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 2024-02-10.

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% 3 6% 7% 4 6% 6% 7% 9% 38%	78677
	Night	9% 6% 3 6% 6% 3 5% 5% 6% 9% 43%	80298
Transparency	Day	42% 7% 14% 18% 14% 5%	78400
	Night	37% 7% 15% 20% 14% 6%	80094
Seeing	Day	9% 4 15% 24% 25% 22%	14491
	Night	10% 4 17% 34% 27% 8%	24645

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	11% 8% 2 5% 6% 3 5% 5% 5% 7% 43%	2836
	11th to 20th	Day	20% 9% 3 6% 5% 3 4 4 5% 7% 35%	2840
	21st to end	Day	10% 6% 3 6% 5% 3 4 5% 6% 8% 45%	3229
February	1st to 10th	Day	12% 6% 3 5% 5% 2 4 6% 6% 8% 42%	1890
	11th to 20th	Day	7% 4 2 5% 4 3 5% 4 5% 6% 54%	1748
	21st to end	Day	10% 5% 2 5% 5% 3 5% 5% 5% 7% 46%	1570
March	1st to 10th	Day	4 3 2 4 6% 3 4 5% 5% 8% 57%	1980
	11th to 20th	Day	8% 4 2 5% 4 3 4 4 7% 8% 50%	1980
	21st to end	Day	7% 4 2 5% 6% 4 4 5% 5% 7% 51%	2178
April	1st to 10th	Day	6% 3 2 4 5% 4 6% 6% 6% 9% 50%	1980
	11th to 20th	Day	7% 5% 3 6% 5% 4 6% 6% 7% 8% 42%	1982
	21st to end	Day	5% 3 2 4 5% 3 5% 5% 7% 10% 51%	2160
May	1st to 10th	Day	4 3 2 4 4 3 5% 5% 6% 9% 56%	2308
	11th to 20th	Day	8% 5% 2 6% 6% 4 5% 7% 6% 9% 42%	2301
	21st to end	Day	6% 4 2 6% 7% 4 8% 7% 9% 8% 37%	2531
June	1st to 10th	Day	12% 7% 4 8% 8% 6% 8% 7% 7% 9% 25%	2340
	11th to 20th	Day	5% 5% 3 7% 9% 6% 9% 8% 9% 11% 28%	2340
	21st to end	Day	8% 6% 4 8% 9% 5% 8% 8% 7% 10% 27%	2340
July	1st to 10th	Day	9% 7% 4 7% 9% 6% 9% 9% 8% 11% 22%	2340
	11th to 20th	Day	8% 6% 4 9% 9% 6% 9% 8% 8% 11% 23%	2340
	21st to end	Day	6% 6% 5% 9% 10% 5% 9% 8% 9% 10% 23%	2547
August	1st to 10th	Day	5% 6% 3 7% 10% 6% 9% 9% 9% 12% 23%	2327
	11th to 20th	Day	6% 5% 3 8% 10% 7% 10% 9% 10% 11% 23%	2196
	21st to end	Day	10% 7% 4 9% 10% 5% 7% 7% 8% 10% 24%	2178
September	1st to 10th	Day	6% 5% 3 9% 11% 6% 9% 10% 8% 11% 23%	1980
	11th to 20th	Day	10% 7% 3 8% 10% 5% 8% 8% 7% 9% 25%	1980
	21st to end	Day	9% 7% 3 8% 9% 6% 7% 6% 7% 11% 27%	1980
October	1st to 10th	Day	10% 5% 3 7% 8% 5% 6% 6% 6% 9% 34%	1966
	11th to 20th	Day	7% 5% 3 5% 5% 4 6% 6% 7% 7% 45%	1800
	21st to end	Day	8% 4 2 5% 4 3 6% 6% 7% 10% 46%	1980
November	1st to 10th	Day	7% 5% 2 5% 5% 3 6% 6% 6% 7% 47%	1857
	11th to 20th	Day	9% 7% 3 6% 6% 3 5% 5% 6% 7% 45%	1900
	21st to end	Day	9% 6% 2 5% 6% 3 5% 4 6% 9% 44%	1786
December	1st to 10th	Day	7% 4 3 6% 4 3 4 5% 7% 9% 48%	1710
	11th to 20th	Day	12% 6% 3 7% 5% 3 4 5% 5% 8% 43%	2153
	21st to end	Day	11% 8% 3 7% 7% 4 6% 5% 5% 7% 37%	3124
January	1st to 10th	Night	11% 7% 3 7% 6% 3 5% 6% 5% 6% 42%	4109
	11th to 20th	Night	17% 10% 3 6% 6% 3 5% 5% 6% 7% 32%	4120
	21st to end	Night	11% 6% 3 6% 5% 3 4 5% 6% 8% 41%	4422
February	1st to 10th	Night	12% 7% 3 4 4 3 4 3 6% 6% 46%	2275
	11th to 20th	Night	9% 5% 2 4 4 3 4 4 6% 7% 52%	2098
	21st to end	Night	8% 6% 3 5% 5% 2 4 5% 4 8% 50%	1683
March	1st to 10th	Night	5% 3 1 3 5% 2 4 3 5% 7% 60%	1980
	11th to 20th	Night	7% 5% 2 4 4 3 4 4 6% 10% 51%	1980
	21st to end	Night	6% 5% 2 5% 4 2 3 5% 6% 8% 54%	2178
April	1st to 10th	Night	7% 4 2 3 4 2 4 5% 6% 9% 53%	1980
	11th to 20th	Night	8% 6% 2 5% 4 3 4 5% 5% 8% 50%	1926
	21st to end	Night	6% 4 2 4 4 2 5% 4 5% 8% 57%	1800
May	1st to 10th	Night	4 3 2 4 4 3 4 4 4 8% 61%	1666
	11th to 20th	Night	6% 4 3 5% 5% 4 4 6% 6% 10% 48%	1592
	21st to end	Night	6% 5% 2 6% 5% 3 5% 6% 8% 8% 46%	1744
June	1st to 10th	Night	11% 7% 3 5% 6% 4 5% 7% 7% 10% 34%	1620
	11th to 20th	Night	5% 6% 3 6% 7% 4 5% 7% 8% 13% 35%	1620
	21st to end	Night	6% 7% 3 7% 7% 4 6% 7% 9% 12% 31%	1620
July	1st to 10th	Night	7% 7% 4 6% 8% 4 6% 6% 9% 13% 29%	1620
	11th to 20th	Night	5% 5% 4 8% 8% 5% 8% 7% 8% 12% 30%	1620
	21st to end	Night	4 6% 4 9% 8% 5% 8% 8% 9% 11% 29%	1771
August	1st to 10th	Night	5% 5% 3 7% 6% 4 7% 7% 9% 12% 35%	1611
	11th to 20th	Night	4 4 3 7% 8% 5% 7% 8% 9% 14% 32%	1682
	21st to end	Night	9% 7% 3 8% 6% 5% 7% 7% 8% 10% 29%	2178
September	1st to 10th	Night	7% 5% 3 7% 8% 5% 7% 7% 8% 12% 31%	1980
	11th to 20th	Night	10% 8% 4 8% 7% 4 6% 7% 7% 10% 30%	1980
	21st to end	Night	9% 6% 4 7% 8% 4 6% 5% 7% 9% 35%	1980
October	1st to 10th	Night	9% 7% 3 6% 6% 4 6% 5% 5% 8% 41%	1993
	11th to 20th	Night	6% 4 2 4 5% 3 5% 5% 6% 8% 51%	2160
	21st to end	Night	9% 4 2 5% 5% 2 4 4 5% 8% 51%	2376
November	1st to 10th	Night	8% 5% 2 5% 5% 3 4 5% 5% 7% 51%	2221
	11th to 20th	Night	9% 6% 3 5% 6% 3 5% 5% 4 7% 46%	2280
	21st to end	Night	9% 5% 3 5% 6% 4 5% 5% 5% 9% 45%	2389
December	1st to 10th	Night	7% 4 2 5% 5% 4 5% 5% 5% 8% 50%	2470
	11th to 20th	Night	12% 7% 3 4 4 3 4 4 5% 6% 49%	3042
	21st to end	Night	14% 8% 3 7% 6% 3 4 5% 6% 7% 37%	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	41% 4 7% 20% 19% 9%	2837
	11th to 20th	Day	50% 3 4 17% 16% 9%	2840
	21st to end	Day	38% 4 8% 20% 21% 8%	3229
February	1st to 10th	Day	38% 5% 9% 17% 20% 11%	1890
	11th to 20th	Day	30% 4 9% 20% 26% 11%	1758
	21st to end	Day	37% 5% 10% 17% 23% 9%	1570
March	1st to 10th	Day	26% 4 10% 25% 25% 9%	1980
	11th to 20th	Day	31% 5% 12% 25% 16% 10%	1980
	21st to end	Day	32% 5% 11% 23% 22% 8%	2178
April	1st to 10th	Day	31% 5% 14% 24% 18% 9%	1980
	11th to 20th	Day	37% 7% 11% 19% 20% 6%	1982
	21st to end	Day	27% 5% 12% 24% 23% 9%	2160
May	1st to 10th	Day	26% 5% 11% 24% 25% 9%	2308
	11th to 20th	Day	39% 7% 16% 21% 15% 3	2305
	21st to end	Day	42% 6% 14% 21% 16% 2	2531
June	1st to 10th	Day	54% 8% 17% 16% 5%	2340
	11th to 20th	Day	46% 10% 22% 19% 2	2340
	21st to end	Day	50% 9% 23% 14% 4	2340
July	1st to 10th	Day	53% 13% 23% 10% 1	2275
	11th to 20th	Day	54% 12% 23% 10%	2271
	21st to end	Day	51% 13% 23% 12% 1	2405
August	1st to 10th	Day	49% 12% 29% 10% 1	2314
	11th to 20th	Day	49% 14% 25% 11% 1	2196
	21st to end	Day	54% 12% 21% 12% 2	2178
September	1st to 10th	Day	52% 13% 20% 11% 4	1980
	11th to 20th	Day	52% 9% 22% 13% 4	1980
	21st to end	Day	50% 8% 15% 15% 9% 3	1980
October	1st to 10th	Day	45% 8% 10% 17% 12% 8%	1966
	11th to 20th	Day	35% 6% 8% 25% 21% 5%	1800
	21st to end	Day	31% 6% 10% 20% 21% 12%	1980
November	1st to 10th	Day	33% 6% 10% 20% 20% 10%	1857
	11th to 20th	Day	38% 4 9% 20% 21% 8%	1900
	21st to end	Day	37% 5% 9% 23% 19% 6%	1786
December	1st to 10th	Day	33% 5% 13% 26% 17% 5%	1710
	11th to 20th	Day	40% 4 10% 18% 18% 11%	2150
	21st to end	Day	46% 5% 10% 19% 13% 6%	3124
January	1st to 10th	Night	43% 4 9% 17% 20% 7%	4109
	11th to 20th	Night	51% 3 7% 20% 13% 6%	4120
	21st to end	Night	40% 6% 11% 17% 17% 9%	4422
February	1st to 10th	Night	37% 4 10% 18% 18% 12%	2275
	11th to 20th	Night	31% 5% 11% 21% 22% 10%	2099
	21st to end	Night	33% 6% 12% 20% 23% 7%	1683
March	1st to 10th	Night	24% 4 13% 27% 24% 8%	1980
	11th to 20th	Night	30% 4 15% 26% 15% 9%	1980
	21st to end	Night	28% 5% 11% 29% 20% 8%	2178
April	1st to 10th	Night	27% 5% 19% 24% 16% 8%	1980
	11th to 20th	Night	33% 5% 13% 25% 19% 6%	1926
	21st to end	Night	26% 4 14% 26% 23% 7%	1800
May	1st to 10th	Night	23% 5% 13% 27% 23% 10%	1666
	11th to 20th	Night	31% 8% 21% 24% 14% 2	1593
	21st to end	Night	32% 8% 17% 24% 18% 1	1745
June	1st to 10th	Night	43% 9% 21% 21% 5%	1620
	11th to 20th	Night	39% 9% 27% 23% 2	1620
	21st to end	Night	41% 13% 25% 15% 5%	1620
July	1st to 10th	Night	44% 17% 27% 10% 2	1575
	11th to 20th	Night	45% 14% 29% 11% 1	1566
	21st to end	Night	43% 15% 29% 12% 1	1672
August	1st to 10th	Night	37% 16% 34% 12% 1	1600
	11th to 20th	Night	37% 15% 33% 13% 1	1682
	21st to end	Night	46% 14% 24% 15% 1	2178
September	1st to 10th	Night	42% 16% 23% 15% 4	1980
	11th to 20th	Night	48% 11% 22% 17% 2 1	1980
	21st to end	Night	44% 8% 19% 17% 10% 3	1980
October	1st to 10th	Night	40% 7% 13% 21% 12% 6%	1993
	11th to 20th	Night	30% 6% 11% 27% 21% 5%	2160
	21st to end	Night	31% 5% 10% 20% 22% 10%	2376
November	1st to 10th	Night	31% 5% 12% 22% 19% 11%	2221
	11th to 20th	Night	37% 5% 10% 20% 19% 9%	2280
	21st to end	Night	36% 4 10% 24% 20% 6%	2389
December	1st to 10th	Night	32% 5% 13% 31% 14% 6%	2470
	11th to 20th	Night	36% 4 10% 21% 17% 11%	3044
	21st to end	Night	45% 6% 11% 20% 14% 4	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	14% 7% 39% 26% 11% 2	270
	11th to 20th	Day	7% 10% 36% 29% 14% 4	270
	21st to end	Day	16% 12% 38% 24% 8% 1	297
February	1st to 10th	Day	13% 9% 30% 27% 18% 4	267
	11th to 20th	Day	10% 5% 32% 32% 18% 5%	240
	21st to end	Day	11% 6% 29% 27% 22% 5%	288
March	1st to 10th	Day	5% 6% 31% 37% 19% 2	420
	11th to 20th	Day	13% 4 25% 30% 24% 4	420
	21st to end	Day	8% 4 26% 32% 25% 5%	462
April	1st to 10th	Day	8% 6% 24% 34% 24% 4	420
	11th to 20th	Day	11% 4 25% 29% 25% 6%	412
	21st to end	Day	7% 3 19% 34% 26% 12%	420
May	1st to 10th	Day	5% 3 19% 26% 32% 15%	423
	11th to 20th	Day	10% 2 10% 23% 33% 22%	447
	21st to end	Day	8% 1 7% 18% 33% 33%	485
June	1st to 10th	Day	15% 2 4 20% 31% 28%	450
	11th to 20th	Day	7% 2 3 24% 23% 40%	450
	21st to end	Day	7% 1 2 11% 26% 53%	450
July	1st to 10th	Day	8% 2 2 7% 25% 57%	445
	11th to 20th	Day	7% 1 9% 27% 56%	444
	21st to end	Day	6% 3 2 14% 25% 50%	488
August	1st to 10th	Day	9% 1 2 13% 26% 49%	446
	11th to 20th	Day	5% 3 1 8% 27% 55%	439
	21st to end	Day	11% 2 2 17% 25% 43%	495
September	1st to 10th	Day	8% 2 4 11% 30% 45%	450
	11th to 20th	Day	10% 3 4 19% 30% 35%	450
	21st to end	Day	10% 3 3 20% 31% 33%	450
October	1st to 10th	Day	14% 1 15% 31% 27% 11%	450
	11th to 20th	Day	7% 2 14% 32% 34% 11%	450
	21st to end	Day	8% 3 16% 36% 30% 7%	495
November	1st to 10th	Day	9% 5% 18% 32% 30% 6%	455
	11th to 20th	Day	15% 4 22% 36% 19% 4	460
	21st to end	Day	11% 4 27% 36% 21% 3	346
December	1st to 10th	Day	7% 6% 31% 33% 16% 7%	270
	11th to 20th	Day	16% 5% 27% 32% 17% 2	270
	21st to end	Day	10% 5% 27% 34% 20% 5%	297
January	1st to 10th	Night	11% 8% 31% 37% 13% 1	760
	11th to 20th	Night	12% 6% 24% 42% 15% 1	760
	21st to end	Night	14% 8% 34% 34% 10%	836
February	1st to 10th	Night	15% 5% 27% 39% 13% 1	759
	11th to 20th	Night	11% 6% 29% 38% 14% 1	703
	21st to end	Night	11% 7% 31% 42% 10%	592
March	1st to 10th	Night	6% 5% 33% 43% 13% 1	720
	11th to 20th	Night	10% 5% 28% 40% 17% 1	720
	21st to end	Night	8% 4 27% 45% 15% 1	792
April	1st to 10th	Night	9% 4 28% 42% 16%	720
	11th to 20th	Night	11% 4 27% 42% 14% 1	711
	21st to end	Night	8% 3 23% 45% 20% 1	720
May	1st to 10th	Night	6% 1 17% 50% 24% 2	586
	11th to 20th	Night	7% 2 13% 42% 31% 5%	531
	21st to end	Night	7% 2 7% 35% 42% 8%	584
June	1st to 10th	Night	12% 2 5% 30% 42% 9%	540
	11th to 20th	Night	6% 1 4 20% 42% 26%	540
	21st to end	Night	9% 1 1 17% 45% 27%	540
July	1st to 10th	Night	9% 2 1 10% 47% 31%	525
	11th to 20th	Night	6% 2 1 10% 45% 36%	522
	21st to end	Night	6% 3 1 11% 49% 30%	591
August	1st to 10th	Night	5% 2 1 11% 53% 28%	537
	11th to 20th	Night	5% 1 1 11% 45% 37%	556
	21st to end	Night	12% 2 3 19% 40% 24%	792
September	1st to 10th	Night	10% 2 4 18% 44% 23%	720
	11th to 20th	Night	13% 3 4 24% 41% 14%	720
	21st to end	Night	13% 2 5% 28% 42% 10%	720
October	1st to 10th	Night	13% 1 12% 38% 29% 6%	720
	11th to 20th	Night	8% 3 12% 39% 35% 4	720
	21st to end	Night	11% 2 15% 40% 29% 3	792
November	1st to 10th	Night	10% 3 22% 36% 27% 3	740
	11th to 20th	Night	12% 4 20% 43% 20% 3	760
	21st to end	Night	12% 3 25% 44% 16% 1	760
December	1st to 10th	Night	8% 3 24% 38% 25% 1	760
	11th to 20th	Night	12% 5% 24% 38% 19% 1	760
	21st to end	Night	12% 4 25% 38% 19% 1	836

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