th to 20th	Day	58% 4 12% 15% 9% 1	1586
	21st to end	Day	61% 5% 10% 13% 9% 1	1480
March	1st to 10th	Day	56% 6% 10% 17% 10% 1	1800
	11th to 20th	Day	55% 5% 12% 14% 11% 3	1971
	21st to end	Day	50% 5% 8% 18% 14% 5%	2297
April	1st to 10th	Day	55% 4 8% 18% 12% 3	2160
	11th to 20th	Day	50% 4 7% 22% 15% 2	2134
	21st to end	Day	56% 4 8% 14% 13% 4	2295
May	1st to 10th	Day	51% 6% 7% 19% 14% 3	2520
	11th to 20th	Day	46% 5% 9% 21% 16% 3	2482
	21st to end	Day	46% 7% 12% 19% 14% 2	2725
June	1st to 10th	Day	53% 6% 11% 18% 11% 1	2520
	11th to 20th	Day	42% 8% 11% 24% 13% 2	2520
	21st to end	Day	48% 9% 13% 20% 10% 1	2520
July	1st to 10th	Day	39% 9% 12% 26% 14% 1	2450
	11th to 20th	Day	40% 8% 15% 26% 11%	2445
	21st to end	Day	40% 9% 16% 24% 11%	2591
August	1st to 10th	Day	41% 9% 16% 25% 9%	2468
	11th to 20th	Day	40% 10% 15% 26% 9% 1	2280
	21st to end	Day	39% 7% 15% 27% 12% 1	2376
September	1st to 10th	Day	39% 7% 14% 27% 12%	2160
	11th to 20th	Day	37% 7% 13% 31% 12% 1	2160
	21st to end	Day	43% 8% 15% 26% 9%	1922
October	1st to 10th	Day	48% 7% 12% 22% 11% 1	1800
	11th to 20th	Day	57% 7% 14% 16% 6% 1	1800
	21st to end	Day	59% 5% 13% 15% 8%	1828
November	1st to 10th	Day	48% 7% 14% 22% 7% 1	1671
	11th to 20th	Day	57% 6% 13% 15% 8% 1	1522
	21st to end	Day	63% 8% 12% 10% 6% 1	1520
December	1st to 10th	Day	67% 7% 12% 11% 3	1520
	11th to 20th	Day	64% 5% 12% 14% 5%	1906
	21st to end	Day	71% 5% 7% 14% 3	2772
January	1st to 10th	Night	69% 5% 9% 13% 5%	2649
	11th to 20th	Night	66% 5% 9% 10% 8% 1	2535
	21st to end	Night	56% 5% 9% 18% 11% 1	2711
February	1st to 10th	Night	63% 5% 9% 16% 7% 1	2340
	11th to 20th	Night	58% 4 10% 19% 8% 1	2271
	21st to end	Night	55% 6% 9% 18% 11% 1	1776
March	1st to 10th	Night	58% 5% 9% 17% 10% 2	2160
	11th to 20th	Night	55% 4 9% 18% 11% 3	1985
	21st to end	Night	48% 4 7% 20% 15% 5%	2058
April	1st to 10th	Night	53% 4 10% 17% 15% 2	1800
	11th to 20th	Night	49% 6% 8% 20% 16% 1	1775
	21st to end	Night	52% 5% 8% 16% 16% 4	1679
May	1st to 10th	Night	47% 6% 7% 19% 16% 5%	1440
	11th to 20th	Night	44% 5% 7% 21% 19% 3	1416
	21st to end	Night	39% 7% 14% 20% 17% 2	1551
June	1st to 10th	Night	46% 7% 13% 24% 9% 1	1440
	11th to 20th	Night	40% 8% 11% 24% 16% 2	1440
	21st to end	Night	45% 9% 12% 19% 14% 1	1440
July	1st to 10th	Night	34% 10% 11% 27% 17% 2	1400
	11th to 20th	Night	37% 8% 11% 31% 13%	1392
	21st to end	Night	37% 8% 16% 24% 15%	1486
August	1st to 10th	Night	35% 11% 18% 23% 12% 1	1443
	11th to 20th	Night	36% 8% 15% 29% 11%	1602
	21st to end	Night	37% 9% 16% 28% 11%	1980
September	1st to 10th	Night	37% 8% 11% 29% 14% 1	1800
	11th to 20th	Night	35% 9% 12% 34% 9% 1	1800
	21st to end	Night	43% 5% 12% 31% 10%	2034
October	1st to 10th	Night	47% 7% 10% 23% 13% 1	2160
	11th to 20th	Night	53% 7% 11% 21% 8%	2160
	21st to end	Night	56% 6% 13% 17% 8%	2526
November	1st to 10th	Night	50% 6% 16% 20% 9%	2407
	11th to 20th	Night	56% 6% 12% 16% 9% 1	2655
	21st to end	Night	61% 6% 13% 15% 5%	2660
December	1st to 10th	Night	67% 6% 10% 12% 4	2660
	11th to 20th	Night	61% 5% 9% 17% 8% 1	3288
	21st to end	Night	71% 5% 8% 13% 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	27% 16% 31% 24% 3 1	180
	11th to 20th	Day	26% 17% 36% 15% 6% 1	162
	21st to end	Day	23% 14% 39% 18% 6%	202
February	1st to 10th	Day	34% 23% 32% 11% 1	240
	11th to 20th	Day	28% 25% 34% 12% 1	244
	21st to end	Day	29% 19% 32% 17% 3	346
March	1st to 10th	Day	23% 17% 35% 19% 6%	420
	11th to 20th	Day	26% 18% 31% 20% 3 1	420
	21st to end	Day	29% 20% 27% 16% 8% 1	462
April	1st to 10th	Day	27% 21% 30% 15% 6%	420
	11th to 20th	Day	30% 19% 22% 18% 10% 1	412
	21st to end	Day	29% 19% 30% 14% 6% 2	420
May	1st to 10th	Day	25% 15% 32% 19% 7% 3	423
	11th to 20th	Day	20% 17% 30% 19% 10% 3	447
	21st to end	Day	19% 14% 32% 23% 10% 2	485
June	1st to 10th	Day	25% 11% 30% 23% 9% 2	450
	11th to 20th	Day	16% 16% 28% 26% 12% 2	450
	21st to end	Day	22% 12% 28% 23% 13% 3	450
July	1st to 10th	Day	12% 8% 26% 36% 13% 4	445
	11th to 20th	Day	13% 9% 31% 28% 16% 3	444
	21st to end	Day	13% 8% 30% 32% 15% 3	488
August	1st to 10th	Day	13% 8% 25% 32% 17% 4	446
	11th to 20th	Day	13% 9% 22% 29% 18% 8%	439
	21st to end	Day	13% 11% 28% 25% 16% 7%	495
September	1st to 10th	Day	16% 9% 29% 21% 16% 8%	450
	11th to 20th	Day	13% 10% 25% 23% 19% 10%	450
	21st to end	Day	20% 13% 26% 22% 14% 5%	450
October	1st to 10th	Day	25% 12% 25% 20% 12% 5%	450
	11th to 20th	Day	29% 16% 30% 18% 7% 1	450
	21st to end	Day	31% 14% 26% 24% 5%	419
November	1st to 10th	Day	25% 17% 28% 19% 10% 1	365
	11th to 20th	Day	26% 20% 35% 13% 4 1	182
	21st to end	Day	29% 19% 29% 17% 4 2	180
December	1st to 10th	Day	28% 14% 36% 19% 3	180
	11th to 20th	Day	26% 24% 33% 16% 1 1	180
	21st to end	Day	32% 20% 32% 13% 2 1	198
January	1st to 10th	Night	28% 13% 33% 22% 4	760
	11th to 20th	Night	29% 14% 29% 22% 6%	725
	21st to end	Night	20% 9% 37% 28% 6% 1	792
February	1st to 10th	Night	23% 13% 36% 25% 3	720
	11th to 20th	Night	20% 17% 36% 22% 4	703
	21st to end	Night	25% 14% 30% 28% 3	592
March	1st to 10th	Night	24% 15% 33% 24% 4	720
	11th to 20th	Night	23% 17% 33% 22% 5%	720
	21st to end	Night	23% 14% 30% 25% 8%	792
April	1st to 10th	Night	28% 13% 30% 25% 5%	720
	11th to 20th	Night	23% 16% 31% 22% 8%	711
	21st to end	Night	27% 12% 27% 25% 8% 1	599
May	1st to 10th	Night	24% 8% 27% 29% 9% 2	360
	11th to 20th	Night	23% 10% 25% 32% 8% 1	354
	21st to end	Night	16% 6% 39% 27% 11% 1	389
June	1st to 10th	Night	20% 4 29% 38% 10%	360
	11th to 20th	Night	17% 6% 28% 33% 14% 2	360
	21st to end	Night	18% 6% 26% 33% 16% 1	360
July	1st to 10th	Night	9% 4 29% 42% 16%	350
	11th to 20th	Night	12% 3 23% 44% 18% 1	348
	21st to end	Night	9% 3 27% 41% 17% 2	394
August	1st to 10th	Night	12% 4 20% 41% 23% 1	379
	11th to 20th	Night	14% 3 19% 37% 25% 3	538
	21st to end	Night	13% 7% 22% 36% 19% 2	792
September	1st to 10th	Night	14% 6% 28% 33% 19% 1	720
	11th to 20th	Night	18% 6% 20% 31% 22% 3	720
	21st to end	Night	22% 8% 24% 26% 20% 2	720
October	1st to 10th	Night	23% 9% 27% 23% 16% 2	720
	11th to 20th	Night	25% 11% 29% 25% 9% 1	720
	21st to end	Night	28% 12% 27% 27% 6%	792
November	1st to 10th	Night	26% 12% 26% 27% 9% 1	740
	11th to 20th	Night	25% 15% 33% 17% 9% 1	760
	21st to end	Night	29% 15% 28% 23% 6%	760
December	1st to 10th	Night	30% 14% 30% 22% 3	760
	11th to 20th	Night	28% 14% 31% 20% 5% 1	760
	21st to end	Night	34% 13% 30% 19% 4	836

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