Observatoire Saint-Sixte
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Saint-Sixte clear sky chart from 2005-11-06 to 2026-06-16.

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% 10% 4 8% 5% 5% 4 4 5% 6% 31%	137976
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 31%	134029
Transparency	Day	53% 5% 12% 20% 9% 1	141430
	Night	54% 5% 11% 19% 10% 1	138664
Seeing	Day	24% 16% 30% 19% 8% 2	16449
	Night	23% 13% 29% 25% 9% 1	25532

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	13% 17% 7% 14% 7% 6% 4 3 4 5% 20%	4661
	11th to 20th	Day	11% 21% 8% 16% 7% 4 3 3 5% 5% 19%	4650
	21st to end	Day	8% 14% 7% 16% 9% 8% 5% 3 5% 5% 21%	5221
February	1st to 10th	Day	12% 17% 5% 13% 7% 5% 4 4 4 5% 24%	1890
	11th to 20th	Day	12% 17% 5% 12% 7% 5% 4 3 4 5% 26%	1872
	21st to end	Day	17% 16% 5% 10% 6% 5% 4 3 5% 5% 24%	1730
March	1st to 10th	Day	19% 16% 5% 11% 6% 4 4 3 4 5% 24%	4576
	11th to 20th	Day	14% 13% 4 9% 6% 5% 5% 4 6% 7% 28%	4995
	21st to end	Day	18% 12% 3 7% 5% 4 4 3 4 5% 35%	5956
April	1st to 10th	Day	22% 12% 4 7% 4 4 4 2 4 5% 32%	2520
	11th to 20th	Day	23% 9% 3 7% 4 4 3 3 5% 5% 34%	2494
	21st to end	Day	22% 11% 3 7% 5% 4 4 3 4 5% 31%	2683
May	1st to 10th	Day	23% 10% 3 8% 5% 6% 5% 4 6% 6% 24%	8237
	11th to 20th	Day	19% 8% 2 8% 5% 5% 5% 4 5% 7% 33%	8221
	21st to end	Day	22% 9% 2 6% 4 4 4 3 5% 6% 33%	9060
June	1st to 10th	Day	24% 7% 3 7% 5% 4 4 4 5% 7% 30%	2940
	11th to 20th	Day	15% 7% 3 6% 5% 5% 5% 4 7% 8% 37%	2870
	21st to end	Day	21% 6% 2 6% 5% 5% 6% 4 7% 7% 30%	2800
July	1st to 10th	Day	15% 6% 2 8% 6% 6% 5% 4 7% 8% 33%	6360
	11th to 20th	Day	13% 5% 2 6% 5% 4 5% 4 6% 8% 40%	6360
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 9% 47%	6926
August	1st to 10th	Day	15% 6% 3 7% 6% 6% 5% 4 6% 9% 35%	4563
	11th to 20th	Day	16% 6% 2 6% 4 4 4 5% 7% 9% 37%	4098
	21st to end	Day	17% 5% 2 6% 4 4 5% 4 7% 9% 38%	4312
September	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 5% 7% 43%	2400
	11th to 20th	Day	14% 4 2 5% 4 4 4 3 5% 7% 48%	2376
	21st to end	Day	20% 6% 2 5% 4 4 3 3 5% 6% 41%	2170
October	1st to 10th	Day	22% 7% 2 6% 4 3 4 3 5% 7% 37%	2000
	11th to 20th	Day	24% 10% 3 7% 5% 5% 4 3 5% 6% 28%	2000
	21st to end	Day	25% 11% 3 7% 5% 4 3 3 5% 5% 27%	2001
November	1st to 10th	Day	17% 11% 3 8% 4 4 4 3 5% 7% 33%	1851
	11th to 20th	Day	19% 13% 4 9% 5% 5% 6% 3 4 5% 26%	1709
	21st to end	Day	23% 17% 5% 9% 4 5% 5% 4 4 4 21%	1680
December	1st to 10th	Day	19% 22% 6% 10% 5% 3 4 3 5% 6% 15%	2680
	11th to 20th	Day	17% 19% 6% 11% 5% 4 4 3 4 4 24%	3066
	21st to end	Day	22% 19% 5% 8% 5% 4 4 3 4 4 23%	4048
January	1st to 10th	Night	13% 18% 8% 14% 6% 4 3 3 4 5% 22%	8234
	11th to 20th	Night	11% 20% 8% 16% 7% 5% 4 3 4 4 19%	8235
	21st to end	Night	9% 15% 7% 13% 7% 7% 6% 3 5% 6% 23%	8959
February	1st to 10th	Night	12% 14% 5% 12% 6% 5% 5% 4 5% 5% 25%	2730
	11th to 20th	Night	12% 13% 5% 12% 7% 5% 5% 3 4 5% 29%	2632
	21st to end	Night	14% 15% 5% 10% 6% 4 4 4 5% 5% 28%	2076
March	1st to 10th	Night	22% 15% 4 10% 4 3 3 3 4 4 28%	5484
	11th to 20th	Night	13% 14% 3 8% 5% 5% 5% 4 6% 8% 30%	5168
	21st to end	Night	18% 11% 4 9% 5% 4 5% 4 4 5% 32%	5217
April	1st to 10th	Night	22% 10% 2 8% 4 5% 4 3 5% 6% 31%	2100
	11th to 20th	Night	20% 9% 3 6% 4 4 4 3 6% 5% 35%	2075
	21st to end	Night	22% 9% 3 7% 3 3 4 3 5% 6% 36%	1954
May	1st to 10th	Night	19% 10% 3 9% 5% 6% 5% 4 6% 7% 26%	4660
	11th to 20th	Night	19% 7% 2 7% 4 5% 5% 4 5% 6% 36%	4639
	21st to end	Night	20% 9% 3 6% 5% 4 5% 4 5% 6% 33%	5114
June	1st to 10th	Night	16% 7% 2 7% 5% 4 5% 5% 7% 9% 33%	1680
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 5% 7% 41%	1640
	21st to end	Night	19% 8% 2 5% 4 4 5% 4 5% 7% 37%	1600
July	1st to 10th	Night	15% 5% 3 7% 5% 5% 6% 5% 7% 8% 35%	3600
	11th to 20th	Night	14% 7% 2 5% 4 4 3 4 5% 7% 45%	3600
	21st to end	Night	9% 5% 2 5% 3 4 4 4 6% 8% 49%	3926
August	1st to 10th	Night	13% 5% 2 7% 5% 5% 4 4 6% 7% 40%	2586
	11th to 20th	Night	17% 6% 2 5% 3 3 4 4 5% 6% 44%	2914
	21st to end	Night	16% 6% 2 6% 4 4 4 4 5% 7% 43%	3586
September	1st to 10th	Night	12% 6% 2 6% 5% 3 4 3 5% 7% 48%	2000
	11th to 20th	Night	15% 4 2 5% 3 3 3 2 5% 7% 51%	2023
	21st to end	Night	17% 6% 3 5% 4 4 3 3 4 6% 45%	2226
October	1st to 10th	Night	19% 8% 3 6% 4 4 4 4 5% 6% 40%	2400
	11th to 20th	Night	21% 8% 3 6% 5% 5% 4 4 5% 6% 34%	2400
	21st to end	Night	24% 10% 3 7% 4 4 4 3 4 6% 31%	2837
November	1st to 10th	Night	20% 10% 3 7% 4 4 4 4 5% 5% 32%	2667
	11th to 20th	Night	20% 11% 3 8% 6% 5% 4 3 5% 5% 30%	2909
	21st to end	Night	20% 15% 5% 10% 6% 4 4 3 4 6% 23%	2940
December	1st to 10th	Night	18% 16% 5% 11% 6% 5% 5% 3 5% 6% 19%	4720
	11th to 20th	Night	24% 13% 4 9% 5% 3 4 3 4 5% 26%	5348
	21st to end	Night	24% 20% 4 9% 4 3 2 2 3 4 24%	7150

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% 4 9% 15% 4	4662
	11th to 20th	Day	70% 4 6% 10% 9% 1	4680
	21st to end	Day	66% 4 9% 10% 10% 1	5221
February	1st to 10th	Day	62% 4 8% 17% 7% 2	1890
	11th to 20th	Day	60% 5% 10% 15% 9% 1	1881
	21st to end	Day	63% 4 9% 13% 10% 1	1730
March	1st to 10th	Day	63% 5% 7% 11% 11% 3	4579
	11th to 20th	Day	55% 5% 16% 13% 10% 1	4995
	21st to end	Day	53% 3 8% 16% 18% 2	5956
April	1st to 10th	Day	55% 3 9% 18% 12% 3	2520
	11th to 20th	Day	53% 5% 7% 19% 14% 2	2494
	21st to end	Day	55% 5% 8% 16% 12% 4	2683
May	1st to 10th	Day	58% 6% 8% 19% 8% 1	8249
	11th to 20th	Day	50% 6% 15% 20% 8% 1	8225
	21st to end	Day	51% 5% 11% 20% 12% 1	9061
June	1st to 10th	Day	53% 6% 10% 18% 12% 1	2940
	11th to 20th	Day	45% 6% 13% 23% 11% 2	2870
	21st to end	Day	51% 8% 13% 18% 9% 1	2800
July	1st to 10th	Day	48% 5% 15% 25% 7%	6290
	11th to 20th	Day	42% 5% 14% 30% 9%	6285
	21st to end	Day	35% 5% 16% 35% 9%	6728
August	1st to 10th	Day	47% 5% 17% 23% 7%	4549
	11th to 20th	Day	43% 6% 16% 28% 7% 1	4137
	21st to end	Day	43% 4 18% 23% 11% 1	4312
September	1st to 10th	Day	39% 7% 14% 28% 12%	2400
	11th to 20th	Day	36% 6% 14% 32% 12% 1	2376
	21st to end	Day	43% 5% 15% 26% 10%	2170
October	1st to 10th	Day	49% 4 11% 28% 8%	3260
	11th to 20th	Day	49% 7% 14% 23% 7% 1	3240
	21st to end	Day	50% 5% 20% 18% 6%	3213
November	1st to 10th	Day	51% 6% 15% 20% 7% 1	1851
	11th to 20th	Day	61% 6% 11% 12% 9% 1	1709
	21st to end	Day	67% 6% 11% 10% 5% 1	1680
December	1st to 10th	Day	72% 4 10% 11% 2	2680
	11th to 20th	Day	66% 5% 8% 14% 6%	3066
	21st to end	Day	66% 4 8% 16% 6%	4048
January	1st to 10th	Night	66% 4 11% 13% 5% 1	8234
	11th to 20th	Night	70% 5% 8% 11% 5%	8280
	21st to end	Night	64% 5% 7% 12% 12%	8959
February	1st to 10th	Night	60% 5% 10% 18% 7% 1	2730
	11th to 20th	Night	59% 4 9% 19% 8% 1	2634
	21st to end	Night	57% 5% 9% 18% 11% 1	2076
March	1st to 10th	Night	61% 3 7% 13% 12% 4	5494
	11th to 20th	Night	51% 6% 15% 16% 9% 3	5169
	21st to end	Night	55% 4 7% 20% 12% 2	5217
April	1st to 10th	Night	54% 4 9% 18% 12% 2	2100
	11th to 20th	Night	49% 5% 8% 21% 14% 1	2075
	21st to end	Night	50% 4 10% 18% 16% 3	1954
May	1st to 10th	Night	57% 5% 11% 18% 8% 2	4666
	11th to 20th	Night	48% 6% 13% 21% 11% 1	4640
	21st to end	Night	53% 4 9% 19% 14% 1	5115
June	1st to 10th	Night	46% 8% 12% 22% 11% 1	1680
	11th to 20th	Night	43% 6% 12% 23% 14% 2	1640
	21st to end	Night	48% 7% 12% 19% 13% 1	1600
July	1st to 10th	Night	47% 4 19% 21% 8%	3560
	11th to 20th	Night	40% 4 17% 31% 8%	3552
	21st to end	Night	33% 5% 16% 32% 14%	3809
August	1st to 10th	Night	43% 6% 16% 27% 8%	2576
	11th to 20th	Night	41% 5% 17% 29% 8%	2941
	21st to end	Night	42% 5% 16% 23% 12% 2	3586
September	1st to 10th	Night	36% 7% 14% 29% 13% 1	2000
	11th to 20th	Night	34% 5% 15% 35% 11% 1	2023
	21st to end	Night	40% 5% 13% 29% 12%	2226
October	1st to 10th	Night	42% 4 14% 27% 11% 1	3920
	11th to 20th	Night	45% 5% 14% 26% 9%	3899
	21st to end	Night	48% 5% 18% 18% 10%	4575
November	1st to 10th	Night	53% 6% 14% 20% 7%	2667
	11th to 20th	Night	56% 6% 13% 15% 9% 1	2909
	21st to end	Night	64% 6% 12% 14% 4	2940
December	1st to 10th	Night	68% 4 13% 12% 3	4720
	11th to 20th	Night	64% 3 6% 15% 11% 1	5348
	21st to end	Night	66% 4 5% 14% 11%	7150

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% 16% 35% 19% 4	220
	11th to 20th	Day	30% 20% 34% 10% 5%	220
	21st to end	Day	20% 19% 34% 21% 5%	259
February	1st to 10th	Day	30% 22% 34% 13% 2	330
	11th to 20th	Day	27% 28% 33% 11% 1	359
	21st to end	Day	28% 20% 36% 15% 2	446
March	1st to 10th	Day	27% 22% 30% 16% 5%	540
	11th to 20th	Day	30% 20% 32% 14% 3 1	540
	21st to end	Day	33% 20% 26% 14% 6% 1	594
April	1st to 10th	Day	34% 18% 29% 13% 5% 1	540
	11th to 20th	Day	34% 22% 21% 14% 8% 1	532
	21st to end	Day	30% 20% 32% 12% 4 1	540
May	1st to 10th	Day	28% 17% 33% 15% 5% 2	543
	11th to 20th	Day	23% 18% 31% 19% 8% 2	567
	21st to end	Day	26% 15% 32% 18% 8% 1	617
June	1st to 10th	Day	26% 14% 30% 23% 8% 1	570
	11th to 20th	Day	19% 18% 32% 20% 9% 2	550
	21st to end	Day	24% 13% 30% 22% 9% 2	530
July	1st to 10th	Day	15% 11% 33% 28% 10% 3	525
	11th to 20th	Day	13% 11% 37% 25% 12% 2	524
	21st to end	Day	13% 8% 36% 28% 12% 2	576
August	1st to 10th	Day	15% 7% 26% 38% 12% 2	526
	11th to 20th	Day	14% 9% 24% 30% 15% 7%	519
	21st to end	Day	18% 11% 29% 22% 13% 6%	583
September	1st to 10th	Day	18% 11% 29% 22% 13% 6%	530
	11th to 20th	Day	13% 10% 23% 25% 20% 9%	530
	21st to end	Day	22% 14% 27% 21% 13% 4	530
October	1st to 10th	Day	26% 15% 23% 20% 13% 3	530
	11th to 20th	Day	28% 21% 28% 15% 7% 1	530
	21st to end	Day	33% 15% 30% 17% 4	473
November	1st to 10th	Day	29% 18% 28% 19% 6% 1	425
	11th to 20th	Day	28% 18% 36% 12% 5% 1	249
	21st to end	Day	35% 21% 30% 12% 2	220
December	1st to 10th	Day	30% 15% 35% 18% 3	220
	11th to 20th	Day	27% 19% 40% 11% 3	220
	21st to end	Day	33% 20% 32% 11% 3 1	242
January	1st to 10th	Night	26% 15% 35% 19% 4	840
	11th to 20th	Night	29% 17% 29% 20% 6%	840
	21st to end	Night	20% 13% 36% 23% 6% 1	924
February	1st to 10th	Night	19% 16% 36% 24% 5%	840
	11th to 20th	Night	20% 19% 37% 20% 3	823
	21st to end	Night	22% 18% 28% 28% 3	692
March	1st to 10th	Night	25% 16% 33% 21% 5%	840
	11th to 20th	Night	27% 16% 31% 21% 4 1	840
	21st to end	Night	26% 15% 31% 21% 7%	924
April	1st to 10th	Night	30% 16% 27% 23% 4	840
	11th to 20th	Night	26% 16% 30% 22% 6%	831
	21st to end	Night	27% 14% 30% 23% 7% 1	694
May	1st to 10th	Night	24% 13% 28% 25% 8% 2	420
	11th to 20th	Night	22% 10% 28% 30% 9%	414
	21st to end	Night	18% 8% 35% 27% 11%	455
June	1st to 10th	Night	17% 7% 31% 36% 9%	420
	11th to 20th	Night	19% 9% 25% 34% 11% 1	410
	21st to end	Night	21% 6% 26% 34% 14% 1	400
July	1st to 10th	Night	10% 6% 29% 39% 16%	390
	11th to 20th	Night	15% 4 19% 44% 18% 1	388
	21st to end	Night	10% 6% 26% 41% 16% 2	438
August	1st to 10th	Night	13% 4 16% 44% 22% 1	398
	11th to 20th	Night	14% 4 19% 34% 26% 3	607
	21st to end	Night	16% 7% 23% 33% 19% 2	880
September	1st to 10th	Night	15% 6% 27% 33% 19% 1	800
	11th to 20th	Night	17% 7% 19% 30% 24% 3	800
	21st to end	Night	20% 7% 26% 28% 18% 2	800
October	1st to 10th	Night	22% 10% 26% 25% 16% 1	800
	11th to 20th	Night	25% 13% 28% 25% 9% 1	800
	21st to end	Night	28% 12% 31% 24% 5%	880
November	1st to 10th	Night	28% 13% 25% 25% 8% 1	820
	11th to 20th	Night	27% 15% 30% 18% 10%	840
	21st to end	Night	28% 19% 27% 21% 5%	840
December	1st to 10th	Night	29% 16% 31% 20% 4	840
	11th to 20th	Night	30% 16% 31% 18% 5% 1	840
	21st to end	Night	35% 15% 29% 17% 4	924

Page generated at 2026-06-13 02:31:39. Version ac07

Copyright © 2026 Attilla Danko.