Royer Oaks Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Royer Oaks 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	6% 5% 2 3 3 2 3 3 4 6% 64%	74816
	Night	7% 4 2 3 3 2 3 3 4 6% 63%	74956
Transparency	Day	22% 5% 8% 20% 27% 18%	74529
	Night	22% 5% 9% 21% 26% 16%	74765
Seeing	Day	8% 12% 31% 35% 11% 3	14182
	Night	8% 3 18% 37% 28% 5%	23901

By Month

Percentage Cloud Cover SW (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	13% 8% 2 4 4 2 4 4 5% 8% 46%	1519
	11th to 20th	Day	10% 6% 2 3 4 3 3 5% 6% 8% 50%	1558
	21st to end	Day	12% 9% 3 6% 4 2 5% 4 3 7% 43%	1787
February	1st to 10th	Day	13% 8% 3 3 3 2 3 4 4 6% 51%	1800
	11th to 20th	Day	11% 10% 3 4 3 2 3 4 4 7% 49%	1750
	21st to end	Day	16% 12% 4 5% 4 3 3 3 3 6% 41%	1480
March	1st to 10th	Day	14% 11% 4 5% 4 3 4 5% 5% 5% 39%	1800
	11th to 20th	Day	10% 9% 4 5% 4 3 4 5% 5% 8% 43%	1876
	21st to end	Day	11% 11% 4 6% 5% 2 4 4 4 5% 45%	2366
April	1st to 10th	Day	11% 7% 4 5% 4 2 3 3 5% 7% 50%	2160
	11th to 20th	Day	10% 7% 2 4 4 3 4 4 5% 7% 52%	2129
	21st to end	Day	8% 7% 2 3 3 2 4 3 5% 8% 56%	2160
May	1st to 10th	Day	6% 5% 2 3 2 2 4 3 4 6% 62%	2240
	11th to 20th	Day	5% 4 2 3 3 2 3 4 4 7% 63%	2301
	21st to end	Day	7% 3 2 3 3 2 2 4 5% 6% 64%	2606
June	1st to 10th	Day	3 3 2 3 3 2 3 3 5% 7% 67%	2520
	11th to 20th	Day	2 1 1 1 1 1 2 2 2 5% 83%	2520
	21st to end	Day	1 2 1 1 1 1 2 2 2 3 84%	2520
July	1st to 10th	Day	1 1 1 1 2 1 2 2 3 4 82%	2520
	11th to 20th	Day	2 1 1 2 2 1 2 3 3 4 80%	2520
	21st to end	Day	2 2 1 2 2 2 2 3 4 5% 75%	2594
August	1st to 10th	Day	1 2 1 1 2 1 2 2 3 2 83%	2149
	11th to 20th	Day	1 1 1 2 1 2 2 3 5% 82%	2122
	21st to end	Day	1 1 1 1 1 1 2 2 4 85%	2376
September	1st to 10th	Day	2 1 1 1 1 2 2 3 4 83%	2160
	11th to 20th	Day	1 1 1 2 1 1 2 2 2 3 84%	2070
	21st to end	Day	1 1 1 2 2 1 2 3 3 4 81%	1980
October	1st to 10th	Day	4 4 1 3 3 1 3 3 4 5% 68%	1952
	11th to 20th	Day	3 4 2 3 3 2 3 3 3 4 72%	1800
	21st to end	Day	5% 3 1 3 2 2 2 3 4 5% 71%	1936
November	1st to 10th	Day	6% 6% 3 4 4 1 2 3 4 7% 60%	1674
	11th to 20th	Day	7% 4 1 3 2 2 2 3 5% 8% 63%	1710
	21st to end	Day	7% 6% 2 3 4 3 3 4 5% 7% 57%	1710
December	1st to 10th	Day	10% 4 2 3 3 2 4 4 6% 9% 53%	1710
	11th to 20th	Day	16% 10% 4 5% 5% 3 4 4 5% 8% 38%	1947
	21st to end	Day	12% 9% 3 5% 6% 3 4 4 5% 9% 40%	2794
January	1st to 10th	Night	15% 6% 2 4 3 2 3 4 6% 8% 47%	2646
	11th to 20th	Night	8% 5% 2 3 4 3 3 4 5% 8% 53%	2472
	21st to end	Night	13% 6% 2 4 3 2 3 3 5% 8% 50%	2566
February	1st to 10th	Night	13% 8% 2 4 4 2 3 3 5% 6% 50%	2160
	11th to 20th	Night	10% 7% 3 4 2 1 3 3 5% 7% 55%	2098
	21st to end	Night	13% 10% 3 4 3 2 3 4 5% 6% 48%	1776
March	1st to 10th	Night	11% 9% 3 5% 4 3 5% 5% 6% 7% 45%	2160
	11th to 20th	Night	11% 7% 1 4 2 3 4 4 5% 8% 50%	2080
	21st to end	Night	12% 10% 3 4 3 3 4 3 3 6% 50%	1990
April	1st to 10th	Night	10% 5% 2 3 2 3 4 4 4 7% 56%	1800
	11th to 20th	Night	9% 4 2 3 3 2 3 4 4 6% 61%	1779
	21st to end	Night	6% 4 2 2 2 1 2 3 4 6% 66%	1800
May	1st to 10th	Night	6% 5% 1 2 2 1 3 3 4 7% 66%	1719
	11th to 20th	Night	6% 3 1 2 2 2 3 2 3 5% 71%	1594
	21st to end	Night	6% 3 1 3 2 2 3 2 3 5% 71%	1665
June	1st to 10th	Night	5% 3 1 2 2 1 2 2 3 7% 72%	1440
	11th to 20th	Night	1 1 1 1 2 1 2 2 3 3 83%	1440
	21st to end	Night	1 1 2 1 2 1 2 3 88%	1440
July	1st to 10th	Night	1 1 1 1 1 2 3 4 86%	1440
	11th to 20th	Night	1 2 1 1 1 2 2 3 5% 82%	1440
	21st to end	Night	2 1 2 3 2 2 3 3 3 3 78%	1719
August	1st to 10th	Night	1 2 1 1 2 1 1 1 2 4 84%	1790
	11th to 20th	Night	1 1 1 1 2 2 2 3 88%	1780
	21st to end	Night	1 1 1 2 1 1 2 2 3 87%	1980
September	1st to 10th	Night	2 1 1 1 1 1 1 2 2 90%	1800
	11th to 20th	Night	1 1 1 1 1 2 2 3 3 85%	1889
	21st to end	Night	1 1 1 2 1 2 2 3 3 83%	1980
October	1st to 10th	Night	3 2 1 2 3 2 3 3 3 4 74%	2004
	11th to 20th	Night	3 2 1 2 3 2 3 4 4 4 73%	2160
	21st to end	Night	4 3 1 2 2 2 2 3 3 4 76%	2418
November	1st to 10th	Night	6% 5% 2 3 3 1 3 3 4 6% 63%	2405
	11th to 20th	Night	5% 3 2 3 3 2 3 3 4 8% 62%	2470
	21st to end	Night	7% 5% 2 5% 3 3 4 4 4 7% 57%	2470
December	1st to 10th	Night	10% 5% 2 5% 4 2 4 4 4 6% 54%	2470
	11th to 20th	Night	17% 7% 2 5% 4 3 4 5% 6% 6% 40%	3254
	21st to end	Night	9% 7% 3 6% 6% 4 5% 5% 5% 9% 40%	4862

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% 6% 13% 24% 18% 4	1519
	11th to 20th	Day	32% 6% 10% 24% 17% 11%	1558
	21st to end	Day	40% 6% 11% 17% 19% 6%	1787
February	1st to 10th	Day	34% 4 10% 23% 20% 9%	1800
	11th to 20th	Day	33% 6% 10% 20% 22% 8%	1760
	21st to end	Day	44% 6% 10% 21% 17% 2	1480
March	1st to 10th	Day	42% 8% 10% 18% 17% 5%	1800
	11th to 20th	Day	35% 8% 14% 20% 18% 4	1876
	21st to end	Day	38% 9% 15% 22% 13% 3	2366
April	1st to 10th	Day	33% 7% 13% 26% 17% 5%	2160
	11th to 20th	Day	30% 7% 14% 23% 21% 6%	2132
	21st to end	Day	26% 7% 11% 35% 19% 2	2160
May	1st to 10th	Day	21% 6% 11% 28% 31% 3	2240
	11th to 20th	Day	19% 6% 11% 30% 29% 5%	2304
	21st to end	Day	19% 7% 10% 36% 25% 3	2606
June	1st to 10th	Day	17% 6% 11% 23% 33% 11%	2520
	11th to 20th	Day	7% 4 6% 18% 42% 24%	2520
	21st to end	Day	7% 3 3 10% 41% 35%	2520
July	1st to 10th	Day	7% 3 4 10% 29% 47%	2451
	11th to 20th	Day	9% 4 4 12% 33% 38%	2445
	21st to end	Day	11% 4 6% 16% 31% 31%	2449
August	1st to 10th	Day	9% 3 4 8% 36% 41%	2135
	11th to 20th	Day	6% 2 4 12% 38% 38%	2122
	21st to end	Day	6% 2 2 10% 38% 41%	2376
September	1st to 10th	Day	6% 2 4 11% 34% 42%	2160
	11th to 20th	Day	7% 3 5% 18% 40% 27%	2070
	21st to end	Day	8% 3 5% 21% 39% 24%	1980
October	1st to 10th	Day	17% 5% 8% 18% 26% 26%	1952
	11th to 20th	Day	17% 3 5% 19% 31% 25%	1800
	21st to end	Day	16% 5% 5% 18% 31% 24%	1936
November	1st to 10th	Day	24% 5% 8% 19% 27% 17%	1674
	11th to 20th	Day	22% 4 10% 23% 27% 14%	1710
	21st to end	Day	28% 5% 8% 16% 26% 16%	1710
December	1st to 10th	Day	28% 6% 11% 20% 21% 13%	1710
	11th to 20th	Day	43% 9% 10% 15% 15% 9%	1947
	21st to end	Day	40% 8% 13% 21% 13% 4	2794
January	1st to 10th	Night	35% 5% 12% 23% 20% 4	2646
	11th to 20th	Night	27% 8% 11% 25% 19% 11%	2472
	21st to end	Night	33% 6% 11% 19% 22% 8%	2566
February	1st to 10th	Night	35% 4 7% 24% 20% 9%	2160
	11th to 20th	Night	28% 5% 10% 22% 25% 9%	2099
	21st to end	Night	38% 6% 12% 23% 18% 3	1776
March	1st to 10th	Night	35% 7% 14% 21% 19% 4	2160
	11th to 20th	Night	32% 7% 14% 23% 19% 5%	2080
	21st to end	Night	35% 8% 15% 26% 13% 3	1990
April	1st to 10th	Night	28% 7% 15% 27% 16% 7%	1800
	11th to 20th	Night	24% 8% 13% 28% 22% 5%	1779
	21st to end	Night	20% 6% 14% 39% 20% 2	1800
May	1st to 10th	Night	20% 6% 11% 31% 28% 4	1719
	11th to 20th	Night	18% 4 9% 32% 33% 4	1595
	21st to end	Night	17% 6% 11% 31% 32% 3	1665
June	1st to 10th	Night	15% 4 10% 24% 37% 10%	1440
	11th to 20th	Night	8% 4 7% 19% 39% 24%	1440
	21st to end	Night	5% 1 4 12% 45% 32%	1440
July	1st to 10th	Night	5% 2 4 10% 32% 47%	1400
	11th to 20th	Night	8% 2 5% 14% 31% 40%	1392
	21st to end	Night	11% 4 6% 16% 28% 34%	1624
August	1st to 10th	Night	8% 2 3 12% 33% 42%	1780
	11th to 20th	Night	4 2 3 17% 38% 36%	1780
	21st to end	Night	6% 2 2 10% 42% 38%	1980
September	1st to 10th	Night	5% 2 3 12% 31% 48%	1800
	11th to 20th	Night	7% 3 5% 17% 43% 26%	1889
	21st to end	Night	8% 2 4 21% 38% 26%	1980
October	1st to 10th	Night	14% 5% 7% 19% 28% 26%	2004
	11th to 20th	Night	14% 5% 5% 20% 31% 25%	2160
	21st to end	Night	15% 4 5% 21% 31% 24%	2418
November	1st to 10th	Night	21% 6% 9% 20% 26% 19%	2405
	11th to 20th	Night	21% 5% 11% 19% 27% 17%	2470
	21st to end	Night	28% 6% 9% 16% 27% 15%	2470
December	1st to 10th	Night	30% 6% 9% 19% 22% 14%	2470
	11th to 20th	Night	40% 9% 10% 17% 15% 8%	3254
	21st to end	Night	37% 10% 12% 23% 13% 4	4862

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	22% 13% 32% 24% 8% 1	180
	11th to 20th	Day	16% 10% 32% 29% 11% 3	260
	21st to end	Day	15% 9% 34% 29% 10% 2	377
February	1st to 10th	Day	15% 10% 33% 27% 12% 2	420
	11th to 20th	Day	16% 15% 38% 22% 6% 2	415
	21st to end	Day	18% 16% 36% 25% 5%	346
March	1st to 10th	Day	19% 19% 35% 20% 6% 1	420
	11th to 20th	Day	16% 20% 33% 23% 7% 1	420
	21st to end	Day	17% 25% 35% 18% 5%	462
April	1st to 10th	Day	14% 20% 31% 25% 8% 2	420
	11th to 20th	Day	14% 24% 32% 24% 5% 1	412
	21st to end	Day	9% 22% 32% 25% 10% 2	420
May	1st to 10th	Day	9% 23% 33% 25% 9% 1	423
	11th to 20th	Day	9% 14% 35% 31% 9% 2	448
	21st to end	Day	11% 15% 30% 31% 8% 4	485
June	1st to 10th	Day	4 12% 31% 37% 13% 4	450
	11th to 20th	Day	2 14% 28% 38% 13% 5%	450
	21st to end	Day	1 11% 32% 38% 14% 4	450
July	1st to 10th	Day	2 2 27% 49% 15% 6%	446
	11th to 20th	Day	1 1 16% 57% 16% 8%	444
	21st to end	Day	2 2 17% 57% 15% 7%	487
August	1st to 10th	Day	2 5% 32% 43% 11% 6%	447
	11th to 20th	Day	1 3 29% 48% 11% 8%	437
	21st to end	Day	1 2 29% 49% 15% 4	495
September	1st to 10th	Day	3 5% 31% 45% 13% 3	450
	11th to 20th	Day	1 11% 36% 41% 9% 2	450
	21st to end	Day	3 11% 32% 40% 14%	450
October	1st to 10th	Day	4 15% 32% 36% 13%	450
	11th to 20th	Day	4 11% 34% 40% 11% 1	450
	21st to end	Day	6% 12% 28% 38% 16% 1	451
November	1st to 10th	Day	7% 12% 30% 33% 16% 1	270
	11th to 20th	Day	7% 10% 31% 32% 17% 3	270
	21st to end	Day	10% 14% 29% 30% 15% 3	270
December	1st to 10th	Day	12% 11% 32% 34% 9% 1	270
	11th to 20th	Day	14% 11% 28% 30% 15% 2	189
	21st to end	Day	24% 12% 29% 22% 11% 2	198
January	1st to 10th	Night	17% 4 26% 30% 18% 5%	760
	11th to 20th	Night	10% 4 27% 38% 19% 3	734
	21st to end	Night	17% 3 26% 34% 17% 3	792
February	1st to 10th	Night	15% 3 25% 37% 18% 3	720
	11th to 20th	Night	14% 4 24% 35% 19% 4	704
	21st to end	Night	18% 5% 22% 38% 14% 3	592
March	1st to 10th	Night	14% 6% 23% 37% 17% 3	720
	11th to 20th	Night	13% 5% 22% 32% 23% 6%	720
	21st to end	Night	15% 6% 22% 37% 19% 1	792
April	1st to 10th	Night	11% 4 23% 40% 20% 2	720
	11th to 20th	Night	10% 6% 24% 32% 24% 4	711
	21st to end	Night	8% 4 23% 35% 26% 4	720
May	1st to 10th	Night	8% 5% 24% 37% 26% 1	639
	11th to 20th	Night	7% 3 21% 38% 28% 2	531
	21st to end	Night	7% 6% 19% 39% 26% 3	503
June	1st to 10th	Night	6% 2 14% 40% 36% 2	360
	11th to 20th	Night	1 1 13% 46% 33% 6%	360
	21st to end	Night	1 1 12% 38% 42% 6%	360
July	1st to 10th	Night	1 6% 33% 54% 6%	350
	11th to 20th	Night	1 2 33% 54% 9%	348
	21st to end	Night	2 1 5% 29% 50% 13%	537
August	1st to 10th	Night	2 1 9% 39% 42% 8%	716
	11th to 20th	Night	1 6% 34% 46% 13%	712
	21st to end	Night	1 5% 30% 53% 10%	792
September	1st to 10th	Night	1 1 6% 34% 46% 11%	720
	11th to 20th	Night	1 1 11% 42% 37% 8%	720
	21st to end	Night	2 1 12% 37% 40% 8%	720
October	1st to 10th	Night	4 1 15% 46% 29% 5%	720
	11th to 20th	Night	3 2 16% 39% 33% 6%	720
	21st to end	Night	4 2 16% 37% 32% 9%	792
November	1st to 10th	Night	9% 3 16% 38% 26% 8%	740
	11th to 20th	Night	8% 2 19% 41% 25% 5%	760
	21st to end	Night	9% 2 23% 39% 21% 5%	760
December	1st to 10th	Night	10% 5% 25% 36% 19% 4	760
	11th to 20th	Night	17% 4 24% 38% 15% 3	760
	21st to end	Night	16% 6% 24% 36% 14% 3	836

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