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 2024-11-12.

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 5% 7% 37%	101464
	Night	13% 10% 4 8% 4 4 4 4 5% 6% 38%	97798
Transparency	Day	43% 7% 12% 23% 13% 1	104995
	Night	44% 6% 12% 22% 14% 2	102408
Seeing	Day	16% 12% 30% 25% 13% 4	15933
	Night	17% 10% 32% 29% 11% 1	25784

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	11% 16% 7% 13% 6% 4 5% 3 4 4 27%	2527
	11th to 20th	Day	11% 13% 5% 17% 5% 4 5% 8% 7% 6% 19%	2530
	21st to end	Day	18% 17% 8% 13% 5% 3 3 3 4 6% 20%	2797
February	1st to 10th	Day	10% 14% 7% 9% 6% 5% 5% 4 5% 7% 30%	1710
	11th to 20th	Day	12% 12% 3 8% 5% 5% 5% 3 5% 5% 36%	1829
	21st to end	Day	11% 15% 5% 10% 5% 6% 5% 3 5% 6% 29%	1570
March	1st to 10th	Day	13% 9% 3 6% 4 4 5% 4 5% 6% 43%	3120
	11th to 20th	Day	10% 7% 3 8% 5% 4 5% 4 6% 6% 43%	3274
	21st to end	Day	21% 14% 4 10% 4 3 4 3 4 5% 29%	4165
April	1st to 10th	Day	18% 13% 4 6% 5% 4 4 4 5% 7% 30%	2280
	11th to 20th	Day	16% 12% 3 7% 5% 4 4 3 5% 5% 34%	2254
	21st to end	Day	18% 11% 3 7% 4 5% 5% 4 5% 7% 32%	2402
May	1st to 10th	Day	15% 8% 2 6% 5% 4 5% 3 5% 7% 39%	4132
	11th to 20th	Day	13% 6% 3 6% 4 3 4 4 5% 7% 44%	4399
	21st to end	Day	13% 7% 2 7% 4 5% 4 4 6% 8% 39%	4824
June	1st to 10th	Day	10% 6% 1 6% 6% 4 4 4 6% 8% 44%	2660
	11th to 20th	Day	11% 6% 3 6% 4 5% 5% 4 6% 8% 42%	2660
	21st to end	Day	11% 6% 2 6% 4 4 5% 5% 6% 10% 41%	2660
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 6% 5% 7% 8% 34%	4440
	11th to 20th	Day	5% 4 2 4 4 5% 6% 5% 7% 10% 49%	4440
	21st to end	Day	9% 6% 2 6% 5% 5% 4 4 6% 9% 44%	4813
August	1st to 10th	Day	10% 5% 3 7% 4 5% 6% 5% 7% 8% 41%	4423
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 4 6% 7% 43%	3807
	21st to end	Day	11% 5% 3 6% 4 4 5% 4 6% 8% 44%	4180
September	1st to 10th	Day	11% 6% 2 7% 4 4 4 4 6% 7% 44%	2280
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 6% 7% 49%	2176
	21st to end	Day	12% 8% 2 7% 4 4 4 4 5% 7% 43%	2090
October	1st to 10th	Day	14% 6% 2 7% 4 4 3 2 5% 8% 45%	2005
	11th to 20th	Day	19% 11% 3 7% 5% 4 4 3 5% 7% 32%	1876
	21st to end	Day	16% 10% 4 7% 5% 4 4 4 5% 5% 36%	1881
November	1st to 10th	Day	12% 10% 3 7% 4 3 4 3 5% 7% 42%	1762
	11th to 20th	Day	11% 13% 3 10% 7% 5% 4 4 5% 7% 30%	1719
	21st to end	Day	17% 11% 3 9% 5% 3 3 3 5% 6% 35%	1558
December	1st to 10th	Day	12% 13% 5% 10% 7% 6% 4 3 5% 7% 28%	1520
	11th to 20th	Day	12% 14% 5% 11% 6% 5% 5% 4 5% 7% 27%	1918
	21st to end	Day	18% 20% 8% 14% 5% 4 4 3 4 5% 15%	2783
January	1st to 10th	Night	13% 15% 7% 11% 5% 4 3 3 4 4 32%	4428
	11th to 20th	Night	12% 14% 6% 15% 6% 5% 5% 6% 7% 5% 18%	4440
	21st to end	Night	15% 16% 8% 13% 5% 5% 4 3 4 4 22%	4855
February	1st to 10th	Night	10% 14% 4 9% 5% 5% 4 4 5% 5% 34%	2470
	11th to 20th	Night	11% 10% 3 8% 5% 5% 5% 3 4 6% 40%	2236
	21st to end	Night	16% 12% 4 8% 6% 4 3 3 5% 5% 35%	1884
March	1st to 10th	Night	16% 9% 2 6% 4 4 4 3 4 6% 42%	3733
	11th to 20th	Night	9% 7% 2 6% 4 4 4 4 7% 8% 44%	3714
	21st to end	Night	20% 12% 5% 10% 5% 4 4 3 4 4 30%	3466
April	1st to 10th	Night	17% 12% 3 7% 4 3 5% 3 6% 6% 35%	1900
	11th to 20th	Night	20% 8% 3 7% 4 4 4 3 4 5% 38%	1875
	21st to end	Night	18% 8% 2 6% 4 4 4 4 5% 7% 38%	1777
May	1st to 10th	Night	15% 8% 3 7% 4 4 5% 3 5% 8% 38%	2761
	11th to 20th	Night	13% 6% 2 6% 5% 5% 5% 4 6% 6% 42%	2506
	21st to end	Night	14% 9% 3 6% 4 4 4 3 5% 7% 41%	2743
June	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 7% 8% 47%	1520
	11th to 20th	Night	11% 4 2 6% 4 4 4 4 7% 9% 46%	1520
	21st to end	Night	10% 6% 2 6% 4 4 4 4 5% 8% 48%	1520
July	1st to 10th	Night	14% 7% 2 7% 4 4 5% 4 6% 7% 41%	2530
	11th to 20th	Night	4 3 1 4 4 3 5% 5% 6% 8% 56%	2530
	21st to end	Night	7% 5% 2 5% 4 4 5% 5% 7% 9% 49%	2750
August	1st to 10th	Night	10% 4 2 6% 4 4 4 4 6% 8% 48%	2517
	11th to 20th	Night	11% 6% 2 5% 4 3 4 4 6% 7% 48%	3018
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 5% 9% 49%	3476
September	1st to 10th	Night	9% 6% 1 5% 3 3 3 3 4 7% 55%	1900
	11th to 20th	Night	12% 4 2 5% 4 3 4 3 5% 7% 52%	1983
	21st to end	Night	12% 7% 2 6% 4 3 4 3 5% 6% 49%	2090
October	1st to 10th	Night	12% 7% 3 6% 4 3 3 3 4 6% 49%	2174
	11th to 20th	Night	16% 9% 3 6% 4 3 4 2 4 6% 41%	2303
	21st to end	Night	18% 11% 3 6% 4 4 3 3 5% 8% 35%	2717
November	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 5% 6% 45%	2536
	11th to 20th	Night	13% 11% 5% 8% 4 5% 4 4 5% 7% 33%	2484
	21st to end	Night	16% 9% 4 7% 5% 4 4 4 4 6% 37%	2619
December	1st to 10th	Night	14% 13% 5% 9% 6% 4 5% 3 5% 6% 30%	2660
	11th to 20th	Night	13% 14% 6% 10% 6% 6% 5% 3 4 4 28%	3279
	21st to end	Night	20% 18% 6% 11% 5% 5% 4 3 4 5% 20%	4884

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	59% 6% 13% 16% 6%	2528
	11th to 20th	Day	61% 6% 11% 16% 5%	2530
	21st to end	Day	66% 5% 9% 15% 5%	2797
February	1st to 10th	Day	53% 5% 13% 19% 9% 1	1710
	11th to 20th	Day	48% 5% 11% 23% 11% 2	1839
	21st to end	Day	55% 5% 11% 17% 10% 3	1570
March	1st to 10th	Day	41% 7% 10% 26% 13% 3	3120
	11th to 20th	Day	39% 7% 12% 26% 14% 2	3274
	21st to end	Day	58% 6% 10% 18% 7% 1	4165
April	1st to 10th	Day	53% 7% 11% 19% 10%	2280
	11th to 20th	Day	50% 6% 10% 19% 12% 2	2254
	21st to end	Day	49% 8% 12% 19% 11% 1	2402
May	1st to 10th	Day	42% 8% 11% 23% 15% 1	4170
	11th to 20th	Day	38% 8% 11% 29% 13% 1	4403
	21st to end	Day	39% 9% 12% 21% 16% 3	4837
June	1st to 10th	Day	35% 7% 14% 26% 16% 2	2660
	11th to 20th	Day	38% 7% 14% 24% 17% 1	2660
	21st to end	Day	37% 8% 15% 23% 15% 2	2660
July	1st to 10th	Day	44% 7% 13% 24% 10% 2	4371
	11th to 20th	Day	27% 8% 18% 30% 17% 1	4365
	21st to end	Day	35% 8% 15% 19% 20% 2	4662
August	1st to 10th	Day	38% 8% 15% 28% 11% 1	4408
	11th to 20th	Day	37% 7% 13% 31% 11% 1	3845
	21st to end	Day	38% 6% 14% 24% 17% 1	4179
September	1st to 10th	Day	37% 9% 11% 26% 17% 1	2280
	11th to 20th	Day	33% 6% 13% 30% 16% 1	2176
	21st to end	Day	40% 6% 12% 28% 13% 1	2090
October	1st to 10th	Day	24% 4 6% 39% 25% 2	3330
	11th to 20th	Day	37% 5% 9% 26% 21% 1	3092
	21st to end	Day	42% 5% 11% 27% 13% 2	3081
November	1st to 10th	Day	42% 5% 15% 23% 14% 2	1762
	11th to 20th	Day	51% 6% 14% 20% 8% 1	1719
	21st to end	Day	50% 4 14% 23% 7%	1558
December	1st to 10th	Day	56% 6% 14% 16% 7% 1	1520
	11th to 20th	Day	55% 5% 16% 16% 7% 2	1915
	21st to end	Day	71% 6% 9% 8% 5%	2783
January	1st to 10th	Night	55% 7% 9% 19% 9% 1	4428
	11th to 20th	Night	63% 6% 10% 12% 8%	4440
	21st to end	Night	64% 6% 10% 13% 6%	4855
February	1st to 10th	Night	52% 5% 10% 18% 12% 2	2470
	11th to 20th	Night	46% 5% 11% 22% 15% 1	2237
	21st to end	Night	50% 5% 12% 19% 12% 3	1884
March	1st to 10th	Night	43% 6% 13% 24% 10% 4	3733
	11th to 20th	Night	34% 6% 13% 26% 19% 3	3714
	21st to end	Night	57% 6% 8% 17% 10% 2	3466
April	1st to 10th	Night	48% 7% 10% 19% 15% 1	1900
	11th to 20th	Night	48% 5% 8% 18% 18% 3	1875
	21st to end	Night	44% 7% 12% 21% 14% 2	1777
May	1st to 10th	Night	42% 8% 12% 18% 17% 2	2788
	11th to 20th	Night	38% 9% 11% 28% 13% 1	2507
	21st to end	Night	41% 8% 10% 18% 15% 7%	2751
June	1st to 10th	Night	33% 8% 14% 27% 17% 2	1520
	11th to 20th	Night	34% 7% 11% 26% 20% 2	1520
	21st to end	Night	34% 6% 16% 24% 18% 2	1520
July	1st to 10th	Night	41% 6% 14% 22% 12% 4	2490
	11th to 20th	Night	23% 7% 18% 33% 18% 2	2482
	21st to end	Night	30% 8% 17% 20% 22% 4	2662
August	1st to 10th	Night	33% 6% 16% 25% 19% 1	2508
	11th to 20th	Night	33% 7% 13% 31% 15%	3046
	21st to end	Night	30% 7% 14% 27% 20% 3	3476
September	1st to 10th	Night	29% 8% 11% 26% 24% 2	1900
	11th to 20th	Night	31% 6% 9% 34% 19% 1	1983
	21st to end	Night	37% 5% 10% 30% 17% 1	2090
October	1st to 10th	Night	26% 4 6% 38% 24% 1	3629
	11th to 20th	Night	34% 4 13% 27% 20% 3	3826
	21st to end	Night	43% 4 9% 26% 16% 1	4467
November	1st to 10th	Night	40% 5% 11% 25% 16% 2	2536
	11th to 20th	Night	49% 6% 14% 19% 10% 1	2484
	21st to end	Night	47% 7% 14% 22% 9% 1	2619
December	1st to 10th	Night	53% 6% 13% 19% 9% 1	2660
	11th to 20th	Night	57% 6% 11% 16% 9% 1	3281
	21st to end	Night	66% 6% 10% 13% 5%	4884

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	16% 17% 37% 23% 6% 1	270
	11th to 20th	Day	16% 19% 34% 24% 6% 1	270
	21st to end	Day	24% 13% 38% 20% 5%	297
February	1st to 10th	Day	13% 20% 44% 17% 5% 1	270
	11th to 20th	Day	20% 25% 35% 14% 6%	270
	21st to end	Day	20% 22% 42% 15%	225
March	1st to 10th	Day	18% 23% 43% 13% 3	270
	11th to 20th	Day	23% 20% 30% 19% 6% 1	281
	21st to end	Day	27% 13% 30% 22% 6% 1	506
April	1st to 10th	Day	27% 13% 36% 20% 5%	460
	11th to 20th	Day	22% 21% 26% 20% 9% 2	452
	21st to end	Day	23% 12% 30% 22% 12% 1	460
May	1st to 10th	Day	23% 13% 32% 22% 8% 2	487
	11th to 20th	Day	19% 12% 32% 23% 11% 3	675
	21st to end	Day	16% 11% 28% 27% 13% 5%	734
June	1st to 10th	Day	8% 10% 35% 28% 15% 4	680
	11th to 20th	Day	13% 11% 29% 30% 13% 5%	680
	21st to end	Day	12% 10% 31% 30% 15% 3	680
July	1st to 10th	Day	9% 4 21% 32% 24% 9%	671
	11th to 20th	Day	7% 7% 29% 32% 20% 6%	668
	21st to end	Day	6% 4 29% 35% 20% 7%	739
August	1st to 10th	Day	8% 4 25% 36% 18% 9%	676
	11th to 20th	Day	12% 5% 18% 28% 24% 13%	488
	21st to end	Day	12% 7% 21% 30% 20% 10%	539
September	1st to 10th	Day	12% 8% 26% 26% 20% 8%	490
	11th to 20th	Day	11% 4 21% 31% 22% 10%	490
	21st to end	Day	16% 12% 26% 24% 17% 5%	490
October	1st to 10th	Day	19% 10% 27% 28% 12% 5%	405
	11th to 20th	Day	21% 20% 31% 19% 9%	300
	21st to end	Day	23% 17% 38% 18% 3 1	330
November	1st to 10th	Day	18% 17% 35% 21% 8% 1	300
	11th to 20th	Day	15% 21% 37% 19% 8%	273
	21st to end	Day	23% 14% 39% 20% 5%	270
December	1st to 10th	Day	19% 21% 40% 13% 6% 1	270
	11th to 20th	Day	19% 19% 39% 15% 7% 1	270
	21st to end	Day	25% 22% 34% 16% 3	297
January	1st to 10th	Night	17% 17% 36% 23% 6%	950
	11th to 20th	Night	20% 14% 39% 20% 6% 1	950
	21st to end	Night	18% 12% 39% 25% 5% 1	1045
February	1st to 10th	Night	18% 14% 37% 25% 5% 1	950
	11th to 20th	Night	17% 16% 36% 25% 5%	761
	21st to end	Night	24% 15% 40% 17% 4	628
March	1st to 10th	Night	20% 14% 34% 24% 8% 1	760
	11th to 20th	Night	16% 14% 37% 26% 7% 1	725
	21st to end	Night	22% 7% 30% 32% 7% 1	627
April	1st to 10th	Night	22% 13% 35% 23% 6%	570
	11th to 20th	Night	23% 11% 33% 25% 6% 1	563
	21st to end	Night	20% 9% 32% 28% 9% 1	570
May	1st to 10th	Night	18% 10% 33% 32% 7% 1	570
	11th to 20th	Night	18% 6% 31% 31% 13%	561
	21st to end	Night	12% 6% 26% 38% 18% 1	617
June	1st to 10th	Night	10% 5% 28% 40% 16% 1	570
	11th to 20th	Night	11% 6% 31% 38% 13% 1	570
	21st to end	Night	11% 5% 26% 42% 16%	570
July	1st to 10th	Night	8% 3 18% 40% 28% 3	555
	11th to 20th	Night	6% 2 25% 41% 23% 3	552
	21st to end	Night	7% 1 20% 45% 24% 2	624
August	1st to 10th	Night	7% 2 19% 45% 25% 3	567
	11th to 20th	Night	10% 3 16% 40% 26% 5%	564
	21st to end	Night	10% 2 19% 41% 25% 3	627
September	1st to 10th	Night	11% 3 22% 35% 24% 5%	570
	11th to 20th	Night	12% 4 22% 32% 26% 5%	653
	21st to end	Night	14% 8% 26% 28% 21% 3	760
October	1st to 10th	Night	16% 8% 28% 31% 16% 1	760
	11th to 20th	Night	21% 12% 32% 26% 8%	760
	21st to end	Night	22% 12% 31% 26% 8% 1	836
November	1st to 10th	Night	16% 12% 38% 23% 10% 1	780
	11th to 20th	Night	19% 17% 36% 22% 6%	765
	21st to end	Night	21% 13% 37% 23% 6%	909
December	1st to 10th	Night	22% 15% 37% 21% 5%	950
	11th to 20th	Night	21% 13% 38% 21% 6% 1	950
	21st to end	Night	28% 14% 36% 19% 3	1045

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