Observatoire de la Pourvoirie St-Zénon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire de la Pourvoirie St-Zénon clear sky chart from 2005-11-06 to 2025-12-03.

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	16% 12% 4 10% 6% 5% 5% 4 5% 6% 28%	127331
	Night	14% 13% 5% 11% 6% 5% 5% 4 5% 6% 27%	123375
Transparency	Day	53% 7% 12% 17% 9% 1	130750
	Night	53% 7% 13% 17% 9% 1	128017
Seeing	Day	25% 20% 31% 17% 6% 2	15818
	Night	22% 17% 31% 23% 7% 1	24751

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	10% 21% 7% 15% 7% 6% 7% 4 4 4 14%	3597
	11th to 20th	Day	7% 15% 7% 16% 12% 6% 4 3 4 5% 20%	3570
	21st to end	Day	7% 11% 8% 18% 7% 5% 6% 4 5% 7% 23%	3960
February	1st to 10th	Day	10% 16% 6% 13% 9% 8% 4 3 4 6% 21%	1720
	11th to 20th	Day	10% 18% 6% 12% 6% 7% 5% 4 5% 5% 23%	1838
	21st to end	Day	12% 19% 6% 11% 7% 5% 4 4 4 4 23%	1650
March	1st to 10th	Day	15% 20% 5% 14% 7% 6% 4 2 3 3 21%	4476
	11th to 20th	Day	10% 16% 6% 12% 6% 4 4 3 5% 6% 28%	4691
	21st to end	Day	13% 16% 5% 11% 4 4 4 3 4 5% 31%	5970
April	1st to 10th	Day	17% 17% 5% 9% 5% 4 3 3 4 5% 27%	2400
	11th to 20th	Day	21% 12% 4 8% 5% 3 4 2 5% 5% 32%	2374
	21st to end	Day	19% 15% 4 8% 5% 5% 4 4 4 4 29%	2585
May	1st to 10th	Day	23% 13% 4 9% 5% 4 4 4 5% 6% 25%	6341
	11th to 20th	Day	22% 9% 3 8% 5% 5% 4 4 5% 6% 30%	6318
	21st to end	Day	22% 10% 3 9% 5% 5% 5% 3 4 5% 27%	6949
June	1st to 10th	Day	23% 9% 3 8% 5% 5% 5% 4 5% 6% 27%	2800
	11th to 20th	Day	17% 8% 4 7% 5% 6% 5% 5% 6% 8% 31%	2800
	21st to end	Day	23% 9% 3 8% 5% 6% 5% 4 5% 7% 25%	2800
July	1st to 10th	Day	17% 8% 4 9% 7% 6% 6% 5% 6% 8% 25%	6360
	11th to 20th	Day	12% 6% 3 7% 5% 6% 6% 5% 8% 8% 34%	6360
	21st to end	Day	11% 6% 2 7% 4 4 5% 4 7% 9% 39%	6926
August	1st to 10th	Day	14% 8% 3 8% 6% 6% 6% 5% 7% 8% 29%	4563
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 5% 7% 7% 32%	4221
	21st to end	Day	15% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	4312
September	1st to 10th	Day	15% 7% 3 7% 5% 4 4 3 6% 8% 39%	2400
	11th to 20th	Day	13% 7% 3 7% 5% 4 4 4 5% 6% 42%	2317
	21st to end	Day	20% 10% 3 5% 4 3 4 3 4 6% 37%	2200
October	1st to 10th	Day	21% 10% 3 7% 5% 5% 4 4 6% 6% 31%	2057
	11th to 20th	Day	20% 15% 4 9% 6% 5% 4 4 4 5% 25%	1939
	21st to end	Day	24% 14% 4 9% 5% 5% 4 3 4 5% 23%	1980
November	1st to 10th	Day	16% 16% 5% 9% 6% 4 4 3 5% 5% 27%	1851
	11th to 20th	Day	16% 17% 5% 11% 6% 5% 5% 3 5% 6% 21%	1764
	21st to end	Day	16% 20% 7% 13% 6% 5% 4 3 4 4 19%	1680
December	1st to 10th	Day	17% 18% 9% 15% 7% 6% 4 3 3 4 15%	2616
	11th to 20th	Day	16% 15% 5% 15% 6% 4 5% 4 4 6% 21%	2986
	21st to end	Day	16% 15% 8% 13% 6% 5% 4 3 4 4 22%	3960
January	1st to 10th	Night	10% 21% 9% 17% 8% 5% 5% 3 3 3 14%	6349
	11th to 20th	Night	7% 13% 7% 19% 12% 8% 5% 4 4 5% 15%	6315
	21st to end	Night	6% 11% 7% 18% 9% 8% 6% 4 5% 6% 21%	6996
February	1st to 10th	Night	9% 15% 5% 13% 7% 6% 6% 4 5% 6% 23%	2675
	11th to 20th	Night	9% 15% 5% 13% 8% 7% 5% 4 6% 6% 23%	2448
	21st to end	Night	11% 14% 5% 11% 7% 5% 5% 5% 5% 6% 27%	1980
March	1st to 10th	Night	17% 17% 5% 12% 7% 4 6% 3 4 4 21%	5364
	11th to 20th	Night	10% 16% 5% 12% 7% 5% 5% 3 5% 7% 26%	5265
	21st to end	Night	13% 12% 5% 11% 6% 6% 5% 4 5% 5% 28%	4962
April	1st to 10th	Night	21% 13% 4 9% 5% 5% 4 3 5% 5% 27%	2000
	11th to 20th	Night	17% 11% 3 7% 5% 5% 4 3 5% 6% 35%	1975
	21st to end	Night	20% 11% 4 8% 4 4 3 4 5% 4 34%	1815
May	1st to 10th	Night	21% 12% 4 8% 5% 5% 3 3 5% 6% 27%	3641
	11th to 20th	Night	23% 10% 3 6% 3 4 4 3 5% 6% 34%	3575
	21st to end	Night	21% 10% 3 9% 5% 5% 5% 4 5% 6% 28%	3926
June	1st to 10th	Night	20% 8% 3 7% 5% 3 5% 4 5% 9% 32%	1600
	11th to 20th	Night	17% 8% 4 6% 5% 4 5% 4 5% 7% 36%	1600
	21st to end	Night	20% 8% 2 6% 4 5% 5% 3 6% 7% 35%	1600
July	1st to 10th	Night	13% 7% 3 8% 6% 6% 6% 5% 8% 9% 31%	3600
	11th to 20th	Night	13% 6% 3 6% 4 4 5% 4 6% 8% 41%	3600
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 8% 47%	3926
August	1st to 10th	Night	11% 8% 3 8% 5% 5% 5% 4 6% 7% 38%	2586
	11th to 20th	Night	16% 8% 3 8% 4 4 5% 3 5% 6% 38%	2776
	21st to end	Night	16% 8% 3 7% 5% 5% 4 3 5% 6% 38%	3586
September	1st to 10th	Night	13% 8% 2 6% 4 4 3 3 5% 7% 44%	2000
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 5% 5% 47%	2082
	21st to end	Night	16% 9% 2 5% 4 3 4 3 4 7% 42%	2200
October	1st to 10th	Night	16% 10% 4 7% 4 4 3 2 4 6% 39%	2340
	11th to 20th	Night	18% 14% 4 9% 4 3 4 2 5% 6% 30%	2460
	21st to end	Night	22% 12% 4 10% 6% 5% 4 3 5% 5% 24%	2860
November	1st to 10th	Night	16% 13% 6% 9% 5% 5% 4 4 5% 6% 27%	2667
	11th to 20th	Night	15% 14% 5% 12% 6% 6% 5% 4 5% 6% 24%	2853
	21st to end	Night	17% 17% 6% 12% 5% 5% 5% 4 5% 5% 20%	2940
December	1st to 10th	Night	13% 17% 6% 15% 9% 7% 6% 4 4 5% 14%	4609
	11th to 20th	Night	19% 17% 6% 11% 7% 5% 4 3 5% 4 20%	5208
	21st to end	Night	17% 18% 7% 12% 7% 4 3 3 4 5% 21%	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	67% 8% 10% 11% 4	3598
	11th to 20th	Day	64% 7% 7% 13% 8% 2	3600
	21st to end	Day	57% 7% 7% 13% 15% 1	3960
February	1st to 10th	Day	63% 5% 7% 15% 8% 2	1720
	11th to 20th	Day	59% 6% 10% 13% 10% 1	1847
	21st to end	Day	61% 6% 7% 14% 9% 3	1650
March	1st to 10th	Day	67% 5% 6% 9% 10% 4	4479
	11th to 20th	Day	53% 7% 14% 11% 12% 3	4691
	21st to end	Day	52% 6% 8% 15% 16% 3	5970
April	1st to 10th	Day	58% 5% 10% 14% 11% 2	2400
	11th to 20th	Day	53% 5% 8% 19% 12% 2	2374
	21st to end	Day	55% 7% 8% 15% 12% 3	2585
May	1st to 10th	Day	58% 7% 9% 16% 10% 1	6352
	11th to 20th	Day	50% 9% 15% 17% 7% 2	6322
	21st to end	Day	56% 7% 9% 14% 12% 1	6949
June	1st to 10th	Day	54% 8% 10% 16% 10% 1	2800
	11th to 20th	Day	48% 8% 13% 20% 10% 2	2800
	21st to end	Day	58% 8% 11% 15% 8%	2800
July	1st to 10th	Day	52% 8% 13% 22% 6%	6290
	11th to 20th	Day	41% 8% 18% 27% 5%	6285
	21st to end	Day	38% 8% 17% 28% 9%	6728
August	1st to 10th	Day	48% 9% 17% 19% 6%	4549
	11th to 20th	Day	46% 9% 18% 21% 5%	4263
	21st to end	Day	46% 8% 13% 20% 12% 1	4312
September	1st to 10th	Day	41% 8% 16% 24% 10% 1	2400
	11th to 20th	Day	39% 8% 14% 28% 10% 1	2317
	21st to end	Day	47% 7% 12% 24% 11%	2200
October	1st to 10th	Day	48% 8% 13% 24% 6% 1	3343
	11th to 20th	Day	53% 6% 14% 21% 6%	3129
	21st to end	Day	51% 7% 20% 15% 6% 1	3180
November	1st to 10th	Day	56% 7% 12% 16% 8% 1	1851
	11th to 20th	Day	59% 8% 11% 13% 8% 1	1764
	21st to end	Day	65% 8% 12% 11% 4 1	1680
December	1st to 10th	Day	70% 5% 9% 11% 4	2616
	11th to 20th	Day	60% 5% 9% 16% 10%	2986
	21st to end	Day	62% 6% 8% 13% 12% 1	3960
January	1st to 10th	Night	69% 7% 10% 10% 4	6349
	11th to 20th	Night	64% 9% 11% 11% 4 1	6360
	21st to end	Night	60% 7% 8% 10% 14% 1	6996
February	1st to 10th	Night	57% 7% 11% 15% 8% 2	2675
	11th to 20th	Night	56% 8% 11% 15% 9% 1	2450
	21st to end	Night	53% 7% 11% 16% 12% 2	1980
March	1st to 10th	Night	64% 6% 9% 9% 10% 2	5374
	11th to 20th	Night	55% 7% 11% 13% 10% 3	5266
	21st to end	Night	53% 6% 9% 20% 9% 3	4962
April	1st to 10th	Night	56% 6% 10% 15% 12% 2	2000
	11th to 20th	Night	47% 7% 10% 20% 13% 2	1975
	21st to end	Night	50% 6% 10% 20% 13% 2	1815
May	1st to 10th	Night	55% 8% 11% 15% 9% 1	3647
	11th to 20th	Night	49% 9% 13% 19% 9% 2	3576
	21st to end	Night	54% 7% 12% 16% 10% 1	3927
June	1st to 10th	Night	47% 9% 13% 20% 11% 1	1600
	11th to 20th	Night	45% 9% 11% 21% 11% 3	1600
	21st to end	Night	48% 8% 14% 17% 12%	1600
July	1st to 10th	Night	46% 8% 22% 19% 5%	3560
	11th to 20th	Night	38% 8% 21% 28% 5%	3552
	21st to end	Night	33% 7% 19% 30% 10%	3809
August	1st to 10th	Night	43% 9% 17% 23% 8%	2576
	11th to 20th	Night	42% 8% 20% 23% 7%	2800
	21st to end	Night	44% 8% 15% 23% 10% 1	3586
September	1st to 10th	Night	37% 9% 16% 25% 13%	2000
	11th to 20th	Night	34% 8% 18% 30% 9% 1	2082
	21st to end	Night	40% 7% 14% 27% 12%	2200
October	1st to 10th	Night	45% 6% 15% 23% 10% 1	3821
	11th to 20th	Night	48% 7% 13% 23% 8%	3996
	21st to end	Night	50% 7% 17% 16% 8% 2	4610
November	1st to 10th	Night	53% 8% 13% 19% 6% 1	2667
	11th to 20th	Night	54% 10% 13% 14% 8% 1	2853
	21st to end	Night	60% 9% 12% 13% 5% 1	2940
December	1st to 10th	Night	64% 8% 14% 10% 3	4609
	11th to 20th	Night	64% 6% 8% 13% 9% 1	5208
	21st to end	Night	63% 5% 6% 11% 13% 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	25% 23% 39% 11% 3	200
	11th to 20th	Day	28% 19% 36% 13% 5%	200
	21st to end	Day	18% 24% 40% 14% 3 1	220
February	1st to 10th	Day	30% 31% 32% 6% 2	260
	11th to 20th	Day	26% 28% 37% 9%	385
	21st to end	Day	28% 27% 31% 12% 2	414
March	1st to 10th	Day	26% 25% 31% 14% 3	500
	11th to 20th	Day	27% 27% 32% 12% 3	500
	21st to end	Day	30% 27% 25% 12% 5% 1	550
April	1st to 10th	Day	32% 21% 29% 13% 4	500
	11th to 20th	Day	36% 20% 23% 12% 8% 1	492
	21st to end	Day	32% 23% 31% 9% 4 1	500
May	1st to 10th	Day	30% 21% 33% 12% 3 1	503
	11th to 20th	Day	24% 21% 31% 17% 5% 1	527
	21st to end	Day	27% 21% 33% 14% 4 1	573
June	1st to 10th	Day	25% 19% 37% 13% 5%	530
	11th to 20th	Day	22% 20% 32% 16% 10% 1	530
	21st to end	Day	29% 16% 30% 18% 5% 2	530
July	1st to 10th	Day	17% 14% 38% 23% 6% 2	525
	11th to 20th	Day	15% 14% 39% 26% 5% 1	524
	21st to end	Day	15% 13% 35% 28% 8% 1	576
August	1st to 10th	Day	16% 12% 34% 27% 8% 2	526
	11th to 20th	Day	16% 9% 32% 26% 12% 4	519
	21st to end	Day	17% 17% 30% 19% 13% 4	583
September	1st to 10th	Day	20% 16% 30% 20% 10% 3	530
	11th to 20th	Day	16% 12% 28% 24% 13% 6%	530
	21st to end	Day	23% 18% 28% 18% 10% 4	530
October	1st to 10th	Day	25% 18% 26% 19% 9% 3	530
	11th to 20th	Day	26% 23% 27% 15% 7% 1	496
	21st to end	Day	32% 18% 31% 15% 3	462
November	1st to 10th	Day	30% 23% 26% 17% 4 1	425
	11th to 20th	Day	29% 20% 33% 14% 3 1	304
	21st to end	Day	29% 31% 29% 8% 3	220
December	1st to 10th	Day	31% 17% 32% 13% 6%	204
	11th to 20th	Day	31% 31% 26% 10% 3	200
	21st to end	Day	29% 24% 31% 10% 5% 1	220
January	1st to 10th	Night	24% 21% 37% 15% 4	800
	11th to 20th	Night	19% 22% 36% 18% 5%	800
	21st to end	Night	14% 19% 40% 23% 4	880
February	1st to 10th	Night	17% 22% 38% 19% 4	800
	11th to 20th	Night	21% 23% 36% 17% 3	783
	21st to end	Night	21% 21% 34% 21% 3	660
March	1st to 10th	Night	26% 20% 33% 19% 3	800
	11th to 20th	Night	25% 25% 29% 16% 5% 1	800
	21st to end	Night	22% 18% 33% 21% 6% 1	880
April	1st to 10th	Night	29% 18% 28% 20% 4	800
	11th to 20th	Night	24% 18% 31% 20% 7%	791
	21st to end	Night	28% 19% 30% 18% 5%	615
May	1st to 10th	Night	27% 14% 31% 20% 8%	421
	11th to 20th	Night	24% 11% 31% 25% 8%	394
	21st to end	Night	21% 13% 38% 20% 8%	433
June	1st to 10th	Night	21% 12% 35% 26% 6%	400
	11th to 20th	Night	20% 11% 27% 31% 11% 1	400
	21st to end	Night	24% 8% 26% 32% 11% 1	400
July	1st to 10th	Night	13% 7% 33% 37% 9% 1	390
	11th to 20th	Night	13% 6% 29% 38% 13% 2	388
	21st to end	Night	13% 7% 29% 37% 13% 1	438
August	1st to 10th	Night	10% 6% 21% 45% 16% 1	398
	11th to 20th	Night	15% 7% 24% 35% 18% 1	531
	21st to end	Night	16% 10% 29% 28% 14% 3	880
September	1st to 10th	Night	16% 11% 29% 33% 11% 1	800
	11th to 20th	Night	16% 9% 27% 29% 17% 2	800
	21st to end	Night	20% 12% 28% 24% 15% 1	800
October	1st to 10th	Night	21% 16% 28% 24% 11% 1	800
	11th to 20th	Night	27% 16% 28% 21% 7% 1	800
	21st to end	Night	27% 19% 28% 21% 4	880
November	1st to 10th	Night	25% 20% 28% 21% 5% 1	820
	11th to 20th	Night	25% 19% 30% 17% 8% 1	840
	21st to end	Night	28% 23% 26% 18% 5% 1	840
December	1st to 10th	Night	26% 18% 31% 20% 4	809
	11th to 20th	Night	27% 19% 32% 15% 6% 1	800
	21st to end	Night	30% 20% 29% 17% 4	880

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