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 2024-11-28.

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% 35%	101396
	Night	16% 10% 3 7% 4 4 4 3 5% 7% 36%	98097
Transparency	Day	46% 7% 11% 20% 13% 3	105125
	Night	45% 7% 11% 20% 14% 3	102507
Seeing	Day	23% 13% 23% 22% 13% 5%	15148
	Night	21% 11% 21% 28% 16% 2	25219

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% 14% 10% 10% 4 3 3 3 4 5% 28%	2836
	11th to 20th	Day	17% 18% 7% 12% 7% 4 3 3 3 3 23%	2840
	21st to end	Day	18% 17% 6% 8% 5% 3 3 2 3 4 28%	3124
February	1st to 10th	Day	18% 14% 3 8% 5% 4 4 3 4 5% 30%	1733
	11th to 20th	Day	24% 13% 5% 8% 4 3 3 2 4 5% 29%	1848
	21st to end	Day	24% 13% 3 7% 3 3 3 2 5% 5% 32%	1570
March	1st to 10th	Day	20% 14% 4 7% 3 3 3 3 4 6% 33%	3391
	11th to 20th	Day	15% 8% 3 7% 4 3 3 3 4 5% 45%	3480
	21st to end	Day	19% 11% 2 6% 4 4 4 3 5% 6% 37%	3815
April	1st to 10th	Day	15% 10% 2 6% 4 4 4 5% 5% 7% 38%	2090
	11th to 20th	Day	16% 8% 2 6% 4 4 4 4 5% 6% 41%	2292
	21st to end	Day	19% 7% 3 5% 5% 4 4 3 6% 7% 38%	2470
May	1st to 10th	Day	22% 9% 3 7% 6% 5% 5% 4 5% 7% 27%	4082
	11th to 20th	Day	20% 9% 3 8% 5% 5% 5% 5% 5% 6% 28%	4081
	21st to end	Day	11% 6% 3 7% 4 5% 6% 5% 8% 11% 35%	4476
June	1st to 10th	Day	13% 7% 2 8% 6% 5% 6% 5% 7% 8% 34%	2470
	11th to 20th	Day	10% 6% 2 9% 6% 7% 6% 5% 8% 9% 32%	2470
	21st to end	Day	11% 6% 3 6% 6% 5% 6% 6% 8% 9% 33%	2470
July	1st to 10th	Day	9% 5% 2 6% 6% 6% 7% 6% 8% 11% 35%	4120
	11th to 20th	Day	14% 5% 3 6% 5% 5% 5% 5% 6% 9% 36%	4120
	21st to end	Day	23% 9% 3 9% 6% 7% 7% 4 6% 7% 19%	4466
August	1st to 10th	Day	15% 7% 2 8% 6% 6% 6% 6% 8% 10% 27%	4104
	11th to 20th	Day	14% 6% 2 6% 6% 5% 7% 5% 8% 10% 32%	4044
	21st to end	Day	7% 4 1 4 3 4 5% 4 6% 9% 52%	4288
September	1st to 10th	Day	12% 6% 2 6% 5% 6% 6% 4 7% 8% 39%	2090
	11th to 20th	Day	15% 8% 2 7% 6% 4 5% 4 6% 7% 37%	2090
	21st to end	Day	16% 6% 2 6% 4 5% 5% 4 6% 7% 38%	2090
October	1st to 10th	Day	15% 6% 2 4 4 3 4 2 5% 6% 48%	2024
	11th to 20th	Day	16% 7% 2 4 3 3 3 3 5% 6% 48%	1900
	21st to end	Day	17% 9% 2 5% 4 3 3 3 4 6% 46%	2090
November	1st to 10th	Day	14% 7% 2 5% 3 3 4 3 4 6% 50%	1872
	11th to 20th	Day	12% 11% 2 6% 4 4 4 4 5% 7% 43%	1800
	21st to end	Day	14% 11% 3 7% 4 3 3 3 5% 5% 41%	1773
December	1st to 10th	Day	20% 15% 4 8% 4 3 4 3 4 5% 30%	1710
	11th to 20th	Day	17% 16% 3 7% 3 2 4 4 5% 7% 34%	2153
	21st to end	Day	15% 16% 5% 8% 6% 5% 5% 4 5% 5% 27%	3124
January	1st to 10th	Night	19% 19% 6% 10% 5% 3 3 3 4 4 25%	4109
	11th to 20th	Night	20% 13% 4 8% 6% 5% 3 2 4 5% 31%	4120
	21st to end	Night	20% 14% 4 8% 5% 4 3 3 4 5% 30%	4532
February	1st to 10th	Night	17% 12% 3 8% 3 4 4 3 5% 7% 34%	2443
	11th to 20th	Night	21% 13% 5% 8% 5% 4 4 3 4 4 29%	2218
	21st to end	Night	22% 14% 3 8% 4 3 3 4 4 6% 30%	1884
March	1st to 10th	Night	24% 14% 3 6% 4 4 3 3 3 5% 33%	3460
	11th to 20th	Night	16% 8% 5% 5% 3 3 3 3 5% 6% 44%	3480
	21st to end	Night	16% 8% 2 5% 4 4 4 4 5% 7% 41%	3817
April	1st to 10th	Night	16% 10% 2 5% 5% 3 4 3 5% 7% 40%	2090
	11th to 20th	Night	15% 8% 2 5% 4 4 4 3 6% 7% 40%	1852
	21st to end	Night	16% 7% 2 6% 5% 4 4 3 4 7% 41%	1710
May	1st to 10th	Night	21% 10% 3 7% 5% 4 5% 4 6% 7% 29%	2813
	11th to 20th	Night	21% 8% 3 6% 4 4 4 4 6% 8% 30%	2812
	21st to end	Night	13% 7% 3 6% 4 5% 5% 5% 7% 10% 35%	3079
June	1st to 10th	Night	11% 7% 2 7% 5% 5% 5% 4 6% 11% 38%	1710
	11th to 20th	Night	7% 6% 2 7% 5% 6% 5% 5% 7% 10% 41%	1710
	21st to end	Night	11% 7% 3 5% 5% 4 6% 4 7% 11% 38%	1710
July	1st to 10th	Night	12% 6% 2 6% 5% 4 7% 5% 8% 12% 33%	2840
	11th to 20th	Night	13% 6% 2 5% 4 4 5% 4 7% 10% 40%	2840
	21st to end	Night	20% 10% 4 9% 6% 5% 4 5% 6% 9% 22%	3087
August	1st to 10th	Night	14% 7% 3 7% 5% 5% 6% 5% 7% 9% 33%	2825
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 4 6% 8% 36%	2785
	21st to end	Night	6% 4 2 4 3 4 4 4 6% 10% 54%	3330
September	1st to 10th	Night	13% 6% 2 6% 5% 4 5% 3 6% 7% 44%	2090
	11th to 20th	Night	15% 6% 3 6% 4 4 4 4 5% 8% 42%	2090
	21st to end	Night	15% 7% 2 6% 4 3 4 3 6% 6% 45%	2090
October	1st to 10th	Night	16% 6% 2 5% 3 3 4 3 4 6% 49%	2154
	11th to 20th	Night	16% 7% 2 4 3 3 3 3 5% 5% 48%	2280
	21st to end	Night	16% 7% 2 5% 3 3 4 3 4 6% 47%	2508
November	1st to 10th	Night	14% 8% 2 5% 3 3 4 3 4 5% 49%	2423
	11th to 20th	Night	15% 10% 3 5% 4 3 3 3 5% 6% 43%	2600
	21st to end	Night	14% 10% 3 5% 4 3 4 3 4 6% 45%	2562
December	1st to 10th	Night	20% 13% 5% 9% 5% 4 4 2 4 6% 29%	2470
	11th to 20th	Night	18% 12% 5% 10% 4 3 4 3 4 5% 33%	3042
	21st to end	Night	15% 17% 8% 8% 5% 4 4 4 6% 7% 23%	4532

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	56% 7% 5% 16% 12% 3	2837
	11th to 20th	Day	64% 6% 5% 12% 11% 2	2840
	21st to end	Day	58% 6% 6% 15% 15% 1	3124
February	1st to 10th	Day	56% 5% 7% 16% 14% 1	1733
	11th to 20th	Day	59% 5% 6% 15% 14% 2	1858
	21st to end	Day	54% 4 8% 15% 15% 5%	1570
March	1st to 10th	Day	52% 5% 6% 18% 15% 3	3391
	11th to 20th	Day	41% 6% 7% 25% 19% 4	3480
	21st to end	Day	47% 5% 8% 25% 12% 2	3815
April	1st to 10th	Day	44% 6% 10% 23% 14% 3	2090
	11th to 20th	Day	43% 5% 8% 21% 18% 4	2292
	21st to end	Day	46% 6% 12% 23% 11% 2	2470
May	1st to 10th	Day	54% 9% 11% 18% 8% 1	4120
	11th to 20th	Day	54% 7% 11% 22% 5% 1	4085
	21st to end	Day	38% 10% 17% 21% 11% 2	4489
June	1st to 10th	Day	44% 9% 16% 20% 11% 1	2470
	11th to 20th	Day	44% 11% 15% 18% 11% 1	2470
	21st to end	Day	44% 9% 15% 18% 13% 1	2470
July	1st to 10th	Day	39% 12% 20% 19% 9% 1	4055
	11th to 20th	Day	43% 9% 18% 23% 6% 1	4051
	21st to end	Day	63% 7% 10% 13% 6%	4324
August	1st to 10th	Day	48% 11% 19% 17% 5%	4091
	11th to 20th	Day	42% 10% 19% 20% 8% 1	4083
	21st to end	Day	29% 7% 19% 33% 10% 1	4288
September	1st to 10th	Day	40% 8% 14% 19% 14% 4	2090
	11th to 20th	Day	46% 6% 10% 22% 15% 1	2090
	21st to end	Day	44% 7% 13% 19% 14% 4	2090
October	1st to 10th	Day	29% 5% 9% 19% 29% 8%	3362
	11th to 20th	Day	28% 3 9% 25% 16% 19%	3140
	21st to end	Day	30% 4 5% 31% 17% 13%	3428
November	1st to 10th	Day	36% 4 9% 20% 26% 5%	1872
	11th to 20th	Day	41% 4 7% 19% 23% 5%	1800
	21st to end	Day	44% 4 8% 22% 19% 4	1773
December	1st to 10th	Day	56% 4 9% 16% 12% 2	1710
	11th to 20th	Day	49% 5% 9% 16% 18% 3	2150
	21st to end	Day	55% 9% 6% 20% 9% 1	3124
January	1st to 10th	Night	62% 4 6% 14% 11% 3	4109
	11th to 20th	Night	55% 7% 5% 14% 17% 3	4120
	21st to end	Night	56% 6% 7% 15% 15% 1	4532
February	1st to 10th	Night	50% 4 9% 20% 15% 1	2443
	11th to 20th	Night	57% 5% 7% 16% 11% 4	2219
	21st to end	Night	55% 5% 7% 18% 11% 4	1884
March	1st to 10th	Night	56% 4 9% 15% 14% 2	3460
	11th to 20th	Night	42% 4 10% 23% 14% 7%	3480
	21st to end	Night	41% 5% 10% 26% 15% 2	3817
April	1st to 10th	Night	43% 6% 13% 22% 13% 3	2090
	11th to 20th	Night	41% 6% 11% 20% 17% 5%	1852
	21st to end	Night	43% 7% 11% 26% 12% 1	1710
May	1st to 10th	Night	52% 9% 13% 18% 7% 1	2840
	11th to 20th	Night	50% 9% 14% 22% 4 1	2813
	21st to end	Night	39% 12% 16% 22% 10% 1	3087
June	1st to 10th	Night	39% 10% 18% 20% 12% 1	1710
	11th to 20th	Night	35% 10% 20% 24% 10% 1	1710
	21st to end	Night	39% 11% 16% 20% 12% 3	1710
July	1st to 10th	Night	41% 12% 20% 19% 8% 1	2795
	11th to 20th	Night	36% 10% 23% 24% 5% 2	2786
	21st to end	Night	58% 9% 12% 16% 4	2988
August	1st to 10th	Night	44% 12% 20% 19% 5%	2814
	11th to 20th	Night	43% 9% 19% 21% 8%	2812
	21st to end	Night	25% 7% 19% 37% 11% 1	3330
September	1st to 10th	Night	38% 8% 15% 20% 15% 3	2090
	11th to 20th	Night	40% 7% 15% 21% 16% 1	2090
	21st to end	Night	39% 7% 14% 22% 17% 2	2090
October	1st to 10th	Night	38% 4 8% 17% 25% 7%	3596
	11th to 20th	Night	29% 4 8% 22% 21% 16%	3779
	21st to end	Night	27% 3 5% 31% 21% 12%	4120
November	1st to 10th	Night	38% 4 9% 21% 23% 5%	2423
	11th to 20th	Night	42% 4 7% 20% 22% 6%	2600
	21st to end	Night	41% 4 7% 24% 20% 4	2562
December	1st to 10th	Night	57% 6% 8% 16% 11% 3	2470
	11th to 20th	Night	55% 3 7% 16% 16% 3	3044
	21st to end	Night	59% 7% 8% 15% 11% 1	4532

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	36% 30% 24% 9% 2	270
	11th to 20th	Day	44% 23% 23% 10%	270
	21st to end	Day	36% 28% 24% 11% 1	297
February	1st to 10th	Day	33% 26% 27% 11% 3	270
	11th to 20th	Day	50% 24% 20% 6% 1	270
	21st to end	Day	48% 22% 24% 6%	225
March	1st to 10th	Day	30% 21% 29% 15% 6% 1	427
	11th to 20th	Day	33% 24% 26% 13% 3 1	460
	21st to end	Day	34% 20% 26% 16% 5%	506
April	1st to 10th	Day	30% 21% 23% 17% 7% 2	460
	11th to 20th	Day	25% 19% 27% 19% 8% 2	452
	21st to end	Day	28% 14% 28% 18% 8% 4	460
May	1st to 10th	Day	29% 17% 24% 18% 8% 4	463
	11th to 20th	Day	22% 10% 25% 25% 13% 5%	487
	21st to end	Day	16% 10% 22% 24% 17% 11%	529
June	1st to 10th	Day	15% 8% 25% 28% 17% 6%	490
	11th to 20th	Day	11% 6% 22% 31% 22% 7%	490
	21st to end	Day	15% 10% 26% 25% 19% 6%	490
July	1st to 10th	Day	14% 7% 17% 27% 24% 12%	485
	11th to 20th	Day	12% 5% 16% 36% 21% 10%	484
	21st to end	Day	17% 5% 18% 27% 23% 11%	532
August	1st to 10th	Day	16% 5% 21% 25% 20% 13%	486
	11th to 20th	Day	14% 4 22% 26% 25% 9%	479
	21st to end	Day	13% 3 12% 29% 27% 16%	539
September	1st to 10th	Day	12% 6% 17% 29% 27% 10%	490
	11th to 20th	Day	18% 6% 16% 25% 26% 10%	490
	21st to end	Day	19% 10% 19% 30% 17% 6%	490
October	1st to 10th	Day	19% 7% 24% 26% 17% 7%	490
	11th to 20th	Day	21% 16% 20% 26% 14% 3	490
	21st to end	Day	23% 12% 25% 26% 12% 3	539
November	1st to 10th	Day	20% 16% 29% 23% 10% 3	410
	11th to 20th	Day	27% 21% 27% 20% 5%	300
	21st to end	Day	26% 18% 32% 19% 5%	291
December	1st to 10th	Day	34% 22% 30% 12% 2	270
	11th to 20th	Day	31% 19% 29% 18% 3	270
	21st to end	Day	33% 20% 30% 14% 3	297
January	1st to 10th	Night	29% 21% 25% 22% 3	760
	11th to 20th	Night	33% 17% 25% 21% 4	760
	21st to end	Night	26% 21% 30% 20% 3	836
February	1st to 10th	Night	26% 17% 29% 25% 3	760
	11th to 20th	Night	33% 23% 29% 15% 1	743
	21st to end	Night	34% 21% 26% 17% 2	628
March	1st to 10th	Night	26% 18% 29% 24% 3	760
	11th to 20th	Night	29% 17% 27% 19% 7% 1	760
	21st to end	Night	23% 16% 30% 25% 7%	836
April	1st to 10th	Night	25% 16% 26% 25% 8%	760
	11th to 20th	Night	21% 15% 27% 26% 10% 1	676
	21st to end	Night	19% 12% 26% 34% 8% 1	570
May	1st to 10th	Night	21% 10% 26% 28% 14% 1	570
	11th to 20th	Night	19% 6% 21% 33% 19% 3	561
	21st to end	Night	14% 4 16% 32% 30% 5%	617
June	1st to 10th	Night	12% 3 15% 37% 28% 4	570
	11th to 20th	Night	8% 5% 13% 38% 30% 5%	570
	21st to end	Night	12% 3 13% 37% 31% 4	570
July	1st to 10th	Night	14% 3 7% 33% 37% 5%	555
	11th to 20th	Night	13% 2 7% 29% 43% 7%	552
	21st to end	Night	12% 4 7% 33% 34% 9%	624
August	1st to 10th	Night	14% 2 6% 34% 39% 5%	567
	11th to 20th	Night	13% 3 7% 39% 33% 6%	564
	21st to end	Night	11% 3 8% 34% 37% 7%	689
September	1st to 10th	Night	14% 3 12% 32% 34% 5%	760
	11th to 20th	Night	17% 4 11% 30% 32% 5%	760
	21st to end	Night	17% 6% 16% 35% 23% 3	760
October	1st to 10th	Night	18% 7% 18% 27% 25% 6%	760
	11th to 20th	Night	20% 9% 23% 32% 14% 2	760
	21st to end	Night	21% 11% 22% 32% 11% 3	836
November	1st to 10th	Night	19% 7% 24% 32% 15% 2	780
	11th to 20th	Night	23% 13% 23% 31% 10% 1	800
	21st to end	Night	23% 13% 26% 28% 10% 1	789
December	1st to 10th	Night	29% 15% 26% 23% 6%	760
	11th to 20th	Night	26% 15% 26% 26% 6% 1	760
	21st to end	Night	30% 14% 25% 27% 4	836

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