Portail Étoilé de Saint-Pierre
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portail Étoilé de Saint-Pierre clear sky chart from 2005-11-06 to 2025-04-26.

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% 12% 4 9% 5% 4 4 3 5% 6% 29%	113042
	Night	17% 14% 5% 10% 6% 4 4 3 4 5% 27%	115609
Transparency	Day	54% 6% 13% 18% 8% 1	116530
	Night	58% 5% 11% 17% 8% 1	120273
Seeing	Day	26% 17% 30% 19% 6% 1	15947
	Night	26% 15% 31% 22% 6%	24490

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	15% 23% 6% 13% 6% 4 3 2 3 4 19%	3573
	11th to 20th	Day	10% 16% 7% 19% 9% 6% 5% 2 3 4 19%	3570
	21st to end	Day	6% 10% 8% 16% 7% 5% 5% 3 5% 5% 30%	3960
February	1st to 10th	Day	12% 16% 5% 11% 9% 5% 4 4 5% 5% 24%	1635
	11th to 20th	Day	14% 17% 6% 13% 6% 5% 4 3 4 4 24%	1895
	21st to end	Day	12% 16% 5% 11% 7% 5% 4 3 4 5% 28%	1650
March	1st to 10th	Day	19% 17% 6% 10% 6% 5% 4 3 4 4 23%	4476
	11th to 20th	Day	15% 16% 4 8% 6% 5% 4 4 6% 6% 27%	4829
	21st to end	Day	19% 17% 4 8% 4 4 3 3 4 4 30%	5712
April	1st to 10th	Day	24% 14% 3 8% 6% 3 4 3 4 6% 28%	2400
	11th to 20th	Day	20% 13% 3 6% 4 3 3 3 4 5% 36%	2502
	21st to end	Day	23% 11% 3 7% 4 4 3 3 4 5% 31%	2535
May	1st to 10th	Day	20% 10% 3 7% 5% 4 4 3 4 8% 32%	4133
	11th to 20th	Day	19% 9% 3 7% 5% 5% 5% 4 6% 7% 28%	4398
	21st to end	Day	18% 9% 3 8% 6% 5% 5% 4 6% 7% 30%	5020
June	1st to 10th	Day	22% 8% 3 6% 5% 4 5% 5% 7% 8% 27%	2850
	11th to 20th	Day	20% 7% 2 5% 5% 5% 5% 4 5% 8% 33%	2850
	21st to end	Day	23% 10% 4 8% 5% 4 4 5% 5% 7% 25%	2850
July	1st to 10th	Day	16% 8% 3 8% 5% 5% 5% 5% 6% 8% 31%	4642
	11th to 20th	Day	17% 8% 2 7% 5% 5% 5% 4 7% 8% 32%	4440
	21st to end	Day	15% 7% 2 6% 4 4 5% 4 6% 7% 40%	4814
August	1st to 10th	Day	17% 7% 2 7% 5% 5% 6% 5% 7% 9% 29%	4339
	11th to 20th	Day	19% 7% 3 7% 6% 5% 5% 4 7% 8% 28%	4045
	21st to end	Day	21% 8% 2 6% 4 4 4 3 4 6% 38%	4412
September	1st to 10th	Day	19% 6% 3 6% 4 4 4 3 5% 8% 38%	2274
	11th to 20th	Day	20% 5% 2 5% 3 3 3 2 4 5% 46%	2090
	21st to end	Day	23% 7% 2 6% 3 3 3 3 5% 7% 38%	2090
October	1st to 10th	Day	20% 10% 3 6% 4 3 5% 3 5% 6% 34%	2088
	11th to 20th	Day	24% 13% 3 8% 5% 5% 4 4 4 5% 25%	1746
	21st to end	Day	28% 14% 3 7% 4 3 4 3 4 6% 24%	1881
November	1st to 10th	Day	19% 14% 5% 9% 5% 4 3 3 5% 4 29%	1761
	11th to 20th	Day	21% 16% 4 10% 5% 4 4 2 4 5% 25%	1765
	21st to end	Day	21% 20% 6% 9% 5% 4 4 3 3 4 21%	1443
December	1st to 10th	Day	18% 17% 6% 12% 5% 6% 5% 3 4 4 20%	2280
	11th to 20th	Day	20% 17% 6% 9% 6% 4 3 3 4 4 26%	2618
	21st to end	Day	20% 16% 5% 11% 5% 4 4 3 4 6% 22%	3476
January	1st to 10th	Night	15% 21% 6% 12% 7% 5% 4 3 3 3 19%	6371
	11th to 20th	Night	10% 19% 8% 20% 8% 5% 4 3 3 3 17%	6315
	21st to end	Night	7% 11% 6% 15% 7% 6% 6% 4 5% 5% 26%	6996
February	1st to 10th	Night	11% 15% 6% 14% 7% 5% 4 3 5% 5% 25%	2763
	11th to 20th	Night	13% 18% 6% 12% 6% 5% 4 3 4 4 24%	2388
	21st to end	Night	14% 18% 5% 10% 6% 5% 4 3 3 5% 29%	1980
March	1st to 10th	Night	21% 16% 5% 11% 4 3 5% 2 3 5% 25%	5364
	11th to 20th	Night	14% 16% 4 10% 6% 6% 6% 4 6% 6% 22%	5129
	21st to end	Night	19% 16% 5% 9% 5% 3 3 2 3 5% 29%	5203
April	1st to 10th	Night	22% 17% 4 8% 4 3 3 3 4 5% 28%	2000
	11th to 20th	Night	21% 12% 2 6% 4 4 4 4 6% 6% 31%	1846
	21st to end	Night	24% 13% 3 7% 4 3 4 3 4 3 33%	1762
May	1st to 10th	Night	23% 9% 2 7% 4 4 4 3 6% 6% 32%	2834
	11th to 20th	Night	20% 10% 3 8% 6% 4 4 4 5% 6% 31%	2497
	21st to end	Night	18% 6% 2 6% 4 5% 5% 4 7% 7% 35%	2558
June	1st to 10th	Night	23% 8% 3 6% 7% 6% 5% 4 6% 6% 26%	1330
	11th to 20th	Night	18% 8% 2 7% 5% 4 4 4 6% 7% 37%	1330
	21st to end	Night	22% 11% 3 6% 5% 5% 4 4 5% 6% 30%	1330
July	1st to 10th	Night	14% 9% 3 10% 6% 5% 5% 4 6% 8% 32%	2320
	11th to 20th	Night	22% 8% 3 6% 4 5% 4 3 6% 7% 31%	2520
	21st to end	Night	14% 7% 2 6% 5% 4 5% 4 6% 8% 39%	2739
August	1st to 10th	Night	21% 8% 3 6% 5% 4 5% 4 5% 7% 33%	2558
	11th to 20th	Night	18% 9% 3 9% 5% 6% 5% 4 6% 7% 28%	2782
	21st to end	Night	18% 9% 3 7% 4 4 5% 3 5% 6% 36%	3242
September	1st to 10th	Night	19% 8% 2 7% 5% 4 4 3 5% 6% 38%	1905
	11th to 20th	Night	18% 8% 3 5% 3 3 3 3 5% 6% 42%	2090
	21st to end	Night	21% 8% 2 6% 4 4 3 3 4 5% 40%	2090
October	1st to 10th	Night	20% 10% 3 6% 4 3 4 3 5% 5% 37%	2091
	11th to 20th	Night	26% 13% 3 7% 4 3 3 2 4 5% 30%	2432
	21st to end	Night	25% 13% 4 6% 4 4 4 3 4 6% 27%	2717
November	1st to 10th	Night	20% 15% 4 8% 5% 4 4 3 4 5% 27%	2537
	11th to 20th	Night	18% 13% 5% 10% 6% 4 4 2 4 5% 29%	2632
	21st to end	Night	20% 21% 6% 9% 4 4 3 3 4 4 22%	2955
December	1st to 10th	Night	15% 17% 8% 15% 7% 5% 4 3 5% 4 16%	4910
	11th to 20th	Night	19% 18% 6% 10% 6% 4 3 3 3 5% 23%	5591
	21st to end	Night	18% 19% 6% 10% 5% 4 3 2 3 3 26%	7502

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	68% 5% 8% 13% 6%	3574
	11th to 20th	Day	68% 4 9% 13% 5% 1	3600
	21st to end	Day	55% 6% 9% 16% 14%	3960
February	1st to 10th	Day	62% 4 11% 14% 8% 1	1635
	11th to 20th	Day	63% 4 11% 15% 7% 1	1904
	21st to end	Day	58% 4 9% 19% 8% 2	1650
March	1st to 10th	Day	63% 6% 7% 10% 10% 3	4479
	11th to 20th	Day	55% 6% 16% 15% 8%	4829
	21st to end	Day	58% 3 7% 17% 12% 3	5712
April	1st to 10th	Day	60% 3 10% 15% 10% 2	2400
	11th to 20th	Day	51% 4 10% 21% 12% 2	2502
	21st to end	Day	55% 4 10% 16% 11% 3	2535
May	1st to 10th	Day	50% 8% 10% 19% 12% 1	4147
	11th to 20th	Day	50% 9% 19% 17% 6%	4402
	21st to end	Day	51% 9% 10% 20% 10%	5020
June	1st to 10th	Day	52% 9% 13% 19% 7% 1	2850
	11th to 20th	Day	48% 7% 15% 20% 9% 1	2850
	21st to end	Day	57% 7% 11% 17% 7%	2850
July	1st to 10th	Day	49% 7% 16% 22% 6%	4570
	11th to 20th	Day	47% 7% 20% 22% 4	4365
	21st to end	Day	43% 7% 19% 23% 8%	4661
August	1st to 10th	Day	49% 8% 19% 20% 4	4325
	11th to 20th	Day	51% 8% 22% 14% 4	4084
	21st to end	Day	48% 6% 17% 18% 11% 1	4412
September	1st to 10th	Day	44% 5% 16% 23% 10% 1	2274
	11th to 20th	Day	41% 4 16% 28% 11%	2090
	21st to end	Day	47% 5% 14% 25% 10%	2090
October	1st to 10th	Day	53% 5% 12% 25% 5%	3465
	11th to 20th	Day	55% 5% 15% 23% 1	2871
	21st to end	Day	53% 5% 16% 20% 6%	3081
November	1st to 10th	Day	58% 6% 13% 17% 6%	1761
	11th to 20th	Day	62% 5% 11% 16% 6%	1765
	21st to end	Day	67% 5% 11% 13% 3	1443
December	1st to 10th	Day	66% 6% 11% 10% 7%	2280
	11th to 20th	Day	64% 3 9% 17% 7%	2618
	21st to end	Day	63% 5% 11% 10% 9% 2	3476
January	1st to 10th	Night	69% 5% 8% 12% 5%	6371
	11th to 20th	Night	72% 4 9% 12% 4	6360
	21st to end	Night	57% 6% 9% 15% 12%	6996
February	1st to 10th	Night	60% 5% 10% 14% 9% 1	2763
	11th to 20th	Night	63% 4 9% 15% 9% 1	2390
	21st to end	Night	58% 4 8% 17% 12% 1	1980
March	1st to 10th	Night	63% 4 8% 12% 12% 1	5374
	11th to 20th	Night	60% 7% 12% 12% 7% 1	5130
	21st to end	Night	60% 3 6% 19% 11% 1	5203
April	1st to 10th	Night	60% 3 8% 14% 13% 2	2000
	11th to 20th	Night	52% 5% 9% 20% 11% 2	1846
	21st to end	Night	56% 4 7% 18% 13% 2	1762
May	1st to 10th	Night	51% 5% 11% 20% 11% 1	2840
	11th to 20th	Night	50% 10% 12% 18% 10%	2498
	21st to end	Night	44% 9% 15% 19% 13% 1	2559
June	1st to 10th	Night	56% 8% 11% 19% 6%	1330
	11th to 20th	Night	46% 8% 11% 23% 10% 3	1330
	21st to end	Night	55% 6% 13% 15% 10%	1330
July	1st to 10th	Night	49% 7% 18% 20% 5%	2282
	11th to 20th	Night	51% 7% 18% 20% 3	2472
	21st to end	Night	41% 9% 21% 21% 8%	2651
August	1st to 10th	Night	50% 7% 16% 23% 4	2548
	11th to 20th	Night	54% 9% 18% 15% 5%	2809
	21st to end	Night	47% 6% 14% 20% 12% 1	3242
September	1st to 10th	Night	46% 6% 13% 22% 11% 1	1905
	11th to 20th	Night	43% 6% 16% 26% 8%	2090
	21st to end	Night	47% 6% 11% 25% 11%	2090
October	1st to 10th	Night	50% 4 12% 27% 6%	3494
	11th to 20th	Night	56% 5% 12% 24% 3	4033
	21st to end	Night	53% 4 18% 17% 8%	4467
November	1st to 10th	Night	59% 5% 12% 18% 5%	2537
	11th to 20th	Night	58% 6% 11% 16% 9%	2632
	21st to end	Night	66% 5% 10% 13% 5%	2955
December	1st to 10th	Night	71% 6% 10% 11% 3	4910
	11th to 20th	Night	66% 4 6% 20% 4	5592
	21st to end	Night	64% 4 9% 14% 9% 1	7502

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	29% 26% 30% 13% 2 1	200
	11th to 20th	Day	31% 25% 31% 11% 3	200
	21st to end	Day	17% 30% 35% 15% 3	220
February	1st to 10th	Day	29% 27% 30% 11% 3	235
	11th to 20th	Day	26% 29% 32% 11% 2	470
	21st to end	Day	25% 23% 32% 17% 3	414
March	1st to 10th	Day	30% 21% 27% 17% 3 1	500
	11th to 20th	Day	26% 22% 30% 17% 5% 1	500
	21st to end	Day	26% 23% 28% 16% 6% 1	550
April	1st to 10th	Day	34% 19% 28% 14% 5%	500
	11th to 20th	Day	32% 21% 25% 18% 4 1	492
	21st to end	Day	28% 23% 30% 14% 4 1	480
May	1st to 10th	Day	30% 15% 35% 15% 4 1	463
	11th to 20th	Day	23% 13% 31% 24% 8% 1	487
	21st to end	Day	23% 17% 37% 17% 6%	634
June	1st to 10th	Day	25% 15% 31% 23% 7%	680
	11th to 20th	Day	22% 14% 29% 26% 9%	680
	21st to end	Day	28% 10% 30% 23% 9% 1	680
July	1st to 10th	Day	21% 12% 33% 26% 8%	607
	11th to 20th	Day	19% 14% 35% 24% 7% 1	484
	21st to end	Day	19% 9% 36% 27% 8% 1	532
August	1st to 10th	Day	20% 11% 32% 25% 11% 2	486
	11th to 20th	Day	20% 10% 28% 28% 13% 1	479
	21st to end	Day	22% 14% 23% 25% 13% 4	539
September	1st to 10th	Day	23% 12% 29% 23% 10% 3	490
	11th to 20th	Day	22% 10% 32% 23% 10% 2	490
	21st to end	Day	23% 13% 29% 21% 9% 5%	490
October	1st to 10th	Day	24% 17% 29% 20% 8% 2	488
	11th to 20th	Day	32% 15% 28% 17% 7% 1	390
	21st to end	Day	36% 19% 29% 13% 4	429
November	1st to 10th	Day	33% 18% 32% 15% 2 1	395
	11th to 20th	Day	29% 21% 28% 16% 7% 1	400
	21st to end	Day	35% 21% 32% 11%	243
December	1st to 10th	Day	33% 17% 28% 19% 3 2	200
	11th to 20th	Day	36% 25% 25% 12% 2 1	200
	21st to end	Day	37% 20% 27% 14% 2	220
January	1st to 10th	Night	28% 22% 33% 15% 2	804
	11th to 20th	Night	25% 21% 33% 18% 3	800
	21st to end	Night	17% 22% 37% 20% 5%	880
February	1st to 10th	Night	19% 24% 34% 18% 5%	800
	11th to 20th	Night	25% 22% 37% 12% 4	783
	21st to end	Night	25% 18% 33% 20% 4	660
March	1st to 10th	Night	25% 15% 37% 19% 4	800
	11th to 20th	Night	23% 16% 37% 18% 5% 1	800
	21st to end	Night	24% 15% 32% 22% 6% 1	880
April	1st to 10th	Night	32% 16% 27% 21% 5%	800
	11th to 20th	Night	28% 13% 32% 21% 5%	662
	21st to end	Night	31% 13% 28% 21% 6%	588
May	1st to 10th	Night	28% 11% 30% 24% 7%	570
	11th to 20th	Night	24% 7% 28% 31% 9% 1	376
	21st to end	Night	26% 9% 33% 25% 7%	411
June	1st to 10th	Night	26% 6% 29% 32% 7%	380
	11th to 20th	Night	20% 7% 25% 37% 11%	380
	21st to end	Night	24% 11% 29% 28% 8%	380
July	1st to 10th	Night	20% 6% 35% 30% 9%	370
	11th to 20th	Night	19% 5% 31% 33% 10% 1	368
	21st to end	Night	17% 7% 30% 36% 11%	416
August	1st to 10th	Night	18% 4 29% 35% 14%	404
	11th to 20th	Night	20% 7% 27% 33% 13% 1	562
	21st to end	Night	20% 8% 29% 30% 12% 2	693
September	1st to 10th	Night	23% 7% 30% 29% 10% 1	760
	11th to 20th	Night	22% 9% 30% 27% 12% 1	760
	21st to end	Night	23% 11% 27% 26% 12%	760
October	1st to 10th	Night	25% 12% 30% 25% 7% 1	760
	11th to 20th	Night	32% 15% 26% 21% 6% 1	760
	21st to end	Night	33% 15% 30% 18% 5%	836
November	1st to 10th	Night	31% 17% 28% 20% 3	780
	11th to 20th	Night	27% 19% 28% 19% 6% 1	817
	21st to end	Night	30% 19% 31% 16% 4 1	900
December	1st to 10th	Night	31% 18% 30% 15% 5%	900
	11th to 20th	Night	29% 22% 28% 14% 6%	900
	21st to end	Night	32% 20% 28% 16% 3	990

Page generated at 2025-04-23 09:25:02. Version ac07

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