ELAC-Stanleigh
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ELAC-Stanleigh clear sky chart from 2005-11-06 to 2026-07-16.

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	15% 10% 4 8% 5% 4 4 4 5% 7% 34%	137911
	Night	14% 12% 4 9% 5% 4 4 3 5% 6% 33%	134980
Transparency	Day	49% 6% 11% 21% 12% 2	141394
	Night	51% 5% 11% 20% 11% 1	139585
Seeing	Day	21% 15% 33% 21% 8% 2	17643
	Night	21% 11% 31% 27% 9% 1	26877

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% 8% 11% 5% 4 4 3 4 6% 25%	4131
	11th to 20th	Day	8% 17% 8% 14% 7% 6% 5% 4 5% 6% 19%	4679
	21st to end	Day	9% 17% 9% 16% 7% 5% 4 3 4 4 21%	5243
February	1st to 10th	Day	9% 15% 6% 12% 7% 6% 5% 4 5% 5% 24%	1890
	11th to 20th	Day	12% 15% 5% 11% 7% 6% 4 4 5% 6% 26%	1848
	21st to end	Day	10% 16% 7% 12% 6% 6% 5% 4 4 6% 24%	1730
March	1st to 10th	Day	15% 13% 4 8% 4 4 4 4 5% 7% 33%	4707
	11th to 20th	Day	15% 13% 4 9% 6% 5% 5% 4 5% 6% 29%	5090
	21st to end	Day	16% 12% 7% 9% 4 4 4 3 4 6% 31%	5586
April	1st to 10th	Day	16% 13% 4 9% 5% 5% 5% 3 5% 6% 28%	2498
	11th to 20th	Day	20% 10% 3 7% 5% 4 4 3 4 6% 34%	2702
	21st to end	Day	19% 11% 3 6% 5% 4 4 3 5% 7% 34%	2730
May	1st to 10th	Day	15% 8% 3 7% 4 5% 4 4 5% 7% 38%	7642
	11th to 20th	Day	19% 8% 3 6% 4 4 4 3 4 6% 39%	7919
	21st to end	Day	15% 8% 2 6% 4 3 4 3 5% 6% 43%	9656
June	1st to 10th	Day	15% 6% 2 6% 4 4 5% 4 6% 8% 39%	3150
	11th to 20th	Day	14% 7% 3 7% 5% 5% 4 4 6% 8% 37%	3150
	21st to end	Day	18% 7% 2 6% 5% 6% 5% 4 5% 7% 35%	3150
July	1st to 10th	Day	15% 7% 3 6% 4 4 5% 5% 7% 9% 36%	6969
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 8% 46%	6767
	21st to end	Day	11% 5% 2 5% 4 3 4 4 6% 9% 48%	6890
August	1st to 10th	Day	10% 6% 3 7% 5% 5% 6% 4 7% 9% 40%	4234
	11th to 20th	Day	15% 5% 2 6% 4 5% 4 4 5% 7% 41%	4174
	21st to end	Day	14% 5% 2 6% 4 4 4 4 6% 7% 43%	4675
September	1st to 10th	Day	14% 7% 3 8% 4 4 4 4 6% 8% 39%	2370
	11th to 20th	Day	14% 5% 2 7% 4 4 5% 3 5% 7% 44%	2200
	21st to end	Day	20% 7% 3 7% 4 4 4 2 4 7% 37%	2200
October	1st to 10th	Day	20% 7% 3 6% 4 4 4 4 5% 7% 36%	2080
	11th to 20th	Day	23% 12% 3 8% 5% 5% 5% 4 5% 6% 25%	2000
	21st to end	Day	22% 14% 4 8% 5% 4 4 4 4 7% 23%	1990
November	1st to 10th	Day	21% 11% 3 9% 5% 5% 5% 4 6% 7% 26%	1831
	11th to 20th	Day	17% 16% 5% 11% 5% 4 4 3 5% 7% 23%	1680
	21st to end	Day	18% 24% 6% 10% 5% 3 3 4 4 4 18%	1680
December	1st to 10th	Day	12% 20% 6% 12% 5% 5% 5% 4 5% 5% 21%	2426
	11th to 20th	Day	15% 18% 8% 14% 6% 4 4 4 4 5% 19%	2691
	21st to end	Day	25% 22% 7% 10% 3 3 3 2 3 3 18%	3553
January	1st to 10th	Night	11% 13% 6% 13% 6% 5% 5% 4 5% 6% 26%	8792
	11th to 20th	Night	9% 19% 8% 15% 6% 4 5% 3 4 5% 20%	8236
	21st to end	Night	8% 13% 7% 14% 8% 7% 6% 3 5% 5% 22%	8963
February	1st to 10th	Night	10% 14% 5% 12% 8% 5% 5% 4 5% 5% 26%	2730
	11th to 20th	Night	10% 14% 5% 11% 7% 6% 4 4 6% 6% 27%	2653
	21st to end	Night	11% 16% 6% 10% 5% 6% 6% 3 4 6% 27%	2076
March	1st to 10th	Night	16% 10% 3 8% 4 3 4 3 4 7% 36%	5353
	11th to 20th	Night	13% 10% 5% 10% 5% 5% 5% 4 5% 7% 31%	5089
	21st to end	Night	17% 13% 5% 9% 4 3 3 3 4 5% 33%	5588
April	1st to 10th	Night	18% 10% 2 8% 4 4 5% 4 5% 6% 33%	2138
	11th to 20th	Night	20% 8% 3 7% 5% 4 3 3 4 5% 38%	1867
	21st to end	Night	19% 9% 3 8% 5% 5% 3 3 5% 6% 35%	1890
May	1st to 10th	Night	14% 7% 3 7% 4 5% 4 4 6% 7% 40%	5257
	11th to 20th	Night	16% 7% 2 6% 4 4 4 3 6% 7% 41%	4941
	21st to end	Night	14% 9% 3 5% 4 4 3 3 5% 6% 45%	4534
June	1st to 10th	Night	13% 7% 3 6% 4 4 4 4 5% 8% 42%	1470
	11th to 20th	Night	12% 6% 2 7% 5% 5% 5% 5% 6% 8% 40%	1470
	21st to end	Night	14% 7% 3 7% 5% 4 5% 5% 5% 6% 40%	1470
July	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 6% 8% 41%	3230
	11th to 20th	Night	11% 5% 2 5% 3 3 4 4 5% 7% 52%	3322
	21st to end	Night	9% 5% 2 5% 3 3 4 3 5% 8% 53%	3981
August	1st to 10th	Night	8% 8% 3 7% 4 5% 4 5% 7% 8% 42%	2915
	11th to 20th	Night	13% 5% 2 6% 4 3 4 4 6% 8% 44%	2875
	21st to end	Night	11% 5% 2 5% 4 4 3 3 4 7% 52%	3223
September	1st to 10th	Night	14% 6% 2 5% 4 4 4 4 5% 8% 44%	2007
	11th to 20th	Night	12% 4 2 4 3 4 3 3 6% 7% 51%	2200
	21st to end	Night	18% 7% 2 7% 4 4 4 3 4 7% 40%	2200
October	1st to 10th	Night	20% 7% 2 6% 4 4 3 3 5% 6% 40%	2320
	11th to 20th	Night	22% 10% 3 8% 5% 4 5% 3 5% 6% 29%	2400
	21st to end	Night	24% 10% 3 7% 5% 4 4 3 5% 6% 28%	2846
November	1st to 10th	Night	17% 11% 4 9% 4 4 3 4 6% 7% 32%	2685
	11th to 20th	Night	18% 13% 4 9% 6% 5% 5% 4 5% 5% 26%	2940
	21st to end	Night	19% 19% 5% 10% 4 4 4 4 5% 5% 22%	2940
December	1st to 10th	Night	13% 18% 6% 10% 6% 5% 5% 4 5% 7% 21%	4976
	11th to 20th	Night	14% 18% 6% 12% 5% 4 4 3 5% 5% 24%	5736
	21st to end	Night	23% 22% 7% 12% 4 2 3 2 3 4 18%	7667

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	62% 4 9% 16% 6% 2	4132
	11th to 20th	Day	65% 5% 8% 10% 10% 1	4710
	21st to end	Day	67% 4 4 13% 12% 1	5243
February	1st to 10th	Day	58% 7% 11% 16% 7% 1	1890
	11th to 20th	Day	57% 8% 10% 14% 10% 1	1857
	21st to end	Day	60% 6% 8% 15% 10% 1	1730
March	1st to 10th	Day	50% 6% 12% 10% 17% 5%	4710
	11th to 20th	Day	55% 5% 11% 16% 10% 2	5090
	21st to end	Day	53% 7% 11% 15% 11% 3	5586
April	1st to 10th	Day	55% 6% 10% 16% 10% 2	2498
	11th to 20th	Day	49% 6% 10% 21% 12% 2	2702
	21st to end	Day	49% 5% 10% 18% 14% 3	2730
May	1st to 10th	Day	44% 6% 9% 22% 15% 3	7651
	11th to 20th	Day	45% 5% 9% 21% 17% 3	7923
	21st to end	Day	42% 5% 9% 23% 17% 5%	9657
June	1st to 10th	Day	42% 7% 12% 22% 15% 2	3150
	11th to 20th	Day	44% 6% 11% 23% 15% 1	3150
	21st to end	Day	47% 7% 11% 21% 12% 2	3150
July	1st to 10th	Day	44% 6% 15% 23% 12% 1	6896
	11th to 20th	Day	34% 6% 17% 34% 9% 1	6686
	21st to end	Day	33% 5% 13% 34% 14%	6692
August	1st to 10th	Day	39% 7% 13% 29% 11% 1	4221
	11th to 20th	Day	42% 6% 17% 25% 10%	4213
	21st to end	Day	39% 7% 14% 29% 10% 1	4674
September	1st to 10th	Day	43% 7% 14% 24% 11% 1	2370
	11th to 20th	Day	39% 6% 16% 28% 11%	2200
	21st to end	Day	47% 5% 15% 24% 9%	2200
October	1st to 10th	Day	38% 5% 13% 27% 17%	3392
	11th to 20th	Day	47% 7% 11% 28% 7%	3240
	21st to end	Day	60% 6% 13% 13% 8% 1	3190
November	1st to 10th	Day	57% 7% 12% 17% 6%	1831
	11th to 20th	Day	60% 7% 13% 14% 6%	1680
	21st to end	Day	69% 5% 11% 10% 4	1680
December	1st to 10th	Day	65% 6% 12% 13% 4	2426
	11th to 20th	Day	67% 5% 8% 13% 7% 1	2691
	21st to end	Day	72% 3 6% 11% 7%	3553
January	1st to 10th	Night	57% 6% 10% 18% 7% 2	8792
	11th to 20th	Night	66% 4 8% 12% 9%	8280
	21st to end	Night	63% 5% 7% 14% 11% 1	8963
February	1st to 10th	Night	59% 6% 9% 16% 8% 1	2730
	11th to 20th	Night	57% 6% 12% 17% 7% 1	2655
	21st to end	Night	57% 7% 10% 16% 9% 1	2076
March	1st to 10th	Night	48% 5% 11% 16% 16% 4	5363
	11th to 20th	Night	51% 7% 15% 18% 9% 1	5090
	21st to end	Night	53% 5% 9% 16% 15% 2	5588
April	1st to 10th	Night	50% 6% 10% 19% 14% 1	2138
	11th to 20th	Night	48% 5% 7% 25% 15% 1	1867
	21st to end	Night	50% 4 10% 19% 13% 3	1890
May	1st to 10th	Night	41% 5% 8% 27% 16% 3	5264
	11th to 20th	Night	42% 6% 9% 26% 15% 2	4942
	21st to end	Night	40% 5% 8% 27% 15% 5%	4535
June	1st to 10th	Night	39% 6% 13% 22% 17% 2	1470
	11th to 20th	Night	40% 7% 13% 23% 16% 2	1470
	21st to end	Night	42% 7% 15% 22% 12% 1	1470
July	1st to 10th	Night	41% 5% 13% 27% 12% 2	3195
	11th to 20th	Night	32% 4 14% 37% 12%	3280
	21st to end	Night	31% 5% 14% 34% 16%	3865
August	1st to 10th	Night	38% 6% 16% 28% 12% 1	2904
	11th to 20th	Night	36% 8% 19% 26% 10% 1	2902
	21st to end	Night	32% 5% 15% 32% 15% 1	3223
September	1st to 10th	Night	38% 8% 16% 25% 12% 2	2007
	11th to 20th	Night	31% 7% 19% 32% 11%	2200
	21st to end	Night	45% 6% 15% 25% 9%	2200
October	1st to 10th	Night	38% 5% 11% 31% 15%	3788
	11th to 20th	Night	43% 5% 14% 29% 8%	3899
	21st to end	Night	52% 6% 19% 17% 7%	4596
November	1st to 10th	Night	51% 7% 13% 21% 8%	2685
	11th to 20th	Night	59% 7% 12% 14% 7%	2940
	21st to end	Night	64% 7% 10% 12% 6% 1	2940
December	1st to 10th	Night	62% 5% 13% 15% 4	4976
	11th to 20th	Night	62% 5% 9% 13% 10% 1	5735
	21st to end	Night	72% 4 6% 11% 7%	7667

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	23% 18% 33% 20% 5%	226
	11th to 20th	Day	20% 15% 38% 21% 5% 1	330
	21st to end	Day	20% 15% 37% 21% 6% 1	363
February	1st to 10th	Day	21% 27% 34% 12% 5%	330
	11th to 20th	Day	22% 23% 38% 12% 5%	330
	21st to end	Day	24% 26% 39% 11%	273
March	1st to 10th	Day	22% 24% 40% 12% 2 1	382
	11th to 20th	Day	23% 17% 37% 18% 4 1	540
	21st to end	Day	27% 18% 32% 18% 4 1	594
April	1st to 10th	Day	23% 17% 34% 20% 6% 1	540
	11th to 20th	Day	31% 18% 24% 16% 8% 3	532
	21st to end	Day	28% 14% 31% 19% 6% 1	540
May	1st to 10th	Day	22% 16% 33% 20% 6% 3	543
	11th to 20th	Day	20% 17% 31% 21% 9% 3	567
	21st to end	Day	18% 15% 37% 22% 8% 1	823
June	1st to 10th	Day	17% 15% 38% 23% 6% 1	780
	11th to 20th	Day	19% 15% 36% 21% 7% 1	780
	21st to end	Day	21% 14% 34% 20% 7% 2	780
July	1st to 10th	Day	12% 12% 40% 28% 7% 2	771
	11th to 20th	Day	13% 10% 36% 27% 10% 4	744
	21st to end	Day	14% 8% 34% 28% 11% 5%	783
August	1st to 10th	Day	14% 9% 26% 30% 17% 5%	526
	11th to 20th	Day	16% 6% 26% 28% 15% 9%	519
	21st to end	Day	16% 9% 28% 28% 13% 6%	583
September	1st to 10th	Day	18% 12% 27% 26% 13% 5%	530
	11th to 20th	Day	16% 7% 25% 31% 14% 7%	530
	21st to end	Day	23% 12% 26% 24% 12% 3	530
October	1st to 10th	Day	26% 15% 25% 21% 11% 2	530
	11th to 20th	Day	26% 16% 35% 18% 4 1	530
	21st to end	Day	28% 20% 33% 18% 1	373
November	1st to 10th	Day	29% 20% 32% 17% 3	319
	11th to 20th	Day	25% 22% 34% 15% 4	220
	21st to end	Day	30% 22% 30% 15% 3	220
December	1st to 10th	Day	23% 17% 31% 20% 7% 1	220
	11th to 20th	Day	30% 21% 33% 12% 3	220
	21st to end	Day	33% 17% 35% 11% 4	242
January	1st to 10th	Night	23% 12% 37% 23% 5%	1050
	11th to 20th	Night	22% 13% 32% 25% 7% 1	1050
	21st to end	Night	20% 10% 34% 27% 9%	1125
February	1st to 10th	Night	18% 14% 41% 23% 5%	840
	11th to 20th	Night	18% 15% 36% 24% 7%	823
	21st to end	Night	18% 16% 36% 26% 4 1	692
March	1st to 10th	Night	17% 14% 40% 24% 4	840
	11th to 20th	Night	21% 14% 37% 23% 5% 1	840
	21st to end	Night	20% 13% 36% 25% 6% 1	924
April	1st to 10th	Night	23% 13% 34% 24% 5%	720
	11th to 20th	Night	25% 10% 29% 25% 9% 1	623
	21st to end	Night	22% 12% 29% 29% 7%	630
May	1st to 10th	Night	16% 8% 29% 36% 9% 1	630
	11th to 20th	Night	19% 8% 29% 34% 9% 1	523
	21st to end	Night	16% 8% 33% 33% 10%	455
June	1st to 10th	Night	13% 8% 40% 31% 8%	420
	11th to 20th	Night	14% 9% 32% 37% 8%	420
	21st to end	Night	15% 6% 26% 40% 11% 1	420
July	1st to 10th	Night	11% 7% 27% 44% 10%	410
	11th to 20th	Night	12% 5% 25% 40% 17% 1	447
	21st to end	Night	14% 4 25% 39% 17% 2	657
August	1st to 10th	Night	12% 2 26% 40% 19% 1	597
	11th to 20th	Night	15% 4 20% 33% 26% 2	594
	21st to end	Night	16% 6% 22% 38% 17% 1	660
September	1st to 10th	Night	17% 7% 27% 27% 19% 3	710
	11th to 20th	Night	15% 6% 26% 31% 19% 4	800
	21st to end	Night	21% 6% 25% 31% 15% 2	800
October	1st to 10th	Night	24% 8% 30% 25% 12% 2	800
	11th to 20th	Night	29% 12% 28% 24% 7%	800
	21st to end	Night	28% 13% 28% 25% 5% 1	880
November	1st to 10th	Night	25% 13% 31% 25% 6% 1	820
	11th to 20th	Night	25% 13% 33% 23% 6%	840
	21st to end	Night	30% 15% 31% 19% 4	840
December	1st to 10th	Night	23% 15% 33% 21% 6%	992
	11th to 20th	Night	27% 16% 33% 19% 4	1050
	21st to end	Night	31% 14% 31% 18% 6%	1155

Page generated at 2026-07-13 03:14:48. Version ac07

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