Ex Nihilo Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ex Nihilo Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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% 6% 2 6% 4 5% 5% 4 6% 8% 41%	148986
	Night	14% 6% 2 6% 4 4 4 4 6% 8% 43%	143062
Transparency	Day	39% 7% 12% 19% 17% 6%	152749
	Night	37% 7% 11% 19% 19% 7%	147371
Seeing	Day	18% 9% 22% 24% 18% 8%	17627
	Night	17% 9% 22% 29% 20% 3	27603

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	17% 9% 3 7% 4 3 3 2 4 4 43%	5258
	11th to 20th	Day	11% 6% 2 4 2 3 2 2 4 5% 60%	5280
	21st to end	Day	24% 10% 3 6% 3 3 3 2 3 4 39%	5858
February	1st to 10th	Day	15% 7% 3 6% 4 4 4 4 5% 6% 42%	2100
	11th to 20th	Day	22% 7% 2 4 3 3 4 3 4 5% 43%	2048
	21st to end	Day	19% 8% 2 6% 3 4 4 3 4 7% 41%	1730
March	1st to 10th	Day	21% 8% 3 8% 4 4 4 3 4 5% 37%	4814
	11th to 20th	Day	10% 5% 2 5% 4 4 3 3 4 7% 53%	5090
	21st to end	Day	17% 6% 2 6% 5% 4 3 3 3 5% 46%	5586
April	1st to 10th	Day	13% 5% 2 6% 5% 4 4 4 6% 7% 45%	2310
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 49%	2483
	21st to end	Day	10% 5% 2 5% 4 4 4 3 6% 8% 49%	2730
May	1st to 10th	Day	14% 7% 3 8% 5% 5% 5% 4 6% 8% 37%	7616
	11th to 20th	Day	14% 7% 3 7% 4 5% 6% 4 7% 8% 35%	7626
	21st to end	Day	18% 7% 3 7% 5% 5% 6% 4 6% 7% 32%	8391
June	1st to 10th	Day	10% 6% 2 8% 6% 6% 7% 4 8% 10% 34%	2730
	11th to 20th	Day	6% 4 2 5% 5% 6% 7% 6% 9% 11% 40%	2730
	21st to end	Day	5% 4 2 7% 5% 6% 7% 5% 8% 11% 40%	2730
July	1st to 10th	Day	5% 4 2 7% 6% 6% 7% 6% 9% 13% 35%	7680
	11th to 20th	Day	7% 5% 2 8% 6% 7% 7% 6% 10% 12% 31%	7680
	21st to end	Day	8% 6% 3 8% 6% 7% 8% 7% 9% 12% 27%	8340
August	1st to 10th	Day	11% 7% 3 7% 6% 6% 7% 6% 10% 12% 25%	6014
	11th to 20th	Day	5% 3 2 5% 4 5% 5% 5% 7% 12% 46%	5746
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	5992
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 8% 41%	2310
	11th to 20th	Day	16% 6% 3 5% 4 4 4 5% 7% 7% 41%	2310
	21st to end	Day	15% 5% 2 5% 3 4 4 3 4 6% 48%	2310
October	1st to 10th	Day	14% 6% 2 4 3 2 3 2 5% 5% 53%	2310
	11th to 20th	Day	12% 4 1 4 3 3 3 2 4 5% 60%	2149
	21st to end	Day	13% 4 2 5% 3 3 3 2 4 6% 57%	2280
November	1st to 10th	Day	13% 4 2 5% 3 2 3 3 4 5% 56%	1927
	11th to 20th	Day	13% 5% 1 4 2 3 3 3 5% 7% 54%	1890
	21st to end	Day	13% 5% 2 5% 3 3 4 4 4 6% 50%	1890
December	1st to 10th	Day	17% 7% 2 5% 3 4 3 3 5% 6% 45%	3020
	11th to 20th	Day	17% 6% 2 7% 4 4 4 3 5% 6% 43%	3463
	21st to end	Day	22% 6% 2 4 3 3 3 3 5% 7% 42%	4565
January	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 4 6% 48%	7635
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 4 5% 51%	7680
	21st to end	Night	21% 8% 2 6% 3 3 3 2 4 4 43%	8304
February	1st to 10th	Night	15% 6% 2 6% 4 4 3 4 4 5% 47%	2520
	11th to 20th	Night	21% 7% 2 5% 4 3 3 3 4 4 44%	2458
	21st to end	Night	20% 6% 2 5% 4 4 4 3 4 6% 43%	2076
March	1st to 10th	Night	22% 7% 2 6% 5% 4 3 4 5% 6% 37%	5258
	11th to 20th	Night	9% 4 1 4 3 3 4 3 6% 8% 54%	5090
	21st to end	Night	15% 6% 2 6% 4 3 4 4 5% 7% 46%	5588
April	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 5% 8% 49%	2310
	11th to 20th	Night	12% 4 2 4 3 3 3 3 5% 7% 53%	2101
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 7% 50%	1890
May	1st to 10th	Night	12% 7% 3 7% 5% 4 5% 4 7% 9% 36%	5237
	11th to 20th	Night	14% 6% 2 7% 4 5% 5% 4 7% 8% 38%	5234
	21st to end	Night	17% 8% 3 7% 4 4 5% 4 6% 8% 34%	5761
June	1st to 10th	Night	8% 5% 2 6% 5% 5% 6% 6% 8% 12% 37%	1890
	11th to 20th	Night	6% 5% 2 5% 5% 5% 5% 5% 9% 11% 41%	1890
	21st to end	Night	7% 4 2 5% 4 4 6% 6% 8% 11% 42%	1890
July	1st to 10th	Night	7% 5% 3 8% 6% 5% 6% 6% 8% 13% 33%	5278
	11th to 20th	Night	7% 5% 2 8% 6% 6% 7% 6% 10% 14% 30%	5277
	21st to end	Night	10% 8% 3 8% 6% 7% 6% 5% 8% 12% 28%	5736
August	1st to 10th	Night	11% 7% 3 8% 6% 5% 6% 6% 9% 13% 26%	4135
	11th to 20th	Night	7% 4 2 5% 4 4 5% 4 8% 13% 43%	4282
	21st to end	Night	9% 6% 2 6% 4 4 6% 4 6% 9% 45%	5085
September	1st to 10th	Night	9% 5% 2 7% 5% 5% 6% 5% 6% 10% 41%	2310
	11th to 20th	Night	14% 6% 2 5% 4 4 4 4 6% 7% 43%	2310
	21st to end	Night	14% 4 2 5% 4 4 4 4 6% 6% 46%	2310
October	1st to 10th	Night	14% 5% 2 5% 3 3 4 2 5% 5% 52%	2310
	11th to 20th	Night	11% 5% 1 4 3 3 3 2 3 4 60%	2469
	21st to end	Night	12% 4 2 3 2 3 3 3 3 6% 59%	2737
November	1st to 10th	Night	14% 6% 2 4 3 3 3 2 4 5% 55%	2586
	11th to 20th	Night	15% 4 1 4 3 3 2 3 4 6% 55%	2730
	21st to end	Night	15% 5% 1 4 3 4 4 3 6% 6% 49%	2730
December	1st to 10th	Night	15% 6% 2 6% 3 3 3 3 3 4 51%	4380
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 4 7% 46%	4952
	21st to end	Night	21% 6% 2 5% 3 4 3 3 5% 6% 42%	6633

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	44% 4 7% 19% 18% 7%	5259
	11th to 20th	Day	28% 2 5% 17% 32% 16%	5280
	21st to end	Day	51% 2 4 15% 18% 11%	5858
February	1st to 10th	Day	43% 4 8% 20% 19% 6%	2100
	11th to 20th	Day	44% 4 6% 19% 22% 6%	2058
	21st to end	Day	44% 4 7% 15% 21% 9%	1730
March	1st to 10th	Day	49% 4 7% 12% 19% 9%	4814
	11th to 20th	Day	32% 4 7% 14% 24% 19%	5090
	21st to end	Day	42% 4 6% 21% 20% 6%	5586
April	1st to 10th	Day	37% 5% 9% 21% 21% 6%	2310
	11th to 20th	Day	35% 4 8% 19% 24% 9%	2483
	21st to end	Day	32% 7% 10% 27% 22% 3	2730
May	1st to 10th	Day	44% 8% 13% 21% 14% 2	7651
	11th to 20th	Day	43% 8% 14% 21% 11% 2	7630
	21st to end	Day	50% 9% 10% 16% 14% 1	8405
June	1st to 10th	Day	41% 11% 15% 20% 10% 2	2730
	11th to 20th	Day	32% 11% 19% 24% 13% 1	2730
	21st to end	Day	34% 11% 18% 21% 13% 3	2730
July	1st to 10th	Day	34% 14% 23% 23% 5% 1	7615
	11th to 20th	Day	38% 15% 27% 17% 2 1	7611
	21st to end	Day	43% 13% 19% 18% 6%	8156
August	1st to 10th	Day	44% 14% 18% 21% 4	6001
	11th to 20th	Day	27% 10% 24% 25% 13% 2	5785
	21st to end	Day	33% 10% 15% 21% 18% 3	5992
September	1st to 10th	Day	37% 10% 14% 18% 17% 4	2310
	11th to 20th	Day	40% 8% 11% 19% 20% 2	2310
	21st to end	Day	37% 5% 12% 20% 19% 6%	2310
October	1st to 10th	Day	27% 4 7% 14% 36% 12%	3700
	11th to 20th	Day	20% 2 5% 15% 29% 30%	3415
	21st to end	Day	25% 4 5% 20% 32% 14%	3618
November	1st to 10th	Day	33% 3 9% 15% 29% 11%	1927
	11th to 20th	Day	31% 3 8% 20% 28% 10%	1890
	21st to end	Day	35% 5% 8% 19% 27% 7%	1890
December	1st to 10th	Day	40% 4 5% 17% 25% 9%	3020
	11th to 20th	Day	43% 5% 7% 18% 20% 8%	3460
	21st to end	Day	41% 5% 8% 25% 19% 2	4565
January	1st to 10th	Night	37% 4 9% 21% 23% 6%	7636
	11th to 20th	Night	35% 3 5% 19% 27% 11%	7680
	21st to end	Night	45% 3 4 15% 24% 9%	8304
February	1st to 10th	Night	39% 4 8% 19% 22% 8%	2520
	11th to 20th	Night	45% 4 6% 17% 21% 8%	2459
	21st to end	Night	44% 3 7% 17% 19% 10%	2076
March	1st to 10th	Night	46% 5% 9% 13% 20% 7%	5258
	11th to 20th	Night	27% 5% 9% 14% 28% 18%	5090
	21st to end	Night	38% 4 6% 21% 21% 10%	5588
April	1st to 10th	Night	34% 5% 10% 20% 23% 8%	2310
	11th to 20th	Night	31% 4 7% 22% 25% 10%	2101
	21st to end	Night	32% 6% 10% 25% 22% 5%	1890
May	1st to 10th	Night	42% 8% 15% 18% 14% 3	5264
	11th to 20th	Night	43% 7% 15% 21% 13% 1	5235
	21st to end	Night	46% 8% 12% 18% 14% 1	5771
June	1st to 10th	Night	36% 12% 17% 20% 13% 2	1890
	11th to 20th	Night	32% 11% 18% 25% 13% 1	1890
	21st to end	Night	31% 12% 19% 21% 13% 4	1890
July	1st to 10th	Night	38% 13% 24% 20% 4 1	5235
	11th to 20th	Night	38% 17% 28% 13% 3 1	5226
	21st to end	Night	46% 11% 20% 14% 6% 2	5606
August	1st to 10th	Night	46% 13% 17% 19% 5%	4124
	11th to 20th	Night	29% 10% 27% 22% 12% 1	4309
	21st to end	Night	34% 9% 13% 23% 19% 2	5085
September	1st to 10th	Night	37% 9% 15% 19% 16% 4	2310
	11th to 20th	Night	39% 8% 12% 19% 19% 3	2310
	21st to end	Night	37% 7% 13% 17% 20% 7%	2310
October	1st to 10th	Night	29% 5% 8% 15% 35% 9%	3700
	11th to 20th	Night	18% 3 5% 17% 30% 28%	3942
	21st to end	Night	23% 3 5% 21% 29% 19%	4349
November	1st to 10th	Night	32% 4 7% 18% 26% 12%	2586
	11th to 20th	Night	32% 3 7% 18% 28% 13%	2730
	21st to end	Night	36% 5% 6% 19% 25% 9%	2730
December	1st to 10th	Night	37% 4 5% 17% 31% 7%	4380
	11th to 20th	Night	39% 5% 10% 16% 22% 7%	4954
	21st to end	Night	43% 4 7% 26% 18% 2	6633

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	28% 21% 35% 14% 2	330
	11th to 20th	Day	31% 20% 30% 15% 4 1	330
	21st to end	Day	31% 19% 37% 10% 2	363
February	1st to 10th	Day	26% 16% 36% 19% 3	330
	11th to 20th	Day	40% 21% 26% 11% 2	330
	21st to end	Day	34% 20% 30% 12% 3	273
March	1st to 10th	Day	26% 15% 28% 20% 9% 1	434
	11th to 20th	Day	26% 15% 34% 18% 6% 1	540
	21st to end	Day	23% 15% 34% 20% 8% 1	594
April	1st to 10th	Day	22% 12% 30% 22% 12% 1	540
	11th to 20th	Day	16% 14% 30% 24% 14% 2	532
	21st to end	Day	17% 9% 28% 26% 15% 4	540
May	1st to 10th	Day	15% 16% 29% 23% 12% 5%	543
	11th to 20th	Day	15% 8% 21% 30% 16% 10%	567
	21st to end	Day	18% 7% 18% 26% 20% 11%	617
June	1st to 10th	Day	14% 4 17% 29% 27% 10%	570
	11th to 20th	Day	8% 4 18% 26% 26% 17%	570
	21st to end	Day	8% 4 16% 31% 26% 14%	570
July	1st to 10th	Day	10% 3 7% 27% 30% 23%	565
	11th to 20th	Day	10% 3 8% 26% 33% 20%	564
	21st to end	Day	7% 2 9% 28% 36% 18%	620
August	1st to 10th	Day	15% 5% 14% 23% 25% 17%	566
	11th to 20th	Day	12% 2 8% 26% 35% 17%	559
	21st to end	Day	15% 3 8% 27% 29% 19%	627
September	1st to 10th	Day	10% 4 13% 29% 29% 15%	570
	11th to 20th	Day	18% 3 9% 25% 30% 15%	570
	21st to end	Day	17% 4 15% 26% 28% 10%	570
October	1st to 10th	Day	20% 4 19% 30% 18% 9%	570
	11th to 20th	Day	12% 8% 26% 24% 25% 5%	570
	21st to end	Day	17% 9% 25% 24% 22% 3	615
November	1st to 10th	Day	20% 8% 26% 29% 14% 2	405
	11th to 20th	Day	18% 16% 35% 21% 8% 2	330
	21st to end	Day	17% 16% 33% 23% 9% 2	330
December	1st to 10th	Day	32% 18% 29% 18% 3	330
	11th to 20th	Day	26% 13% 33% 21% 6% 2	330
	21st to end	Day	32% 15% 30% 14% 6% 2	363
January	1st to 10th	Night	26% 17% 28% 22% 7%	840
	11th to 20th	Night	22% 17% 31% 25% 6%	840
	21st to end	Night	22% 18% 34% 23% 3	924
February	1st to 10th	Night	21% 13% 31% 30% 5%	840
	11th to 20th	Night	26% 17% 32% 22% 3	823
	21st to end	Night	25% 16% 32% 25% 2	692
March	1st to 10th	Night	20% 14% 29% 31% 6%	840
	11th to 20th	Night	18% 12% 34% 27% 9%	840
	21st to end	Night	18% 14% 32% 28% 9%	924
April	1st to 10th	Night	15% 14% 30% 29% 11%	840
	11th to 20th	Night	14% 11% 28% 31% 15%	655
	21st to end	Night	12% 7% 27% 34% 20% 1	630
May	1st to 10th	Night	13% 8% 26% 33% 19% 1	630
	11th to 20th	Night	13% 7% 21% 33% 22% 4	621
	21st to end	Night	16% 3 13% 32% 31% 5%	683
June	1st to 10th	Night	10% 2 16% 35% 31% 6%	630
	11th to 20th	Night	7% 2 17% 29% 35% 11%	630
	21st to end	Night	8% 1 11% 34% 37% 9%	630
July	1st to 10th	Night	10% 2 8% 27% 40% 13%	615
	11th to 20th	Night	9% 2 5% 23% 49% 12%	612
	21st to end	Night	8% 4 7% 32% 42% 7%	690
August	1st to 10th	Night	15% 3 6% 29% 38% 10%	627
	11th to 20th	Night	14% 2 7% 25% 40% 11%	624
	21st to end	Night	15% 4 7% 27% 37% 10%	706
September	1st to 10th	Night	10% 3 10% 27% 39% 10%	840
	11th to 20th	Night	18% 2 10% 27% 38% 5%	840
	21st to end	Night	16% 3 13% 32% 32% 3	840
October	1st to 10th	Night	15% 4 17% 31% 27% 5%	840
	11th to 20th	Night	14% 6% 23% 32% 22% 3	840
	21st to end	Night	14% 8% 21% 33% 22% 2	913
November	1st to 10th	Night	18% 7% 21% 33% 20% 1	820
	11th to 20th	Night	18% 10% 27% 30% 14%	840
	21st to end	Night	19% 11% 26% 30% 14% 1	840
December	1st to 10th	Night	21% 14% 28% 30% 8%	840
	11th to 20th	Night	21% 13% 26% 30% 10%	840
	21st to end	Night	27% 11% 27% 29% 6%	924

Page generated at 2026-10-26 04:37:50. Version ac07

Copyright © 2026 Attilla Danko.