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 2025-07-21.

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	13% 8% 3 7% 5% 4 5% 4 6% 7% 39%	120121
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	119052
Transparency	Day	42% 5% 10% 23% 17% 3	123841
	Night	41% 4 9% 23% 18% 4	123458
Seeing	Day	16% 10% 23% 23% 19% 8%	16163
	Night	17% 9% 24% 29% 18% 3	25869

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	14% 11% 9% 11% 4 4 4 3 4 4 30%	3617
	11th to 20th	Day	15% 9% 4 12% 4 3 4 2 3 5% 40%	3726
	21st to end	Day	19% 9% 5% 5% 3 3 4 3 5% 6% 37%	4834
February	1st to 10th	Day	18% 11% 4 8% 5% 4 4 3 4 7% 33%	2000
	11th to 20th	Day	21% 10% 4 7% 4 4 3 3 4 6% 33%	1948
	21st to end	Day	18% 10% 3 6% 4 3 5% 3 4 6% 37%	1650
March	1st to 10th	Day	16% 13% 4 8% 5% 4 4 3 4 5% 35%	4480
	11th to 20th	Day	13% 9% 3 6% 4 3 3 3 5% 5% 46%	4768
	21st to end	Day	12% 8% 3 7% 5% 5% 4 3 5% 7% 43%	5770
April	1st to 10th	Day	13% 7% 2 7% 5% 4 4 4 5% 7% 43%	2400
	11th to 20th	Day	13% 7% 3 7% 5% 3 4 4 5% 6% 43%	2374
	21st to end	Day	13% 8% 3 6% 5% 6% 4 4 5% 7% 39%	2459
May	1st to 10th	Day	18% 10% 3 8% 5% 5% 5% 4 7% 8% 27%	5862
	11th to 20th	Day	15% 8% 3 10% 6% 5% 5% 4 6% 7% 31%	5861
	21st to end	Day	13% 8% 3 7% 5% 5% 4 4 5% 7% 38%	6433
June	1st to 10th	Day	10% 6% 3 6% 5% 5% 5% 6% 7% 7% 41%	2636
	11th to 20th	Day	6% 4 2 6% 5% 5% 7% 4 7% 11% 43%	2800
	21st to end	Day	5% 4 2 5% 5% 6% 7% 6% 8% 12% 42%	2800
July	1st to 10th	Day	7% 6% 2 7% 6% 6% 6% 5% 8% 11% 37%	4580
	11th to 20th	Day	8% 6% 2 6% 5% 6% 5% 5% 8% 12% 38%	4520
	21st to end	Day	17% 10% 4 9% 7% 6% 5% 5% 6% 8% 24%	4466
August	1st to 10th	Day	4 4 2 5% 3 4 5% 5% 7% 10% 53%	3953
	11th to 20th	Day	7% 6% 2 7% 5% 5% 6% 5% 8% 10% 38%	3729
	21st to end	Day	5% 3 2 5% 4 4 4 4 7% 9% 51%	4180
September	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 7% 8% 40%	2252
	11th to 20th	Day	12% 6% 3 5% 5% 5% 4 3 5% 6% 45%	2090
	21st to end	Day	11% 6% 2 6% 5% 3 4 3 4 6% 49%	2090
October	1st to 10th	Day	10% 4 1 4 4 4 4 3 5% 6% 56%	2090
	11th to 20th	Day	12% 5% 2 5% 3 4 3 3 4 7% 52%	2047
	21st to end	Day	17% 6% 2 5% 4 4 3 3 4 6% 46%	1881
November	1st to 10th	Day	12% 6% 2 5% 3 3 4 2 4 7% 53%	1762
	11th to 20th	Day	14% 8% 3 5% 3 4 4 3 4 7% 47%	1800
	21st to end	Day	15% 9% 3 6% 3 3 3 3 5% 5% 44%	1800
December	1st to 10th	Day	15% 8% 2 6% 5% 4 4 3 4 5% 43%	2930
	11th to 20th	Day	15% 11% 5% 9% 5% 4 4 3 5% 7% 32%	3373
	21st to end	Day	15% 13% 5% 8% 6% 5% 5% 4 6% 7% 27%	4160
January	1st to 10th	Night	15% 13% 4 11% 4 4 3 3 5% 6% 34%	6348
	11th to 20th	Night	16% 11% 3 9% 5% 4 3 2 3 4 39%	6243
	21st to end	Night	18% 10% 4 7% 4 4 3 2 3 4 40%	6104
February	1st to 10th	Night	21% 11% 4 8% 5% 4 4 3 4 5% 32%	2400
	11th to 20th	Night	22% 9% 4 8% 4 3 4 2 4 5% 35%	2338
	21st to end	Night	21% 9% 3 7% 4 4 4 4 4 5% 34%	1980
March	1st to 10th	Night	17% 11% 3 9% 4 4 4 3 5% 6% 35%	5373
	11th to 20th	Night	11% 6% 2 5% 4 3 4 4 6% 6% 50%	5177
	21st to end	Night	12% 8% 3 7% 5% 5% 4 3 5% 7% 40%	5202
April	1st to 10th	Night	12% 6% 3 6% 5% 4 4 4 5% 7% 46%	2000
	11th to 20th	Night	15% 7% 3 5% 4 3 4 3 5% 6% 46%	1975
	21st to end	Night	13% 8% 3 6% 4 4 4 3 6% 9% 40%	1940
May	1st to 10th	Night	13% 11% 3 9% 6% 5% 5% 4 7% 7% 31%	4033
	11th to 20th	Night	12% 10% 4 9% 5% 5% 5% 4 6% 9% 31%	4032
	21st to end	Night	12% 8% 3 7% 4 5% 5% 4 6% 8% 37%	4421
June	1st to 10th	Night	9% 6% 2 6% 4 5% 4 4 7% 9% 45%	1757
	11th to 20th	Night	4 4 1 4 4 4 5% 4 6% 11% 53%	1600
	21st to end	Night	3 3 1 3 4 5% 6% 5% 7% 13% 49%	1600
July	1st to 10th	Night	5% 5% 2 7% 5% 5% 5% 5% 7% 12% 42%	2610
	11th to 20th	Night	5% 6% 2 6% 4 4 4 3 7% 10% 49%	2664
	21st to end	Night	10% 8% 5% 10% 7% 6% 6% 5% 7% 8% 28%	3087
August	1st to 10th	Night	4 3 1 4 3 4 3 3 5% 7% 63%	2972
	11th to 20th	Night	4 4 2 5% 4 5% 6% 4 8% 11% 48%	3100
	21st to end	Night	4 3 1 4 4 3 4 3 5% 9% 60%	3476
September	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 3 5% 7% 49%	1909
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 6% 7% 49%	2090
	21st to end	Night	10% 5% 2 5% 4 3 3 3 5% 6% 53%	2090
October	1st to 10th	Night	8% 4 2 4 2 3 3 2 5% 5% 62%	2090
	11th to 20th	Night	15% 6% 2 5% 3 3 3 2 4 6% 52%	2128
	21st to end	Night	15% 6% 2 5% 4 3 3 3 4 6% 49%	2716
November	1st to 10th	Night	13% 5% 1 4 3 4 4 3 5% 6% 52%	2536
	11th to 20th	Night	13% 7% 2 5% 4 3 4 3 5% 7% 49%	2600
	21st to end	Night	16% 8% 3 6% 4 3 4 3 4 5% 46%	2600
December	1st to 10th	Night	17% 11% 3 8% 4 3 3 3 4 4 40%	4250
	11th to 20th	Night	17% 10% 4 7% 5% 4 4 3 4 5% 38%	4822
	21st to end	Night	15% 13% 6% 11% 6% 5% 5% 3 4 5% 26%	6789

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	57% 3 9% 18% 11% 2	3618
	11th to 20th	Day	48% 3 5% 23% 19% 1	3726
	21st to end	Day	48% 3 6% 22% 17% 4	4834
February	1st to 10th	Day	52% 4 8% 21% 13% 2	2000
	11th to 20th	Day	54% 3 6% 19% 14% 3	1958
	21st to end	Day	48% 4 8% 19% 16% 6%	1650
March	1st to 10th	Day	53% 3 4 15% 18% 7%	4480
	11th to 20th	Day	40% 4 8% 23% 19% 6%	4768
	21st to end	Day	42% 3 9% 29% 15% 1	5772
April	1st to 10th	Day	40% 5% 10% 23% 19% 3	2400
	11th to 20th	Day	41% 4 8% 22% 21% 4	2374
	21st to end	Day	44% 5% 9% 26% 14% 2	2459
May	1st to 10th	Day	53% 5% 14% 18% 9% 1	5897
	11th to 20th	Day	52% 4 13% 20% 10%	5865
	21st to end	Day	45% 5% 9% 22% 16% 3	6447
June	1st to 10th	Day	39% 6% 11% 24% 17% 3	2636
	11th to 20th	Day	32% 7% 16% 23% 18% 3	2800
	21st to end	Day	31% 10% 16% 23% 16% 4	2800
July	1st to 10th	Day	39% 6% 19% 25% 10% 2	4511
	11th to 20th	Day	37% 7% 15% 30% 9% 2	4445
	21st to end	Day	57% 6% 12% 18% 6% 1	4324
August	1st to 10th	Day	25% 7% 14% 33% 20% 1	3940
	11th to 20th	Day	37% 6% 13% 26% 17% 1	3765
	21st to end	Day	27% 6% 10% 36% 19% 2	4179
September	1st to 10th	Day	41% 6% 11% 20% 19% 4	2252
	11th to 20th	Day	39% 6% 9% 23% 21% 2	2090
	21st to end	Day	36% 4 10% 24% 22% 4	2090
October	1st to 10th	Day	19% 4 6% 18% 44% 9%	3480
	11th to 20th	Day	24% 2 8% 25% 28% 13%	3378
	21st to end	Day	31% 3 6% 25% 27% 8%	3081
November	1st to 10th	Day	32% 4 9% 24% 25% 5%	1762
	11th to 20th	Day	38% 3 8% 23% 22% 6%	1800
	21st to end	Day	42% 3 7% 25% 19% 4	1800
December	1st to 10th	Day	44% 3 6% 27% 19% 1	2930
	11th to 20th	Day	53% 3 7% 21% 13% 2	3370
	21st to end	Day	56% 5% 11% 21% 7% 1	4160
January	1st to 10th	Night	51% 3 9% 21% 14% 1	6348
	11th to 20th	Night	50% 3 4 20% 19% 4	6243
	21st to end	Night	49% 3 5% 21% 16% 5%	6104
February	1st to 10th	Night	55% 3 8% 18% 14% 1	2400
	11th to 20th	Night	53% 3 6% 19% 15% 4	2339
	21st to end	Night	54% 4 6% 15% 17% 4	1980
March	1st to 10th	Night	49% 5% 6% 15% 19% 6%	5373
	11th to 20th	Night	35% 3 9% 21% 20% 12%	5177
	21st to end	Night	44% 3 9% 26% 15% 2	5202
April	1st to 10th	Night	37% 5% 10% 25% 20% 3	2000
	11th to 20th	Night	38% 3 9% 21% 22% 7%	1975
	21st to end	Night	41% 4 10% 26% 17% 1	1940
May	1st to 10th	Night	52% 5% 12% 20% 10% 1	4060
	11th to 20th	Night	50% 6% 14% 17% 12%	4033
	21st to end	Night	44% 6% 14% 22% 14% 1	4429
June	1st to 10th	Night	35% 6% 13% 27% 16% 4	1757
	11th to 20th	Night	25% 6% 17% 30% 21% 2	1600
	21st to end	Night	26% 9% 17% 28% 16% 5%	1600
July	1st to 10th	Night	34% 6% 18% 30% 11% 2	2570
	11th to 20th	Night	30% 5% 17% 35% 11% 2	2616
	21st to end	Night	51% 7% 14% 19% 8% 1	2988
August	1st to 10th	Night	21% 6% 10% 39% 23% 1	2961
	11th to 20th	Night	28% 5% 19% 31% 16% 1	3130
	21st to end	Night	22% 5% 11% 36% 24% 3	3476
September	1st to 10th	Night	34% 6% 10% 24% 20% 6%	1909
	11th to 20th	Night	32% 6% 10% 23% 24% 4	2090
	21st to end	Night	32% 5% 8% 23% 24% 7%	2090
October	1st to 10th	Night	19% 3 5% 19% 42% 11%	3480
	11th to 20th	Night	21% 2 5% 24% 33% 14%	3523
	21st to end	Night	26% 2 7% 24% 28% 12%	4466
November	1st to 10th	Night	34% 4 7% 24% 25% 7%	2536
	11th to 20th	Night	36% 4 6% 23% 25% 7%	2600
	21st to end	Night	41% 4 7% 25% 19% 4	2600
December	1st to 10th	Night	49% 2 6% 19% 22% 1	4250
	11th to 20th	Night	49% 3 6% 23% 17% 2	4824
	21st to end	Night	59% 4 8% 19% 8% 1	6789

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% 34% 14% 6% 1	300
	11th to 20th	Day	30% 25% 28% 13% 3 1	300
	21st to end	Day	22% 21% 44% 11% 2	330
February	1st to 10th	Day	24% 22% 34% 16% 3 1	300
	11th to 20th	Day	37% 23% 28% 8% 3	300
	21st to end	Day	32% 19% 38% 10% 2	249
March	1st to 10th	Day	29% 20% 31% 15% 4	300
	11th to 20th	Day	25% 21% 31% 16% 6%	405
	21st to end	Day	23% 18% 35% 16% 7% 1	550
April	1st to 10th	Day	23% 16% 28% 24% 8% 1	500
	11th to 20th	Day	19% 17% 29% 23% 10% 2	492
	21st to end	Day	20% 15% 32% 20% 11% 3	500
May	1st to 10th	Day	18% 17% 28% 21% 12% 4	503
	11th to 20th	Day	18% 6% 27% 23% 19% 6%	527
	21st to end	Day	12% 6% 21% 28% 20% 13%	573
June	1st to 10th	Day	14% 6% 18% 23% 24% 15%	566
	11th to 20th	Day	7% 3 19% 27% 30% 14%	730
	21st to end	Day	6% 2 18% 33% 26% 15%	730
July	1st to 10th	Day	9% 3 12% 28% 30% 17%	721
	11th to 20th	Day	7% 2 11% 25% 35% 19%	684
	21st to end	Day	11% 2 12% 27% 27% 21%	532
August	1st to 10th	Day	9% 3 12% 27% 30% 20%	486
	11th to 20th	Day	9% 3 14% 31% 31% 11%	479
	21st to end	Day	11% 2 9% 20% 35% 22%	539
September	1st to 10th	Day	12% 3 12% 26% 32% 14%	490
	11th to 20th	Day	13% 2 12% 24% 33% 16%	490
	21st to end	Day	16% 6% 17% 26% 26% 8%	490
October	1st to 10th	Day	11% 3 21% 30% 26% 8%	490
	11th to 20th	Day	13% 15% 27% 28% 15% 3	447
	21st to end	Day	23% 13% 27% 27% 9% 2	330
November	1st to 10th	Day	18% 11% 31% 24% 15% 1	300
	11th to 20th	Day	17% 20% 33% 24% 5%	300
	21st to end	Day	23% 14% 30% 22% 10% 2	300
December	1st to 10th	Day	25% 20% 28% 21% 6% 1	300
	11th to 20th	Day	24% 18% 32% 18% 9%	300
	21st to end	Day	25% 21% 30% 20% 4	330
January	1st to 10th	Night	23% 17% 29% 26% 5%	1000
	11th to 20th	Night	28% 16% 30% 20% 5%	948
	21st to end	Night	22% 14% 38% 23% 2	880
February	1st to 10th	Night	26% 15% 34% 21% 4	800
	11th to 20th	Night	28% 16% 33% 20% 3	783
	21st to end	Night	27% 17% 33% 22% 1	660
March	1st to 10th	Night	23% 13% 33% 26% 5%	800
	11th to 20th	Night	22% 14% 33% 25% 6% 1	800
	21st to end	Night	19% 14% 33% 27% 6%	770
April	1st to 10th	Night	15% 13% 33% 30% 9% 1	600
	11th to 20th	Night	18% 10% 33% 28% 11% 1	593
	21st to end	Night	16% 9% 33% 29% 11% 2	600
May	1st to 10th	Night	12% 7% 34% 31% 16% 2	600
	11th to 20th	Night	12% 6% 24% 34% 20% 4	591
	21st to end	Night	7% 3 18% 36% 31% 5%	650
June	1st to 10th	Night	11% 3 17% 39% 28% 4	600
	11th to 20th	Night	6% 1 11% 40% 37% 5%	600
	21st to end	Night	5% 2 11% 39% 38% 6%	600
July	1st to 10th	Night	9% 2 7% 32% 40% 10%	585
	11th to 20th	Night	6% 2 7% 30% 45% 9%	582
	21st to end	Night	9% 1 10% 30% 42% 9%	624
August	1st to 10th	Night	8% 1 7% 31% 45% 8%	567
	11th to 20th	Night	6% 2 9% 38% 38% 5%	564
	21st to end	Night	8% 1 7% 29% 46% 9%	627
September	1st to 10th	Night	10% 2 12% 33% 38% 6%	579
	11th to 20th	Night	11% 3 14% 30% 36% 6%	760
	21st to end	Night	11% 4 18% 29% 34% 4	760
October	1st to 10th	Night	10% 4 19% 33% 29% 5%	760
	11th to 20th	Night	18% 7% 21% 35% 17% 2	760
	21st to end	Night	17% 8% 29% 28% 16% 1	836
November	1st to 10th	Night	15% 7% 24% 33% 18% 2	780
	11th to 20th	Night	17% 11% 32% 30% 9% 1	800
	21st to end	Night	20% 12% 27% 31% 10% 1	800
December	1st to 10th	Night	25% 15% 28% 26% 7%	800
	11th to 20th	Night	24% 10% 31% 28% 7% 1	800
	21st to end	Night	26% 12% 30% 27% 4	1010

Page generated at 2025-07-18 00:09:57. Version ac07

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