Ancient Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ancient Sky Observatory clear sky chart from 2005-11-06 to 2025-01-22.

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	11% 6% 2 5% 4 4 4 3 5% 7% 49%	105301
	Night	12% 6% 2 5% 3 3 3 3 5% 6% 53%	104211
Transparency	Day	33% 5% 10% 21% 21% 10%	109096
	Night	32% 4 10% 21% 21% 12%	108530
Seeing	Day	13% 7% 22% 30% 22% 6%	15145
	Night	14% 7% 28% 35% 15% 1	25169

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% 10% 3 5% 2 2 3 2 4 6% 52%	2937
	11th to 20th	Day	15% 5% 2 3 2 2 2 2 4 6% 56%	2940
	21st to end	Day	16% 9% 3 7% 3 3 2 2 4 6% 46%	3281
February	1st to 10th	Day	18% 7% 3 5% 3 3 3 3 4 6% 47%	1900
	11th to 20th	Day	15% 7% 2 5% 3 3 3 3 5% 6% 47%	1849
	21st to end	Day	18% 7% 2 4 2 3 3 3 3 4 51%	1570
March	1st to 10th	Day	15% 8% 3 7% 5% 4 3 3 4 6% 43%	3420
	11th to 20th	Day	19% 8% 2 7% 5% 5% 5% 4 6% 6% 35%	3480
	21st to end	Day	11% 8% 3 6% 4 4 4 4 6% 7% 45%	3814
April	1st to 10th	Day	12% 8% 3 6% 5% 3 4 4 5% 6% 44%	2090
	11th to 20th	Day	16% 7% 2 6% 4 4 3 2 4 6% 44%	2066
	21st to end	Day	13% 7% 2 5% 4 4 4 3 5% 6% 48%	2446
May	1st to 10th	Day	20% 9% 3 7% 5% 4 5% 3 6% 7% 32%	4082
	11th to 20th	Day	14% 7% 2 6% 4 4 4 3 5% 6% 45%	4082
	21st to end	Day	11% 6% 2 7% 5% 6% 6% 5% 8% 9% 34%	4477
June	1st to 10th	Day	5% 4 2 5% 4 5% 5% 5% 7% 8% 51%	2470
	11th to 20th	Day	8% 3 1 5% 3 4 4 4 6% 10% 51%	2470
	21st to end	Day	6% 5% 2 4 5% 6% 6% 5% 8% 10% 44%	2470
July	1st to 10th	Day	6% 4 2 4 5% 6% 5% 4 6% 8% 51%	4120
	11th to 20th	Day	4 3 1 4 3 4 5% 4 7% 10% 56%	4120
	21st to end	Day	9% 5% 2 5% 5% 5% 4 5% 7% 9% 42%	4465
August	1st to 10th	Day	1 1 1 2 3 3 3 3 4 8% 72%	4104
	11th to 20th	Day	3 2 1 3 2 3 4 3 6% 10% 62%	3822
	21st to end	Day	4 3 2 5% 4 4 6% 4 7% 11% 51%	3993
September	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 4 6% 7% 49%	2090
	11th to 20th	Day	10% 5% 2 6% 4 6% 5% 4 4 9% 44%	2090
	21st to end	Day	6% 4 2 6% 4 4 5% 4 6% 8% 54%	2090
October	1st to 10th	Day	7% 3 2 5% 3 3 3 4 4 6% 61%	2090
	11th to 20th	Day	12% 4 2 5% 2 3 3 2 4 5% 58%	1937
	21st to end	Day	11% 4 2 5% 3 3 2 2 4 5% 60%	2090
November	1st to 10th	Day	14% 6% 2 5% 3 3 4 2 4 5% 51%	1958
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 4 6% 54%	1869
	21st to end	Day	15% 6% 3 5% 4 3 3 3 4 6% 48%	1800
December	1st to 10th	Day	21% 6% 2 6% 3 4 3 3 4 4 45%	2940
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 5% 6% 45%	3391
	21st to end	Day	14% 6% 2 4 2 2 2 2 3 4 59%	4488
January	1st to 10th	Night	16% 6% 2 3 2 2 2 2 3 5% 56%	4238
	11th to 20th	Night	12% 4 1 3 2 2 2 1 3 5% 64%	4250
	21st to end	Night	18% 7% 3 7% 4 3 3 2 3 6% 44%	4436
February	1st to 10th	Night	16% 8% 2 6% 3 3 2 3 4 5% 50%	2280
	11th to 20th	Night	17% 6% 2 5% 3 2 3 3 4 5% 50%	2218
	21st to end	Night	17% 7% 2 4 4 4 4 1 3 6% 49%	1884
March	1st to 10th	Night	14% 7% 3 6% 4 4 4 3 5% 6% 43%	3433
	11th to 20th	Night	18% 8% 3 6% 4 5% 5% 4 6% 6% 36%	3480
	21st to end	Night	11% 9% 3 7% 5% 4 4 3 5% 6% 45%	3817
April	1st to 10th	Night	11% 7% 2 4 4 3 3 4 4 7% 49%	2090
	11th to 20th	Night	16% 6% 2 5% 4 3 3 3 6% 6% 45%	2063
	21st to end	Night	13% 7% 2 5% 3 3 3 3 5% 7% 49%	1730
May	1st to 10th	Night	22% 9% 2 5% 4 3 3 3 5% 7% 37%	2823
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 7% 50%	2823
	21st to end	Night	11% 6% 2 6% 4 4 5% 5% 7% 8% 43%	3090
June	1st to 10th	Night	4 4 1 4 3 3 3 4 6% 8% 60%	1710
	11th to 20th	Night	5% 2 1 4 3 4 4 4 6% 8% 60%	1710
	21st to end	Night	5% 4 2 3 4 3 5% 3 7% 8% 55%	1710
July	1st to 10th	Night	5% 3 1 4 3 3 4 4 6% 9% 59%	2850
	11th to 20th	Night	3 3 1 4 3 3 3 3 6% 10% 62%	2850
	21st to end	Night	6% 7% 2 5% 4 4 4 5% 7% 10% 45%	3098
August	1st to 10th	Night	1 1 1 1 1 1 1 2 4 7% 78%	2836
	11th to 20th	Night	3 2 1 3 3 2 3 3 6% 10% 63%	3012
	21st to end	Night	4 3 1 4 3 3 3 4 6% 10% 58%	3666
September	1st to 10th	Night	9% 5% 2 4 3 4 4 3 6% 7% 52%	2090
	11th to 20th	Night	10% 5% 2 6% 4 4 3 3 6% 7% 48%	2090
	21st to end	Night	7% 4 1 4 3 3 3 3 6% 7% 59%	2090
October	1st to 10th	Night	6% 3 1 4 2 3 3 2 4 6% 66%	2090
	11th to 20th	Night	11% 4 2 4 3 3 2 3 4 6% 59%	2223
	21st to end	Night	12% 5% 2 3 3 2 3 2 3 4 60%	2508
November	1st to 10th	Night	13% 5% 2 5% 3 3 3 2 4 6% 55%	2340
	11th to 20th	Night	7% 6% 2 4 3 3 4 2 4 5% 59%	2527
	21st to end	Night	15% 6% 2 5% 3 3 4 3 4 5% 50%	2600
December	1st to 10th	Night	20% 6% 2 5% 3 3 3 3 5% 6% 44%	4250
	11th to 20th	Night	18% 9% 2 6% 3 3 3 2 3 4 48%	4816
	21st to end	Night	15% 7% 2 4 2 2 2 2 3 4 58%	6490

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	35% 3 6% 19% 24% 14%	2938
	11th to 20th	Day	31% 3 6% 14% 31% 14%	2940
	21st to end	Day	43% 3 4 11% 27% 12%	3281
February	1st to 10th	Day	42% 3 6% 12% 22% 15%	1900
	11th to 20th	Day	39% 3 7% 18% 20% 13%	1859
	21st to end	Day	38% 4 5% 16% 21% 16%	1570
March	1st to 10th	Day	43% 3 6% 14% 21% 12%	3420
	11th to 20th	Day	49% 8% 9% 10% 16% 8%	3480
	21st to end	Day	38% 6% 10% 15% 23% 8%	3814
April	1st to 10th	Day	41% 4 8% 18% 14% 14%	2090
	11th to 20th	Day	42% 4 8% 17% 19% 11%	2066
	21st to end	Day	38% 4 7% 21% 22% 8%	2446
May	1st to 10th	Day	50% 6% 11% 14% 14% 5%	4120
	11th to 20th	Day	39% 6% 9% 18% 24% 5%	4086
	21st to end	Day	39% 10% 13% 24% 10% 3	4489
June	1st to 10th	Day	28% 7% 13% 29% 20% 4	2470
	11th to 20th	Day	28% 6% 14% 28% 19% 6%	2470
	21st to end	Day	33% 8% 12% 24% 20% 3	2470
July	1st to 10th	Day	29% 7% 18% 38% 9% 1	4056
	11th to 20th	Day	22% 6% 15% 35% 18% 3	4051
	21st to end	Day	34% 8% 18% 29% 11% 1	4325
August	1st to 10th	Day	12% 4 11% 37% 34% 1	4090
	11th to 20th	Day	17% 6% 20% 40% 15% 3	3861
	21st to end	Day	25% 10% 18% 26% 19% 2	3992
September	1st to 10th	Day	35% 6% 10% 23% 17% 10%	2090
	11th to 20th	Day	37% 8% 11% 20% 19% 6%	2090
	21st to end	Day	27% 6% 11% 21% 21% 14%	2090
October	1st to 10th	Day	15% 3 6% 13% 24% 39%	3480
	11th to 20th	Day	19% 2 8% 19% 30% 22%	3190
	21st to end	Day	23% 3 5% 20% 36% 13%	3428
November	1st to 10th	Day	37% 3 9% 18% 22% 11%	1958
	11th to 20th	Day	32% 4 8% 18% 21% 16%	1869
	21st to end	Day	39% 3 7% 17% 20% 14%	1800
December	1st to 10th	Day	44% 3 7% 16% 21% 9%	2940
	11th to 20th	Day	39% 5% 7% 12% 23% 13%	3389
	21st to end	Day	30% 3 5% 16% 28% 17%	4488
January	1st to 10th	Night	34% 2 4 18% 26% 17%	4238
	11th to 20th	Night	27% 2 4 16% 29% 22%	4250
	21st to end	Night	43% 3 4 13% 23% 13%	4436
February	1st to 10th	Night	39% 3 6% 13% 21% 18%	2280
	11th to 20th	Night	39% 3 6% 17% 20% 16%	2218
	21st to end	Night	41% 2 6% 14% 19% 18%	1884
March	1st to 10th	Night	41% 5% 8% 15% 20% 11%	3433
	11th to 20th	Night	49% 7% 10% 12% 14% 9%	3480
	21st to end	Night	40% 6% 9% 16% 21% 8%	3817
April	1st to 10th	Night	36% 5% 9% 18% 17% 15%	2090
	11th to 20th	Night	39% 5% 10% 14% 18% 14%	2063
	21st to end	Night	37% 4 10% 20% 20% 9%	1730
May	1st to 10th	Night	46% 6% 10% 17% 14% 6%	2850
	11th to 20th	Night	33% 7% 10% 24% 18% 8%	2824
	21st to end	Night	36% 8% 14% 28% 9% 5%	3099
June	1st to 10th	Night	20% 8% 16% 29% 23% 4	1710
	11th to 20th	Night	22% 5% 15% 31% 20% 7%	1710
	21st to end	Night	26% 6% 15% 30% 19% 4	1710
July	1st to 10th	Night	21% 8% 24% 39% 7% 1	2805
	11th to 20th	Night	18% 6% 19% 38% 18% 1	2796
	21st to end	Night	32% 8% 22% 29% 8%	2999
August	1st to 10th	Night	8% 3 13% 41% 33% 2	2826
	11th to 20th	Night	17% 5% 28% 34% 13% 2	3039
	21st to end	Night	22% 8% 21% 28% 18% 3	3666
September	1st to 10th	Night	30% 7% 12% 24% 17% 10%	2090
	11th to 20th	Night	34% 6% 15% 22% 16% 7%	2090
	21st to end	Night	25% 5% 13% 24% 19% 14%	2090
October	1st to 10th	Night	14% 2 7% 15% 21% 42%	3480
	11th to 20th	Night	18% 3 9% 16% 32% 21%	3683
	21st to end	Night	24% 3 5% 19% 33% 16%	4120
November	1st to 10th	Night	34% 3 8% 19% 22% 14%	2340
	11th to 20th	Night	28% 3 8% 20% 23% 17%	2527
	21st to end	Night	38% 3 7% 16% 20% 15%	2600
December	1st to 10th	Night	42% 3 7% 18% 20% 10%	4250
	11th to 20th	Night	42% 2 6% 10% 23% 17%	4817
	21st to end	Night	33% 2 4 16% 27% 18%	6490

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	21% 15% 36% 24% 4	300
	11th to 20th	Day	22% 14% 33% 24% 6% 1	300
	21st to end	Day	18% 19% 38% 20% 5%	300
February	1st to 10th	Day	27% 16% 34% 19% 5%	270
	11th to 20th	Day	24% 18% 37% 17% 4	270
	21st to end	Day	30% 15% 38% 17%	225
March	1st to 10th	Day	23% 13% 40% 20% 4	270
	11th to 20th	Day	17% 20% 37% 18% 7%	270
	21st to end	Day	17% 15% 43% 19% 6%	297
April	1st to 10th	Day	22% 18% 38% 19% 3	270
	11th to 20th	Day	19% 18% 37% 21% 6%	264
	21st to end	Day	18% 13% 39% 22% 8%	626
May	1st to 10th	Day	16% 11% 38% 28% 7%	653
	11th to 20th	Day	18% 6% 31% 31% 13% 2	675
	21st to end	Day	16% 6% 27% 34% 15% 3	734
June	1st to 10th	Day	5% 3 17% 42% 26% 7%	680
	11th to 20th	Day	7% 1 18% 47% 23% 4	680
	21st to end	Day	6% 2 10% 40% 31% 10%	680
July	1st to 10th	Day	7% 3 8% 38% 33% 11%	671
	11th to 20th	Day	3 1 5% 37% 45% 9%	668
	21st to end	Day	4 1 4 37% 45% 9%	739
August	1st to 10th	Day	1 4 34% 46% 14%	676
	11th to 20th	Day	4 1 3 38% 41% 13%	537
	21st to end	Day	5% 1 4 26% 40% 23%	430
September	1st to 10th	Day	8% 1 5% 25% 39% 21%	300
	11th to 20th	Day	7% 1 7% 26% 45% 15%	300
	21st to end	Day	9% 2 6% 35% 33% 15%	300
October	1st to 10th	Day	4 1 14% 38% 33% 9%	300
	11th to 20th	Day	10% 5% 24% 31% 26% 4	300
	21st to end	Day	20% 10% 24% 28% 17% 2	330
November	1st to 10th	Day	20% 8% 29% 29% 11% 3	300
	11th to 20th	Day	14% 12% 30% 28% 12% 3	300
	21st to end	Day	23% 15% 32% 19% 10% 1	300
December	1st to 10th	Day	18% 8% 34% 27% 10% 3	300
	11th to 20th	Day	19% 12% 38% 23% 7%	300
	21st to end	Day	19% 11% 39% 24% 5% 2	330
January	1st to 10th	Night	19% 10% 36% 29% 7% 1	1000
	11th to 20th	Night	19% 11% 33% 30% 7%	1000
	21st to end	Night	20% 10% 35% 28% 6%	980
February	1st to 10th	Night	22% 12% 34% 28% 4	760
	11th to 20th	Night	22% 12% 40% 24% 2	744
	21st to end	Night	23% 14% 42% 19% 2	628
March	1st to 10th	Night	20% 13% 37% 26% 4	584
	11th to 20th	Night	17% 12% 36% 30% 5%	570
	21st to end	Night	15% 11% 44% 26% 4	627
April	1st to 10th	Night	15% 11% 44% 29% 2	570
	11th to 20th	Night	19% 12% 40% 24% 6%	563
	21st to end	Night	17% 8% 43% 24% 8% 1	570
May	1st to 10th	Night	14% 10% 42% 26% 7%	570
	11th to 20th	Night	13% 5% 37% 33% 11% 1	561
	21st to end	Night	11% 6% 35% 34% 13%	617
June	1st to 10th	Night	5% 2 23% 42% 25% 2	570
	11th to 20th	Night	6% 2 27% 50% 14% 2	570
	21st to end	Night	6% 2 16% 46% 26% 3	570
July	1st to 10th	Night	5% 3 12% 48% 29% 4	555
	11th to 20th	Night	4 1 9% 55% 29% 3	552
	21st to end	Night	3 1 8% 52% 33% 3	624
August	1st to 10th	Night	2 8% 53% 34% 4	567
	11th to 20th	Night	7% 6% 50% 33% 4	564
	21st to end	Night	5% 1 4 46% 40% 5%	627
September	1st to 10th	Night	9% 2 10% 41% 34% 4	570
	11th to 20th	Night	12% 2 7% 40% 37% 3	570
	21st to end	Night	9% 1 12% 38% 36% 4	570
October	1st to 10th	Night	7% 2 12% 48% 26% 5%	570
	11th to 20th	Night	13% 3 24% 39% 18% 2	703
	21st to end	Night	14% 8% 28% 31% 18% 1	836
November	1st to 10th	Night	15% 5% 25% 39% 15% 1	780
	11th to 20th	Night	10% 7% 31% 37% 14% 1	927
	21st to end	Night	18% 11% 31% 28% 11% 1	1000
December	1st to 10th	Night	16% 8% 32% 34% 10% 1	1000
	11th to 20th	Night	21% 10% 30% 33% 6% 1	1000
	21st to end	Night	19% 9% 33% 31% 7%	1100

Page generated at 2025-01-19 02:06:48. Version ac07

Copyright © 2025 Attilla Danko.