Rough Acre Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rough Acre 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	17% 8% 2 6% 4 4 5% 4 6% 7% 37%	130816
	Night	17% 8% 3 6% 4 4 4 4 5% 7% 38%	130679
Transparency	Day	29% 38% 8% 14% 10% 2	134323
	Night	29% 37% 7% 14% 10% 2	135259
Seeing	Day	23% 16% 30% 21% 8% 2	16563
	Night	21% 11% 29% 29% 9% 1	25543

By Month

Percentage Cloud Cover NE (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	11% 7% 3 8% 5% 6% 6% 5% 7% 6% 36%	4661
	11th to 20th	Day	12% 10% 3 7% 4 4 4 3 5% 6% 41%	4650
	21st to end	Day	10% 10% 2 6% 4 4 4 3 5% 6% 45%	5124
February	1st to 10th	Day	14% 10% 3 6% 5% 5% 5% 4 5% 7% 37%	1865
	11th to 20th	Day	11% 8% 2 8% 5% 5% 5% 5% 6% 7% 38%	2048
	21st to end	Day	15% 11% 3 7% 5% 4 5% 4 4 7% 35%	1730
March	1st to 10th	Day	19% 12% 3 7% 5% 5% 5% 4 4 5% 32%	4576
	11th to 20th	Day	17% 9% 2 5% 5% 5% 4 3 5% 6% 38%	4860
	21st to end	Day	25% 9% 2 6% 4 3 3 3 5% 8% 33%	5757
April	1st to 10th	Day	19% 10% 4 7% 4 3 3 3 5% 6% 36%	2400
	11th to 20th	Day	17% 8% 2 6% 5% 4 5% 3 5% 6% 38%	2423
	21st to end	Day	22% 7% 2 8% 5% 5% 5% 3 4 4 35%	2600
May	1st to 10th	Day	34% 10% 3 7% 4 4 4 3 4 5% 22%	5886
	11th to 20th	Day	20% 8% 2 8% 5% 5% 5% 4 5% 8% 29%	6087
	21st to end	Day	20% 7% 2 6% 4 4 5% 4 6% 8% 33%	6949
June	1st to 10th	Day	21% 8% 2 6% 4 5% 5% 3 6% 8% 31%	2800
	11th to 20th	Day	17% 7% 2 6% 4 5% 5% 4 6% 8% 37%	2800
	21st to end	Day	19% 7% 3 7% 4 4 5% 4 6% 7% 34%	2800
July	1st to 10th	Day	16% 6% 2 6% 5% 5% 6% 5% 6% 10% 33%	6360
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 5% 7% 10% 39%	6360
	21st to end	Day	15% 5% 2 5% 4 4 4 4 6% 8% 43%	6926
August	1st to 10th	Day	14% 5% 2 8% 5% 5% 5% 4 7% 8% 36%	4344
	11th to 20th	Day	18% 7% 3 7% 4 4 5% 5% 6% 7% 34%	4175
	21st to end	Day	9% 4 2 5% 4 4 5% 4 6% 9% 47%	4350
September	1st to 10th	Day	12% 4 2 4 4 4 4 4 6% 8% 48%	2395
	11th to 20th	Day	13% 5% 1 5% 3 3 3 3 4 5% 54%	2200
	21st to end	Day	17% 4 2 5% 4 4 3 4 5% 6% 47%	2200
October	1st to 10th	Day	21% 7% 2 5% 4 3 4 3 5% 5% 41%	2200
	11th to 20th	Day	17% 6% 2 5% 4 5% 4 3 5% 7% 42%	1915
	21st to end	Day	24% 7% 2 5% 4 4 4 3 5% 5% 37%	1980
November	1st to 10th	Day	14% 6% 2 5% 4 3 3 3 5% 7% 46%	1851
	11th to 20th	Day	18% 4 1 4 3 4 4 4 6% 7% 44%	1890
	21st to end	Day	20% 8% 2 5% 3 4 4 4 6% 6% 38%	1860
December	1st to 10th	Day	16% 9% 3 6% 4 5% 5% 5% 6% 8% 33%	2680
	11th to 20th	Day	18% 8% 2 6% 3 4 3 3 5% 7% 41%	3066
	21st to end	Day	20% 10% 2 7% 4 4 5% 3 5% 5% 35%	4048
January	1st to 10th	Night	13% 7% 3 7% 4 5% 5% 4 6% 6% 41%	8234
	11th to 20th	Night	12% 10% 3 10% 6% 5% 5% 4 6% 7% 32%	8235
	21st to end	Night	12% 7% 2 7% 4 4 5% 4 5% 7% 42%	9057
February	1st to 10th	Night	13% 10% 3 7% 4 4 5% 4 5% 7% 37%	2751
	11th to 20th	Night	11% 9% 3 6% 5% 5% 5% 4 6% 6% 40%	2458
	21st to end	Night	13% 9% 3 6% 3 5% 5% 4 5% 7% 39%	2076
March	1st to 10th	Night	21% 9% 2 7% 5% 5% 4 4 5% 6% 32%	5484
	11th to 20th	Night	14% 9% 3 7% 5% 5% 6% 4 5% 6% 35%	5253
	21st to end	Night	25% 8% 2 5% 3 3 4 3 6% 7% 33%	5160
April	1st to 10th	Night	20% 9% 3 7% 5% 4 4 3 5% 6% 36%	2000
	11th to 20th	Night	17% 9% 3 6% 5% 4 4 4 5% 5% 38%	1926
	21st to end	Night	22% 8% 2 5% 4 4 4 3 5% 6% 38%	1800
May	1st to 10th	Night	35% 9% 3 6% 4 3 4 3 4 6% 22%	4054
	11th to 20th	Night	15% 9% 3 9% 6% 6% 6% 4 6% 7% 28%	3836
	21st to end	Night	20% 7% 2 7% 4 4 5% 4 5% 8% 33%	3926
June	1st to 10th	Night	21% 7% 2 7% 4 4 4 4 6% 7% 34%	1600
	11th to 20th	Night	18% 7% 2 5% 3 4 5% 4 5% 8% 39%	1600
	21st to end	Night	19% 6% 2 6% 4 4 4 5% 5% 8% 38%	1600
July	1st to 10th	Night	14% 6% 2 6% 4 5% 4 5% 7% 10% 38%	3600
	11th to 20th	Night	13% 7% 3 7% 5% 4 5% 4 6% 8% 39%	3600
	21st to end	Night	11% 5% 2 5% 4 4 4 4 5% 7% 50%	3926
August	1st to 10th	Night	22% 7% 3 7% 5% 4 4 3 4 6% 34%	2772
	11th to 20th	Night	17% 8% 3 6% 5% 4 4 4 6% 6% 38%	2872
	21st to end	Night	10% 5% 2 4 4 3 4 4 6% 9% 49%	3543
September	1st to 10th	Night	13% 5% 1 5% 3 3 3 3 5% 7% 52%	2005
	11th to 20th	Night	14% 6% 1 4 3 3 3 3 5% 6% 52%	2200
	21st to end	Night	16% 6% 2 5% 4 3 4 3 5% 6% 47%	2200
October	1st to 10th	Night	19% 6% 3 6% 4 4 3 4 4 6% 41%	2200
	11th to 20th	Night	16% 6% 1 5% 5% 4 5% 4 5% 7% 42%	2480
	21st to end	Night	23% 8% 2 5% 3 4 4 4 5% 5% 37%	2860
November	1st to 10th	Night	15% 6% 2 7% 3 4 4 4 5% 6% 45%	2667
	11th to 20th	Night	16% 7% 2 5% 4 4 4 4 5% 6% 44%	2730
	21st to end	Night	16% 8% 2 5% 4 5% 4 4 5% 6% 41%	2756
December	1st to 10th	Night	15% 10% 3 6% 5% 4 4 4 5% 7% 37%	4720
	11th to 20th	Night	23% 8% 2 5% 4 4 3 3 4 6% 39%	5348
	21st to end	Night	20% 10% 3 7% 4 4 4 3 4 5% 37%	7150

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	35% 23% 10% 19% 12%	4662
	11th to 20th	Day	33% 23% 8% 19% 14% 2	4680
	21st to end	Day	30% 24% 7% 19% 15% 4	5124
February	1st to 10th	Day	18% 60% 5% 9% 6% 2	1865
	11th to 20th	Day	19% 59% 4 8% 9% 2	2057
	21st to end	Day	18% 60% 4 8% 8% 2	1730
March	1st to 10th	Day	41% 29% 5% 8% 10% 7%	4579
	11th to 20th	Day	32% 31% 11% 13% 9% 3	4860
	21st to end	Day	39% 29% 4 11% 14% 3	5757
April	1st to 10th	Day	20% 62% 3 7% 6% 2	2400
	11th to 20th	Day	17% 63% 4 8% 6% 1	2423
	21st to end	Day	21% 62% 3 7% 7%	2600
May	1st to 10th	Day	50% 31% 7% 9% 3	5897
	11th to 20th	Day	40% 33% 10% 12% 4 1	6091
	21st to end	Day	35% 32% 11% 14% 8%	6949
June	1st to 10th	Day	18% 62% 4 9% 7%	2800
	11th to 20th	Day	16% 62% 5% 8% 8% 1	2800
	21st to end	Day	17% 61% 4 10% 7% 1	2800
July	1st to 10th	Day	35% 28% 11% 16% 11%	6290
	11th to 20th	Day	28% 29% 12% 18% 12% 1	6285
	21st to end	Day	29% 27% 9% 20% 14%	6728
August	1st to 10th	Day	29% 38% 9% 16% 7%	4330
	11th to 20th	Day	33% 38% 9% 15% 4	4214
	21st to end	Day	20% 38% 11% 17% 13% 1	4350
September	1st to 10th	Day	12% 58% 7% 15% 8% 1	2395
	11th to 20th	Day	15% 57% 4 13% 10% 2	2200
	21st to end	Day	17% 57% 4 13% 8% 1	2200
October	1st to 10th	Day	29% 37% 5% 15% 12% 1	3590
	11th to 20th	Day	20% 36% 8% 18% 16% 2	3092
	21st to end	Day	21% 36% 6% 20% 14% 3	3180
November	1st to 10th	Day	18% 54% 7% 11% 8% 1	1851
	11th to 20th	Day	17% 54% 4 9% 13% 2	1890
	21st to end	Day	20% 55% 7% 10% 8% 1	1860
December	1st to 10th	Day	29% 38% 10% 17% 6%	2680
	11th to 20th	Day	28% 32% 7% 13% 15% 4	3066
	21st to end	Day	38% 27% 5% 14% 11% 6%	4048
January	1st to 10th	Night	33% 23% 9% 23% 10% 2	8234
	11th to 20th	Night	39% 24% 9% 14% 12% 3	8280
	21st to end	Night	32% 24% 6% 17% 15% 5%	9057
February	1st to 10th	Night	19% 60% 4 9% 7% 1	2751
	11th to 20th	Night	17% 58% 5% 9% 10% 2	2460
	21st to end	Night	17% 59% 3 10% 7% 2	2076
March	1st to 10th	Night	40% 29% 7% 8% 11% 5%	5494
	11th to 20th	Night	34% 32% 12% 10% 10% 2	5254
	21st to end	Night	38% 30% 4 9% 16% 4	5160
April	1st to 10th	Night	21% 62% 3 6% 7% 1	2000
	11th to 20th	Night	20% 63% 3 7% 7% 1	1926
	21st to end	Night	18% 63% 4 7% 7% 1	1800
May	1st to 10th	Night	49% 32% 7% 8% 4	4060
	11th to 20th	Night	38% 34% 9% 13% 5%	3837
	21st to end	Night	36% 32% 12% 14% 6% 1	3927
June	1st to 10th	Night	18% 63% 4 8% 6%	1600
	11th to 20th	Night	15% 63% 6% 9% 6% 1	1600
	21st to end	Night	17% 61% 4 13% 5% 1	1600
July	1st to 10th	Night	29% 29% 13% 19% 10%	3560
	11th to 20th	Night	33% 29% 10% 15% 13%	3552
	21st to end	Night	24% 27% 10% 28% 10%	3809
August	1st to 10th	Night	36% 38% 8% 12% 5%	2762
	11th to 20th	Night	32% 39% 10% 13% 6%	2899
	21st to end	Night	18% 38% 11% 21% 12% 1	3543
September	1st to 10th	Night	13% 58% 6% 15% 8%	2005
	11th to 20th	Night	15% 57% 5% 12% 10% 2	2200
	21st to end	Night	16% 58% 4 14% 7% 1	2200
October	1st to 10th	Night	28% 38% 8% 15% 10% 1	3590
	11th to 20th	Night	16% 36% 7% 18% 19% 3	4042
	21st to end	Night	23% 36% 5% 21% 12% 2	4610
November	1st to 10th	Night	19% 54% 5% 14% 7% 1	2667
	11th to 20th	Night	18% 54% 4 11% 12% 1	2730
	21st to end	Night	19% 54% 9% 11% 7%	2756
December	1st to 10th	Night	30% 35% 6% 23% 7%	4720
	11th to 20th	Night	34% 31% 7% 10% 15% 3	5348
	21st to end	Night	38% 27% 5% 15% 13% 2	7150

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	24% 23% 29% 20% 4 1	220
	11th to 20th	Day	25% 25% 25% 20% 3 1	220
	21st to end	Day	22% 21% 35% 17% 5%	242
February	1st to 10th	Day	22% 22% 35% 18% 3	371
	11th to 20th	Day	24% 21% 34% 18% 3	536
	21st to end	Day	27% 18% 32% 20% 4	446
March	1st to 10th	Day	29% 20% 29% 19% 3	540
	11th to 20th	Day	26% 26% 29% 14% 5% 1	528
	21st to end	Day	28% 18% 32% 13% 7% 3	550
April	1st to 10th	Day	31% 22% 26% 13% 6% 2	500
	11th to 20th	Day	29% 23% 26% 13% 7% 2	492
	21st to end	Day	27% 22% 29% 15% 6% 1	500
May	1st to 10th	Day	36% 15% 28% 15% 4 2	503
	11th to 20th	Day	20% 19% 35% 17% 6% 2	527
	21st to end	Day	22% 14% 33% 22% 8% 1	573
June	1st to 10th	Day	21% 15% 34% 20% 8% 1	530
	11th to 20th	Day	19% 15% 33% 24% 7% 3	530
	21st to end	Day	21% 14% 37% 16% 10% 2	530
July	1st to 10th	Day	18% 10% 33% 25% 12% 2	525
	11th to 20th	Day	14% 8% 34% 29% 12% 3	524
	21st to end	Day	16% 8% 34% 29% 11% 3	576
August	1st to 10th	Day	13% 9% 28% 30% 16% 4	526
	11th to 20th	Day	15% 11% 27% 25% 15% 7%	519
	21st to end	Day	12% 9% 29% 29% 15% 4	583
September	1st to 10th	Day	16% 9% 27% 25% 16% 7%	530
	11th to 20th	Day	17% 11% 24% 22% 16% 10%	530
	21st to end	Day	18% 12% 30% 23% 12% 6%	530
October	1st to 10th	Day	25% 14% 22% 22% 13% 5%	530
	11th to 20th	Day	21% 14% 28% 21% 13% 3	453
	21st to end	Day	31% 13% 27% 22% 6%	462
November	1st to 10th	Day	22% 17% 28% 24% 7% 1	425
	11th to 20th	Day	26% 17% 29% 21% 6% 1	430
	21st to end	Day	30% 18% 29% 18% 5% 1	400
December	1st to 10th	Day	25% 26% 23% 18% 7%	220
	11th to 20th	Day	30% 23% 29% 15% 4	220
	21st to end	Day	29% 19% 30% 16% 6%	242
January	1st to 10th	Night	20% 14% 36% 27% 3	840
	11th to 20th	Night	25% 15% 31% 25% 4	840
	21st to end	Night	17% 16% 36% 26% 5%	924
February	1st to 10th	Night	20% 14% 37% 28% 2	840
	11th to 20th	Night	18% 14% 42% 22% 4	823
	21st to end	Night	21% 14% 35% 27% 4	692
March	1st to 10th	Night	24% 15% 35% 23% 3	840
	11th to 20th	Night	22% 16% 31% 25% 6%	829
	21st to end	Night	26% 12% 31% 22% 8% 1	880
April	1st to 10th	Night	26% 14% 30% 26% 4	800
	11th to 20th	Night	21% 15% 34% 24% 5%	742
	21st to end	Night	24% 15% 30% 26% 5%	600
May	1st to 10th	Night	27% 10% 29% 28% 6% 1	600
	11th to 20th	Night	20% 12% 32% 26% 9% 1	512
	21st to end	Night	22% 8% 23% 36% 10% 1	433
June	1st to 10th	Night	20% 8% 28% 34% 10% 1	400
	11th to 20th	Night	19% 7% 26% 34% 14% 1	400
	21st to end	Night	19% 4 26% 40% 11% 1	400
July	1st to 10th	Night	16% 3 23% 42% 16%	390
	11th to 20th	Night	15% 3 19% 42% 19% 1	388
	21st to end	Night	15% 2 19% 48% 15% 1	438
August	1st to 10th	Night	17% 5% 17% 42% 20%	506
	11th to 20th	Night	16% 6% 25% 33% 18% 2	592
	21st to end	Night	16% 4 21% 38% 19% 2	850
September	1st to 10th	Night	15% 5% 25% 34% 19% 1	800
	11th to 20th	Night	16% 7% 20% 32% 22% 4	800
	21st to end	Night	17% 7% 27% 30% 17% 2	800
October	1st to 10th	Night	22% 9% 21% 30% 17% 2	800
	11th to 20th	Night	19% 10% 29% 29% 12% 1	800
	21st to end	Night	28% 11% 25% 29% 7%	880
November	1st to 10th	Night	19% 13% 31% 29% 7% 1	820
	11th to 20th	Night	20% 15% 31% 26% 6% 1	840
	21st to end	Night	21% 17% 28% 28% 6%	840
December	1st to 10th	Night	25% 12% 33% 24% 6%	840
	11th to 20th	Night	24% 16% 29% 27% 3	840
	21st to end	Night	26% 13% 31% 27% 3	924

Page generated at 2026-03-15 02:23:51. Version ac07

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