Romulus Observatory
Clear Sky Chart
History

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

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	16% 9% 3 7% 5% 4 5% 4 6% 7% 34%	127557
	Night	16% 10% 3 7% 4 4 4 3 5% 7% 36%	124478
Transparency	Day	46% 7% 12% 19% 13% 3	131242
	Night	46% 6% 11% 20% 13% 3	128862
Seeing	Day	24% 14% 23% 22% 13% 5%	16688
	Night	22% 11% 21% 28% 16% 2	26806

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% 17% 9% 9% 6% 3 2 2 3 5% 28%	4146
	11th to 20th	Day	14% 17% 8% 11% 5% 4 3 2 2 2 30%	4150
	21st to end	Day	15% 14% 6% 9% 5% 3 3 2 3 4 35%	4565
February	1st to 10th	Day	17% 14% 3 8% 5% 4 5% 3 4 5% 31%	1825
	11th to 20th	Day	24% 13% 5% 8% 4 3 3 2 4 5% 28%	1948
	21st to end	Day	23% 12% 3 7% 3 3 3 2 5% 5% 34%	1650
March	1st to 10th	Day	17% 14% 5% 7% 4 4 3 3 4 5% 34%	4877
	11th to 20th	Day	13% 10% 3 6% 3 3 3 3 4 6% 44%	4980
	21st to end	Day	18% 9% 2 6% 4 4 4 3 5% 7% 39%	5465
April	1st to 10th	Day	15% 11% 2 6% 4 4 4 5% 5% 7% 37%	2200
	11th to 20th	Day	16% 8% 2 6% 4 4 4 4 5% 6% 41%	2414
	21st to end	Day	19% 7% 3 5% 5% 4 4 3 5% 6% 37%	2600
May	1st to 10th	Day	19% 10% 4 8% 6% 6% 6% 4 6% 8% 24%	5862
	11th to 20th	Day	20% 9% 3 9% 6% 5% 6% 5% 6% 8% 24%	5861
	21st to end	Day	18% 7% 3 7% 4 5% 6% 5% 7% 10% 29%	6433
June	1st to 10th	Day	13% 7% 2 7% 6% 5% 6% 5% 7% 9% 34%	2600
	11th to 20th	Day	10% 6% 2 8% 6% 7% 6% 5% 8% 9% 32%	2600
	21st to end	Day	11% 6% 3 6% 5% 5% 6% 6% 8% 9% 34%	2600
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 6% 6% 7% 11% 37%	5900
	11th to 20th	Day	16% 6% 2 7% 5% 5% 6% 5% 7% 10% 30%	5900
	21st to end	Day	20% 8% 3 8% 6% 6% 6% 4 6% 9% 23%	6424
August	1st to 10th	Day	15% 7% 2 7% 6% 6% 6% 6% 8% 10% 27%	4234
	11th to 20th	Day	14% 6% 2 6% 6% 5% 7% 5% 8% 10% 32%	4174
	21st to end	Day	7% 4 1 5% 3 4 5% 4 6% 9% 52%	4423
September	1st to 10th	Day	12% 6% 2 6% 5% 6% 5% 4 7% 8% 39%	2200
	11th to 20th	Day	14% 8% 2 7% 6% 4 5% 4 6% 7% 38%	2200
	21st to end	Day	16% 6% 2 7% 4 5% 6% 4 6% 8% 37%	2200
October	1st to 10th	Day	14% 6% 2 4 4 3 3 3 5% 6% 50%	2130
	11th to 20th	Day	15% 7% 2 4 3 3 3 3 5% 6% 49%	2000
	21st to end	Day	17% 9% 2 5% 4 3 3 3 4 6% 45%	2200
November	1st to 10th	Day	13% 7% 2 5% 3 3 4 3 4 6% 50%	1968
	11th to 20th	Day	12% 10% 2 6% 4 4 4 4 4 7% 43%	1890
	21st to end	Day	15% 11% 3 7% 4 3 3 3 5% 6% 40%	1890
December	1st to 10th	Day	16% 13% 4 11% 5% 3 3 3 3 5% 35%	3020
	11th to 20th	Day	20% 15% 4 8% 4 3 4 4 5% 6% 28%	3463
	21st to end	Day	15% 13% 6% 7% 5% 5% 5% 4 6% 7% 28%	4565
January	1st to 10th	Night	16% 16% 5% 14% 6% 3 3 2 3 4 27%	6019
	11th to 20th	Night	17% 13% 5% 11% 7% 4 3 2 3 4 32%	6030
	21st to end	Night	16% 11% 4 9% 4 4 3 3 4 5% 37%	6633
February	1st to 10th	Night	18% 12% 3 7% 3 4 4 3 5% 7% 34%	2572
	11th to 20th	Night	21% 13% 5% 9% 4 4 4 3 4 4 28%	2338
	21st to end	Night	21% 13% 3 8% 4 3 3 4 4 6% 31%	1980
March	1st to 10th	Night	22% 13% 3 5% 3 3 3 2 3 5% 37%	4974
	11th to 20th	Night	15% 7% 3 4 2 2 3 3 5% 7% 48%	4980
	21st to end	Night	13% 9% 3 6% 4 4 5% 4 5% 7% 40%	5467
April	1st to 10th	Night	16% 10% 2 4 5% 3 5% 3 5% 7% 39%	2200
	11th to 20th	Night	15% 8% 2 5% 4 4 4 3 6% 7% 41%	1951
	21st to end	Night	16% 8% 2 7% 5% 4 4 4 5% 7% 39%	1800
May	1st to 10th	Night	21% 10% 3 7% 5% 4 5% 4 7% 9% 26%	4033
	11th to 20th	Night	19% 10% 4 9% 6% 4 5% 4 6% 8% 26%	4032
	21st to end	Night	21% 9% 3 6% 4 4 4 4 6% 8% 30%	4421
June	1st to 10th	Night	12% 7% 2 7% 5% 4 5% 4 6% 11% 38%	1800
	11th to 20th	Night	8% 7% 2 7% 5% 6% 5% 5% 7% 10% 40%	1800
	21st to end	Night	10% 7% 3 5% 5% 4 5% 4 7% 11% 38%	1800
July	1st to 10th	Night	13% 6% 2 6% 5% 4 6% 5% 8% 11% 35%	4058
	11th to 20th	Night	16% 6% 2 6% 4 5% 5% 5% 7% 11% 32%	4057
	21st to end	Night	19% 9% 4 9% 6% 5% 5% 5% 6% 9% 23%	4427
August	1st to 10th	Night	15% 7% 3 7% 5% 5% 6% 5% 7% 9% 33%	2915
	11th to 20th	Night	14% 7% 2 7% 5% 5% 5% 4 6% 8% 36%	2875
	21st to end	Night	7% 4 2 4 3 4 4 4 6% 10% 53%	3436
September	1st to 10th	Night	13% 6% 2 6% 5% 4 5% 3 5% 7% 44%	2200
	11th to 20th	Night	14% 6% 3 6% 4 4 4 4 5% 8% 43%	2200
	21st to end	Night	16% 7% 2 6% 4 3 4 3 6% 6% 43%	2200
October	1st to 10th	Night	16% 6% 2 5% 3 3 4 3 4 6% 49%	2268
	11th to 20th	Night	16% 7% 2 4 3 3 3 3 5% 5% 49%	2400
	21st to end	Night	16% 8% 3 5% 3 3 3 3 4 6% 47%	2640
November	1st to 10th	Night	14% 8% 2 5% 3 3 4 3 4 5% 49%	2547
	11th to 20th	Night	15% 10% 3 6% 3 3 3 3 5% 6% 43%	2730
	21st to end	Night	14% 10% 3 5% 4 3 4 3 4 5% 44%	2730
December	1st to 10th	Night	16% 12% 5% 9% 4 3 4 3 4 5% 36%	4380
	11th to 20th	Night	21% 13% 7% 10% 4 3 3 3 3 5% 28%	4952
	21st to end	Night	15% 15% 7% 7% 4 4 4 3 5% 7% 28%	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	57% 5% 6% 17% 12% 2	4147
	11th to 20th	Day	60% 5% 4 11% 18% 2	4150
	21st to end	Day	52% 4 7% 21% 14% 2	4565
February	1st to 10th	Day	55% 5% 8% 17% 14% 2	1825
	11th to 20th	Day	60% 4 6% 15% 14% 2	1958
	21st to end	Day	53% 3 8% 16% 16% 5%	1650
March	1st to 10th	Day	52% 4 6% 14% 17% 7%	4877
	11th to 20th	Day	40% 4 6% 20% 24% 7%	4980
	21st to end	Day	44% 5% 7% 28% 13% 2	5465
April	1st to 10th	Day	44% 6% 11% 22% 14% 3	2200
	11th to 20th	Day	43% 5% 9% 21% 18% 4	2414
	21st to end	Day	47% 6% 12% 23% 11% 2	2600
May	1st to 10th	Day	54% 9% 12% 17% 7%	5897
	11th to 20th	Day	55% 9% 14% 18% 4 1	5865
	21st to end	Day	46% 9% 14% 18% 11% 2	6447
June	1st to 10th	Day	44% 9% 15% 20% 11% 1	2600
	11th to 20th	Day	44% 11% 15% 18% 11% 1	2600
	21st to end	Day	42% 9% 16% 19% 13% 1	2600
July	1st to 10th	Day	38% 12% 19% 21% 10% 1	5835
	11th to 20th	Day	46% 11% 21% 17% 4 1	5831
	21st to end	Day	56% 9% 13% 14% 8%	6240
August	1st to 10th	Day	48% 11% 19% 17% 5%	4221
	11th to 20th	Day	42% 10% 19% 20% 8% 1	4213
	21st to end	Day	29% 7% 19% 33% 11% 1	4423
September	1st to 10th	Day	40% 8% 14% 19% 15% 4	2200
	11th to 20th	Day	45% 6% 10% 23% 15% 1	2200
	21st to end	Day	44% 7% 14% 19% 13% 4	2200
October	1st to 10th	Day	29% 5% 9% 20% 30% 8%	3468
	11th to 20th	Day	28% 3 9% 26% 16% 18%	3240
	21st to end	Day	31% 4 5% 31% 17% 13%	3538
November	1st to 10th	Day	36% 4 9% 20% 26% 5%	1968
	11th to 20th	Day	40% 4 8% 20% 23% 5%	1890
	21st to end	Day	45% 4 7% 22% 18% 4	1890
December	1st to 10th	Day	52% 4 8% 14% 20% 2	3020
	11th to 20th	Day	55% 5% 9% 15% 13% 2	3460
	21st to end	Day	51% 8% 11% 19% 9% 1	4565
January	1st to 10th	Night	62% 3 6% 16% 11% 2	6019
	11th to 20th	Night	56% 5% 5% 13% 17% 4	6030
	21st to end	Night	50% 5% 7% 20% 16% 2	6633
February	1st to 10th	Night	51% 4 10% 19% 15% 1	2572
	11th to 20th	Night	58% 5% 8% 15% 11% 4	2339
	21st to end	Night	54% 5% 7% 18% 12% 4	1980
March	1st to 10th	Night	51% 4 7% 19% 14% 6%	4974
	11th to 20th	Night	36% 3 9% 20% 23% 9%	4980
	21st to end	Night	42% 5% 11% 26% 13% 2	5467
April	1st to 10th	Night	43% 6% 13% 22% 13% 3	2200
	11th to 20th	Night	41% 6% 10% 21% 17% 5%	1951
	21st to end	Night	44% 7% 10% 25% 12% 1	1800
May	1st to 10th	Night	52% 10% 14% 16% 8% 1	4060
	11th to 20th	Night	54% 9% 14% 19% 3	4033
	21st to end	Night	49% 9% 14% 18% 9% 1	4429
June	1st to 10th	Night	40% 10% 18% 20% 12% 1	1800
	11th to 20th	Night	37% 10% 19% 23% 10% 1	1800
	21st to end	Night	38% 10% 16% 21% 12% 2	1800
July	1st to 10th	Night	40% 10% 19% 22% 8% 1	4015
	11th to 20th	Night	44% 12% 22% 17% 4 1	4006
	21st to end	Night	56% 11% 12% 14% 7%	4297
August	1st to 10th	Night	44% 12% 20% 19% 5%	2904
	11th to 20th	Night	43% 9% 19% 20% 8%	2902
	21st to end	Night	25% 7% 19% 37% 11% 1	3436
September	1st to 10th	Night	38% 8% 16% 19% 16% 3	2200
	11th to 20th	Night	39% 7% 15% 22% 16% 1	2200
	21st to end	Night	40% 7% 14% 21% 16% 2	2200
October	1st to 10th	Night	37% 4 8% 18% 26% 7%	3710
	11th to 20th	Night	29% 4 9% 21% 21% 16%	3899
	21st to end	Night	27% 3 5% 30% 21% 12%	4252
November	1st to 10th	Night	38% 4 9% 21% 22% 5%	2547
	11th to 20th	Night	41% 4 7% 20% 22% 6%	2730
	21st to end	Night	42% 4 7% 24% 19% 3	2730
December	1st to 10th	Night	51% 5% 7% 19% 16% 2	4380
	11th to 20th	Night	61% 3 8% 16% 11% 2	4954
	21st to end	Night	54% 6% 11% 17% 11% 1	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	40% 28% 22% 8% 2	330
	11th to 20th	Day	47% 22% 22% 9%	330
	21st to end	Day	38% 28% 23% 10% 1	363
February	1st to 10th	Day	32% 27% 27% 10% 3	300
	11th to 20th	Day	52% 22% 20% 5% 1	300
	21st to end	Day	46% 22% 24% 6% 1	249
March	1st to 10th	Day	31% 20% 29% 14% 5% 1	466
	11th to 20th	Day	33% 23% 27% 13% 2 1	500
	21st to end	Day	34% 19% 28% 15% 4	550
April	1st to 10th	Day	29% 22% 25% 17% 6% 1	500
	11th to 20th	Day	25% 20% 28% 18% 8% 1	492
	21st to end	Day	27% 14% 29% 18% 8% 4	500
May	1st to 10th	Day	29% 18% 24% 18% 8% 4	503
	11th to 20th	Day	23% 11% 25% 24% 12% 4	527
	21st to end	Day	19% 11% 22% 23% 16% 10%	573
June	1st to 10th	Day	16% 8% 25% 27% 17% 6%	530
	11th to 20th	Day	11% 7% 22% 31% 22% 7%	530
	21st to end	Day	14% 9% 24% 26% 21% 6%	530
July	1st to 10th	Day	14% 6% 17% 28% 24% 11%	525
	11th to 20th	Day	12% 5% 15% 35% 22% 10%	524
	21st to end	Day	16% 4 18% 27% 24% 11%	576
August	1st to 10th	Day	17% 5% 20% 25% 20% 13%	526
	11th to 20th	Day	14% 4 21% 27% 25% 9%	519
	21st to end	Day	12% 4 14% 29% 25% 15%	583
September	1st to 10th	Day	12% 6% 18% 29% 26% 10%	530
	11th to 20th	Day	16% 6% 16% 26% 26% 10%	530
	21st to end	Day	18% 11% 19% 30% 16% 5%	530
October	1st to 10th	Day	18% 7% 24% 28% 17% 6%	530
	11th to 20th	Day	20% 15% 23% 26% 14% 2	530
	21st to end	Day	26% 12% 25% 24% 11% 2	583
November	1st to 10th	Day	20% 16% 30% 23% 9% 3	446
	11th to 20th	Day	28% 21% 28% 18% 5%	330
	21st to end	Day	30% 20% 29% 17% 5%	330
December	1st to 10th	Day	35% 23% 29% 11% 2	330
	11th to 20th	Day	34% 18% 29% 17% 3	330
	21st to end	Day	33% 20% 29% 13% 4	363
January	1st to 10th	Night	31% 20% 25% 21% 3	840
	11th to 20th	Night	33% 18% 26% 20% 3	840
	21st to end	Night	27% 21% 31% 19% 2	924
February	1st to 10th	Night	27% 16% 30% 24% 3	801
	11th to 20th	Night	33% 23% 29% 14% 1	783
	21st to end	Night	33% 21% 28% 17% 2	660
March	1st to 10th	Night	27% 18% 29% 23% 3	800
	11th to 20th	Night	29% 17% 28% 19% 7% 1	800
	21st to end	Night	23% 16% 31% 24% 6%	880
April	1st to 10th	Night	25% 17% 27% 25% 7%	800
	11th to 20th	Night	21% 15% 27% 26% 10% 1	712
	21st to end	Night	19% 13% 25% 34% 8% 1	600
May	1st to 10th	Night	21% 10% 27% 28% 13% 1	600
	11th to 20th	Night	19% 6% 22% 32% 18% 3	591
	21st to end	Night	15% 4 16% 31% 28% 5%	650
June	1st to 10th	Night	13% 4 15% 36% 28% 4	600
	11th to 20th	Night	10% 5% 13% 39% 30% 5%	600
	21st to end	Night	12% 3 13% 38% 31% 3	600
July	1st to 10th	Night	14% 3 8% 34% 37% 5%	585
	11th to 20th	Night	14% 2 7% 27% 43% 7%	582
	21st to end	Night	12% 4 7% 33% 35% 9%	657
August	1st to 10th	Night	15% 2 7% 34% 38% 5%	597
	11th to 20th	Night	12% 3 7% 38% 34% 6%	594
	21st to end	Night	11% 4 8% 35% 36% 6%	726
September	1st to 10th	Night	14% 3 12% 33% 33% 5%	800
	11th to 20th	Night	17% 4 11% 31% 33% 5%	800
	21st to end	Night	17% 7% 15% 36% 23% 3	800
October	1st to 10th	Night	17% 6% 18% 29% 24% 6%	800
	11th to 20th	Night	19% 9% 23% 34% 14% 2	800
	21st to end	Night	22% 11% 22% 31% 11% 2	880
November	1st to 10th	Night	19% 8% 25% 32% 14% 2	820
	11th to 20th	Night	23% 13% 24% 30% 9% 1	840
	21st to end	Night	23% 14% 26% 27% 9% 1	840
December	1st to 10th	Night	29% 15% 28% 22% 6%	840
	11th to 20th	Night	27% 15% 27% 25% 5% 1	840
	21st to end	Night	30% 14% 25% 27% 4	924

Page generated at 2026-01-29 01:21:46. Version ac07

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