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 2024-01-14.

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 6% 5% 3 5% 5% 5% 7% 35%	74913
	Night	16% 9% 4 6% 5% 3 4 4 5% 7% 37%	74859
Transparency	Day	47% 7% 11% 19% 13% 2	74636
	Night	46% 7% 11% 20% 14% 3	74655
Seeing	Day	23% 13% 22% 22% 14% 6%	13942
	Night	21% 11% 20% 28% 17% 3	24080

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	19% 12% 4 8% 7% 3 4 4 3 5% 31%	1706
	11th to 20th	Day	19% 15% 5% 10% 5% 3 4 4 3 4 27%	1647
	21st to end	Day	16% 13% 7% 6% 5% 3 4 3 4 5% 33%	1782
February	1st to 10th	Day	18% 13% 5% 8% 6% 3 5% 4 4 5% 29%	1642
	11th to 20th	Day	25% 12% 6% 8% 4 2 3 3 3 6% 28%	1748
	21st to end	Day	24% 12% 4 6% 3 2 3 3 5% 5% 32%	1480
March	1st to 10th	Day	16% 11% 4 5% 3 3 4 4 4 6% 40%	1948
	11th to 20th	Day	21% 10% 4 7% 5% 2 4 5% 4 7% 31%	1980
	21st to end	Day	20% 12% 3 5% 5% 3 5% 4 5% 6% 33%	2178
April	1st to 10th	Day	15% 9% 3 5% 5% 3 4 5% 5% 7% 39%	1980
	11th to 20th	Day	17% 7% 3 5% 4 3 4 5% 5% 7% 41%	2169
	21st to end	Day	19% 6% 3 5% 6% 3 4 4 5% 7% 37%	2340
May	1st to 10th	Day	18% 7% 3 5% 7% 4 6% 6% 5% 8% 33%	2340
	11th to 20th	Day	21% 7% 3 7% 6% 3 5% 6% 6% 6% 30%	2301
	21st to end	Day	13% 5% 3 6% 6% 4 6% 7% 7% 10% 34%	2531
June	1st to 10th	Day	13% 6% 2 7% 6% 4 6% 6% 7% 9% 34%	2340
	11th to 20th	Day	10% 5% 3 7% 7% 5% 8% 6% 8% 10% 32%	2340
	21st to end	Day	12% 6% 3 6% 6% 4 7% 7% 8% 9% 32%	2340
July	1st to 10th	Day	12% 7% 3 7% 6% 5% 7% 6% 7% 9% 31%	2340
	11th to 20th	Day	14% 6% 3 6% 6% 4 8% 7% 6% 9% 30%	2340
	21st to end	Day	15% 6% 3 7% 7% 5% 7% 6% 7% 8% 28%	2547
August	1st to 10th	Day	13% 6% 2 7% 7% 4 7% 7% 8% 10% 28%	2327
	11th to 20th	Day	15% 6% 3 6% 8% 5% 8% 7% 8% 8% 28%	2303
	21st to end	Day	12% 6% 2 5% 6% 3 6% 6% 7% 9% 37%	2442
September	1st to 10th	Day	12% 5% 2 5% 6% 4 6% 5% 7% 9% 38%	1980
	11th to 20th	Day	15% 7% 2 6% 6% 3 5% 6% 6% 7% 37%	1980
	21st to end	Day	16% 6% 2 6% 4 4 5% 5% 5% 7% 39%	1980
October	1st to 10th	Day	15% 6% 2 4 5% 3 4 3 5% 6% 48%	1918
	11th to 20th	Day	17% 7% 2 3 4 2 3 4 5% 6% 47%	1800
	21st to end	Day	18% 9% 2 4 4 2 3 3 4 6% 44%	1980
November	1st to 10th	Day	14% 6% 2 4 3 2 4 4 4 6% 51%	1777
	11th to 20th	Day	12% 10% 3 5% 4 3 4 4 4 7% 44%	1710
	21st to end	Day	15% 9% 3 6% 5% 3 3 4 5% 6% 42%	1710
December	1st to 10th	Day	20% 15% 4 7% 5% 2 4 4 4 6% 30%	1710
	11th to 20th	Day	17% 15% 3 6% 4 2 3 5% 5% 7% 34%	2153
	21st to end	Day	15% 15% 6% 7% 6% 4 6% 5% 5% 5% 27%	3124
January	1st to 10th	Night	19% 12% 4 8% 7% 2 3 4 4 6% 30%	2459
	11th to 20th	Night	20% 14% 6% 7% 5% 3 4 4 5% 5% 27%	2391
	21st to end	Night	16% 14% 6% 8% 6% 3 3 3 4 4 34%	2574
February	1st to 10th	Night	18% 11% 4 7% 4 3 4 4 4 8% 33%	2314
	11th to 20th	Night	21% 12% 5% 8% 5% 3 4 5% 3 5% 29%	2098
	21st to end	Night	23% 13% 3 8% 4 2 3 4 4 6% 30%	1776
March	1st to 10th	Night	17% 12% 4 6% 4 3 4 3 3 5% 40%	2010
	11th to 20th	Night	20% 12% 4 6% 4 3 4 4 5% 7% 31%	1980
	21st to end	Night	16% 11% 4 5% 5% 3 5% 4 5% 7% 36%	2178
April	1st to 10th	Night	16% 9% 2 4 5% 3 4 4 5% 7% 40%	1980
	11th to 20th	Night	15% 7% 2 4 5% 3 5% 4 6% 7% 42%	1755
	21st to end	Night	17% 7% 3 5% 5% 3 4 4 4 7% 40%	1620
May	1st to 10th	Night	19% 7% 4 5% 5% 3 4 4 6% 8% 35%	1620
	11th to 20th	Night	16% 6% 3 5% 6% 4 6% 6% 6% 10% 32%	1592
	21st to end	Night	11% 7% 3 5% 5% 3 6% 6% 7% 10% 37%	1744
June	1st to 10th	Night	11% 5% 2 6% 5% 3 5% 5% 6% 12% 39%	1620
	11th to 20th	Night	7% 6% 3 6% 6% 4 5% 5% 7% 10% 42%	1620
	21st to end	Night	10% 6% 3 5% 5% 3 6% 5% 7% 12% 37%	1620
July	1st to 10th	Night	12% 6% 2 6% 6% 3 6% 6% 6% 11% 36%	1620
	11th to 20th	Night	11% 5% 2 5% 5% 5% 6% 6% 8% 12% 35%	1620
	21st to end	Night	10% 6% 3 6% 6% 4 6% 7% 8% 11% 34%	1771
August	1st to 10th	Night	12% 6% 2 6% 6% 4 5% 6% 7% 10% 38%	1611
	11th to 20th	Night	12% 7% 2 8% 7% 3 6% 6% 6% 8% 34%	1598
	21st to end	Night	11% 5% 3 5% 5% 3 5% 6% 7% 11% 40%	1890
September	1st to 10th	Night	13% 5% 2 5% 6% 3 5% 4 5% 8% 44%	1980
	11th to 20th	Night	15% 6% 3 5% 4 3 4 5% 5% 8% 43%	1980
	21st to end	Night	14% 6% 2 5% 4 2 5% 4 6% 7% 46%	1980
October	1st to 10th	Night	16% 6% 2 4 3 2 4 3 4 6% 48%	2040
	11th to 20th	Night	17% 7% 2 4 3 2 3 4 5% 5% 47%	2160
	21st to end	Night	16% 7% 3 4 4 3 4 4 4 6% 46%	2376
November	1st to 10th	Night	14% 8% 2 4 3 2 4 3 4 5% 50%	2298
	11th to 20th	Night	15% 9% 3 5% 4 3 3 4 5% 6% 44%	2470
	21st to end	Night	14% 9% 3 4 4 3 4 4 4 6% 45%	2470
December	1st to 10th	Night	20% 12% 5% 8% 6% 3 4 3 4 6% 29%	2470
	11th to 20th	Night	19% 12% 5% 9% 5% 2 4 4 4 5% 33%	3042
	21st to end	Night	15% 17% 8% 7% 6% 3 5% 5% 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	55% 6% 8% 16% 11% 4	1707
	11th to 20th	Day	61% 4 7% 15% 10% 3	1647
	21st to end	Day	53% 3 8% 21% 13% 1	1782
February	1st to 10th	Day	57% 5% 7% 17% 12% 1	1642
	11th to 20th	Day	60% 5% 6% 16% 13% 1	1758
	21st to end	Day	54% 3 8% 15% 15% 5%	1480
March	1st to 10th	Day	45% 4 7% 22% 18% 4	1948
	11th to 20th	Day	53% 5% 9% 18% 11% 3	1980
	21st to end	Day	52% 5% 11% 21% 9% 2	2178
April	1st to 10th	Day	43% 6% 11% 24% 14% 3	1980
	11th to 20th	Day	43% 5% 8% 20% 19% 5%	2169
	21st to end	Day	46% 6% 12% 22% 11% 2	2340
May	1st to 10th	Day	48% 7% 14% 21% 10% 1	2340
	11th to 20th	Day	53% 7% 14% 19% 6% 1	2305
	21st to end	Day	43% 10% 16% 23% 8%	2531
June	1st to 10th	Day	44% 9% 16% 20% 11% 1	2340
	11th to 20th	Day	45% 11% 15% 16% 11% 2	2340
	21st to end	Day	45% 9% 15% 18% 13% 1	2340
July	1st to 10th	Day	46% 11% 16% 16% 9% 1	2275
	11th to 20th	Day	47% 11% 17% 13% 11% 2	2271
	21st to end	Day	50% 9% 15% 17% 7% 1	2405
August	1st to 10th	Day	48% 11% 17% 17% 7%	2314
	11th to 20th	Day	50% 11% 15% 15% 9%	2303
	21st to end	Day	42% 9% 17% 19% 11% 2	2442
September	1st to 10th	Day	41% 8% 15% 20% 12% 4	1980
	11th to 20th	Day	45% 6% 10% 22% 16% 1	1980
	21st to end	Day	43% 6% 13% 20% 14% 4	1980
October	1st to 10th	Day	37% 5% 11% 22% 20% 4	1918
	11th to 20th	Day	38% 4 9% 26% 19% 5%	1800
	21st to end	Day	41% 4 7% 24% 18% 5%	1980
November	1st to 10th	Day	35% 4 8% 20% 27% 6%	1777
	11th to 20th	Day	41% 4 7% 20% 23% 6%	1710
	21st to end	Day	43% 4 8% 22% 19% 4	1710
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	56% 4 8% 15% 12% 4	2459
	11th to 20th	Night	60% 5% 7% 13% 13% 3	2391
	21st to end	Night	55% 4 8% 17% 14% 2	2574
February	1st to 10th	Night	52% 4 10% 20% 14% 1	2314
	11th to 20th	Night	58% 5% 7% 17% 10% 4	2099
	21st to end	Night	55% 5% 7% 18% 10% 4	1776
March	1st to 10th	Night	48% 4 8% 17% 19% 4	2010
	11th to 20th	Night	52% 5% 11% 20% 10% 2	1980
	21st to end	Night	47% 7% 12% 21% 12% 2	2178
April	1st to 10th	Night	42% 5% 13% 23% 13% 3	1980
	11th to 20th	Night	40% 6% 11% 20% 17% 5%	1755
	21st to end	Night	44% 7% 10% 26% 12% 1	1620
May	1st to 10th	Night	46% 8% 15% 21% 8% 1	1620
	11th to 20th	Night	46% 11% 14% 21% 7% 1	1593
	21st to end	Night	40% 11% 18% 21% 9% 1	1745
June	1st to 10th	Night	39% 9% 19% 20% 12%	1620
	11th to 20th	Night	36% 10% 20% 23% 10% 1	1620
	21st to end	Night	39% 11% 16% 20% 11% 3	1620
July	1st to 10th	Night	41% 9% 18% 18% 13% 1	1575
	11th to 20th	Night	37% 13% 21% 17% 9% 3	1566
	21st to end	Night	40% 12% 18% 22% 8%	1672
August	1st to 10th	Night	39% 12% 21% 20% 8% 1	1600
	11th to 20th	Night	45% 11% 16% 18% 10% 1	1598
	21st to end	Night	36% 8% 20% 24% 11% 1	1890
September	1st to 10th	Night	38% 9% 16% 20% 14% 3	1980
	11th to 20th	Night	39% 7% 14% 21% 17% 1	1980
	21st to end	Night	37% 7% 13% 23% 18% 2	1980
October	1st to 10th	Night	37% 5% 11% 22% 21% 4	2040
	11th to 20th	Night	38% 5% 9% 26% 18% 4	2160
	21st to end	Night	40% 5% 4 23% 21% 7%	2376
November	1st to 10th	Night	37% 4 9% 21% 23% 6%	2298
	11th to 20th	Night	41% 4 7% 20% 22% 6%	2470
	21st to end	Night	41% 4 7% 24% 20% 4	2470
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	43% 22% 24% 11%	249
	21st to end	Day	38% 29% 22% 10% 1	264
February	1st to 10th	Day	35% 28% 25% 9% 3	240
	11th to 20th	Day	51% 23% 21% 4	240
	21st to end	Day	49% 21% 23% 6% 1	198
March	1st to 10th	Day	28% 23% 30% 15% 5%	388
	11th to 20th	Day	35% 23% 25% 13% 3 1	420
	21st to end	Day	34% 18% 26% 16% 5%	462
April	1st to 10th	Day	28% 20% 24% 19% 7% 2	420
	11th to 20th	Day	24% 19% 26% 20% 9% 2	412
	21st to end	Day	29% 13% 28% 18% 8% 4	420
May	1st to 10th	Day	29% 16% 26% 16% 9% 4	423
	11th to 20th	Day	23% 9% 25% 26% 13% 5%	447
	21st to end	Day	17% 10% 20% 24% 18% 12%	485
June	1st to 10th	Day	16% 6% 24% 28% 18% 7%	450
	11th to 20th	Day	12% 7% 22% 29% 22% 8%	450
	21st to end	Day	16% 10% 25% 24% 18% 6%	450
July	1st to 10th	Day	15% 7% 16% 24% 24% 13%	445
	11th to 20th	Day	11% 5% 16% 35% 22% 11%	444
	21st to end	Day	16% 4 17% 28% 24% 11%	488
August	1st to 10th	Day	16% 5% 20% 24% 21% 14%	446
	11th to 20th	Day	15% 3 19% 26% 27% 10%	439
	21st to end	Day	14% 4 13% 28% 24% 17%	495
September	1st to 10th	Day	13% 5% 15% 29% 27% 11%	450
	11th to 20th	Day	17% 6% 14% 26% 27% 10%	450
	21st to end	Day	18% 9% 18% 31% 18% 6%	450
October	1st to 10th	Day	21% 7% 23% 25% 18% 7%	450
	11th to 20th	Day	22% 15% 21% 26% 14% 2	450
	21st to end	Day	25% 13% 25% 24% 11% 3	495
November	1st to 10th	Day	19% 16% 27% 24% 10% 3	375
	11th to 20th	Day	26% 21% 28% 19% 6%	270
	21st to end	Day	25% 19% 31% 19% 6%	270
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% 22% 4	736
	21st to end	Night	26% 22% 29% 20% 3	792
February	1st to 10th	Night	27% 17% 28% 25% 3	720
	11th to 20th	Night	33% 23% 29% 15% 1	703
	21st to end	Night	34% 21% 25% 18% 2	592
March	1st to 10th	Night	26% 19% 28% 24% 3	720
	11th to 20th	Night	29% 16% 27% 19% 8% 1	720
	21st to end	Night	23% 15% 29% 26% 7%	792
April	1st to 10th	Night	25% 16% 25% 26% 8%	720
	11th to 20th	Night	20% 14% 27% 27% 11% 1	641
	21st to end	Night	19% 12% 24% 34% 9% 1	540
May	1st to 10th	Night	21% 9% 26% 28% 14% 1	540
	11th to 20th	Night	18% 6% 20% 33% 19% 3	531
	21st to end	Night	14% 3 14% 33% 31% 5%	584
June	1st to 10th	Night	13% 3 14% 36% 30% 4	540
	11th to 20th	Night	8% 5% 13% 36% 31% 6%	540
	21st to end	Night	12% 3 14% 37% 31% 4	540
July	1st to 10th	Night	14% 3 7% 32% 38% 6%	525
	11th to 20th	Night	13% 2 7% 29% 43% 7%	522
	21st to end	Night	9% 4 7% 33% 36% 10%	591
August	1st to 10th	Night	13% 2 5% 33% 41% 5%	537
	11th to 20th	Night	13% 3 6% 38% 34% 6%	534
	21st to end	Night	12% 4 8% 33% 37% 7%	652
September	1st to 10th	Night	14% 3 11% 31% 36% 6%	720
	11th to 20th	Night	17% 4 10% 30% 34% 5%	720
	21st to end	Night	16% 6% 15% 35% 24% 3	720
October	1st to 10th	Night	18% 7% 18% 26% 25% 6%	720
	11th to 20th	Night	20% 9% 23% 31% 14% 2	720
	21st to end	Night	22% 12% 22% 31% 11% 3	792
November	1st to 10th	Night	20% 7% 23% 33% 16% 2	740
	11th to 20th	Night	22% 13% 23% 31% 10% 1	760
	21st to end	Night	22% 13% 26% 28% 10% 1	760
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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