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 2024-11-12.

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	5% 5% 2 3 2 2 3 2 4 5% 66%	104970
	Night	7% 5% 2 3 2 2 3 2 4 6% 65%	102979
Transparency	Day	19% 6% 8% 19% 26% 23%	104720
	Night	21% 6% 9% 20% 25% 20%	102819
Seeing	Day	8% 12% 31% 35% 11% 3	15360
	Night	8% 3 19% 37% 27% 5%	24967

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	9% 10% 4 7% 4 3 3 3 5% 8% 42%	2539
	11th to 20th	Day	8% 8% 3 7% 5% 5% 5% 5% 7% 9% 38%	2704
	21st to end	Day	14% 11% 3 7% 4 3 4 4 6% 9% 34%	3151
February	1st to 10th	Day	14% 10% 2 4 3 2 3 3 4 6% 49%	1900
	11th to 20th	Day	11% 11% 2 5% 3 3 3 3 4 7% 48%	1850
	21st to end	Day	15% 13% 4 6% 4 3 3 2 4 6% 41%	1570
March	1st to 10th	Day	13% 16% 6% 7% 4 3 4 3 5% 6% 34%	3138
	11th to 20th	Day	6% 7% 2 5% 3 3 4 3 5% 9% 53%	3321
	21st to end	Day	11% 16% 4 7% 4 4 4 4 5% 6% 36%	4155
April	1st to 10th	Day	10% 8% 3 6% 3 3 2 3 5% 6% 50%	2280
	11th to 20th	Day	9% 8% 2 5% 3 4 4 3 5% 6% 52%	2249
	21st to end	Day	8% 8% 2 4 2 3 3 3 5% 7% 56%	2280
May	1st to 10th	Day	5% 5% 2 3 2 2 3 2 3 4 69%	3945
	11th to 20th	Day	3 2 1 2 2 2 3 3 4 5% 72%	4071
	21st to end	Day	4 2 1 2 1 1 2 2 3 4 78%	4618
June	1st to 10th	Day	3 3 2 3 3 2 3 3 5% 7% 67%	2660
	11th to 20th	Day	1 1 1 1 1 2 1 2 2 5% 83%	2660
	21st to end	Day	1 2 1 1 1 1 2 2 2 3 85%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 2 89%	4420
	11th to 20th	Day	1 1 2 2 1 2 2 3 3 83%	4420
	21st to end	Day	1 1 2 1 1 1 2 3 3 84%	4531
August	1st to 10th	Day	1 1 1 1 1 1 1 1 2 2 86%	3783
	11th to 20th	Day	1 1 1 1 1 1 2 3 90%	3724
	21st to end	Day	1 1 1 1 1 1 1 2 3 90%	4180
September	1st to 10th	Day	2 1 1 1 2 2 1 2 4 84%	2280
	11th to 20th	Day	1 2 1 2 1 2 2 2 2 3 84%	2185
	21st to end	Day	1 1 1 2 1 1 2 2 3 4 81%	2090
October	1st to 10th	Day	3 3 1 2 2 1 2 2 4 4 76%	3436
	11th to 20th	Day	1 3 1 3 2 2 2 2 3 3 76%	3161
	21st to end	Day	6% 4 2 3 2 3 3 2 5% 6% 63%	3365
November	1st to 10th	Day	5% 7% 3 4 3 2 2 3 4 6% 60%	1764
	11th to 20th	Day	7% 4 1 3 2 3 2 2 6% 7% 63%	1719
	21st to end	Day	7% 7% 1 3 3 4 3 3 5% 7% 56%	1710
December	1st to 10th	Day	10% 5% 2 4 3 3 4 3 6% 8% 52%	1710
	11th to 20th	Day	16% 10% 3 5% 4 5% 3 3 5% 8% 37%	1947
	21st to end	Day	12% 10% 3 6% 5% 4 4 3 5% 8% 40%	2794
January	1st to 10th	Night	12% 9% 3 6% 3 3 3 3 5% 8% 45%	4406
	11th to 20th	Night	11% 10% 3 5% 3 4 4 4 6% 10% 40%	4259
	21st to end	Night	17% 9% 3 5% 3 4 4 4 6% 10% 36%	4513
February	1st to 10th	Night	14% 10% 2 5% 4 2 3 2 5% 6% 47%	2280
	11th to 20th	Night	11% 9% 2 4 2 2 2 2 5% 7% 53%	2218
	21st to end	Night	13% 11% 3 5% 3 2 3 3 5% 6% 48%	1884
March	1st to 10th	Night	14% 12% 4 6% 3 3 4 3 6% 7% 38%	3736
	11th to 20th	Night	6% 6% 2 4 2 3 4 3 5% 7% 58%	3655
	21st to end	Night	15% 12% 3 5% 3 4 4 3 4 6% 41%	3498
April	1st to 10th	Night	9% 7% 2 4 2 4 3 3 5% 6% 56%	1900
	11th to 20th	Night	8% 5% 2 4 3 2 3 3 4 6% 60%	1879
	21st to end	Night	7% 5% 1 3 3 1 2 3 4 6% 65%	1900
May	1st to 10th	Night	7% 4 1 2 1 1 2 1 3 5% 73%	3029
	11th to 20th	Night	4 2 1 2 1 1 2 2 3 5% 78%	2834
	21st to end	Night	3 2 1 2 1 1 2 1 2 3 82%	2959
June	1st to 10th	Night	5% 3 1 2 2 2 2 2 3 7% 72%	1520
	11th to 20th	Night	1 1 1 1 2 1 2 1 3 3 84%	1520
	21st to end	Night	1 1 1 1 1 1 2 2 88%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 2 91%	2540
	11th to 20th	Night	1 2 1 1 1 2 1 2 4 85%	2540
	21st to end	Night	1 1 1 2 1 1 1 1 2 2 87%	3025
August	1st to 10th	Night	1 1 1 1 1 2 2 2 4 84%	3166
	11th to 20th	Night	1 1 1 1 2 2 92%	3126
	21st to end	Night	1 1 1 1 1 1 2 92%	3498
September	1st to 10th	Night	2 1 1 1 1 1 1 2 2 90%	1900
	11th to 20th	Night	1 1 1 1 1 1 2 2 2 3 84%	1994
	21st to end	Night	1 1 2 2 1 2 2 2 3 84%	2090
October	1st to 10th	Night	2 2 1 2 1 2 2 1 2 3 83%	3540
	11th to 20th	Night	2 1 1 2 2 2 3 2 3 4 78%	3777
	21st to end	Night	3 3 1 3 2 2 2 2 2 5% 75%	4199
November	1st to 10th	Night	6% 5% 2 4 3 2 3 2 4 6% 63%	2535
	11th to 20th	Night	5% 3 2 4 3 2 3 3 5% 8% 62%	2483
	21st to end	Night	7% 5% 2 5% 3 3 4 3 5% 7% 56%	2470
December	1st to 10th	Night	10% 6% 2 6% 3 3 4 3 4 6% 54%	2470
	11th to 20th	Night	17% 7% 2 5% 4 4 4 4 6% 6% 40%	3254
	21st to end	Night	9% 8% 3 7% 5% 6% 5% 4 6% 9% 39%	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	32% 9% 17% 28% 12% 2	2539
	11th to 20th	Day	34% 11% 17% 22% 10% 6%	2704
	21st to end	Day	41% 9% 15% 19% 13% 4	3151
February	1st to 10th	Day	35% 5% 10% 23% 19% 8%	1900
	11th to 20th	Day	34% 7% 10% 20% 22% 8%	1860
	21st to end	Day	43% 7% 11% 21% 16% 2	1570
March	1st to 10th	Day	42% 13% 10% 16% 14% 5%	3138
	11th to 20th	Day	22% 12% 18% 30% 12% 7%	3321
	21st to end	Day	39% 14% 14% 20% 12% 2	4155
April	1st to 10th	Day	32% 7% 12% 26% 18% 5%	2280
	11th to 20th	Day	29% 8% 14% 23% 21% 5%	2252
	21st to end	Day	25% 7% 11% 35% 20% 2	2280
May	1st to 10th	Day	15% 7% 9% 32% 31% 6%	3945
	11th to 20th	Day	11% 5% 9% 26% 45% 4	4074
	21st to end	Day	11% 4 6% 33% 42% 3	4618
June	1st to 10th	Day	16% 5% 11% 23% 34% 10%	2660
	11th to 20th	Day	6% 4 6% 18% 43% 23%	2660
	21st to end	Day	7% 3 3 10% 40% 37%	2660
July	1st to 10th	Day	4 2 2 5% 16% 70%	4351
	11th to 20th	Day	8% 3 3 7% 20% 59%	4345
	21st to end	Day	7% 2 4 13% 29% 46%	4387
August	1st to 10th	Day	7% 2 2 6% 38% 44%	3770
	11th to 20th	Day	4 1 2 7% 27% 59%	3760
	21st to end	Day	4 2 2 11% 31% 51%	4179
September	1st to 10th	Day	6% 2 4 10% 34% 43%	2280
	11th to 20th	Day	7% 3 5% 19% 41% 26%	2185
	21st to end	Day	8% 3 5% 20% 38% 27%	2090
October	1st to 10th	Day	11% 3 4 12% 20% 50%	3436
	11th to 20th	Day	14% 3 4 22% 35% 21%	3161
	21st to end	Day	19% 5% 5% 14% 33% 23%	3365
November	1st to 10th	Day	24% 5% 8% 20% 26% 17%	1764
	11th to 20th	Day	22% 4 10% 23% 27% 14%	1719
	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% 7% 15% 23% 18% 2	4406
	11th to 20th	Night	35% 10% 14% 24% 11% 6%	4259
	21st to end	Night	39% 10% 13% 19% 14% 5%	4513
February	1st to 10th	Night	37% 5% 7% 23% 19% 9%	2280
	11th to 20th	Night	29% 6% 11% 22% 24% 9%	2219
	21st to end	Night	37% 6% 12% 24% 18% 3	1884
March	1st to 10th	Night	37% 11% 12% 18% 19% 3	3736
	11th to 20th	Night	21% 11% 17% 31% 13% 7%	3655
	21st to end	Night	39% 11% 15% 22% 10% 1	3498
April	1st to 10th	Night	28% 7% 15% 27% 16% 6%	1900
	11th to 20th	Night	23% 9% 13% 28% 22% 5%	1879
	21st to end	Night	20% 6% 13% 38% 21% 1	1900
May	1st to 10th	Night	16% 5% 7% 37% 29% 5%	3029
	11th to 20th	Night	10% 3 7% 35% 43% 2	2835
	21st to end	Night	10% 4 7% 36% 42% 2	2959
June	1st to 10th	Night	14% 4 11% 24% 38% 9%	1520
	11th to 20th	Night	7% 4 7% 18% 40% 24%	1520
	21st to end	Night	5% 1 4 11% 44% 35%	1520
July	1st to 10th	Night	3 1 2 6% 18% 70%	2500
	11th to 20th	Night	7% 2 3 9% 20% 60%	2492
	21st to end	Night	6% 2 3 12% 26% 50%	2931
August	1st to 10th	Night	7% 2 3 9% 37% 43%	3156
	11th to 20th	Night	2 1 2 10% 23% 62%	3156
	21st to end	Night	4 1 2 9% 33% 52%	3498
September	1st to 10th	Night	5% 1 3 11% 31% 49%	1900
	11th to 20th	Night	7% 3 5% 17% 43% 25%	1994
	21st to end	Night	8% 2 4 21% 37% 29%	2090
October	1st to 10th	Night	9% 3 4 11% 23% 51%	3540
	11th to 20th	Night	10% 3 4 25% 39% 19%	3777
	21st to end	Night	14% 3 5% 18% 37% 23%	4199
November	1st to 10th	Night	21% 5% 9% 21% 25% 18%	2535
	11th to 20th	Night	20% 5% 11% 19% 28% 17%	2483
	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	15% 11% 31% 29% 11% 3	279
	21st to end	Day	15% 11% 33% 29% 10% 2	410
February	1st to 10th	Day	17% 11% 33% 25% 11% 2	460
	11th to 20th	Day	15% 16% 38% 24% 6% 2	455
	21st to end	Day	16% 18% 36% 24% 5%	382
March	1st to 10th	Day	19% 20% 36% 20% 6% 1	460
	11th to 20th	Day	15% 20% 34% 22% 7% 1	460
	21st to end	Day	17% 27% 34% 17% 4	506
April	1st to 10th	Day	14% 21% 30% 25% 8% 2	460
	11th to 20th	Day	14% 23% 32% 25% 6% 1	452
	21st to end	Day	9% 22% 33% 24% 10% 2	460
May	1st to 10th	Day	10% 23% 33% 24% 9% 1	463
	11th to 20th	Day	8% 14% 35% 32% 9% 2	488
	21st to end	Day	10% 14% 31% 32% 9% 4	529
June	1st to 10th	Day	3 11% 30% 38% 12% 4	490
	11th to 20th	Day	2 14% 31% 37% 13% 4	490
	21st to end	Day	1 10% 31% 39% 14% 4	490
July	1st to 10th	Day	1 2 27% 49% 15% 5%	486
	11th to 20th	Day	1 1 16% 58% 16% 8%	484
	21st to end	Day	2 2 17% 57% 16% 7%	531
August	1st to 10th	Day	3 5% 31% 43% 11% 7%	487
	11th to 20th	Day	1 3 30% 47% 11% 7%	477
	21st to end	Day	1 3 29% 47% 15% 5%	539
September	1st to 10th	Day	2 5% 31% 45% 14% 3	490
	11th to 20th	Day	1 12% 36% 39% 9% 2	490
	21st to end	Day	3 12% 32% 39% 14%	490
October	1st to 10th	Day	4 14% 30% 38% 14%	490
	11th to 20th	Day	4 11% 34% 40% 11% 1	490
	21st to end	Day	6% 12% 28% 38% 15% 1	492
November	1st to 10th	Day	7% 13% 31% 32% 16% 1	300
	11th to 20th	Day	7% 11% 31% 32% 17% 3	273
	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 28% 37% 18% 3	760
	21st to end	Night	16% 4 26% 34% 16% 3	836
February	1st to 10th	Night	17% 4 24% 35% 17% 3	760
	11th to 20th	Night	14% 5% 24% 35% 18% 4	744
	21st to end	Night	18% 6% 24% 37% 13% 2	628
March	1st to 10th	Night	14% 7% 24% 36% 16% 3	760
	11th to 20th	Night	12% 5% 25% 31% 22% 5%	760
	21st to end	Night	15% 7% 23% 37% 18% 1	836
April	1st to 10th	Night	11% 6% 23% 39% 19% 2	760
	11th to 20th	Night	10% 7% 24% 32% 24% 3	751
	21st to end	Night	8% 5% 23% 35% 25% 4	760
May	1st to 10th	Night	8% 5% 25% 37% 25% 1	674
	11th to 20th	Night	7% 3 21% 39% 28% 2	561
	21st to end	Night	6% 6% 20% 40% 25% 2	531
June	1st to 10th	Night	6% 2 14% 42% 35% 2	380
	11th to 20th	Night	1 1 16% 46% 31% 6%	380
	21st to end	Night	1 1 12% 40% 40% 6%	380
July	1st to 10th	Night	1 6% 36% 52% 6%	370
	11th to 20th	Night	1 2 35% 52% 9%	368
	21st to end	Night	2 1 5% 31% 49% 13%	568
August	1st to 10th	Night	2 1 9% 40% 41% 7%	756
	11th to 20th	Night	1 7% 36% 44% 12%	752
	21st to end	Night	1 6% 31% 52% 10%	836
September	1st to 10th	Night	1 1 7% 36% 44% 11%	760
	11th to 20th	Night	1 1 12% 42% 35% 8%	760
	21st to end	Night	1 1 13% 38% 38% 8%	760
October	1st to 10th	Night	4 1 15% 48% 28% 5%	760
	11th to 20th	Night	3 2 18% 39% 32% 6%	760
	21st to end	Night	4 3 18% 37% 30% 9%	836
November	1st to 10th	Night	8% 3 18% 38% 25% 8%	780
	11th to 20th	Night	8% 2 19% 41% 25% 5%	764
	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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