Observatoire de Mirabilis
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire de Mirabilis clear sky chart from 2005-11-06 to 2025-10-19.

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% 28%	126923
	Night	17% 14% 5% 10% 5% 4 4 3 4 5% 28%	122787
Transparency	Day	56% 4 11% 19% 9% 1	130354
	Night	57% 4 11% 18% 9% 1	127443
Seeing	Day	28% 17% 30% 17% 6% 2	15699
	Night	25% 14% 28% 24% 8% 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% 22% 6% 18% 8% 5% 5% 2 2 3 14%	3597
	11th to 20th	Day	12% 17% 8% 18% 12% 4 3 2 3 5% 17%	3570
	21st to end	Day	8% 17% 10% 15% 6% 5% 6% 4 4 5% 20%	3960
February	1st to 10th	Day	12% 18% 6% 13% 9% 4 3 3 4 5% 22%	1781
	11th to 20th	Day	12% 20% 5% 13% 6% 5% 4 2 4 4 23%	1807
	21st to end	Day	16% 19% 5% 10% 7% 5% 3 3 4 4 23%	1650
March	1st to 10th	Day	18% 16% 6% 14% 6% 4 4 3 4 4 21%	4476
	11th to 20th	Day	14% 16% 6% 11% 5% 4 4 3 4 6% 27%	4794
	21st to end	Day	19% 13% 4 9% 5% 4 3 3 4 5% 32%	5896
April	1st to 10th	Day	20% 15% 4 8% 5% 4 3 3 4 5% 29%	2400
	11th to 20th	Day	22% 11% 4 7% 4 5% 3 3 4 5% 32%	2374
	21st to end	Day	22% 11% 4 8% 5% 4 4 4 4 4 30%	2565
May	1st to 10th	Day	24% 12% 4 9% 5% 5% 5% 3 5% 6% 22%	6346
	11th to 20th	Day	20% 9% 3 8% 5% 4 5% 4 6% 8% 29%	6318
	21st to end	Day	23% 10% 3 7% 5% 5% 4 3 5% 6% 29%	6949
June	1st to 10th	Day	24% 9% 3 6% 5% 5% 5% 4 6% 6% 27%	2800
	11th to 20th	Day	17% 6% 3 7% 5% 5% 5% 4 6% 8% 33%	2800
	21st to end	Day	24% 8% 3 7% 5% 5% 5% 4 6% 6% 27%	2800
July	1st to 10th	Day	17% 8% 3 8% 5% 5% 6% 4 6% 9% 28%	6360
	11th to 20th	Day	15% 6% 3 6% 5% 5% 5% 4 6% 9% 37%	6360
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 7% 10% 42%	6926
August	1st to 10th	Day	16% 8% 3 7% 5% 6% 5% 4 6% 9% 30%	4563
	11th to 20th	Day	18% 7% 2 7% 4 4 5% 4 6% 8% 35%	4152
	21st to end	Day	17% 6% 2 6% 5% 4 5% 4 5% 9% 36%	4312
September	1st to 10th	Day	18% 5% 2 5% 4 5% 4 3 6% 7% 40%	2400
	11th to 20th	Day	15% 6% 2 6% 3 4 4 3 5% 8% 44%	2350
	21st to end	Day	22% 7% 2 5% 4 3 4 3 6% 6% 39%	2200
October	1st to 10th	Day	23% 10% 3 6% 4 4 4 3 6% 6% 33%	2010
	11th to 20th	Day	25% 13% 3 9% 4 4 4 3 5% 5% 26%	1966
	21st to end	Day	26% 14% 3 8% 4 4 4 3 4 6% 23%	1881
November	1st to 10th	Day	18% 14% 4 7% 5% 4 4 3 5% 6% 31%	1761
	11th to 20th	Day	21% 16% 5% 10% 6% 4 4 3 5% 5% 22%	1653
	21st to end	Day	22% 18% 6% 11% 6% 4 4 3 3 4 19%	1600
December	1st to 10th	Day	20% 23% 10% 9% 7% 4 4 3 4 4 13%	2600
	11th to 20th	Day	18% 21% 6% 13% 6% 3 4 3 3 3 20%	2986
	21st to end	Day	19% 18% 5% 10% 5% 4 5% 3 4 4 22%	3960
January	1st to 10th	Night	12% 25% 10% 14% 6% 4 4 2 3 3 16%	6349
	11th to 20th	Night	10% 18% 8% 19% 11% 5% 3 2 4 4 15%	6315
	21st to end	Night	8% 16% 9% 17% 8% 6% 6% 3 4 4 19%	6996
February	1st to 10th	Night	12% 17% 6% 12% 8% 5% 5% 4 5% 5% 23%	2615
	11th to 20th	Night	13% 17% 7% 12% 7% 5% 4 3 4 4 24%	2478
	21st to end	Night	14% 18% 6% 10% 5% 4 5% 3 6% 5% 25%	1980
March	1st to 10th	Night	21% 15% 5% 11% 5% 4 4 2 3 4 26%	5364
	11th to 20th	Night	14% 15% 4 10% 6% 4 4 4 5% 7% 28%	5163
	21st to end	Night	18% 15% 4 9% 5% 5% 4 3 5% 5% 28%	5036
April	1st to 10th	Night	22% 12% 5% 7% 5% 4 3 3 4 6% 30%	2000
	11th to 20th	Night	21% 10% 3 7% 5% 3 3 3 4 6% 35%	1975
	21st to end	Night	21% 11% 3 7% 4 4 4 3 5% 5% 34%	1852
May	1st to 10th	Night	21% 11% 3 9% 5% 5% 4 4 6% 6% 26%	3594
	11th to 20th	Night	20% 9% 3 7% 4 3 5% 3 5% 7% 34%	3575
	21st to end	Night	22% 10% 3 7% 5% 4 5% 4 5% 7% 28%	3926
June	1st to 10th	Night	20% 8% 3 7% 5% 4 5% 3 8% 8% 30%	1600
	11th to 20th	Night	18% 7% 3 6% 5% 4 4 4 5% 7% 37%	1600
	21st to end	Night	22% 7% 2 6% 5% 4 4 4 6% 5% 36%	1600
July	1st to 10th	Night	17% 7% 3 7% 5% 5% 6% 5% 7% 8% 30%	3600
	11th to 20th	Night	18% 8% 2 5% 3 4 4 3 5% 7% 40%	3600
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 8% 46%	3926
August	1st to 10th	Night	14% 8% 3 6% 5% 4 4 4 5% 7% 39%	2586
	11th to 20th	Night	18% 7% 2 6% 5% 4 4 3 5% 5% 40%	2860
	21st to end	Night	18% 6% 3 6% 4 4 4 4 5% 7% 40%	3586
September	1st to 10th	Night	15% 7% 2 6% 4 3 4 3 5% 6% 45%	2000
	11th to 20th	Night	16% 6% 2 4 3 3 3 3 6% 7% 48%	2050
	21st to end	Night	18% 7% 2 5% 3 3 4 3 4 7% 43%	2200
October	1st to 10th	Night	20% 7% 3 6% 4 5% 3 3 5% 6% 39%	2387
	11th to 20th	Night	23% 13% 3 6% 4 4 4 3 4 5% 30%	2391
	21st to end	Night	25% 13% 3 7% 5% 4 4 3 4 6% 26%	2717
November	1st to 10th	Night	20% 13% 4 8% 4 4 4 4 5% 6% 28%	2537
	11th to 20th	Night	18% 16% 5% 11% 5% 5% 4 3 4 5% 25%	2745
	21st to end	Night	22% 19% 6% 10% 5% 4 4 3 4 4 20%	2800
December	1st to 10th	Night	19% 20% 7% 15% 7% 5% 4 3 4 4 13%	4580
	11th to 20th	Night	23% 18% 6% 11% 5% 3 3 2 3 3 22%	5208
	21st to end	Night	21% 21% 4 11% 5% 3 3 2 3 4 24%	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	76% 2 7% 11% 3	3598
	11th to 20th	Day	73% 2 4 13% 8% 1	3600
	21st to end	Day	65% 3 8% 12% 11% 1	3960
February	1st to 10th	Day	65% 2 7% 16% 7% 2	1781
	11th to 20th	Day	64% 3 9% 13% 10% 1	1816
	21st to end	Day	65% 2 7% 14% 10% 2	1650
March	1st to 10th	Day	67% 2 7% 10% 12% 3	4479
	11th to 20th	Day	59% 3 14% 12% 11% 2	4794
	21st to end	Day	55% 3 6% 17% 17% 2	5896
April	1st to 10th	Day	57% 3 8% 17% 12% 3	2400
	11th to 20th	Day	55% 3 8% 20% 12% 2	2374
	21st to end	Day	58% 3 8% 16% 12% 3	2565
May	1st to 10th	Day	62% 5% 7% 18% 7% 1	6357
	11th to 20th	Day	54% 4 15% 16% 8% 2	6322
	21st to end	Day	56% 4 10% 16% 12% 1	6949
June	1st to 10th	Day	56% 5% 9% 18% 11% 1	2800
	11th to 20th	Day	49% 5% 12% 21% 11% 3	2800
	21st to end	Day	57% 6% 11% 18% 9% 1	2800
July	1st to 10th	Day	51% 6% 14% 22% 6%	6290
	11th to 20th	Day	42% 6% 16% 29% 7%	6285
	21st to end	Day	36% 6% 18% 32% 8%	6728
August	1st to 10th	Day	50% 5% 17% 22% 6%	4549
	11th to 20th	Day	46% 6% 17% 26% 5%	4194
	21st to end	Day	45% 6% 16% 22% 12% 1	4312
September	1st to 10th	Day	43% 4 14% 27% 12%	2400
	11th to 20th	Day	39% 4 13% 31% 12% 1	2350
	21st to end	Day	45% 5% 13% 25% 11%	2200
October	1st to 10th	Day	49% 4 12% 27% 7%	3270
	11th to 20th	Day	53% 5% 12% 22% 7% 1	3194
	21st to end	Day	54% 5% 20% 14% 7% 1	3081
November	1st to 10th	Day	54% 4 13% 20% 7% 1	1761
	11th to 20th	Day	64% 3 11% 12% 9% 1	1653
	21st to end	Day	69% 4 11% 11% 4 1	1600
December	1st to 10th	Day	75% 4 8% 10% 3	2600
	11th to 20th	Day	70% 2 7% 15% 6%	2986
	21st to end	Day	65% 3 8% 15% 9%	3960
January	1st to 10th	Night	73% 3 10% 11% 3	6349
	11th to 20th	Night	72% 2 9% 11% 4 1	6360
	21st to end	Night	68% 3 8% 8% 12% 1	6996
February	1st to 10th	Night	63% 4 8% 17% 7% 1	2615
	11th to 20th	Night	63% 3 9% 16% 8% 1	2480
	21st to end	Night	60% 3 9% 17% 10% 2	1980
March	1st to 10th	Night	63% 3 9% 11% 12% 3	5374
	11th to 20th	Night	55% 5% 12% 15% 10% 3	5164
	21st to end	Night	58% 4 6% 19% 10% 3	5036
April	1st to 10th	Night	56% 3 7% 18% 13% 2	2000
	11th to 20th	Night	51% 4 8% 22% 14% 2	1975
	21st to end	Night	53% 3 8% 19% 13% 3	1852
May	1st to 10th	Night	58% 4 12% 18% 7% 2	3600
	11th to 20th	Night	50% 5% 15% 17% 10% 2	3576
	21st to end	Night	56% 4 10% 16% 12% 1	3927
June	1st to 10th	Night	51% 4 12% 22% 11%	1600
	11th to 20th	Night	46% 6% 11% 22% 13% 2	1600
	21st to end	Night	49% 6% 11% 21% 12% 1	1600
July	1st to 10th	Night	49% 6% 19% 19% 7%	3560
	11th to 20th	Night	42% 6% 18% 28% 5%	3552
	21st to end	Night	34% 6% 18% 30% 11%	3809
August	1st to 10th	Night	43% 6% 17% 27% 7%	2576
	11th to 20th	Night	44% 5% 19% 25% 7%	2884
	21st to end	Night	43% 5% 15% 24% 10% 2	3586
September	1st to 10th	Night	38% 5% 15% 28% 13% 1	2000
	11th to 20th	Night	37% 4 15% 33% 11% 1	2050
	21st to end	Night	42% 5% 13% 29% 12%	2200
October	1st to 10th	Night	45% 5% 14% 23% 11% 1	3907
	11th to 20th	Night	52% 4 13% 21% 9%	3902
	21st to end	Night	54% 3 17% 15% 9% 1	4467
November	1st to 10th	Night	56% 5% 13% 19% 7% 1	2537
	11th to 20th	Night	61% 4 10% 16% 8% 2	2745
	21st to end	Night	69% 3 10% 14% 5%	2800
December	1st to 10th	Night	73% 5% 11% 9% 3	4580
	11th to 20th	Night	69% 2 5% 14% 9% 1	5208
	21st to end	Night	66% 2 6% 12% 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% 22% 39% 12% 4	200
	11th to 20th	Day	29% 22% 33% 11% 5% 1	200
	21st to end	Day	27% 20% 36% 12% 5%	220
February	1st to 10th	Day	35% 27% 30% 6% 1	291
	11th to 20th	Day	30% 30% 34% 6% 1	354
	21st to end	Day	33% 23% 31% 12% 2	414
March	1st to 10th	Day	29% 21% 34% 13% 2 1	500
	11th to 20th	Day	31% 23% 30% 13% 2 1	500
	21st to end	Day	34% 21% 24% 15% 5% 1	550
April	1st to 10th	Day	35% 19% 27% 14% 3 1	500
	11th to 20th	Day	37% 18% 24% 12% 8% 1	492
	21st to end	Day	34% 19% 30% 12% 4 1	500
May	1st to 10th	Day	31% 19% 30% 14% 5%	503
	11th to 20th	Day	26% 17% 32% 18% 6% 1	527
	21st to end	Day	27% 18% 31% 18% 5%	573
June	1st to 10th	Day	30% 15% 33% 15% 6%	530
	11th to 20th	Day	22% 19% 32% 18% 8% 1	530
	21st to end	Day	29% 14% 33% 16% 6% 2	530
July	1st to 10th	Day	19% 12% 34% 24% 8% 2	525
	11th to 20th	Day	18% 13% 35% 27% 6% 1	524
	21st to end	Day	16% 13% 37% 26% 8% 2	576
August	1st to 10th	Day	18% 11% 28% 31% 10% 2	526
	11th to 20th	Day	17% 8% 28% 28% 13% 6%	519
	21st to end	Day	20% 15% 26% 22% 10% 6%	583
September	1st to 10th	Day	23% 10% 30% 21% 11% 5%	530
	11th to 20th	Day	15% 11% 25% 24% 16% 9%	530
	21st to end	Day	23% 15% 25% 22% 10% 5%	530
October	1st to 10th	Day	29% 15% 24% 18% 12% 2	530
	11th to 20th	Day	33% 18% 26% 16% 6% 1	515
	21st to end	Day	38% 12% 30% 15% 5%	429
November	1st to 10th	Day	39% 14% 25% 18% 4 1	395
	11th to 20th	Day	32% 22% 29% 13% 5%	253
	21st to end	Day	33% 26% 29% 12% 1 1	200
December	1st to 10th	Day	38% 17% 27% 14% 5%	200
	11th to 20th	Day	34% 27% 29% 8% 2 1	200
	21st to end	Day	35% 22% 27% 11% 4 1	220
January	1st to 10th	Night	29% 18% 34% 16% 3	800
	11th to 20th	Night	29% 21% 28% 17% 6%	800
	21st to end	Night	18% 19% 37% 20% 6% 1	880
February	1st to 10th	Night	22% 21% 32% 20% 4 1	800
	11th to 20th	Night	23% 23% 34% 16% 3	783
	21st to end	Night	27% 17% 32% 21% 3	660
March	1st to 10th	Night	27% 17% 33% 20% 3	800
	11th to 20th	Night	26% 20% 31% 19% 4 1	800
	21st to end	Night	28% 15% 29% 22% 6% 1	880
April	1st to 10th	Night	31% 16% 26% 22% 4	800
	11th to 20th	Night	29% 14% 29% 23% 6%	791
	21st to end	Night	30% 16% 28% 18% 8%	652
May	1st to 10th	Night	27% 12% 29% 22% 8% 1	400
	11th to 20th	Night	23% 10% 30% 28% 9% 1	394
	21st to end	Night	21% 11% 35% 24% 8%	433
June	1st to 10th	Night	23% 9% 32% 28% 9% 1	400
	11th to 20th	Night	21% 10% 25% 33% 11% 1	400
	21st to end	Night	22% 8% 22% 33% 15% 1	400
July	1st to 10th	Night	15% 7% 25% 38% 15% 1	390
	11th to 20th	Night	16% 6% 22% 39% 16% 1	388
	21st to end	Night	13% 7% 24% 40% 14% 1	438
August	1st to 10th	Night	14% 4 17% 46% 18% 1	398
	11th to 20th	Night	15% 5% 20% 34% 24% 2	576
	21st to end	Night	20% 6% 24% 31% 16% 2	880
September	1st to 10th	Night	18% 7% 26% 34% 14% 1	800
	11th to 20th	Night	19% 6% 19% 33% 21% 1	800
	21st to end	Night	21% 10% 25% 27% 17% 2	800
October	1st to 10th	Night	23% 11% 25% 28% 12% 1	800
	11th to 20th	Night	30% 12% 27% 23% 7% 1	793
	21st to end	Night	31% 13% 27% 23% 5%	836
November	1st to 10th	Night	30% 16% 26% 22% 6% 1	780
	11th to 20th	Night	28% 18% 29% 16% 8% 1	800
	21st to end	Night	34% 19% 24% 17% 6% 1	800
December	1st to 10th	Night	31% 20% 27% 17% 5%	800
	11th to 20th	Night	32% 20% 26% 16% 5% 1	800
	21st to end	Night	36% 17% 26% 17% 4	880

Page generated at 2025-10-16 01:50:23. Version ac07

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