ELAC-Stanleigh
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ELAC-Stanleigh clear sky chart from 2005-11-05 to 2024-10-14.

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%	101335
	Night	15% 12% 4 8% 5% 4 4 3 5% 6% 33%	97300
Transparency	Day	48% 7% 12% 20% 11% 1	101083
	Night	50% 7% 12% 20% 10% 1	97176
Seeing	Day	20% 15% 33% 22% 8% 2	15356
	Night	21% 11% 31% 27% 9% 1	24584

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% 21% 8% 11% 5% 4 3 3 5% 5% 21%	2217
	11th to 20th	Day	10% 15% 6% 13% 10% 7% 6% 4 6% 6% 18%	2530
	21st to end	Day	14% 23% 11% 15% 6% 4 4 3 4 3 13%	2818
February	1st to 10th	Day	10% 14% 6% 13% 7% 6% 5% 5% 5% 5% 24%	1710
	11th to 20th	Day	11% 16% 4 11% 6% 6% 4 4 5% 6% 27%	1667
	21st to end	Day	10% 16% 7% 11% 6% 6% 5% 4 4 6% 24%	1570
March	1st to 10th	Day	14% 14% 4 8% 4 4 4 4 5% 7% 31%	3203
	11th to 20th	Day	10% 15% 5% 9% 5% 4 5% 4 5% 6% 32%	3480
	21st to end	Day	13% 11% 9% 10% 5% 4 4 4 5% 6% 30%	3815
April	1st to 10th	Day	17% 13% 4 9% 5% 5% 5% 4 5% 6% 29%	2260
	11th to 20th	Day	20% 10% 3 7% 5% 4 4 3 4 7% 33%	2442
	21st to end	Day	19% 11% 3 6% 5% 3 4 3 5% 7% 35%	2470
May	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 6% 7% 38%	4108
	11th to 20th	Day	13% 8% 3 7% 4 4 4 3 5% 7% 41%	4234
	21st to end	Day	15% 7% 2 6% 4 3 3 3 6% 7% 44%	5153
June	1st to 10th	Day	16% 6% 2 7% 4 4 4 4 6% 8% 38%	2850
	11th to 20th	Day	14% 7% 2 6% 5% 5% 4 4 5% 8% 38%	2850
	21st to end	Day	18% 7% 2 5% 5% 5% 5% 4 5% 7% 36%	2850
July	1st to 10th	Day	16% 8% 3 6% 4 4 5% 4 6% 8% 35%	4760
	11th to 20th	Day	11% 5% 2 5% 5% 5% 5% 4 6% 8% 44%	4673
	21st to end	Day	12% 5% 2 5% 4 3 4 4 5% 8% 49%	4792
August	1st to 10th	Day	10% 6% 3 7% 5% 5% 6% 4 7% 9% 40%	4104
	11th to 20th	Day	15% 5% 2 7% 4 5% 4 4 6% 7% 41%	4044
	21st to end	Day	15% 5% 2 6% 4 4 4 4 5% 7% 43%	4532
September	1st to 10th	Day	14% 7% 3 8% 4 4 4 4 5% 8% 40%	2252
	11th to 20th	Day	15% 5% 2 7% 4 4 5% 3 5% 7% 43%	2090
	21st to end	Day	20% 7% 3 7% 4 4 4 2 4 7% 37%	2090
October	1st to 10th	Day	21% 7% 3 6% 4 4 4 4 5% 7% 35%	1976
	11th to 20th	Day	23% 13% 3 8% 5% 5% 5% 3 5% 6% 23%	1830
	21st to end	Day	23% 14% 3 8% 5% 4 4 4 4 7% 23%	1792
November	1st to 10th	Day	21% 11% 3 9% 5% 5% 4 4 6% 7% 26%	1653
	11th to 20th	Day	16% 17% 5% 11% 5% 4 4 3 5% 6% 24%	1520
	21st to end	Day	19% 25% 5% 9% 5% 3 3 3 4 4 19%	1520
December	1st to 10th	Day	12% 21% 5% 11% 4 4 5% 4 5% 4 23%	1378
	11th to 20th	Day	16% 20% 6% 10% 6% 4 4 3 4 5% 22%	1671
	21st to end	Day	24% 24% 5% 10% 4 3 4 3 3 3 18%	2431
January	1st to 10th	Night	14% 16% 6% 12% 6% 4 4 3 5% 6% 24%	4737
	11th to 20th	Night	10% 15% 6% 15% 9% 4 5% 4 5% 6% 21%	4440
	21st to end	Night	14% 18% 9% 13% 7% 5% 5% 3 4 4 19%	4845
February	1st to 10th	Night	11% 15% 5% 11% 8% 6% 4 4 5% 5% 26%	2470
	11th to 20th	Night	11% 14% 4 11% 7% 5% 4 4 5% 6% 28%	2395
	21st to end	Night	11% 16% 6% 10% 5% 5% 6% 3 4 6% 28%	1884
March	1st to 10th	Night	16% 10% 3 8% 4 4 4 3 4 7% 38%	3637
	11th to 20th	Night	9% 11% 5% 10% 5% 5% 4 3 4 6% 38%	3479
	21st to end	Night	13% 10% 6% 9% 5% 4 4 3 4 6% 37%	3817
April	1st to 10th	Night	19% 11% 2 7% 4 4 5% 4 5% 7% 33%	1935
	11th to 20th	Night	20% 7% 2 6% 5% 4 4 3 4 5% 39%	1687
	21st to end	Night	19% 9% 3 7% 5% 5% 3 3 5% 5% 36%	1710
May	1st to 10th	Night	17% 7% 2 7% 4 5% 4 4 5% 6% 39%	2833
	11th to 20th	Night	11% 9% 3 7% 6% 5% 4 4 7% 8% 37%	2659
	21st to end	Night	14% 8% 3 5% 3 4 4 3 6% 7% 42%	2418
June	1st to 10th	Night	13% 7% 3 6% 4 4 4 4 5% 8% 41%	1330
	11th to 20th	Night	12% 6% 2 6% 5% 5% 5% 5% 5% 8% 40%	1330
	21st to end	Night	14% 7% 3 7% 5% 4 5% 4 5% 6% 41%	1330
July	1st to 10th	Night	15% 7% 2 5% 4 4 4 4 6% 8% 42%	2210
	11th to 20th	Night	11% 5% 2 5% 4 3 4 4 4 7% 52%	2295
	21st to end	Night	9% 5% 2 5% 3 4 4 4 5% 7% 51%	2770
August	1st to 10th	Night	8% 8% 3 7% 4 5% 4 5% 7% 8% 42%	2825
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 6% 8% 45%	2785
	21st to end	Night	11% 5% 2 5% 4 4 3 3 4 6% 52%	3124
September	1st to 10th	Night	14% 6% 3 5% 4 4 4 4 4 7% 44%	1906
	11th to 20th	Night	13% 4 1 4 3 4 3 3 6% 7% 51%	2090
	21st to end	Night	18% 7% 2 7% 4 4 4 3 4 7% 40%	2090
October	1st to 10th	Night	21% 7% 2 6% 4 5% 3 3 4 5% 38%	2204
	11th to 20th	Night	23% 10% 2 8% 5% 4 5% 3 5% 6% 29%	2203
	21st to end	Night	26% 11% 3 6% 5% 4 4 3 5% 6% 28%	2560
November	1st to 10th	Night	17% 11% 4 9% 4 4 3 4 6% 7% 32%	2423
	11th to 20th	Night	19% 13% 4 9% 6% 5% 5% 4 5% 5% 25%	2660
	21st to end	Night	19% 19% 5% 9% 4 4 4 4 5% 5% 23%	2660
December	1st to 10th	Night	15% 19% 5% 8% 6% 4 4 4 5% 5% 24%	2797
	11th to 20th	Night	17% 17% 5% 9% 5% 4 4 3 4 5% 25%	3526
	21st to end	Night	22% 23% 6% 10% 4 3 3 3 3 4 18%	5236

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	66% 6% 13% 11% 4	2218
	11th to 20th	Day	64% 8% 12% 10% 5% 1	2530
	21st to end	Day	75% 6% 5% 7% 6% 1	2818
February	1st to 10th	Day	59% 7% 12% 14% 7% 1	1710
	11th to 20th	Day	55% 8% 10% 15% 10% 1	1676
	21st to end	Day	59% 7% 8% 15% 10% 1	1570
March	1st to 10th	Day	50% 8% 15% 12% 14% 2	3206
	11th to 20th	Day	50% 6% 11% 22% 9% 2	3480
	21st to end	Day	51% 9% 11% 14% 11% 4	3815
April	1st to 10th	Day	55% 6% 10% 16% 11% 2	2260
	11th to 20th	Day	50% 6% 10% 21% 11% 2	2442
	21st to end	Day	48% 6% 9% 18% 15% 4	2470
May	1st to 10th	Day	42% 8% 11% 23% 15% 2	4120
	11th to 20th	Day	40% 7% 13% 23% 15% 2	4238
	21st to end	Day	38% 7% 13% 18% 18% 6%	5153
June	1st to 10th	Day	42% 7% 12% 23% 14% 2	2850
	11th to 20th	Day	43% 6% 11% 22% 16% 1	2850
	21st to end	Day	46% 7% 11% 21% 13% 2	2850
July	1st to 10th	Day	45% 7% 15% 22% 11% 1	4686
	11th to 20th	Day	36% 6% 15% 32% 10% 1	4592
	21st to end	Day	33% 6% 11% 33% 17%	4640
August	1st to 10th	Day	39% 7% 13% 29% 11% 1	4091
	11th to 20th	Day	41% 6% 17% 25% 10%	4083
	21st to end	Day	40% 7% 14% 29% 10% 1	4531
September	1st to 10th	Day	42% 7% 14% 24% 12% 1	2252
	11th to 20th	Day	40% 6% 16% 27% 11%	2090
	21st to end	Day	48% 5% 15% 23% 9%	2090
October	1st to 10th	Day	48% 7% 12% 23% 10%	1976
	11th to 20th	Day	59% 9% 10% 16% 5%	1831
	21st to end	Day	61% 8% 12% 11% 7% 1	1792
November	1st to 10th	Day	56% 7% 13% 17% 7%	1653
	11th to 20th	Day	60% 8% 13% 14% 6%	1520
	21st to end	Day	69% 5% 12% 10% 5%	1520
December	1st to 10th	Day	62% 7% 10% 14% 6% 1	1378
	11th to 20th	Day	64% 6% 10% 13% 5% 1	1671
	21st to end	Day	72% 5% 7% 11% 6%	2431
January	1st to 10th	Night	60% 8% 12% 12% 7% 1	4737
	11th to 20th	Night	62% 7% 12% 13% 5%	4440
	21st to end	Night	69% 6% 7% 12% 6%	4845
February	1st to 10th	Night	59% 6% 10% 16% 8% 1	2470
	11th to 20th	Night	56% 7% 13% 17% 8% 1	2397
	21st to end	Night	56% 7% 11% 16% 9% 1	1884
March	1st to 10th	Night	46% 7% 12% 20% 15% 1	3647
	11th to 20th	Night	45% 8% 14% 22% 10% 2	3480
	21st to end	Night	47% 6% 13% 18% 14% 2	3817
April	1st to 10th	Night	50% 7% 10% 20% 13% 1	1935
	11th to 20th	Night	46% 5% 7% 25% 15% 2	1687
	21st to end	Night	50% 5% 9% 19% 14% 4	1710
May	1st to 10th	Night	44% 7% 9% 26% 13% 2	2840
	11th to 20th	Night	41% 7% 10% 26% 14% 1	2660
	21st to end	Night	39% 9% 12% 21% 13% 6%	2419
June	1st to 10th	Night	39% 7% 15% 23% 15% 2	1330
	11th to 20th	Night	39% 7% 13% 22% 17% 2	1330
	21st to end	Night	42% 8% 14% 22% 13% 2	1330
July	1st to 10th	Night	40% 6% 14% 25% 14% 2	2175
	11th to 20th	Night	32% 5% 15% 34% 14%	2253
	21st to end	Night	33% 6% 12% 30% 20%	2682
August	1st to 10th	Night	38% 6% 15% 28% 12% 1	2814
	11th to 20th	Night	36% 9% 19% 26% 10% 1	2812
	21st to end	Night	33% 5% 14% 32% 16% 1	3124
September	1st to 10th	Night	38% 7% 15% 25% 12% 2	1906
	11th to 20th	Night	32% 7% 19% 32% 10%	2090
	21st to end	Night	45% 7% 15% 25% 9%	2090
October	1st to 10th	Night	47% 6% 12% 24% 11%	2204
	11th to 20th	Night	55% 6% 14% 20% 5%	2207
	21st to end	Night	57% 6% 12% 16% 7% 1	2560
November	1st to 10th	Night	51% 7% 14% 20% 8%	2423
	11th to 20th	Night	60% 7% 12% 14% 7%	2660
	21st to end	Night	63% 7% 11% 12% 6% 1	2660
December	1st to 10th	Night	61% 6% 12% 17% 4	2797
	11th to 20th	Night	60% 7% 11% 12% 7% 2	3525
	21st to end	Night	71% 5% 8% 11% 5%	5236

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	24% 21% 31% 18% 6% 1	184
	11th to 20th	Day	19% 15% 36% 23% 6% 1	270
	21st to end	Day	22% 16% 35% 20% 7% 1	297
February	1st to 10th	Day	21% 26% 35% 13% 5%	270
	11th to 20th	Day	20% 24% 40% 12% 4	270
	21st to end	Day	25% 27% 39% 8%	225
March	1st to 10th	Day	20% 24% 40% 13% 3 1	317
	11th to 20th	Day	20% 16% 38% 20% 5% 1	460
	21st to end	Day	25% 17% 34% 18% 5% 1	506
April	1st to 10th	Day	21% 16% 36% 21% 6% 1	460
	11th to 20th	Day	30% 19% 23% 17% 8% 2	452
	21st to end	Day	29% 14% 30% 19% 7% 2	460
May	1st to 10th	Day	21% 15% 35% 20% 6% 3	463
	11th to 20th	Day	17% 15% 32% 22% 10% 3	487
	21st to end	Day	17% 14% 36% 23% 8% 2	715
June	1st to 10th	Day	17% 15% 38% 23% 6% 1	680
	11th to 20th	Day	19% 14% 36% 22% 8% 1	680
	21st to end	Day	21% 14% 33% 22% 8% 3	680
July	1st to 10th	Day	11% 11% 38% 29% 8% 2	671
	11th to 20th	Day	13% 9% 34% 29% 10% 4	668
	21st to end	Day	15% 8% 33% 28% 11% 5%	729
August	1st to 10th	Day	14% 9% 27% 29% 16% 5%	486
	11th to 20th	Day	15% 6% 25% 28% 16% 10%	479
	21st to end	Day	17% 8% 28% 28% 13% 6%	539
September	1st to 10th	Day	17% 12% 27% 26% 12% 5%	490
	11th to 20th	Day	17% 8% 26% 32% 12% 5%	490
	21st to end	Day	23% 12% 27% 24% 12% 3	490
October	1st to 10th	Day	27% 15% 25% 20% 10% 2	490
	11th to 20th	Day	27% 17% 34% 18% 4 1	462
	21st to end	Day	28% 20% 34% 17% 1	307
November	1st to 10th	Day	28% 20% 33% 17% 3	261
	11th to 20th	Day	25% 22% 35% 15% 3	180
	21st to end	Day	31% 22% 29% 14% 3	180
December	1st to 10th	Day	22% 16% 33% 22% 7% 1	180
	11th to 20th	Day	29% 23% 32% 12% 3	180
	21st to end	Day	32% 19% 37% 9% 3 1	198
January	1st to 10th	Night	24% 13% 35% 23% 5%	950
	11th to 20th	Night	23% 13% 32% 24% 7% 1	950
	21st to end	Night	21% 9% 32% 27% 10%	1019
February	1st to 10th	Night	18% 14% 41% 23% 4	760
	11th to 20th	Night	19% 14% 36% 23% 7%	743
	21st to end	Night	18% 16% 37% 25% 4 1	628
March	1st to 10th	Night	16% 14% 40% 26% 4 1	760
	11th to 20th	Night	19% 13% 37% 23% 6% 1	760
	21st to end	Night	19% 12% 36% 26% 6% 1	836
April	1st to 10th	Night	23% 12% 33% 26% 5%	652
	11th to 20th	Night	25% 9% 31% 25% 9% 1	563
	21st to end	Night	21% 13% 29% 28% 8% 1	570
May	1st to 10th	Night	17% 9% 30% 34% 10% 1	570
	11th to 20th	Night	18% 8% 29% 35% 9% 1	473
	21st to end	Night	16% 8% 34% 32% 11%	411
June	1st to 10th	Night	13% 7% 42% 31% 7%	380
	11th to 20th	Night	15% 7% 32% 37% 9%	380
	21st to end	Night	15% 7% 26% 39% 12% 1	380
July	1st to 10th	Night	11% 7% 28% 45% 9%	370
	11th to 20th	Night	12% 5% 26% 40% 17% 1	414
	21st to end	Night	14% 4 25% 39% 17% 2	624
August	1st to 10th	Night	12% 2 27% 39% 19% 1	567
	11th to 20th	Night	14% 3 21% 33% 26% 2	564
	21st to end	Night	17% 6% 22% 37% 17% 1	627
September	1st to 10th	Night	18% 7% 26% 28% 19% 3	674
	11th to 20th	Night	15% 6% 27% 30% 19% 3	760
	21st to end	Night	21% 7% 25% 31% 14% 2	760
October	1st to 10th	Night	24% 8% 30% 24% 11% 2	760
	11th to 20th	Night	29% 12% 28% 24% 6%	735
	21st to end	Night	30% 14% 28% 23% 4 1	792
November	1st to 10th	Night	23% 13% 31% 25% 7% 1	740
	11th to 20th	Night	25% 13% 33% 24% 5%	760
	21st to end	Night	30% 14% 31% 19% 5% 1	760
December	1st to 10th	Night	25% 15% 34% 20% 6%	897
	11th to 20th	Night	27% 16% 32% 19% 4 1	950
	21st to end	Night	31% 14% 32% 18% 5%	1045

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