The Rose Dorothea Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Rose Dorothea Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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% 9% 3 7% 4 4 4 3 5% 7% 38%	126070
	Night	18% 10% 4 8% 4 4 4 3 5% 6% 35%	123270
Transparency	Day	46% 4 12% 22% 13% 2	129620
	Night	50% 3 11% 20% 14% 2	127807
Seeing	Day	20% 7% 25% 29% 15% 3	16391
	Night	22% 10% 25% 28% 13% 2	25154

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	12% 9% 4 13% 5% 4 4 3 4 5% 37%	3597
	11th to 20th	Day	16% 12% 6% 7% 3 3 3 2 4 5% 39%	3570
	21st to end	Day	19% 10% 2 12% 6% 4 4 3 4 5% 32%	4072
February	1st to 10th	Day	20% 9% 5% 8% 5% 4 4 3 5% 6% 29%	1865
	11th to 20th	Day	15% 9% 4 7% 6% 5% 5% 4 5% 6% 35%	1948
	21st to end	Day	17% 10% 3 10% 5% 4 4 3 6% 6% 33%	1650
March	1st to 10th	Day	19% 13% 4 8% 5% 3 3 3 5% 6% 32%	4476
	11th to 20th	Day	16% 11% 4 7% 4 4 3 3 4 6% 39%	4888
	21st to end	Day	27% 11% 3 7% 4 4 3 3 3 4 32%	5588
April	1st to 10th	Day	20% 9% 3 6% 3 3 3 3 5% 6% 40%	2400
	11th to 20th	Day	18% 8% 2 5% 3 3 4 3 5% 7% 42%	2439
	21st to end	Day	17% 7% 1 6% 4 4 4 3 5% 6% 42%	2600
May	1st to 10th	Day	24% 12% 4 8% 4 3 4 3 5% 6% 26%	5886
	11th to 20th	Day	20% 12% 3 7% 4 4 3 3 4 7% 33%	5861
	21st to end	Day	18% 8% 3 7% 4 4 4 3 5% 7% 39%	6781
June	1st to 10th	Day	18% 7% 3 5% 3 3 5% 4 7% 8% 36%	2800
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 9% 41%	2800
	21st to end	Day	15% 6% 2 5% 4 4 4 4 6% 8% 41%	2800
July	1st to 10th	Day	14% 8% 3 7% 4 4 4 4 6% 9% 36%	6360
	11th to 20th	Day	9% 8% 3 7% 5% 5% 5% 4 7% 11% 36%	6360
	21st to end	Day	14% 6% 2 6% 4 3 4 3 5% 7% 47%	6757
August	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 7% 38%	4235
	11th to 20th	Day	12% 8% 3 7% 4 5% 5% 4 5% 7% 41%	4175
	21st to end	Day	8% 4 2 4 3 3 4 4 6% 8% 54%	4384
September	1st to 10th	Day	13% 6% 2 5% 3 3 4 4 5% 6% 49%	2340
	11th to 20th	Day	13% 6% 2 4 3 4 3 3 4 5% 53%	2200
	21st to end	Day	18% 7% 3 6% 4 4 4 3 5% 7% 41%	2200
October	1st to 10th	Day	20% 6% 2 5% 3 4 3 3 4 6% 44%	2123
	11th to 20th	Day	14% 7% 2 5% 4 3 3 3 5% 7% 47%	1886
	21st to end	Day	18% 8% 2 6% 3 3 3 3 6% 7% 41%	1881
November	1st to 10th	Day	18% 7% 2 5% 4 2 3 3 5% 7% 43%	1761
	11th to 20th	Day	17% 10% 2 5% 3 2 4 4 5% 7% 41%	1800
	21st to end	Day	18% 8% 3 6% 4 3 4 3 5% 8% 39%	1800
December	1st to 10th	Day	14% 12% 4 6% 3 4 2 3 5% 6% 41%	2841
	11th to 20th	Day	18% 10% 4 7% 3 3 3 2 4 6% 39%	2986
	21st to end	Day	21% 17% 7% 13% 4 3 3 2 4 4 23%	3960
January	1st to 10th	Night	16% 8% 4 10% 3 3 3 2 3 4 43%	6349
	11th to 20th	Night	17% 13% 6% 11% 4 3 4 3 4 4 32%	6315
	21st to end	Night	17% 12% 5% 10% 6% 4 4 3 4 5% 30%	6881
February	1st to 10th	Night	18% 12% 4 8% 6% 5% 5% 3 6% 5% 29%	2535
	11th to 20th	Night	19% 11% 3 8% 5% 4 4 3 4 6% 32%	2338
	21st to end	Night	18% 11% 3 7% 5% 4 4 3 5% 7% 35%	1980
March	1st to 10th	Night	26% 10% 3 6% 4 3 3 3 4 5% 32%	5364
	11th to 20th	Night	16% 12% 4 7% 5% 4 4 3 5% 5% 34%	5055
	21st to end	Night	26% 12% 3 6% 4 4 4 3 4 4 30%	5340
April	1st to 10th	Night	20% 10% 3 6% 5% 4 4 3 5% 6% 35%	2000
	11th to 20th	Night	19% 9% 2 7% 3 4 4 3 5% 7% 37%	1908
	21st to end	Night	18% 9% 2 7% 5% 5% 5% 3 5% 5% 36%	1800
May	1st to 10th	Night	26% 12% 3 9% 4 4 4 3 4 6% 26%	4054
	11th to 20th	Night	20% 11% 4 8% 4 5% 5% 4 5% 7% 27%	4033
	21st to end	Night	20% 10% 3 8% 5% 4 5% 4 5% 6% 28%	4122
June	1st to 10th	Night	17% 8% 2 6% 4 4 6% 4 6% 7% 34%	1600
	11th to 20th	Night	17% 9% 2 6% 4 4 4 4 6% 8% 37%	1600
	21st to end	Night	16% 8% 3 6% 5% 4 4 3 6% 10% 35%	1600
July	1st to 10th	Night	13% 11% 4 9% 5% 5% 4 4 6% 8% 31%	3600
	11th to 20th	Night	11% 9% 4 7% 5% 4 5% 4 6% 9% 35%	3600
	21st to end	Night	14% 7% 3 7% 5% 5% 4 4 6% 7% 40%	4034
August	1st to 10th	Night	16% 8% 3 7% 5% 5% 6% 5% 7% 8% 30%	2915
	11th to 20th	Night	11% 9% 2 7% 5% 5% 5% 5% 6% 8% 37%	2872
	21st to end	Night	8% 4 2 4 3 3 4 3 6% 9% 54%	3510
September	1st to 10th	Night	11% 6% 2 6% 5% 4 4 4 5% 7% 45%	2060
	11th to 20th	Night	13% 6% 2 5% 3 3 2 3 5% 6% 51%	2200
	21st to end	Night	17% 7% 2 7% 4 4 4 4 6% 4 41%	2200
October	1st to 10th	Night	21% 7% 2 5% 4 4 3 3 5% 5% 39%	2124
	11th to 20th	Night	16% 5% 2 5% 4 4 4 4 5% 5% 45%	2289
	21st to end	Night	21% 9% 3 7% 4 3 3 3 5% 6% 35%	2717
November	1st to 10th	Night	16% 7% 3 6% 4 3 4 3 4 6% 43%	2537
	11th to 20th	Night	17% 10% 3 5% 4 4 4 3 5% 7% 40%	2600
	21st to end	Night	17% 9% 3 7% 4 4 4 3 4 7% 39%	2600
December	1st to 10th	Night	20% 10% 4 7% 4 4 4 3 5% 5% 35%	4334
	11th to 20th	Night	19% 12% 4 7% 4 3 3 3 4 6% 34%	5208
	21st to end	Night	18% 14% 8% 13% 4 3 3 2 4 5% 25%	6996

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	50% 3 9% 22% 15% 1	3598
	11th to 20th	Day	49% 3 12% 21% 12% 3	3600
	21st to end	Day	55% 4 9% 17% 14% 1	4072
February	1st to 10th	Day	56% 4 11% 16% 11% 1	1865
	11th to 20th	Day	51% 4 11% 21% 11% 2	1957
	21st to end	Day	52% 4 10% 19% 13% 2	1650
March	1st to 10th	Day	55% 2 10% 14% 11% 8%	4479
	11th to 20th	Day	48% 3 14% 23% 10% 2	4888
	21st to end	Day	58% 2 6% 14% 14% 6%	5588
April	1st to 10th	Day	46% 3 9% 19% 20% 4	2400
	11th to 20th	Day	42% 3 11% 24% 18% 3	2439
	21st to end	Day	42% 4 9% 23% 18% 3	2600
May	1st to 10th	Day	61% 3 12% 15% 7% 1	5897
	11th to 20th	Day	53% 3 13% 19% 10% 2	5865
	21st to end	Day	46% 4 12% 23% 15% 1	6781
June	1st to 10th	Day	45% 5% 13% 23% 13%	2800
	11th to 20th	Day	41% 4 14% 24% 15% 1	2800
	21st to end	Day	40% 6% 16% 24% 13% 2	2800
July	1st to 10th	Day	45% 7% 14% 20% 13% 1	6290
	11th to 20th	Day	41% 7% 17% 24% 11% 1	6285
	21st to end	Day	37% 4 13% 31% 13% 1	6562
August	1st to 10th	Day	45% 5% 17% 23% 8% 1	4222
	11th to 20th	Day	42% 5% 14% 29% 9%	4214
	21st to end	Day	27% 4 16% 31% 20% 1	4384
September	1st to 10th	Day	36% 4 10% 27% 21% 1	2340
	11th to 20th	Day	35% 3 9% 29% 22% 3	2200
	21st to end	Day	43% 4 8% 24% 18% 2	2200
October	1st to 10th	Day	41% 3 13% 25% 17% 2	3513
	11th to 20th	Day	33% 2 8% 31% 23% 3	3102
	21st to end	Day	32% 3 13% 34% 16% 2	3081
November	1st to 10th	Day	42% 4 13% 24% 15% 2	1761
	11th to 20th	Day	44% 4 10% 22% 19% 2	1800
	21st to end	Day	45% 5% 14% 26% 10% 1	1800
December	1st to 10th	Day	45% 3 14% 26% 11%	2841
	11th to 20th	Day	48% 4 15% 16% 14% 4	2986
	21st to end	Day	66% 3 9% 13% 7% 2	3960
January	1st to 10th	Night	47% 2 9% 28% 13% 1	6349
	11th to 20th	Night	58% 3 10% 17% 10% 2	6360
	21st to end	Night	58% 2 9% 17% 13% 2	6881
February	1st to 10th	Night	56% 3 11% 19% 9% 1	2535
	11th to 20th	Night	54% 3 8% 20% 13% 1	2340
	21st to end	Night	50% 3 9% 24% 12% 3	1980
March	1st to 10th	Night	56% 2 9% 15% 13% 5%	5374
	11th to 20th	Night	53% 2 14% 17% 13% 1	5056
	21st to end	Night	60% 2 5% 11% 17% 5%	5340
April	1st to 10th	Night	51% 3 8% 19% 16% 4	2000
	11th to 20th	Night	48% 3 7% 21% 18% 2	1908
	21st to end	Night	50% 3 8% 19% 18% 3	1800
May	1st to 10th	Night	61% 3 12% 14% 7% 3	4060
	11th to 20th	Night	57% 4 14% 15% 8% 2	4034
	21st to end	Night	56% 3 10% 18% 12%	4123
June	1st to 10th	Night	49% 5% 13% 21% 12% 1	1600
	11th to 20th	Night	47% 3 11% 23% 16% 1	1600
	21st to end	Night	45% 5% 13% 23% 13% 1	1600
July	1st to 10th	Night	51% 5% 10% 19% 15%	3560
	11th to 20th	Night	45% 6% 16% 22% 10% 1	3552
	21st to end	Night	43% 5% 12% 27% 12% 1	3915
August	1st to 10th	Night	50% 6% 14% 19% 11% 1	2903
	11th to 20th	Night	43% 7% 14% 26% 9%	2899
	21st to end	Night	28% 4 16% 29% 23% 1	3510
September	1st to 10th	Night	39% 4 10% 26% 20% 2	2060
	11th to 20th	Night	35% 4 9% 26% 24% 2	2200
	21st to end	Night	44% 4 7% 22% 20% 2	2200
October	1st to 10th	Night	44% 4 11% 27% 13% 1	3514
	11th to 20th	Night	31% 2 9% 25% 31% 2	3812
	21st to end	Night	37% 2 10% 29% 20% 1	4467
November	1st to 10th	Night	43% 3 11% 24% 15% 3	2537
	11th to 20th	Night	46% 3 10% 21% 17% 3	2600
	21st to end	Night	46% 3 13% 26% 10% 2	2600
December	1st to 10th	Night	52% 3 12% 22% 11%	4334
	11th to 20th	Night	53% 3 13% 15% 15% 1	5208
	21st to end	Night	62% 4 9% 15% 9%	6996

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	28% 17% 31% 20% 6%	200
	11th to 20th	Day	26% 17% 31% 21% 6%	200
	21st to end	Day	21% 13% 39% 20% 5% 1	267
February	1st to 10th	Day	26% 16% 34% 21% 3	433
	11th to 20th	Day	25% 19% 37% 17% 2	496
	21st to end	Day	21% 14% 36% 22% 5% 1	414
March	1st to 10th	Day	24% 14% 35% 21% 6%	500
	11th to 20th	Day	19% 13% 35% 25% 7% 1	500
	21st to end	Day	26% 10% 33% 20% 8% 2	550
April	1st to 10th	Day	27% 10% 30% 25% 6% 2	500
	11th to 20th	Day	23% 7% 33% 29% 8% 1	492
	21st to end	Day	23% 4 27% 33% 11% 2	500
May	1st to 10th	Day	29% 4 28% 29% 9% 2	503
	11th to 20th	Day	20% 2 27% 34% 14% 2	527
	21st to end	Day	18% 3 21% 34% 20% 4	573
June	1st to 10th	Day	20% 2 18% 35% 22% 3	530
	11th to 20th	Day	16% 2 21% 40% 18% 4	530
	21st to end	Day	15% 3 18% 38% 22% 4	530
July	1st to 10th	Day	17% 2 12% 36% 27% 6%	525
	11th to 20th	Day	13% 1 12% 35% 32% 6%	524
	21st to end	Day	14% 1 12% 41% 27% 5%	576
August	1st to 10th	Day	13% 1 11% 41% 28% 6%	526
	11th to 20th	Day	15% 2 11% 33% 32% 8%	519
	21st to end	Day	12% 2 14% 38% 24% 10%	583
September	1st to 10th	Day	19% 2 15% 34% 23% 8%	530
	11th to 20th	Day	18% 2 18% 28% 24% 9%	530
	21st to end	Day	22% 3 21% 30% 18% 6%	530
October	1st to 10th	Day	24% 4 19% 28% 20% 5%	502
	11th to 20th	Day	17% 7% 29% 27% 18% 3	407
	21st to end	Day	25% 7% 30% 24% 12% 3	429
November	1st to 10th	Day	18% 10% 27% 30% 12% 3	395
	11th to 20th	Day	24% 14% 30% 24% 8% 1	400
	21st to end	Day	21% 16% 29% 26% 8% 1	400
December	1st to 10th	Day	23% 12% 38% 19% 7%	350
	11th to 20th	Day	26% 20% 32% 16% 7%	200
	21st to end	Day	30% 10% 33% 17% 8% 1	220
January	1st to 10th	Night	25% 18% 35% 19% 4	800
	11th to 20th	Night	28% 18% 31% 19% 4	800
	21st to end	Night	24% 18% 34% 19% 4 1	880
February	1st to 10th	Night	26% 17% 31% 24% 3	800
	11th to 20th	Night	27% 18% 37% 16% 3	783
	21st to end	Night	23% 16% 34% 23% 5%	660
March	1st to 10th	Night	25% 15% 31% 23% 6%	800
	11th to 20th	Night	23% 13% 33% 22% 7% 1	800
	21st to end	Night	25% 10% 30% 25% 9% 1	880
April	1st to 10th	Night	26% 10% 31% 27% 7% 1	800
	11th to 20th	Night	23% 7% 32% 30% 7%	724
	21st to end	Night	23% 6% 26% 32% 12% 1	600
May	1st to 10th	Night	30% 2 24% 33% 10% 2	600
	11th to 20th	Night	22% 3 29% 33% 11% 2	592
	21st to end	Night	24% 3 16% 39% 16% 1	525
June	1st to 10th	Night	23% 4 17% 39% 16% 2	400
	11th to 20th	Night	21% 3 14% 43% 16% 3	400
	21st to end	Night	20% 3 14% 42% 20% 1	400
July	1st to 10th	Night	15% 3 11% 42% 27% 2	390
	11th to 20th	Night	15% 2 11% 44% 25% 2	388
	21st to end	Night	16% 1 12% 43% 26% 2	483
August	1st to 10th	Night	11% 2 13% 42% 29% 2	598
	11th to 20th	Night	13% 2 12% 44% 27% 2	592
	21st to end	Night	14% 2 11% 39% 29% 5%	830
September	1st to 10th	Night	15% 2 17% 36% 27% 4	800
	11th to 20th	Night	16% 3 16% 31% 26% 7%	800
	21st to end	Night	21% 3 16% 33% 24% 4	800
October	1st to 10th	Night	25% 5% 20% 27% 19% 4	773
	11th to 20th	Night	20% 7% 28% 28% 15% 3	760
	21st to end	Night	27% 9% 27% 25% 10% 1	836
November	1st to 10th	Night	21% 7% 29% 30% 12% 2	780
	11th to 20th	Night	24% 16% 29% 23% 9% 1	800
	21st to end	Night	22% 21% 27% 23% 7% 1	800
December	1st to 10th	Night	28% 14% 30% 21% 6%	800
	11th to 20th	Night	27% 20% 31% 19% 4	800
	21st to end	Night	26% 16% 34% 19% 6%	880

Page generated at 2025-10-01 01:39:01. Version ac07

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