Easlick Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Easlick Observatory clear sky chart from 2005-11-06 to 2026-11-13.

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	12% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	153277
	Night	12% 8% 3 7% 4 4 4 3 5% 7% 42%	147825
Transparency	Day	42% 4 11% 23% 16% 3	156954
	Night	41% 4 10% 23% 18% 4	152208
Seeing	Day	16% 10% 23% 23% 18% 8%	17950
	Night	17% 9% 24% 29% 18% 3	27594

By Month

Percentage Cloud Cover SE (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	15% 11% 8% 10% 4 5% 4 3 4 4 31%	4691
	11th to 20th	Day	13% 8% 3 10% 4 2 4 2 3 5% 47%	4855
	21st to end	Day	21% 9% 5% 6% 4 4 4 3 4 5% 35%	6256
February	1st to 10th	Day	17% 11% 4 8% 5% 4 4 3 4 7% 33%	2100
	11th to 20th	Day	21% 10% 4 7% 4 4 3 3 4 6% 33%	2048
	21st to end	Day	18% 9% 3 7% 5% 3 5% 3 4 6% 37%	1730
March	1st to 10th	Day	16% 13% 4 8% 5% 4 4 3 4 5% 35%	4580
	11th to 20th	Day	12% 9% 3 6% 4 3 3 3 5% 5% 46%	4873
	21st to end	Day	11% 8% 3 7% 5% 5% 4 3 5% 7% 43%	5897
April	1st to 10th	Day	13% 7% 2 6% 5% 4 4 4 5% 8% 44%	2520
	11th to 20th	Day	13% 7% 3 6% 5% 3 4 4 5% 7% 43%	2494
	21st to end	Day	13% 8% 3 6% 4 6% 4 4 5% 7% 40%	2582
May	1st to 10th	Day	16% 9% 3 7% 4 5% 5% 4 6% 8% 32%	7616
	11th to 20th	Day	13% 7% 2 8% 5% 5% 5% 4 6% 8% 37%	7626
	21st to end	Day	17% 10% 4 9% 6% 5% 5% 4 5% 7% 30%	8391
June	1st to 10th	Day	10% 6% 3 6% 5% 5% 5% 6% 7% 8% 41%	2768
	11th to 20th	Day	6% 4 2 5% 5% 5% 7% 4 7% 11% 44%	2940
	21st to end	Day	5% 4 2 5% 5% 6% 6% 6% 8% 12% 42%	2940
July	1st to 10th	Day	7% 6% 3 8% 6% 6% 6% 6% 8% 11% 34%	8280
	11th to 20th	Day	7% 6% 3 8% 6% 7% 7% 6% 9% 13% 29%	8166
	21st to end	Day	10% 7% 3 7% 5% 5% 5% 5% 8% 13% 31%	8340
August	1st to 10th	Day	6% 4 2 6% 4 5% 6% 5% 8% 10% 44%	5788
	11th to 20th	Day	6% 4 2 6% 4 5% 6% 5% 7% 11% 44%	5486
	21st to end	Day	4 3 1 4 3 4 4 4 7% 10% 55%	6080
September	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 8% 41%	2488
	11th to 20th	Day	12% 6% 3 5% 5% 5% 4 3 5% 6% 46%	2310
	21st to end	Day	11% 6% 2 6% 5% 4 5% 3 4 6% 49%	2310
October	1st to 10th	Day	13% 5% 2 5% 4 3 3 2 4 5% 54%	3700
	11th to 20th	Day	10% 5% 2 4 3 3 3 3 4 7% 56%	3625
	21st to end	Day	12% 6% 2 4 3 3 3 2 4 5% 56%	3322
November	1st to 10th	Day	11% 6% 2 6% 3 3 4 2 4 6% 52%	1942
	11th to 20th	Day	13% 8% 2 5% 3 4 4 3 4 7% 47%	1908
	21st to end	Day	14% 9% 3 7% 3 3 3 3 5% 5% 44%	1890
December	1st to 10th	Day	15% 8% 3 7% 5% 4 4 3 4 5% 42%	3020
	11th to 20th	Day	15% 11% 5% 9% 5% 4 4 3 5% 7% 32%	3463
	21st to end	Day	15% 13% 5% 8% 6% 5% 5% 4 6% 7% 27%	4252
January	1st to 10th	Night	15% 11% 4 9% 4 4 3 3 5% 6% 37%	8232
	11th to 20th	Night	14% 10% 3 8% 5% 4 3 2 4 4 42%	8121
	21st to end	Night	18% 11% 5% 8% 4 4 3 2 3 4 37%	7907
February	1st to 10th	Night	20% 10% 4 9% 5% 4 4 3 4 5% 32%	2520
	11th to 20th	Night	22% 9% 4 8% 4 3 4 2 4 5% 35%	2458
	21st to end	Night	21% 9% 3 8% 4 3 4 4 4 5% 34%	2076
March	1st to 10th	Night	17% 11% 3 8% 4 4 4 3 5% 6% 35%	5493
	11th to 20th	Night	11% 6% 2 5% 4 3 4 4 5% 6% 50%	5292
	21st to end	Night	12% 8% 3 7% 5% 5% 4 4 5% 7% 40%	5317
April	1st to 10th	Night	12% 5% 3 6% 5% 4 4 4 5% 8% 46%	2100
	11th to 20th	Night	14% 7% 2 5% 4 3 4 3 5% 7% 46%	2075
	21st to end	Night	13% 8% 3 6% 4 4 4 3 6% 9% 41%	2037
May	1st to 10th	Night	13% 10% 3 9% 5% 4 4 4 6% 7% 34%	5237
	11th to 20th	Night	10% 8% 3 8% 5% 5% 5% 4 6% 9% 36%	5234
	21st to end	Night	15% 10% 3 9% 5% 5% 5% 4 6% 8% 30%	5761
June	1st to 10th	Night	8% 6% 2 6% 4 5% 4 4 7% 10% 45%	1845
	11th to 20th	Night	4 4 1 4 4 4 5% 4 6% 11% 53%	1680
	21st to end	Night	4 3 1 3 4 5% 6% 5% 8% 13% 48%	1680
July	1st to 10th	Night	6% 5% 2 7% 5% 5% 5% 5% 7% 11% 42%	4708
	11th to 20th	Night	5% 6% 3 7% 5% 6% 5% 5% 9% 13% 36%	4798
	21st to end	Night	7% 6% 3 8% 6% 5% 6% 5% 8% 12% 34%	5736
August	1st to 10th	Night	5% 4 2 6% 4 5% 4 4 6% 8% 52%	4343
	11th to 20th	Night	3 3 1 4 3 4 5% 4 7% 11% 55%	4560
	21st to end	Night	3 2 1 4 3 3 4 3 5% 9% 63%	5046
September	1st to 10th	Night	9% 5% 2 5% 4 4 5% 4 5% 7% 50%	2110
	11th to 20th	Night	9% 4 2 6% 4 4 4 3 6% 7% 50%	2310
	21st to end	Night	10% 6% 2 5% 4 3 3 3 5% 6% 52%	2310
October	1st to 10th	Night	10% 5% 2 4 2 3 3 2 4 5% 59%	3700
	11th to 20th	Night	12% 6% 2 5% 3 3 2 2 3 5% 58%	3768
	21st to end	Night	10% 5% 2 5% 3 3 2 2 4 5% 58%	4817
November	1st to 10th	Night	13% 5% 2 4 3 4 4 3 5% 6% 52%	2796
	11th to 20th	Night	12% 7% 2 5% 4 3 4 3 5% 7% 49%	2757
	21st to end	Night	16% 8% 3 6% 4 3 4 3 4 5% 45%	2730
December	1st to 10th	Night	17% 11% 3 8% 4 3 3 3 4 4 39%	4380
	11th to 20th	Night	17% 10% 4 7% 5% 4 4 3 4 5% 38%	4952
	21st to end	Night	15% 13% 6% 12% 5% 5% 5% 3 4 5% 26%	6939

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	56% 3 10% 20% 9% 1	4692
	11th to 20th	Day	42% 2 4 27% 23% 2	4855
	21st to end	Day	52% 2 6% 22% 14% 4	6256
February	1st to 10th	Day	52% 4 8% 21% 13% 2	2100
	11th to 20th	Day	53% 3 7% 20% 14% 3	2058
	21st to end	Day	48% 4 8% 19% 16% 6%	1730
March	1st to 10th	Day	53% 3 4 15% 18% 7%	4580
	11th to 20th	Day	40% 4 8% 23% 19% 6%	4873
	21st to end	Day	42% 3 9% 29% 15% 1	5899
April	1st to 10th	Day	40% 5% 11% 23% 19% 3	2520
	11th to 20th	Day	41% 4 8% 23% 20% 4	2494
	21st to end	Day	43% 5% 9% 26% 15% 2	2582
May	1st to 10th	Day	49% 4 14% 21% 11% 1	7651
	11th to 20th	Day	47% 4 13% 22% 14% 1	7630
	21st to end	Day	55% 5% 9% 18% 12% 2	8405
June	1st to 10th	Day	39% 6% 11% 24% 17% 3	2768
	11th to 20th	Day	32% 7% 16% 24% 18% 3	2940
	21st to end	Day	31% 9% 16% 25% 15% 4	2940
July	1st to 10th	Day	41% 5% 18% 25% 10% 1	8211
	11th to 20th	Day	44% 7% 21% 22% 5% 1	8091
	21st to end	Day	43% 4 22% 22% 7% 1	8156
August	1st to 10th	Day	32% 7% 15% 29% 17% 1	5775
	11th to 20th	Day	33% 4 17% 30% 15% 1	5522
	21st to end	Day	23% 5% 11% 37% 22% 2	6079
September	1st to 10th	Day	40% 6% 11% 21% 19% 3	2488
	11th to 20th	Day	37% 6% 9% 25% 21% 2	2310
	21st to end	Day	37% 4 9% 23% 23% 4	2310
October	1st to 10th	Day	26% 3 4 19% 36% 12%	5089
	11th to 20th	Day	24% 2 6% 21% 30% 17%	4958
	21st to end	Day	28% 2 4 28% 24% 14%	4520
November	1st to 10th	Day	34% 4 9% 24% 25% 5%	1942
	11th to 20th	Day	37% 2 9% 24% 22% 6%	1908
	21st to end	Day	42% 3 7% 25% 19% 4	1890
December	1st to 10th	Day	45% 3 6% 26% 19% 1	3020
	11th to 20th	Day	52% 3 8% 21% 13% 2	3460
	21st to end	Day	56% 5% 11% 21% 7% 1	4252
January	1st to 10th	Night	49% 3 12% 23% 12% 1	8233
	11th to 20th	Night	46% 2 4 22% 22% 3	8121
	21st to end	Night	52% 2 7% 19% 15% 5%	7907
February	1st to 10th	Night	55% 3 8% 18% 14% 1	2520
	11th to 20th	Night	53% 3 6% 20% 15% 4	2459
	21st to end	Night	54% 4 6% 16% 17% 4	2076
March	1st to 10th	Night	49% 4 7% 15% 18% 6%	5493
	11th to 20th	Night	35% 3 9% 22% 20% 12%	5292
	21st to end	Night	44% 3 9% 26% 15% 2	5317
April	1st to 10th	Night	37% 5% 11% 25% 20% 3	2100
	11th to 20th	Night	37% 3 11% 20% 22% 6%	2075
	21st to end	Night	41% 4 10% 27% 17% 1	2037
May	1st to 10th	Night	50% 4 12% 21% 12% 1	5264
	11th to 20th	Night	46% 5% 14% 17% 17% 1	5235
	21st to end	Night	52% 5% 13% 18% 11% 1	5771
June	1st to 10th	Night	35% 6% 13% 26% 16% 4	1845
	11th to 20th	Night	25% 5% 16% 30% 21% 2	1680
	21st to end	Night	26% 8% 17% 28% 15% 5%	1680
July	1st to 10th	Night	36% 6% 18% 27% 12% 1	4670
	11th to 20th	Night	39% 7% 24% 23% 6% 1	4753
	21st to end	Night	41% 5% 24% 22% 7% 1	5606
August	1st to 10th	Night	29% 6% 14% 31% 20% 1	4332
	11th to 20th	Night	23% 4 21% 37% 15% 1	4590
	21st to end	Night	20% 4 10% 38% 25% 2	5046
September	1st to 10th	Night	33% 6% 10% 25% 21% 5%	2110
	11th to 20th	Night	32% 6% 10% 24% 24% 3	2310
	21st to end	Night	33% 5% 8% 22% 25% 7%	2310
October	1st to 10th	Night	24% 3 4 17% 38% 14%	5090
	11th to 20th	Night	23% 2 4 18% 36% 17%	5163
	21st to end	Night	25% 2 5% 25% 25% 18%	6567
November	1st to 10th	Night	35% 3 7% 24% 24% 7%	2796
	11th to 20th	Night	35% 3 6% 24% 24% 7%	2757
	21st to end	Night	42% 4 7% 25% 19% 4	2730
December	1st to 10th	Night	49% 2 7% 19% 22% 1	4380
	11th to 20th	Night	49% 3 6% 23% 18% 2	4954
	21st to end	Night	59% 4 8% 19% 8% 1	6939

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% 35% 14% 6% 1	330
	11th to 20th	Day	30% 25% 29% 13% 2 1	330
	21st to end	Day	23% 21% 43% 10% 2	363
February	1st to 10th	Day	24% 21% 35% 16% 2 1	330
	11th to 20th	Day	37% 24% 28% 8% 3	330
	21st to end	Day	32% 19% 37% 11% 1	273
March	1st to 10th	Day	28% 21% 33% 15% 4	330
	11th to 20th	Day	25% 22% 31% 15% 6%	440
	21st to end	Day	22% 19% 37% 16% 6% 1	594
April	1st to 10th	Day	22% 16% 28% 26% 7% 1	540
	11th to 20th	Day	19% 17% 30% 23% 10% 2	532
	21st to end	Day	19% 16% 31% 21% 10% 3	540
May	1st to 10th	Day	18% 17% 30% 21% 11% 3	543
	11th to 20th	Day	17% 7% 29% 23% 19% 6%	567
	21st to end	Day	14% 6% 21% 27% 20% 12%	617
June	1st to 10th	Day	13% 6% 19% 24% 24% 14%	608
	11th to 20th	Day	7% 4 20% 27% 29% 13%	780
	21st to end	Day	6% 2 19% 33% 26% 15%	780
July	1st to 10th	Day	9% 3 12% 28% 31% 18%	771
	11th to 20th	Day	8% 3 11% 26% 35% 18%	732
	21st to end	Day	10% 3 12% 27% 28% 20%	620
August	1st to 10th	Day	9% 2 11% 28% 31% 19%	566
	11th to 20th	Day	9% 3 13% 31% 32% 12%	559
	21st to end	Day	10% 2 11% 22% 34% 21%	627
September	1st to 10th	Day	11% 4 13% 26% 31% 15%	570
	11th to 20th	Day	12% 2 11% 25% 35% 16%	570
	21st to end	Day	16% 6% 17% 29% 25% 8%	570
October	1st to 10th	Day	14% 3 21% 31% 25% 7%	570
	11th to 20th	Day	13% 13% 26% 31% 15% 2	523
	21st to end	Day	22% 12% 27% 28% 10% 2	396
November	1st to 10th	Day	18% 10% 33% 24% 14% 1	360
	11th to 20th	Day	17% 21% 34% 24% 5%	336
	21st to end	Day	24% 15% 31% 20% 9% 2	330
December	1st to 10th	Day	24% 21% 30% 20% 5% 1	330
	11th to 20th	Day	24% 17% 32% 18% 8%	330
	21st to end	Day	26% 21% 31% 18% 4	363
January	1st to 10th	Night	23% 17% 30% 26% 4	1050
	11th to 20th	Night	28% 16% 31% 20% 5%	995
	21st to end	Night	22% 15% 38% 23% 2	924
February	1st to 10th	Night	25% 14% 35% 21% 4	840
	11th to 20th	Night	29% 15% 34% 20% 3	823
	21st to end	Night	27% 17% 33% 22% 1	692
March	1st to 10th	Night	23% 13% 33% 25% 5%	840
	11th to 20th	Night	22% 13% 33% 25% 6% 1	840
	21st to end	Night	19% 14% 34% 27% 6%	808
April	1st to 10th	Night	15% 13% 34% 30% 9%	630
	11th to 20th	Night	17% 10% 34% 27% 11%	623
	21st to end	Night	17% 9% 32% 30% 11% 1	630
May	1st to 10th	Night	12% 6% 34% 31% 15% 1	630
	11th to 20th	Night	12% 6% 25% 33% 19% 4	621
	21st to end	Night	8% 4 18% 35% 30% 4	683
June	1st to 10th	Night	11% 3 17% 39% 27% 3	630
	11th to 20th	Night	6% 1 12% 41% 35% 5%	630
	21st to end	Night	5% 2 11% 40% 37% 6%	630
July	1st to 10th	Night	10% 3 7% 31% 40% 10%	615
	11th to 20th	Night	6% 2 7% 30% 45% 10%	612
	21st to end	Night	9% 1 10% 30% 41% 9%	690
August	1st to 10th	Night	8% 2 7% 32% 44% 7%	627
	11th to 20th	Night	6% 2 9% 38% 39% 6%	624
	21st to end	Night	8% 1 8% 31% 44% 8%	693
September	1st to 10th	Night	9% 2 12% 34% 38% 6%	640
	11th to 20th	Night	11% 3 13% 29% 38% 6%	840
	21st to end	Night	12% 3 18% 30% 33% 4	840
October	1st to 10th	Night	10% 4 19% 34% 28% 5%	840
	11th to 20th	Night	18% 7% 20% 37% 17% 1	840
	21st to end	Night	17% 8% 29% 30% 16% 1	924
November	1st to 10th	Night	15% 7% 24% 35% 17% 2	860
	11th to 20th	Night	16% 11% 33% 30% 9% 1	849
	21st to end	Night	21% 12% 26% 30% 10% 1	840
December	1st to 10th	Night	25% 15% 28% 26% 7%	840
	11th to 20th	Night	24% 10% 31% 28% 6% 1	840
	21st to end	Night	27% 13% 30% 26% 4	1061

Page generated at 2026-11-10 02:55:23. Version ac07

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