Rolling Hills Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rolling Hills Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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	8% 6% 2 7% 5% 6% 6% 5% 7% 10% 39%	109179
	Night	9% 6% 3 7% 5% 5% 5% 4 6% 9% 42%	109854
Transparency	Day	39% 7% 15% 20% 14% 5%	112946
	Night	37% 7% 16% 21% 14% 5%	114226
Seeing	Day	9% 4 16% 25% 24% 21%	15970
	Night	10% 4 18% 34% 27% 8%	26121

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	8% 6% 2 4 4 3 4 3 5% 8% 54%	4056
	11th to 20th	Day	19% 10% 3 8% 5% 4 4 3 5% 7% 31%	4060
	21st to end	Day	13% 6% 2 6% 3 3 3 3 6% 9% 46%	4627
February	1st to 10th	Day	12% 7% 3 6% 4 3 4 5% 6% 8% 42%	2000
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 3 5% 6% 53%	1948
	21st to end	Day	9% 6% 2 6% 4 5% 4 4 5% 7% 48%	1751
March	1st to 10th	Day	6% 7% 3 9% 6% 6% 6% 4 6% 9% 39%	3545
	11th to 20th	Day	6% 4 2 6% 4 4 4 3 8% 10% 50%	3491
	21st to end	Day	10% 5% 2 6% 4 4 4 4 5% 7% 50%	3815
April	1st to 10th	Day	6% 3 1 5% 5% 5% 5% 5% 6% 9% 49%	2090
	11th to 20th	Day	7% 5% 3 6% 4 6% 5% 5% 7% 8% 44%	2094
	21st to end	Day	5% 3 2 5% 4 4 6% 4 8% 9% 51%	2280
May	1st to 10th	Day	3 3 1 4 3 4 5% 4 7% 10% 57%	4037
	11th to 20th	Day	8% 5% 2 6% 6% 6% 5% 6% 8% 9% 37%	4081
	21st to end	Day	4 3 2 5% 4 5% 6% 5% 8% 10% 50%	4476
June	1st to 10th	Day	11% 7% 3 9% 7% 7% 7% 5% 8% 9% 25%	2470
	11th to 20th	Day	6% 6% 2 9% 8% 8% 8% 6% 9% 11% 28%	2470
	21st to end	Day	8% 7% 3 10% 8% 7% 7% 6% 7% 10% 26%	2470
July	1st to 10th	Day	6% 6% 3 8% 8% 8% 8% 8% 10% 14% 22%	4120
	11th to 20th	Day	5% 6% 3 8% 7% 8% 9% 6% 10% 13% 26%	4120
	21st to end	Day	5% 6% 3 8% 8% 8% 8% 7% 9% 13% 24%	4466
August	1st to 10th	Day	5% 7% 4 8% 7% 7% 8% 6% 9% 13% 27%	4104
	11th to 20th	Day	4 4 2 7% 6% 7% 7% 6% 9% 13% 35%	3839
	21st to end	Day	7% 7% 3 10% 8% 7% 7% 6% 8% 11% 25%	3828
September	1st to 10th	Day	6% 6% 3 10% 10% 8% 9% 7% 8% 10% 21%	2090
	11th to 20th	Day	10% 7% 3 10% 9% 7% 7% 6% 7% 9% 25%	2090
	21st to end	Day	9% 7% 3 9% 7% 8% 6% 5% 7% 10% 28%	2090
October	1st to 10th	Day	11% 6% 4 8% 7% 7% 5% 5% 6% 9% 33%	2075
	11th to 20th	Day	6% 5% 2 5% 5% 6% 6% 4 7% 7% 46%	1900
	21st to end	Day	7% 5% 2 6% 3 5% 6% 4 7% 10% 47%	2090
November	1st to 10th	Day	7% 6% 2 6% 4 5% 6% 5% 6% 7% 46%	1957
	11th to 20th	Day	8% 7% 2 6% 5% 4 5% 4 6% 7% 46%	2000
	21st to end	Day	9% 6% 2 6% 5% 4 5% 3 6% 9% 45%	1880
December	1st to 10th	Day	5% 4 1 5% 3 3 3 4 6% 8% 58%	2930
	11th to 20th	Day	10% 6% 3 7% 4 4 5% 5% 6% 9% 41%	3373
	21st to end	Day	10% 8% 3 8% 6% 5% 5% 4 6% 8% 39%	4466
January	1st to 10th	Night	7% 6% 2 5% 4 3 3 3 4 7% 54%	5889
	11th to 20th	Night	18% 10% 3 8% 5% 4 5% 4 6% 7% 29%	5900
	21st to end	Night	15% 6% 2 6% 4 3 4 3 6% 9% 42%	6324
February	1st to 10th	Night	11% 7% 3 5% 4 4 4 3 6% 7% 46%	2400
	11th to 20th	Night	10% 5% 1 5% 4 5% 4 3 6% 7% 49%	2338
	21st to end	Night	8% 6% 2 5% 4 3 3 4 5% 7% 51%	1876
March	1st to 10th	Night	14% 10% 3 8% 5% 4 4 3 5% 6% 38%	3528
	11th to 20th	Night	4 4 2 5% 4 4 4 4 8% 12% 48%	3498
	21st to end	Night	10% 7% 3 6% 4 3 4 4 5% 7% 49%	3817
April	1st to 10th	Night	6% 4 2 4 3 4 4 4 7% 10% 52%	2090
	11th to 20th	Night	7% 6% 2 6% 4 4 4 4 5% 8% 50%	2034
	21st to end	Night	6% 4 2 5% 3 3 4 3 5% 8% 57%	1900
May	1st to 10th	Night	3 3 2 5% 4 4 4 4 6% 8% 56%	2886
	11th to 20th	Night	6% 5% 3 6% 5% 4 5% 5% 7% 12% 45%	2812
	21st to end	Night	3 3 1 4 4 3 4 3 6% 8% 60%	3079
June	1st to 10th	Night	10% 7% 3 6% 5% 5% 6% 5% 7% 11% 36%	1710
	11th to 20th	Night	5% 7% 2 7% 7% 5% 6% 6% 9% 12% 33%	1710
	21st to end	Night	6% 8% 3 8% 6% 6% 6% 6% 9% 12% 30%	1710
July	1st to 10th	Night	7% 8% 3 9% 7% 7% 7% 5% 9% 12% 26%	2840
	11th to 20th	Night	4 6% 3 9% 7% 7% 9% 6% 10% 13% 27%	2840
	21st to end	Night	6% 8% 4 13% 8% 8% 8% 6% 9% 11% 20%	3087
August	1st to 10th	Night	5% 5% 3 9% 6% 7% 6% 5% 8% 12% 35%	2825
	11th to 20th	Night	3 4 2 7% 6% 6% 7% 6% 9% 12% 38%	2953
	21st to end	Night	6% 6% 3 8% 6% 6% 7% 7% 10% 12% 30%	3828
September	1st to 10th	Night	8% 5% 3 9% 7% 7% 7% 5% 8% 11% 30%	2090
	11th to 20th	Night	10% 9% 3 9% 7% 5% 5% 5% 7% 10% 29%	2090
	21st to end	Night	9% 6% 3 8% 6% 6% 6% 3 7% 9% 36%	2090
October	1st to 10th	Night	10% 7% 3 8% 5% 5% 6% 4 6% 8% 38%	2104
	11th to 20th	Night	6% 5% 2 5% 4 5% 5% 4 6% 8% 51%	2280
	21st to end	Night	9% 4 2 6% 3 3 4 4 5% 8% 51%	2508
November	1st to 10th	Night	8% 6% 1 5% 4 5% 4 4 6% 7% 49%	2341
	11th to 20th	Night	9% 7% 3 6% 5% 5% 4 4 5% 6% 47%	2400
	21st to end	Night	9% 5% 3 5% 5% 5% 4 4 6% 8% 46%	2515
December	1st to 10th	Night	4 3 1 4 3 4 4 4 5% 7% 59%	4250
	11th to 20th	Night	10% 7% 3 5% 4 4 4 4 5% 7% 49%	4822
	21st to end	Night	12% 8% 3 8% 5% 4 5% 4 5% 7% 38%	6490

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	30% 3 8% 17% 28% 13%	4057
	11th to 20th	Day	54% 3 5% 16% 15% 7%	4060
	21st to end	Day	36% 3 8% 28% 19% 7%	4627
February	1st to 10th	Day	38% 5% 9% 17% 21% 10%	2000
	11th to 20th	Day	31% 5% 8% 21% 25% 10%	1958
	21st to end	Day	35% 5% 10% 18% 22% 10%	1751
March	1st to 10th	Day	42% 6% 13% 20% 15% 5%	3545
	11th to 20th	Day	29% 4 12% 29% 20% 6%	3491
	21st to end	Day	33% 4 9% 30% 16% 8%	3815
April	1st to 10th	Day	30% 5% 13% 25% 18% 9%	2090
	11th to 20th	Day	36% 7% 10% 20% 21% 7%	2094
	21st to end	Day	27% 5% 12% 24% 23% 9%	2280
May	1st to 10th	Day	21% 6% 14% 34% 19% 5%	4074
	11th to 20th	Day	39% 8% 17% 23% 12% 2	4085
	21st to end	Day	27% 6% 13% 35% 18% 1	4489
June	1st to 10th	Day	52% 8% 17% 17% 5%	2470
	11th to 20th	Day	46% 10% 21% 20% 2	2470
	21st to end	Day	51% 10% 23% 13% 4	2470
July	1st to 10th	Day	47% 13% 28% 11% 1	4055
	11th to 20th	Day	45% 14% 32% 8%	4051
	21st to end	Day	46% 16% 28% 9% 1	4324
August	1st to 10th	Day	45% 12% 29% 11% 3	4091
	11th to 20th	Day	37% 13% 26% 19% 5%	3878
	21st to end	Day	49% 13% 23% 13% 1	3828
September	1st to 10th	Day	53% 14% 19% 11% 4	2090
	11th to 20th	Day	53% 9% 21% 13% 4	2090
	21st to end	Day	49% 8% 15% 16% 9% 3	2090
October	1st to 10th	Day	58% 10% 11% 10% 7% 5%	3452
	11th to 20th	Day	28% 5% 8% 22% 31% 6%	3140
	21st to end	Day	28% 4 8% 22% 30% 7%	3428
November	1st to 10th	Day	33% 6% 10% 20% 20% 10%	1957
	11th to 20th	Day	37% 4 9% 21% 20% 9%	2000
	21st to end	Day	36% 5% 9% 23% 20% 8%	1880
December	1st to 10th	Day	25% 4 15% 28% 19% 9%	2930
	11th to 20th	Day	38% 5% 11% 18% 19% 9%	3370
	21st to end	Day	42% 5% 11% 22% 15% 5%	4466
January	1st to 10th	Night	31% 3 7% 19% 28% 12%	5889
	11th to 20th	Night	54% 4 8% 16% 13% 4	5900
	21st to end	Night	40% 5% 9% 22% 16% 8%	6324
February	1st to 10th	Night	37% 4 10% 18% 19% 12%	2400
	11th to 20th	Night	33% 5% 11% 21% 21% 10%	2339
	21st to end	Night	31% 5% 12% 20% 23% 8%	1876
March	1st to 10th	Night	48% 4 12% 17% 15% 5%	3528
	11th to 20th	Night	26% 6% 21% 27% 14% 5%	3498
	21st to end	Night	33% 5% 9% 26% 21% 6%	3817
April	1st to 10th	Night	27% 5% 19% 25% 16% 8%	2090
	11th to 20th	Night	32% 5% 12% 25% 19% 7%	2034
	21st to end	Night	25% 4 15% 26% 23% 7%	1900
May	1st to 10th	Night	24% 7% 18% 31% 15% 5%	2914
	11th to 20th	Night	31% 7% 21% 28% 11% 1	2813
	21st to end	Night	21% 6% 15% 39% 18% 1	3087
June	1st to 10th	Night	41% 9% 22% 22% 5%	1710
	11th to 20th	Night	40% 9% 26% 22% 2	1710
	21st to end	Night	43% 12% 25% 15% 5%	1710
July	1st to 10th	Night	45% 15% 29% 10% 1	2795
	11th to 20th	Night	44% 16% 32% 7%	2786
	21st to end	Night	54% 13% 25% 8% 1	2988
August	1st to 10th	Night	39% 13% 33% 12% 2	2814
	11th to 20th	Night	34% 13% 29% 19% 6%	2980
	21st to end	Night	42% 15% 28% 13% 2	3828
September	1st to 10th	Night	44% 16% 22% 14% 4	2090
	11th to 20th	Night	48% 11% 22% 17% 2 1	2090
	21st to end	Night	43% 8% 19% 19% 10% 2	2090
October	1st to 10th	Night	56% 9% 12% 12% 7% 4	3507
	11th to 20th	Night	28% 5% 10% 28% 25% 4	3779
	21st to end	Night	29% 4 9% 20% 31% 6%	4120
November	1st to 10th	Night	32% 5% 12% 22% 19% 10%	2341
	11th to 20th	Night	37% 5% 10% 20% 19% 9%	2400
	21st to end	Night	35% 4 10% 23% 20% 8%	2515
December	1st to 10th	Night	24% 4 15% 31% 13% 13%	4250
	11th to 20th	Night	34% 6% 12% 23% 16% 10%	4824
	21st to end	Night	44% 5% 12% 21% 13% 4	6490

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	13% 8% 42% 25% 10% 2	300
	11th to 20th	Day	8% 11% 37% 28% 13% 4	300
	21st to end	Day	16% 13% 38% 25% 8% 1	330
February	1st to 10th	Day	13% 8% 27% 28% 21% 3	300
	11th to 20th	Day	11% 5% 33% 33% 15% 4	300
	21st to end	Day	12% 5% 30% 28% 21% 4	350
March	1st to 10th	Day	7% 7% 32% 36% 17% 1	500
	11th to 20th	Day	12% 4 26% 31% 23% 4	464
	21st to end	Day	8% 5% 28% 31% 24% 4	506
April	1st to 10th	Day	8% 6% 26% 34% 22% 4	460
	11th to 20th	Day	10% 5% 25% 30% 24% 6%	452
	21st to end	Day	7% 3 18% 34% 26% 12%	460
May	1st to 10th	Day	5% 2 19% 27% 31% 16%	463
	11th to 20th	Day	10% 2 12% 24% 32% 20%	487
	21st to end	Day	7% 1 7% 19% 34% 32%	529
June	1st to 10th	Day	14% 2 4 20% 33% 27%	490
	11th to 20th	Day	8% 3 5% 24% 22% 38%	490
	21st to end	Day	7% 2 2 11% 26% 52%	490
July	1st to 10th	Day	7% 2 1 7% 26% 56%	485
	11th to 20th	Day	7% 1 9% 26% 57%	484
	21st to end	Day	6% 3 2 14% 24% 51%	532
August	1st to 10th	Day	9% 2 3 13% 25% 47%	486
	11th to 20th	Day	5% 3 2 9% 26% 55%	479
	21st to end	Day	11% 2 3 17% 25% 42%	539
September	1st to 10th	Day	8% 3 4 12% 30% 44%	490
	11th to 20th	Day	9% 3 4 19% 31% 33%	490
	21st to end	Day	10% 3 4 20% 30% 32%	490
October	1st to 10th	Day	16% 2 15% 30% 26% 11%	490
	11th to 20th	Day	7% 2 18% 31% 32% 10%	490
	21st to end	Day	7% 3 17% 37% 29% 7%	539
November	1st to 10th	Day	9% 4 19% 32% 31% 6%	495
	11th to 20th	Day	14% 4 21% 37% 20% 4	500
	21st to end	Day	10% 5% 27% 34% 20% 4	380
December	1st to 10th	Day	7% 5% 34% 32% 15% 7%	300
	11th to 20th	Day	16% 5% 27% 34% 16% 2	300
	21st to end	Day	10% 5% 29% 34% 18% 4	330
January	1st to 10th	Night	11% 8% 32% 37% 12% 1	800
	11th to 20th	Night	12% 7% 26% 41% 14% 1	800
	21st to end	Night	15% 8% 34% 34% 9%	880
February	1st to 10th	Night	15% 5% 27% 39% 14% 1	800
	11th to 20th	Night	12% 6% 30% 37% 13% 1	783
	21st to end	Night	10% 6% 32% 43% 9%	660
March	1st to 10th	Night	7% 6% 33% 42% 12% 1	800
	11th to 20th	Night	9% 6% 28% 40% 16% 1	767
	21st to end	Night	9% 5% 27% 45% 14% 1	836
April	1st to 10th	Night	9% 4 30% 42% 15%	760
	11th to 20th	Night	11% 5% 27% 42% 14% 1	751
	21st to end	Night	8% 4 23% 45% 20% 1	760
May	1st to 10th	Night	5% 1 16% 50% 24% 2	618
	11th to 20th	Night	7% 2 14% 42% 30% 5%	561
	21st to end	Night	7% 2 7% 35% 42% 7%	617
June	1st to 10th	Night	12% 2 5% 32% 41% 8%	570
	11th to 20th	Night	7% 1 5% 21% 41% 25%	570
	21st to end	Night	9% 2 1 18% 45% 26%	570
July	1st to 10th	Night	9% 2 1 11% 47% 31%	555
	11th to 20th	Night	7% 2 1 10% 45% 36%	552
	21st to end	Night	6% 3 2 11% 48% 30%	624
August	1st to 10th	Night	5% 2 2 11% 51% 28%	567
	11th to 20th	Night	5% 1 1 11% 45% 37%	588
	21st to end	Night	12% 2 3 19% 40% 23%	836
September	1st to 10th	Night	10% 2 4 18% 44% 22%	760
	11th to 20th	Night	14% 3 5% 24% 41% 14%	760
	21st to end	Night	13% 2 5% 28% 42% 10%	760
October	1st to 10th	Night	15% 2 12% 37% 29% 6%	760
	11th to 20th	Night	8% 3 14% 38% 33% 4	760
	21st to end	Night	11% 2 15% 41% 28% 3	836
November	1st to 10th	Night	10% 3 21% 36% 27% 3	780
	11th to 20th	Night	12% 4 20% 43% 20% 2	800
	21st to end	Night	11% 3 25% 43% 17% 1	800
December	1st to 10th	Night	8% 3 25% 39% 24% 1	800
	11th to 20th	Night	12% 6% 24% 39% 19% 1	800
	21st to end	Night	12% 5% 25% 39% 18% 1	880

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