TWASC Eggshatch Observatorium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the TWASC Eggshatch Observatorium clear sky chart from 2005-11-06 to 2026-01-02.

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	13% 9% 3 8% 5% 4 4 4 6% 7% 38%	127368
	Night	13% 10% 4 8% 5% 4 4 4 5% 6% 39%	124239
Transparency	Day	43% 6% 12% 24% 13% 1	130854
	Night	44% 6% 12% 22% 15% 2	128827
Seeing	Day	16% 13% 31% 25% 12% 4	17474
	Night	17% 11% 32% 29% 11% 1	27387

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	8% 12% 5% 15% 8% 6% 5% 4 5% 5% 27%	3625
	11th to 20th	Day	8% 10% 7% 13% 5% 4 6% 7% 6% 6% 27%	3620
	21st to end	Day	13% 14% 7% 10% 4 3 3 3 4 6% 33%	4010
February	1st to 10th	Day	10% 13% 7% 9% 6% 5% 5% 4 5% 7% 29%	1800
	11th to 20th	Day	12% 12% 3 8% 5% 5% 5% 3 5% 5% 36%	1928
	21st to end	Day	11% 15% 4 10% 5% 6% 5% 3 5% 6% 30%	1650
March	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 5% 6% 43%	4480
	11th to 20th	Day	11% 7% 3 7% 5% 4 5% 4 6% 6% 43%	4690
	21st to end	Day	22% 13% 4 10% 4 3 4 3 4 5% 28%	5967
April	1st to 10th	Day	18% 13% 4 7% 5% 4 4 4 5% 7% 30%	2400
	11th to 20th	Day	16% 12% 3 7% 5% 4 4 3 5% 6% 34%	2374
	21st to end	Day	17% 11% 3 7% 4 5% 5% 4 5% 7% 32%	2529
May	1st to 10th	Day	14% 9% 2 6% 4 4 4 3 5% 7% 42%	5926
	11th to 20th	Day	13% 6% 3 7% 4 4 4 4 6% 8% 42%	6319
	21st to end	Day	16% 7% 2 6% 4 5% 4 4 6% 9% 39%	6935
June	1st to 10th	Day	10% 6% 2 6% 6% 4 4 4 6% 8% 44%	2800
	11th to 20th	Day	11% 6% 3 7% 4 5% 5% 4 6% 9% 42%	2800
	21st to end	Day	11% 6% 2 6% 4 4 5% 5% 6% 10% 42%	2800
July	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 7% 9% 39%	6360
	11th to 20th	Day	7% 5% 2 5% 4 5% 6% 4 7% 9% 46%	6360
	21st to end	Day	9% 6% 2 6% 4 5% 4 4 6% 10% 44%	6924
August	1st to 10th	Day	10% 5% 3 7% 4 5% 6% 5% 7% 9% 41%	4563
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 6% 7% 43%	3929
	21st to end	Day	11% 5% 3 6% 4 4 5% 4 6% 8% 44%	4312
September	1st to 10th	Day	11% 6% 2 7% 5% 4 4 4 5% 7% 44%	2400
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 6% 8% 49%	2290
	21st to end	Day	12% 8% 2 7% 4 4 4 4 5% 7% 43%	2200
October	1st to 10th	Day	13% 6% 3 7% 4 4 3 3 5% 7% 45%	2110
	11th to 20th	Day	18% 11% 3 8% 5% 4 5% 3 5% 7% 32%	1975
	21st to end	Day	16% 10% 4 7% 5% 4 4 4 5% 5% 37%	1980
November	1st to 10th	Day	11% 10% 3 7% 4 4 4 3 5% 6% 42%	1852
	11th to 20th	Day	11% 13% 3 10% 7% 5% 4 4 5% 6% 31%	1890
	21st to end	Day	16% 12% 4 10% 5% 3 3 3 5% 6% 33%	1722
December	1st to 10th	Day	7% 9% 5% 11% 6% 7% 5% 4 7% 8% 32%	2690
	11th to 20th	Day	11% 16% 5% 12% 6% 5% 5% 4 5% 7% 24%	3088
	21st to end	Day	18% 22% 8% 13% 5% 4 3 3 4 5% 15%	4070
January	1st to 10th	Night	9% 12% 5% 12% 7% 5% 3 3 4 5% 34%	6363
	11th to 20th	Night	10% 12% 6% 15% 6% 6% 6% 6% 6% 5% 23%	6360
	21st to end	Night	11% 13% 7% 12% 5% 5% 4 3 4 5% 30%	6953
February	1st to 10th	Night	11% 14% 4 9% 5% 5% 4 4 5% 5% 34%	2600
	11th to 20th	Night	11% 10% 3 9% 5% 5% 5% 3 4 5% 39%	2356
	21st to end	Night	15% 12% 4 8% 5% 4 3 3 5% 5% 35%	1980
March	1st to 10th	Night	15% 8% 2 6% 4 4 4 3 4 6% 44%	5373
	11th to 20th	Night	10% 7% 2 6% 4 4 4 4 6% 9% 45%	5312
	21st to end	Night	19% 13% 5% 9% 4 4 4 3 4 5% 31%	4962
April	1st to 10th	Night	17% 11% 3 7% 4 3 4 3 6% 6% 36%	2000
	11th to 20th	Night	20% 8% 3 6% 4 4 4 3 4 5% 37%	1975
	21st to end	Night	17% 8% 3 6% 4 4 4 4 5% 7% 38%	1870
May	1st to 10th	Night	15% 7% 3 7% 4 4 5% 3 5% 8% 40%	3955
	11th to 20th	Night	10% 5% 2 7% 5% 5% 6% 4 6% 6% 43%	3596
	21st to end	Night	16% 9% 3 6% 4 4 4 3 5% 6% 40%	3942
June	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 6% 8% 48%	1600
	11th to 20th	Night	12% 5% 2 6% 4 4 4 4 7% 9% 44%	1600
	21st to end	Night	10% 7% 2 6% 4 5% 4 4 5% 7% 47%	1600
July	1st to 10th	Night	12% 6% 1 6% 4 4 5% 3 6% 8% 45%	3618
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 5% 8% 53%	3617
	21st to end	Night	6% 5% 2 5% 4 4 4 4 6% 8% 51%	3947
August	1st to 10th	Night	10% 4 2 6% 4 4 4 4 6% 8% 49%	2597
	11th to 20th	Night	11% 7% 2 5% 3 3 4 4 6% 7% 47%	3116
	21st to end	Night	7% 5% 2 5% 4 4 5% 3 5% 9% 50%	3586
September	1st to 10th	Night	9% 6% 1 5% 3 3 3 3 5% 7% 55%	2000
	11th to 20th	Night	11% 4 2 5% 4 3 4 4 5% 7% 52%	2088
	21st to end	Night	12% 7% 2 6% 4 3 4 3 4 6% 49%	2200
October	1st to 10th	Night	11% 7% 3 6% 4 3 3 3 5% 6% 49%	2289
	11th to 20th	Night	16% 9% 3 6% 4 4 4 2 5% 6% 41%	2424
	21st to end	Night	17% 11% 3 6% 4 4 3 3 5% 8% 36%	2860
November	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 5% 6% 44%	2666
	11th to 20th	Night	13% 11% 5% 9% 4 5% 4 4 5% 7% 34%	2730
	21st to end	Night	16% 11% 4 8% 5% 4 4 4 4 6% 35%	2895
December	1st to 10th	Night	9% 10% 4 7% 5% 4 5% 3 5% 7% 41%	4720
	11th to 20th	Night	14% 16% 6% 10% 6% 5% 4 3 4 4 27%	5339
	21st to end	Night	19% 19% 6% 12% 5% 5% 4 3 3 5% 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	57% 6% 13% 18% 6%	3626
	11th to 20th	Day	53% 6% 11% 14% 15% 1	3620
	21st to end	Day	53% 4 11% 19% 13%	4010
February	1st to 10th	Day	53% 5% 12% 19% 9% 1	1800
	11th to 20th	Day	49% 5% 10% 23% 11% 2	1938
	21st to end	Day	54% 5% 11% 17% 10% 2	1650
March	1st to 10th	Day	41% 6% 11% 25% 12% 5%	4480
	11th to 20th	Day	39% 6% 14% 25% 14% 1	4690
	21st to end	Day	58% 5% 10% 15% 10% 1	5969
April	1st to 10th	Day	53% 7% 11% 19% 10%	2400
	11th to 20th	Day	50% 6% 11% 19% 12% 2	2374
	21st to end	Day	49% 8% 12% 19% 11% 1	2529
May	1st to 10th	Day	40% 7% 11% 25% 16% 2	5961
	11th to 20th	Day	39% 8% 12% 25% 15% 1	6323
	21st to end	Day	40% 8% 11% 26% 14% 2	6949
June	1st to 10th	Day	35% 7% 14% 26% 15% 2	2800
	11th to 20th	Day	38% 7% 14% 24% 16% 1	2800
	21st to end	Day	37% 8% 15% 23% 15% 2	2800
July	1st to 10th	Day	39% 7% 13% 29% 10% 1	6291
	11th to 20th	Day	32% 7% 15% 30% 14% 1	6285
	21st to end	Day	35% 8% 16% 22% 18% 2	6728
August	1st to 10th	Day	37% 8% 15% 28% 11% 1	4548
	11th to 20th	Day	37% 7% 13% 31% 11% 1	3967
	21st to end	Day	37% 6% 14% 25% 17% 1	4311
September	1st to 10th	Day	37% 8% 11% 26% 16% 1	2400
	11th to 20th	Day	33% 6% 14% 31% 16% 1	2290
	21st to end	Day	40% 6% 11% 29% 13% 1	2200
October	1st to 10th	Day	24% 4 6% 39% 26% 2	3435
	11th to 20th	Day	38% 5% 9% 26% 21% 1	3191
	21st to end	Day	42% 5% 11% 28% 13% 1	3180
November	1st to 10th	Day	42% 5% 14% 23% 14% 2	1852
	11th to 20th	Day	51% 6% 13% 21% 9% 1	1890
	21st to end	Day	53% 4 13% 22% 7%	1722
December	1st to 10th	Day	46% 8% 19% 21% 6% 1	2690
	11th to 20th	Day	57% 5% 12% 16% 8% 1	3085
	21st to end	Day	72% 5% 7% 9% 7%	4070
January	1st to 10th	Night	52% 6% 8% 21% 13% 1	6363
	11th to 20th	Night	60% 6% 9% 10% 16% 1	6360
	21st to end	Night	55% 6% 12% 15% 12%	6953
February	1st to 10th	Night	52% 5% 10% 18% 13% 2	2600
	11th to 20th	Night	47% 5% 10% 22% 15% 1	2357
	21st to end	Night	49% 5% 13% 19% 12% 3	1980
March	1st to 10th	Night	41% 6% 14% 23% 13% 4	5373
	11th to 20th	Night	34% 5% 14% 29% 14% 4	5312
	21st to end	Night	56% 5% 9% 16% 12% 2	4962
April	1st to 10th	Night	47% 7% 10% 20% 16% 1	2000
	11th to 20th	Night	48% 5% 9% 19% 17% 3	1975
	21st to end	Night	44% 7% 13% 22% 13% 2	1870
May	1st to 10th	Night	41% 7% 12% 19% 19% 2	3982
	11th to 20th	Night	37% 9% 14% 22% 16% 3	3597
	21st to end	Night	43% 7% 9% 23% 12% 5%	3950
June	1st to 10th	Night	33% 8% 13% 27% 18% 1	1600
	11th to 20th	Night	35% 7% 11% 25% 20% 2	1600
	21st to end	Night	36% 6% 16% 23% 18% 2	1600
July	1st to 10th	Night	37% 5% 13% 31% 10% 3	3580
	11th to 20th	Night	28% 6% 16% 30% 18% 2	3572
	21st to end	Night	29% 7% 17% 23% 22% 3	3832
August	1st to 10th	Night	33% 6% 16% 25% 19% 1	2588
	11th to 20th	Night	33% 7% 14% 31% 14%	3144
	21st to end	Night	30% 7% 14% 27% 20% 3	3586
September	1st to 10th	Night	28% 8% 11% 27% 24% 2	2000
	11th to 20th	Night	31% 6% 10% 33% 19% 1	2088
	21st to end	Night	37% 5% 10% 31% 17% 1	2200
October	1st to 10th	Night	26% 4 6% 38% 24% 2	3744
	11th to 20th	Night	35% 4 12% 27% 19% 3	3947
	21st to end	Night	43% 4 9% 26% 17% 1	4610
November	1st to 10th	Night	40% 5% 11% 25% 17% 2	2666
	11th to 20th	Night	48% 6% 13% 20% 10% 2	2730
	21st to end	Night	50% 6% 13% 21% 8% 1	2895
December	1st to 10th	Night	42% 5% 17% 25% 8% 3	4720
	11th to 20th	Night	60% 5% 9% 14% 11% 1	5341
	21st to end	Night	68% 5% 8% 13% 6%	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	15% 18% 37% 24% 5% 1	303
	11th to 20th	Day	15% 22% 34% 23% 5% 1	300
	21st to end	Day	22% 16% 39% 18% 5%	330
February	1st to 10th	Day	14% 21% 44% 15% 5% 1	300
	11th to 20th	Day	19% 26% 35% 15% 5%	300
	21st to end	Day	22% 23% 41% 14%	249
March	1st to 10th	Day	19% 23% 42% 13% 2	300
	11th to 20th	Day	24% 21% 30% 18% 6% 1	312
	21st to end	Day	27% 14% 30% 21% 6% 1	550
April	1st to 10th	Day	27% 14% 35% 19% 4	500
	11th to 20th	Day	23% 21% 26% 20% 9% 2	492
	21st to end	Day	22% 13% 31% 22% 11% 1	500
May	1st to 10th	Day	22% 13% 34% 21% 8% 2	528
	11th to 20th	Day	19% 13% 32% 23% 11% 3	725
	21st to end	Day	17% 11% 29% 26% 12% 5%	789
June	1st to 10th	Day	8% 10% 35% 29% 14% 4	730
	11th to 20th	Day	13% 11% 29% 30% 12% 5%	730
	21st to end	Day	11% 11% 31% 29% 14% 3	730
July	1st to 10th	Day	9% 4 21% 33% 25% 8%	721
	11th to 20th	Day	8% 7% 29% 31% 19% 6%	718
	21st to end	Day	6% 4 30% 35% 19% 7%	794
August	1st to 10th	Day	8% 3 25% 36% 18% 9%	726
	11th to 20th	Day	13% 5% 18% 27% 24% 12%	528
	21st to end	Day	11% 7% 21% 30% 21% 10%	583
September	1st to 10th	Day	12% 8% 26% 26% 19% 9%	530
	11th to 20th	Day	11% 4 21% 31% 23% 10%	530
	21st to end	Day	15% 11% 26% 24% 17% 6%	530
October	1st to 10th	Day	17% 10% 27% 28% 13% 5%	440
	11th to 20th	Day	21% 20% 33% 18% 8%	330
	21st to end	Day	22% 17% 37% 19% 4 1	363
November	1st to 10th	Day	17% 19% 36% 20% 8% 1	330
	11th to 20th	Day	18% 22% 34% 18% 8%	330
	21st to end	Day	23% 15% 39% 18% 5%	330
December	1st to 10th	Day	17% 21% 41% 14% 6% 1	330
	11th to 20th	Day	18% 22% 38% 14% 6% 1	330
	21st to end	Day	26% 21% 35% 14% 3 1	363
January	1st to 10th	Night	17% 18% 36% 23% 5%	1006
	11th to 20th	Night	20% 15% 40% 19% 6% 1	1000
	21st to end	Night	18% 13% 39% 24% 5% 1	1100
February	1st to 10th	Night	19% 14% 37% 24% 5% 1	1000
	11th to 20th	Night	17% 17% 36% 25% 5%	801
	21st to end	Night	24% 14% 41% 17% 4	660
March	1st to 10th	Night	20% 14% 34% 24% 7% 1	800
	11th to 20th	Night	16% 15% 36% 25% 7% 1	763
	21st to end	Night	22% 8% 29% 32% 7% 1	660
April	1st to 10th	Night	22% 13% 35% 23% 7%	600
	11th to 20th	Night	23% 11% 33% 26% 6% 1	593
	21st to end	Night	20% 10% 33% 28% 9% 1	600
May	1st to 10th	Night	18% 10% 33% 32% 8% 1	600
	11th to 20th	Night	17% 7% 31% 31% 13% 1	591
	21st to end	Night	12% 6% 26% 38% 17% 1	650
June	1st to 10th	Night	10% 5% 28% 40% 16% 1	600
	11th to 20th	Night	12% 6% 31% 37% 13% 1	600
	21st to end	Night	11% 5% 27% 42% 15%	600
July	1st to 10th	Night	8% 3 18% 40% 28% 3	585
	11th to 20th	Night	6% 2 25% 41% 23% 2	582
	21st to end	Night	7% 1 20% 46% 24% 2	657
August	1st to 10th	Night	7% 2 18% 45% 25% 3	597
	11th to 20th	Night	10% 3 16% 40% 26% 5%	594
	21st to end	Night	9% 2 19% 41% 25% 3	660
September	1st to 10th	Night	11% 3 23% 35% 23% 6%	600
	11th to 20th	Night	11% 4 21% 31% 28% 5%	688
	21st to end	Night	14% 8% 26% 29% 21% 3	800
October	1st to 10th	Night	15% 8% 29% 31% 16% 1	800
	11th to 20th	Night	21% 12% 33% 26% 8%	800
	21st to end	Night	21% 12% 31% 27% 8% 1	880
November	1st to 10th	Night	16% 12% 38% 23% 10% 1	820
	11th to 20th	Night	18% 18% 36% 22% 6%	840
	21st to end	Night	21% 14% 37% 22% 6%	1005
December	1st to 10th	Night	20% 16% 37% 22% 4	1050
	11th to 20th	Night	22% 15% 37% 21% 5% 1	1050
	21st to end	Night	28% 14% 36% 19% 3	1155

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