Eridanus Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eridanus Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	12% 7% 2 6% 4 4 4 4 5% 7% 46%	130903
	Night	12% 6% 2 5% 4 3 4 3 5% 6% 49%	130848
Transparency	Day	37% 5% 10% 25% 19% 4	134509
	Night	35% 4 10% 25% 21% 5%	135319
Seeing	Day	17% 10% 24% 25% 18% 6%	17221
	Night	17% 9% 25% 31% 16% 2	27166

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	15% 9% 2 6% 4 4 3 3 4 5% 45%	5289
	11th to 20th	Day	8% 8% 2 5% 3 3 3 3 4 5% 56%	5310
	21st to end	Day	18% 8% 3 5% 3 2 3 2 3 5% 49%	5812
February	1st to 10th	Day	16% 9% 4 7% 3 3 3 3 3 5% 43%	1890
	11th to 20th	Day	16% 8% 2 6% 5% 4 4 3 4 6% 42%	1974
	21st to end	Day	15% 9% 4 6% 4 4 4 3 5% 6% 40%	1730
March	1st to 10th	Day	17% 11% 2 5% 3 3 4 3 4 5% 42%	5046
	11th to 20th	Day	10% 5% 2 5% 4 3 4 3 5% 6% 54%	5057
	21st to end	Day	13% 7% 3 7% 4 4 4 3 5% 7% 41%	5525
April	1st to 10th	Day	13% 7% 3 7% 4 3 4 3 5% 5% 46%	2400
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 6% 6% 42%	2374
	21st to end	Day	17% 8% 3 8% 4 4 4 4 4 7% 38%	2400
May	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 8% 35%	5419
	11th to 20th	Day	13% 7% 3 7% 5% 4 4 4 6% 7% 39%	5862
	21st to end	Day	19% 7% 2 7% 5% 5% 4 4 5% 6% 36%	6817
June	1st to 10th	Day	8% 5% 2 6% 6% 5% 5% 5% 7% 9% 43%	2800
	11th to 20th	Day	5% 4 1 5% 5% 4 6% 6% 7% 10% 46%	2800
	21st to end	Day	4 4 2 5% 5% 4 5% 5% 7% 10% 50%	2800
July	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 4 7% 9% 42%	6049
	11th to 20th	Day	4 3 1 5% 4 4 5% 4 7% 11% 49%	5900
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	6422
August	1st to 10th	Day	4 3 1 4 3 3 4 4 5% 9% 61%	4234
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 10% 43%	3889
	21st to end	Day	6% 3 2 5% 4 5% 4 4 6% 9% 54%	4312
September	1st to 10th	Day	10% 6% 2 6% 4 5% 4 4 5% 8% 47%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 8% 45%	2400
	21st to end	Day	10% 5% 2 4 4 3 4 3 5% 8% 53%	2104
October	1st to 10th	Day	8% 4 2 5% 3 4 4 3 5% 6% 57%	2000
	11th to 20th	Day	13% 5% 2 5% 2 3 3 2 4 5% 57%	2000
	21st to end	Day	19% 6% 2 5% 3 3 2 3 4 6% 49%	2200
November	1st to 10th	Day	12% 6% 2 5% 2 3 2 2 3 5% 58%	2058
	11th to 20th	Day	10% 6% 3 5% 4 4 4 3 5% 6% 52%	1907
	21st to end	Day	18% 7% 3 5% 3 3 3 3 3 5% 47%	1804
December	1st to 10th	Day	12% 7% 2 5% 3 3 4 3 3 4 53%	2690
	11th to 20th	Day	14% 8% 2 7% 5% 3 3 3 3 5% 46%	3088
	21st to end	Day	17% 14% 3 7% 5% 4 4 4 5% 5% 33%	4141
January	1st to 10th	Night	14% 7% 3 6% 3 3 4 3 5% 6% 45%	7634
	11th to 20th	Night	10% 5% 2 6% 3 3 3 2 4 5% 57%	7680
	21st to end	Night	18% 9% 3 6% 3 4 3 2 4 5% 43%	8402
February	1st to 10th	Night	16% 9% 2 6% 4 4 4 3 4 6% 42%	2730
	11th to 20th	Night	16% 8% 3 6% 4 4 3 3 4 6% 44%	2531
	21st to end	Night	16% 7% 2 3 4 4 3 3 5% 5% 48%	2076
March	1st to 10th	Night	18% 9% 2 6% 3 3 3 3 4 4 43%	5058
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 6% 52%	5057
	21st to end	Night	12% 6% 2 6% 4 4 5% 3 5% 7% 45%	5404
April	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 6% 50%	2000
	11th to 20th	Night	14% 5% 2 5% 4 3 3 3 5% 6% 49%	1975
	21st to end	Night	15% 7% 3 5% 3 3 3 3 6% 7% 43%	2000
May	1st to 10th	Night	16% 7% 2 6% 4 3 4 4 6% 6% 41%	4473
	11th to 20th	Night	16% 9% 3 7% 4 4 4 3 5% 8% 39%	4053
	21st to end	Night	19% 7% 2 6% 4 4 4 3 5% 7% 39%	4044
June	1st to 10th	Night	7% 4 2 4 3 3 4 4 7% 9% 52%	1600
	11th to 20th	Night	4 3 1 5% 2 3 4 4 7% 10% 57%	1600
	21st to end	Night	3 3 1 4 4 3 4 3 7% 9% 60%	1600
July	1st to 10th	Night	6% 4 2 6% 4 5% 5% 4 7% 9% 46%	3909
	11th to 20th	Night	3 2 1 4 3 4 5% 3 7% 9% 59%	4077
	21st to end	Night	4 5% 2 5% 4 4 5% 4 7% 11% 48%	4449
August	1st to 10th	Night	3 3 1 3 2 3 3 4 5% 8% 64%	2926
	11th to 20th	Night	5% 3 2 5% 4 5% 5% 4 7% 9% 52%	3159
	21st to end	Night	5% 3 1 4 3 3 4 3 5% 9% 60%	3586
September	1st to 10th	Night	8% 4 2 4 4 3 3 3 5% 8% 55%	2000
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 6% 7% 48%	2000
	21st to end	Night	7% 6% 2 5% 3 3 3 3 5% 6% 58%	2270
October	1st to 10th	Night	7% 4 1 3 3 2 3 3 6% 6% 63%	2400
	11th to 20th	Night	11% 4 1 3 3 2 2 2 3 6% 61%	2400
	21st to end	Night	19% 5% 2 4 2 2 4 3 4 4 52%	2640
November	1st to 10th	Night	12% 5% 2 4 3 3 3 2 4 6% 57%	2460
	11th to 20th	Night	11% 5% 1 4 3 2 3 3 5% 6% 57%	2709
	21st to end	Night	14% 6% 2 4 4 4 3 3 5% 6% 49%	2813
December	1st to 10th	Night	13% 7% 2 5% 3 3 3 3 4 6% 53%	4720
	11th to 20th	Night	15% 8% 3 6% 3 4 3 3 3 5% 47%	5339
	21st to end	Night	20% 10% 3 7% 4 4 4 3 4 5% 35%	7074

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	43% 3 9% 28% 15% 2	5290
	11th to 20th	Day	31% 3 5% 26% 30% 5%	5310
	21st to end	Day	40% 2 6% 20% 24% 7%	5812
February	1st to 10th	Day	44% 4 10% 24% 15% 3	1890
	11th to 20th	Day	44% 4 9% 21% 17% 4	1983
	21st to end	Day	45% 4 8% 18% 17% 8%	1730
March	1st to 10th	Day	45% 4 6% 20% 13% 12%	5046
	11th to 20th	Day	32% 4 6% 23% 23% 13%	5057
	21st to end	Day	42% 4 9% 28% 16% 1	5527
April	1st to 10th	Day	39% 5% 8% 23% 22% 3	2400
	11th to 20th	Day	40% 5% 11% 22% 15% 6%	2374
	21st to end	Day	46% 6% 9% 21% 16% 2	2400
May	1st to 10th	Day	46% 6% 12% 22% 13% 1	5454
	11th to 20th	Day	43% 7% 13% 24% 12% 2	5866
	21st to end	Day	48% 6% 10% 19% 16% 2	6831
June	1st to 10th	Day	36% 9% 14% 24% 17% 1	2800
	11th to 20th	Day	29% 9% 15% 27% 17% 3	2800
	21st to end	Day	28% 7% 16% 27% 20% 4	2800
July	1st to 10th	Day	36% 6% 14% 26% 16% 2	5985
	11th to 20th	Day	27% 6% 16% 33% 17% 2	5831
	21st to end	Day	35% 5% 16% 31% 12% 1	6240
August	1st to 10th	Day	21% 5% 17% 33% 21% 4	4220
	11th to 20th	Day	36% 5% 14% 30% 13% 1	3925
	21st to end	Day	27% 5% 13% 35% 17% 3	4311
September	1st to 10th	Day	35% 5% 11% 25% 18% 6%	2400
	11th to 20th	Day	37% 5% 10% 22% 24% 3	2400
	21st to end	Day	31% 4 8% 25% 29% 4	2104
October	1st to 10th	Day	17% 3 5% 17% 44% 14%	3260
	11th to 20th	Day	21% 2 3 23% 41% 10%	3240
	21st to end	Day	32% 3 6% 32% 23% 4	3538
November	1st to 10th	Day	31% 3 8% 27% 24% 7%	2058
	11th to 20th	Day	34% 4 7% 24% 24% 8%	1907
	21st to end	Day	42% 3 8% 20% 23% 4	1804
December	1st to 10th	Day	36% 4 7% 30% 21% 2	2690
	11th to 20th	Day	42% 3 7% 26% 20% 2	3085
	21st to end	Day	52% 4 9% 20% 11% 3	4141
January	1st to 10th	Night	40% 3 11% 30% 13% 3	7635
	11th to 20th	Night	30% 3 7% 21% 35% 5%	7680
	21st to end	Night	45% 3 8% 19% 21% 5%	8402
February	1st to 10th	Night	44% 4 9% 24% 16% 3	2730
	11th to 20th	Night	42% 3 9% 23% 19% 4	2532
	21st to end	Night	38% 4 9% 21% 21% 8%	2076
March	1st to 10th	Night	45% 4 6% 16% 18% 12%	5058
	11th to 20th	Night	33% 4 7% 26% 19% 11%	5057
	21st to end	Night	38% 5% 10% 29% 17% 2	5404
April	1st to 10th	Night	35% 5% 12% 22% 21% 6%	2000
	11th to 20th	Night	36% 4 10% 24% 20% 7%	1975
	21st to end	Night	40% 5% 11% 25% 17% 3	2000
May	1st to 10th	Night	44% 6% 15% 23% 11% 2	4503
	11th to 20th	Night	44% 6% 13% 23% 11% 3	4054
	21st to end	Night	44% 6% 11% 22% 13% 3	4052
June	1st to 10th	Night	25% 7% 14% 30% 21% 2	1600
	11th to 20th	Night	20% 6% 14% 35% 20% 4	1600
	21st to end	Night	20% 6% 16% 31% 21% 7%	1600
July	1st to 10th	Night	32% 6% 17% 22% 21% 1	3866
	11th to 20th	Night	21% 6% 18% 36% 17% 2	4026
	21st to end	Night	29% 5% 17% 32% 16%	4320
August	1st to 10th	Night	17% 5% 16% 37% 24% 1	2916
	11th to 20th	Night	28% 5% 20% 30% 16% 1	3189
	21st to end	Night	21% 5% 14% 33% 23% 3	3586
September	1st to 10th	Night	27% 5% 11% 29% 19% 8%	2000
	11th to 20th	Night	33% 6% 9% 22% 25% 5%	2000
	21st to end	Night	28% 4 8% 23% 31% 6%	2270
October	1st to 10th	Night	13% 3 7% 16% 40% 21%	3920
	11th to 20th	Night	18% 2 4 17% 38% 19%	3899
	21st to end	Night	29% 3 5% 27% 29% 7%	4252
November	1st to 10th	Night	30% 3 8% 26% 26% 7%	2460
	11th to 20th	Night	29% 3 7% 27% 26% 8%	2709
	21st to end	Night	36% 4 8% 24% 24% 4	2813
December	1st to 10th	Night	35% 2 9% 35% 17% 2	4720
	11th to 20th	Night	42% 2 7% 24% 21% 4	5341
	21st to end	Night	50% 4 10% 22% 10% 3	7074

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	29% 20% 33% 13% 5%	330
	11th to 20th	Day	27% 22% 35% 12% 3	330
	21st to end	Day	29% 19% 33% 16% 3	363
February	1st to 10th	Day	28% 16% 37% 12% 6%	330
	11th to 20th	Day	33% 20% 32% 10% 4 1	330
	21st to end	Day	33% 20% 32% 11% 3 1	273
March	1st to 10th	Day	32% 18% 32% 13% 4 1	330
	11th to 20th	Day	32% 21% 33% 12% 2	321
	21st to end	Day	28% 22% 34% 14% 2	355
April	1st to 10th	Day	26% 17% 30% 17% 10% 1	500
	11th to 20th	Day	23% 18% 33% 19% 6% 1	492
	21st to end	Day	24% 14% 33% 21% 6% 2	500
May	1st to 10th	Day	23% 15% 30% 25% 7% 1	504
	11th to 20th	Day	19% 9% 33% 23% 13% 4	725
	21st to end	Day	15% 7% 32% 26% 15% 4	789
June	1st to 10th	Day	10% 5% 22% 29% 25% 9%	730
	11th to 20th	Day	7% 2 20% 39% 25% 6%	730
	21st to end	Day	5% 4 21% 38% 26% 6%	730
July	1st to 10th	Day	8% 1 15% 33% 31% 12%	721
	11th to 20th	Day	5% 2 14% 34% 32% 13%	718
	21st to end	Day	7% 2 12% 30% 34% 15%	794
August	1st to 10th	Day	7% 2 10% 40% 32% 8%	726
	11th to 20th	Day	12% 2 10% 29% 33% 13%	547
	21st to end	Day	9% 2 9% 30% 34% 15%	583
September	1st to 10th	Day	10% 3 9% 34% 32% 11%	530
	11th to 20th	Day	11% 2 10% 32% 29% 15%	530
	21st to end	Day	13% 6% 20% 31% 22% 7%	374
October	1st to 10th	Day	8% 8% 22% 34% 24% 4	330
	11th to 20th	Day	17% 13% 32% 24% 12% 2	330
	21st to end	Day	32% 11% 30% 21% 6% 1	363
November	1st to 10th	Day	18% 11% 34% 23% 12% 2	330
	11th to 20th	Day	18% 15% 35% 22% 9% 1	330
	21st to end	Day	31% 13% 29% 18% 7% 2	330
December	1st to 10th	Day	22% 22% 32% 15% 7% 2	330
	11th to 20th	Day	27% 19% 31% 16% 7%	330
	21st to end	Day	27% 20% 35% 15% 3	363
January	1st to 10th	Night	23% 16% 32% 25% 4	1050
	11th to 20th	Night	23% 18% 34% 21% 4	1050
	21st to end	Night	22% 13% 37% 25% 3	1155
February	1st to 10th	Night	24% 13% 35% 25% 4	1050
	11th to 20th	Night	23% 16% 36% 22% 2	896
	21st to end	Night	24% 17% 35% 23% 2	692
March	1st to 10th	Night	24% 12% 33% 28% 3	646
	11th to 20th	Night	24% 12% 31% 28% 4	621
	21st to end	Night	22% 12% 34% 26% 6%	660
April	1st to 10th	Night	18% 14% 36% 27% 5%	600
	11th to 20th	Night	17% 14% 31% 30% 8%	593
	21st to end	Night	17% 10% 37% 27% 8% 1	600
May	1st to 10th	Night	14% 10% 30% 38% 9% 1	600
	11th to 20th	Night	18% 5% 28% 27% 19% 3	591
	21st to end	Night	13% 4 22% 36% 21% 4	650
June	1st to 10th	Night	9% 3 16% 35% 34% 4	600
	11th to 20th	Night	5% 1 15% 44% 33% 3	600
	21st to end	Night	4 1 15% 42% 35% 4	600
July	1st to 10th	Night	6% 2 7% 36% 42% 7%	585
	11th to 20th	Night	5% 1 7% 36% 43% 8%	582
	21st to end	Night	6% 1 5% 39% 39% 9%	657
August	1st to 10th	Night	8% 1 7% 40% 39% 6%	597
	11th to 20th	Night	8% 1 8% 40% 38% 5%	594
	21st to end	Night	7% 2 8% 36% 42% 6%	660
September	1st to 10th	Night	10% 9% 36% 41% 4	600
	11th to 20th	Night	12% 2 8% 34% 38% 7%	600
	21st to end	Night	10% 5% 15% 35% 32% 3	719
October	1st to 10th	Night	9% 3 18% 36% 31% 3	800
	11th to 20th	Night	13% 9% 26% 34% 18% 1	800
	21st to end	Night	23% 9% 26% 29% 11% 1	880
November	1st to 10th	Night	15% 8% 30% 33% 14%	820
	11th to 20th	Night	15% 9% 32% 34% 10%	840
	21st to end	Night	20% 12% 29% 29% 9%	923
December	1st to 10th	Night	20% 11% 32% 30% 6%	1050
	11th to 20th	Night	24% 12% 27% 31% 5%	1050
	21st to end	Night	25% 14% 30% 27% 5%	1155

Page generated at 2026-03-15 01:23:51. Version ac07

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