ObservÉtoiles
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ObservÉtoiles clear sky chart from 2005-11-06 to 2026-05-02.

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% 13% 5% 10% 6% 5% 5% 3 5% 6% 25%	131641
	Night	17% 15% 5% 12% 6% 5% 4 3 4 5% 24%	130846
Transparency	Day	57% 6% 11% 17% 8% 1	135109
	Night	61% 6% 10% 15% 7% 1	135465
Seeing	Day	27% 19% 28% 17% 7% 2	16489
	Night	24% 17% 27% 23% 8% 1	25521

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	11% 19% 11% 19% 9% 8% 4 2 3 4 10%	4661
	11th to 20th	Day	7% 17% 8% 24% 14% 8% 5% 3 3 3 8%	4650
	21st to end	Day	8% 17% 8% 13% 9% 14% 6% 3 5% 4 11%	5124
February	1st to 10th	Day	13% 16% 7% 17% 10% 6% 4 3 4 4 17%	1795
	11th to 20th	Day	11% 18% 8% 17% 9% 5% 5% 3 4 4 16%	2027
	21st to end	Day	12% 18% 6% 13% 10% 6% 4 3 4 5% 19%	1730
March	1st to 10th	Day	18% 16% 7% 14% 6% 5% 5% 3 4 4 18%	4576
	11th to 20th	Day	13% 18% 5% 12% 5% 4 4 3 5% 5% 25%	4746
	21st to end	Day	20% 14% 6% 12% 5% 3 3 2 3 4 26%	6102
April	1st to 10th	Day	20% 20% 4 10% 5% 3 3 3 4 4 24%	2520
	11th to 20th	Day	20% 16% 4 8% 4 4 4 3 4 5% 30%	2500
	21st to end	Day	24% 13% 3 8% 5% 4 4 3 4 5% 27%	2730
May	1st to 10th	Day	30% 15% 3 7% 5% 4 4 3 4 5% 19%	6054
	11th to 20th	Day	21% 9% 3 8% 6% 5% 5% 4 5% 7% 25%	6318
	21st to end	Day	21% 10% 3 7% 4 4 5% 4 5% 7% 29%	6949
June	1st to 10th	Day	28% 8% 2 6% 5% 4 5% 4 5% 7% 27%	2800
	11th to 20th	Day	19% 7% 3 6% 5% 5% 4 4 6% 8% 34%	2800
	21st to end	Day	25% 8% 3 6% 5% 5% 5% 4 5% 7% 27%	2800
July	1st to 10th	Day	17% 7% 3 8% 5% 6% 6% 5% 7% 9% 27%	6360
	11th to 20th	Day	16% 7% 2 6% 5% 5% 6% 5% 7% 8% 33%	6360
	21st to end	Day	16% 7% 2 5% 4 4 5% 4 6% 9% 38%	6926
August	1st to 10th	Day	20% 8% 3 8% 5% 5% 5% 4 6% 7% 29%	4563
	11th to 20th	Day	20% 8% 3 7% 5% 5% 5% 4 6% 7% 30%	4115
	21st to end	Day	20% 7% 2 5% 4 4 4 4 6% 8% 37%	4312
September	1st to 10th	Day	17% 6% 2 6% 4 4 4 4 5% 6% 41%	2400
	11th to 20th	Day	18% 6% 2 5% 3 3 3 2 5% 6% 45%	2265
	21st to end	Day	19% 7% 3 6% 4 3 4 4 7% 9% 35%	2200
October	1st to 10th	Day	23% 11% 2 7% 4 4 4 4 5% 6% 30%	2143
	11th to 20th	Day	21% 14% 2 7% 5% 5% 4 4 5% 5% 28%	1910
	21st to end	Day	26% 20% 4 7% 3 4 4 2 3 4 23%	1980
November	1st to 10th	Day	19% 20% 4 8% 4 3 4 3 3 6% 26%	1851
	11th to 20th	Day	16% 20% 7% 10% 5% 4 3 3 4 5% 22%	1890
	21st to end	Day	18% 21% 7% 13% 5% 5% 3 3 5% 4 17%	1690
December	1st to 10th	Day	14% 24% 7% 21% 9% 4 4 2 3 3 9%	2680
	11th to 20th	Day	17% 19% 8% 16% 5% 4 3 2 3 4 19%	3066
	21st to end	Day	17% 19% 6% 14% 7% 5% 6% 3 3 4 18%	4048
January	1st to 10th	Night	10% 19% 9% 21% 11% 6% 4 2 3 3 11%	8234
	11th to 20th	Night	9% 19% 9% 22% 12% 6% 5% 3 3 3 10%	8235
	21st to end	Night	9% 15% 6% 15% 12% 11% 6% 4 4 4 15%	9057
February	1st to 10th	Night	13% 19% 6% 18% 7% 6% 4 3 3 4 16%	2824
	11th to 20th	Night	12% 17% 7% 16% 9% 6% 4 4 3 3 19%	2479
	21st to end	Night	12% 19% 6% 13% 7% 6% 4 3 4 4 23%	2076
March	1st to 10th	Night	18% 17% 6% 13% 5% 6% 4 3 4 4 22%	5484
	11th to 20th	Night	13% 18% 5% 12% 6% 5% 4 3 4 6% 24%	5414
	21st to end	Night	16% 20% 6% 12% 6% 4 3 2 3 3 25%	5072
April	1st to 10th	Night	22% 15% 4 10% 5% 3 4 2 4 5% 26%	2100
	11th to 20th	Night	23% 15% 3 6% 5% 4 3 2 4 5% 29%	2067
	21st to end	Night	21% 13% 3 8% 4 4 3 3 4 5% 32%	1890
May	1st to 10th	Night	28% 15% 4 8% 4 3 4 3 4 5% 21%	3939
	11th to 20th	Night	22% 10% 3 7% 5% 4 4 4 5% 6% 29%	3575
	21st to end	Night	21% 10% 3 8% 5% 4 4 3 5% 6% 31%	3926
June	1st to 10th	Night	27% 7% 3 6% 4 4 4 4 4 7% 30%	1600
	11th to 20th	Night	20% 7% 2 5% 4 5% 4 4 5% 8% 36%	1600
	21st to end	Night	21% 8% 2 6% 5% 5% 5% 5% 8% 7% 29%	1600
July	1st to 10th	Night	16% 7% 2 7% 6% 5% 6% 4 6% 8% 30%	3600
	11th to 20th	Night	21% 8% 2 6% 4 4 4 3 5% 6% 37%	3600
	21st to end	Night	15% 7% 2 6% 5% 5% 5% 4 6% 7% 39%	3926
August	1st to 10th	Night	24% 8% 3 6% 3 3 4 3 4 6% 34%	2586
	11th to 20th	Night	18% 8% 3 7% 6% 5% 5% 5% 5% 5% 33%	2897
	21st to end	Night	17% 8% 2 6% 4 4 5% 3 5% 6% 41%	3586
September	1st to 10th	Night	17% 7% 2 7% 4 4 4 3 5% 5% 42%	2000
	11th to 20th	Night	16% 5% 2 4 3 4 3 4 5% 7% 46%	2135
	21st to end	Night	21% 7% 2 5% 4 3 4 3 5% 6% 41%	2200
October	1st to 10th	Night	21% 9% 3 7% 4 4 4 4 5% 6% 35%	2255
	11th to 20th	Night	22% 13% 3 8% 4 4 4 4 4 5% 30%	2490
	21st to end	Night	26% 15% 3 7% 4 4 4 2 5% 6% 24%	2860
November	1st to 10th	Night	21% 16% 4 8% 4 4 4 4 4 5% 26%	2667
	11th to 20th	Night	17% 18% 7% 10% 5% 4 3 3 3 5% 25%	2730
	21st to end	Night	18% 20% 7% 13% 6% 4 4 3 4 4 19%	2924
December	1st to 10th	Night	15% 24% 11% 20% 6% 4 3 2 3 3 9%	4720
	11th to 20th	Night	18% 23% 7% 17% 6% 5% 4 2 2 2 15%	5348
	21st to end	Night	17% 19% 6% 14% 6% 5% 5% 2 3 3 19%	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	78% 5% 9% 7% 1	4662
	11th to 20th	Day	78% 5% 8% 6% 3 1	4680
	21st to end	Day	72% 5% 8% 8% 7%	5124
February	1st to 10th	Day	71% 5% 6% 12% 5% 1	1795
	11th to 20th	Day	70% 5% 9% 10% 4 1	2036
	21st to end	Day	67% 5% 9% 12% 7% 2	1730
March	1st to 10th	Day	69% 6% 7% 8% 7% 3	4579
	11th to 20th	Day	58% 6% 13% 12% 9% 2	4746
	21st to end	Day	61% 4 7% 13% 14% 1	6102
April	1st to 10th	Day	63% 4 8% 16% 8% 1	2520
	11th to 20th	Day	57% 5% 7% 18% 12% 1	2500
	21st to end	Day	59% 5% 8% 16% 9% 2	2730
May	1st to 10th	Day	66% 6% 8% 15% 5% 1	6065
	11th to 20th	Day	55% 9% 13% 15% 7% 1	6322
	21st to end	Day	51% 9% 11% 20% 9% 1	6949
June	1st to 10th	Day	55% 7% 11% 17% 8% 1	2800
	11th to 20th	Day	47% 7% 13% 22% 11% 1	2800
	21st to end	Day	54% 8% 11% 18% 8% 1	2800
July	1st to 10th	Day	48% 8% 15% 21% 7%	6290
	11th to 20th	Day	43% 9% 13% 27% 8%	6285
	21st to end	Day	40% 7% 13% 32% 7%	6728
August	1st to 10th	Day	51% 8% 15% 19% 6%	4549
	11th to 20th	Day	49% 8% 18% 19% 5%	4154
	21st to end	Day	44% 8% 15% 19% 13% 1	4312
September	1st to 10th	Day	41% 7% 13% 28% 10% 1	2400
	11th to 20th	Day	39% 4 13% 30% 13% 1	2265
	21st to end	Day	44% 7% 15% 22% 11% 1	2200
October	1st to 10th	Day	47% 6% 14% 24% 9%	3494
	11th to 20th	Day	52% 5% 15% 20% 8%	3087
	21st to end	Day	54% 4 14% 20% 8% 1	3180
November	1st to 10th	Day	59% 7% 10% 16% 7%	1851
	11th to 20th	Day	65% 6% 10% 11% 7% 2	1890
	21st to end	Day	68% 7% 13% 9% 3	1690
December	1st to 10th	Day	81% 4 7% 6% 2	2680
	11th to 20th	Day	70% 3 7% 14% 5%	3066
	21st to end	Day	70% 5% 7% 10% 8% 1	4048
January	1st to 10th	Night	76% 5% 10% 7% 2	8234
	11th to 20th	Night	77% 5% 8% 6% 3	8280
	21st to end	Night	70% 4 10% 8% 8%	9057
February	1st to 10th	Night	70% 5% 7% 12% 5%	2824
	11th to 20th	Night	69% 5% 9% 12% 5%	2481
	21st to end	Night	63% 4 10% 13% 8% 1	2076
March	1st to 10th	Night	65% 5% 8% 12% 8% 2	5494
	11th to 20th	Night	60% 6% 13% 13% 7% 1	5415
	21st to end	Night	63% 4 7% 15% 10% 1	5072
April	1st to 10th	Night	60% 5% 8% 15% 10% 1	2100
	11th to 20th	Night	56% 5% 9% 17% 12%	2067
	21st to end	Night	53% 6% 9% 18% 12% 2	1890
May	1st to 10th	Night	64% 6% 9% 14% 6% 1	3945
	11th to 20th	Night	52% 9% 12% 19% 7% 1	3576
	21st to end	Night	52% 8% 10% 18% 11% 1	3927
June	1st to 10th	Night	53% 6% 11% 20% 9% 1	1600
	11th to 20th	Night	45% 8% 13% 21% 11% 2	1600
	21st to end	Night	50% 10% 13% 18% 9% 1	1600
July	1st to 10th	Night	47% 9% 16% 20% 8%	3560
	11th to 20th	Night	44% 8% 13% 28% 8%	3552
	21st to end	Night	41% 8% 15% 27% 9%	3809
August	1st to 10th	Night	49% 7% 15% 24% 6%	2576
	11th to 20th	Night	47% 9% 18% 19% 7%	2924
	21st to end	Night	42% 7% 14% 24% 13% 1	3586
September	1st to 10th	Night	41% 8% 12% 27% 11% 1	2000
	11th to 20th	Night	37% 7% 15% 29% 12% 1	2135
	21st to end	Night	43% 7% 13% 24% 13% 1	2200
October	1st to 10th	Night	51% 6% 11% 23% 9%	3684
	11th to 20th	Night	50% 5% 12% 22% 11%	4052
	21st to end	Night	53% 5% 14% 18% 9% 1	4610
November	1st to 10th	Night	58% 6% 12% 17% 6% 1	2667
	11th to 20th	Night	61% 6% 9% 14% 8% 2	2730
	21st to end	Night	68% 6% 11% 12% 3	2924
December	1st to 10th	Night	80% 6% 7% 6% 1	4720
	11th to 20th	Night	76% 4 5% 9% 6%	5348
	21st to end	Night	70% 5% 6% 11% 8% 1	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	30% 21% 31% 14% 3	220
	11th to 20th	Day	28% 29% 30% 9% 3	220
	21st to end	Day	27% 24% 33% 11% 5%	242
February	1st to 10th	Day	33% 31% 26% 9% 1	281
	11th to 20th	Day	29% 27% 32% 10% 2	515
	21st to end	Day	30% 24% 30% 14% 2	446
March	1st to 10th	Day	31% 23% 30% 13% 3	540
	11th to 20th	Day	31% 26% 26% 14% 4	540
	21st to end	Day	36% 23% 22% 12% 5% 1	594
April	1st to 10th	Day	37% 18% 26% 14% 4	540
	11th to 20th	Day	34% 24% 22% 13% 5% 2	532
	21st to end	Day	35% 19% 28% 11% 5% 2	540
May	1st to 10th	Day	36% 20% 23% 14% 6% 1	507
	11th to 20th	Day	24% 18% 31% 18% 8% 2	527
	21st to end	Day	21% 20% 31% 20% 6% 1	573
June	1st to 10th	Day	31% 14% 33% 15% 6% 1	530
	11th to 20th	Day	21% 15% 33% 21% 8% 2	530
	21st to end	Day	25% 16% 35% 16% 7% 1	530
July	1st to 10th	Day	18% 12% 34% 25% 10% 1	525
	11th to 20th	Day	16% 13% 37% 25% 7% 2	524
	21st to end	Day	17% 14% 35% 25% 8% 1	576
August	1st to 10th	Day	16% 13% 26% 29% 14% 2	526
	11th to 20th	Day	20% 9% 29% 24% 15% 4	519
	21st to end	Day	19% 16% 26% 23% 10% 5%	583
September	1st to 10th	Day	19% 13% 28% 21% 13% 5%	530
	11th to 20th	Day	18% 11% 24% 21% 20% 7%	530
	21st to end	Day	19% 14% 26% 24% 12% 5%	530
October	1st to 10th	Day	24% 18% 25% 16% 12% 4	530
	11th to 20th	Day	28% 22% 23% 18% 9% 1	480
	21st to end	Day	36% 19% 25% 14% 5%	462
November	1st to 10th	Day	34% 20% 27% 14% 4	425
	11th to 20th	Day	28% 24% 27% 14% 6% 2	430
	21st to end	Day	30% 26% 30% 14%	230
December	1st to 10th	Day	28% 25% 30% 12% 5%	220
	11th to 20th	Day	34% 28% 29% 8% 1	220
	21st to end	Day	32% 24% 28% 12% 3	242
January	1st to 10th	Night	25% 26% 30% 17% 2	840
	11th to 20th	Night	26% 24% 29% 16% 5%	840
	21st to end	Night	19% 21% 33% 21% 5% 1	924
February	1st to 10th	Night	22% 24% 31% 19% 4	840
	11th to 20th	Night	22% 23% 33% 18% 3	823
	21st to end	Night	24% 20% 32% 22% 2	692
March	1st to 10th	Night	27% 20% 30% 20% 3	840
	11th to 20th	Night	28% 21% 25% 20% 5%	840
	21st to end	Night	26% 20% 27% 19% 6% 1	924
April	1st to 10th	Night	31% 18% 26% 21% 4	840
	11th to 20th	Night	30% 19% 25% 20% 7%	823
	21st to end	Night	30% 15% 30% 19% 6%	630
May	1st to 10th	Night	27% 14% 28% 22% 8% 1	557
	11th to 20th	Night	21% 13% 29% 26% 9% 1	394
	21st to end	Night	20% 12% 27% 31% 8% 1	433
June	1st to 10th	Night	29% 9% 26% 28% 8% 1	400
	11th to 20th	Night	21% 9% 26% 32% 13% 1	400
	21st to end	Night	20% 10% 26% 35% 9% 1	400
July	1st to 10th	Night	13% 5% 31% 37% 14%	390
	11th to 20th	Night	13% 4 25% 41% 15% 2	388
	21st to end	Night	13% 5% 26% 41% 15%	438
August	1st to 10th	Night	15% 6% 19% 37% 22% 1	398
	11th to 20th	Night	14% 8% 23% 34% 20% 1	603
	21st to end	Night	15% 8% 24% 34% 18% 2	880
September	1st to 10th	Night	18% 8% 27% 32% 15% 1	800
	11th to 20th	Night	18% 8% 19% 29% 24% 3	800
	21st to end	Night	21% 10% 28% 23% 16% 2	800
October	1st to 10th	Night	25% 12% 23% 25% 13% 1	800
	11th to 20th	Night	26% 15% 26% 23% 9% 1	800
	21st to end	Night	33% 16% 28% 20% 4	880
November	1st to 10th	Night	30% 17% 28% 19% 5% 1	820
	11th to 20th	Night	27% 20% 28% 16% 8% 1	840
	21st to end	Night	28% 24% 27% 16% 5%	840
December	1st to 10th	Night	30% 24% 27% 15% 4	840
	11th to 20th	Night	30% 22% 27% 16% 4	840
	21st to end	Night	30% 22% 27% 18% 3	924

Page generated at 2026-04-29 02:21:47. Version ac07

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