Sierra Stars Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sierra Stars Observatory clear sky chart from 2005-11-06 to 2025-02-25.

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	1 3 1 4 3 3 3 3 5% 7% 67%	112575
	Night	1 3 1 4 3 3 3 3 5% 7% 67%	114901
Transparency	Day	18% 6% 11% 18% 27% 22%	112320
	Night	17% 6% 12% 21% 27% 17%	114745
Seeing	Day	9% 21% 31% 23% 11% 3	15397
	Night	5% 14% 26% 32% 19% 4	24903

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	2 1 4 2 3 4 4 8% 10% 61%	3639
	11th to 20th	Day	1 3 2 4 3 4 5% 4 6% 7% 63%	3757
	21st to end	Day	2 7% 3 6% 4 4 4 4 6% 9% 50%	4510
February	1st to 10th	Day	1 5% 1 4 3 3 4 4 7% 10% 58%	1825
	11th to 20th	Day	4 2 5% 4 4 4 5% 7% 11% 52%	1950
	21st to end	Day	1 3 2 6% 3 3 5% 4 7% 9% 57%	1611
March	1st to 10th	Day	1 4 4 8% 5% 6% 5% 6% 9% 10% 42%	3138
	11th to 20th	Day	4 2 4 3 3 3 4 6% 8% 64%	3423
	21st to end	Day	1 8% 2 6% 4 3 4 4 7% 9% 52%	4087
April	1st to 10th	Day	2 4 2 5% 4 4 5% 4 6% 8% 56%	2280
	11th to 20th	Day	2 5% 1 3 4 3 4 5% 6% 8% 59%	2249
	21st to end	Day	3 4 1 3 2 3 3 3 6% 8% 63%	2280
May	1st to 10th	Day	2 2 1 3 3 3 3 3 4 7% 69%	3979
	11th to 20th	Day	2 2 1 3 3 3 3 3 4 5% 71%	4253
	21st to end	Day	4 4 2 4 3 4 4 3 5% 6% 62%	4813
June	1st to 10th	Day	3 3 1 4 3 4 4 4 5% 7% 63%	2660
	11th to 20th	Day	2 2 1 2 1 2 2 2 3 5% 77%	2660
	21st to end	Day	2 2 1 3 3 2 4 3 3 4 74%	2660
July	1st to 10th	Day	1 1 1 1 2 1 1 2 3 87%	4420
	11th to 20th	Day	3 3 1 4 3 3 4 3 4 6% 66%	4420
	21st to end	Day	1 2 1 2 2 3 3 3 4 5% 74%	4673
August	1st to 10th	Day	2 2 1 3 2 2 3 3 3 4 75%	4009
	11th to 20th	Day	1 1 1 1 1 1 2 2 2 3 84%	3724
	21st to end	Day	1 1 1 2 1 1 2 1 2 2 86%	4180
September	1st to 10th	Day	1 1 1 1 2 2 2 2 4 5% 79%	2280
	11th to 20th	Day	2 2 1 3 3 2 3 2 4 5% 73%	2242
	21st to end	Day	2 2 1 3 2 2 2 2 3 4 77%	2090
October	1st to 10th	Day	2 3 1 3 2 2 3 3 5% 7% 71%	3252
	11th to 20th	Day	1 2 1 3 2 1 2 1 3 4 81%	3161
	21st to end	Day	1 1 1 3 2 3 3 3 5% 7% 72%	3365
November	1st to 10th	Day	1 3 1 3 3 3 4 3 7% 8% 63%	1764
	11th to 20th	Day	1 3 1 2 2 3 3 3 6% 8% 67%	1800
	21st to end	Day	1 2 1 3 3 4 3 4 6% 10% 63%	1769
December	1st to 10th	Day	1 3 1 2 2 2 3 4 5% 8% 69%	2620
	11th to 20th	Day	1 4 2 5% 4 5% 5% 4 8% 9% 52%	3028
	21st to end	Day	1 4 3 6% 4 4 6% 4 7% 13% 48%	4004
January	1st to 10th	Night	1 4 2 4 3 3 4 3 6% 11% 60%	6306
	11th to 20th	Night	3 2 5% 4 3 4 3 6% 7% 62%	6205
	21st to end	Night	1 6% 3 6% 4 4 5% 4 6% 8% 53%	6468
February	1st to 10th	Night	1 4 2 6% 3 3 5% 4 6% 8% 58%	2575
	11th to 20th	Night	1 4 2 4 3 3 4 5% 8% 11% 55%	2338
	21st to end	Night	1 4 2 4 3 3 4 4 7% 9% 60%	1937
March	1st to 10th	Night	6% 4 6% 4 4 5% 5% 8% 10% 48%	3736
	11th to 20th	Night	1 4 1 5% 3 3 4 4 6% 8% 60%	3554
	21st to end	Night	1 7% 3 6% 5% 5% 4 4 7% 10% 48%	3565
April	1st to 10th	Night	2 4 1 4 3 3 4 4 7% 10% 58%	1900
	11th to 20th	Night	1 4 1 3 3 3 3 3 6% 8% 64%	1879
	21st to end	Night	3 3 1 3 2 2 2 2 5% 8% 68%	1900
May	1st to 10th	Night	2 2 1 3 2 3 3 2 3 6% 74%	2993
	11th to 20th	Night	1 3 1 3 2 2 3 2 4 5% 75%	2627
	21st to end	Night	3 3 1 3 2 2 3 2 3 6% 72%	2762
June	1st to 10th	Night	4 3 1 2 2 3 3 3 5% 5% 69%	1520
	11th to 20th	Night	1 1 2 2 2 2 3 4 82%	1520
	21st to end	Night	2 2 1 2 1 2 2 2 2 3 82%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 91%	2540
	11th to 20th	Night	2 2 1 4 2 2 3 2 4 5% 73%	2540
	21st to end	Night	2 1 1 2 2 2 2 2 3 4 81%	2877
August	1st to 10th	Night	2 1 1 2 2 2 2 2 4 5% 77%	2933
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 88%	3126
	21st to end	Night	1 1 1 1 1 1 1 2 2 89%	3498
September	1st to 10th	Night	1 1 1 1 1 1 3 3 86%	1900
	11th to 20th	Night	1 2 1 2 1 2 2 1 3 5% 79%	1938
	21st to end	Night	1 1 1 3 2 2 2 3 3 4 78%	2090
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 80%	3725
	11th to 20th	Night	1 2 1 2 1 1 1 2 3 4 82%	3777
	21st to end	Night	1 2 1 3 2 3 3 2 4 7% 72%	4201
November	1st to 10th	Night	1 3 1 3 2 3 3 3 6% 9% 67%	2535
	11th to 20th	Night	2 1 2 2 3 4 3 5% 10% 67%	2600
	21st to end	Night	2 3 2 3 2 3 3 3 6% 8% 66%	2628
December	1st to 10th	Night	1 2 1 3 2 2 3 3 6% 8% 69%	4560
	11th to 20th	Night	1 3 1 4 3 3 4 4 8% 13% 53%	5176
	21st to end	Night	1 4 2 7% 5% 5% 6% 5% 9% 12% 45%	6952

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	17% 9% 24% 26% 22% 2	3639
	11th to 20th	Day	19% 8% 12% 20% 30% 11%	3757
	21st to end	Day	30% 9% 13% 15% 23% 11%	4510
February	1st to 10th	Day	23% 6% 16% 25% 21% 9%	1825
	11th to 20th	Day	25% 8% 14% 26% 22% 7%	1960
	21st to end	Day	24% 9% 16% 25% 20% 6%	1611
March	1st to 10th	Day	29% 9% 22% 21% 16% 3	3138
	11th to 20th	Day	19% 7% 15% 30% 26% 4	3423
	21st to end	Day	29% 9% 19% 23% 17% 3	4087
April	1st to 10th	Day	26% 8% 16% 24% 20% 7%	2280
	11th to 20th	Day	21% 7% 16% 26% 22% 8%	2252
	21st to end	Day	20% 7% 17% 27% 23% 5%	2280
May	1st to 10th	Day	16% 5% 9% 25% 36% 9%	3979
	11th to 20th	Day	16% 5% 8% 20% 38% 14%	4256
	21st to end	Day	20% 7% 10% 22% 36% 6%	4814
June	1st to 10th	Day	20% 7% 12% 20% 31% 11%	2660
	11th to 20th	Day	12% 3 7% 20% 35% 23%	2660
	21st to end	Day	15% 3 6% 10% 30% 37%	2660
July	1st to 10th	Day	6% 2 3 5% 13% 71%	4351
	11th to 20th	Day	21% 4 5% 7% 13% 50%	4345
	21st to end	Day	14% 4 5% 9% 20% 49%	4525
August	1st to 10th	Day	15% 3 4 7% 23% 47%	3994
	11th to 20th	Day	8% 1 4 4 16% 66%	3760
	21st to end	Day	9% 2 3 7% 35% 44%	4179
September	1st to 10th	Day	11% 2 4 8% 32% 43%	2280
	11th to 20th	Day	14% 3 7% 17% 35% 24%	2242
	21st to end	Day	13% 2 6% 17% 37% 26%	2090
October	1st to 10th	Day	14% 4 4 11% 34% 32%	3252
	11th to 20th	Day	12% 2 4 16% 51% 16%	3161
	21st to end	Day	14% 5% 7% 24% 36% 14%	3365
November	1st to 10th	Day	18% 7% 13% 22% 29% 11%	1764
	11th to 20th	Day	15% 5% 16% 23% 32% 8%	1800
	21st to end	Day	18% 7% 13% 25% 26% 11%	1769
December	1st to 10th	Day	15% 5% 15% 24% 31% 10%	2620
	11th to 20th	Day	28% 10% 16% 18% 19% 8%	3028
	21st to end	Day	27% 9% 22% 23% 16% 3	4004
January	1st to 10th	Night	21% 6% 21% 28% 21% 3	6306
	11th to 20th	Night	20% 7% 13% 21% 32% 7%	6205
	21st to end	Night	28% 9% 12% 18% 25% 8%	6468
February	1st to 10th	Night	25% 7% 15% 25% 19% 10%	2575
	11th to 20th	Night	22% 7% 15% 28% 23% 5%	2339
	21st to end	Night	21% 7% 18% 28% 21% 4	1937
March	1st to 10th	Night	26% 8% 21% 22% 19% 3	3736
	11th to 20th	Night	22% 8% 15% 38% 14% 3	3554
	21st to end	Night	27% 10% 23% 24% 14% 2	3565
April	1st to 10th	Night	21% 10% 18% 28% 18% 6%	1900
	11th to 20th	Night	20% 7% 15% 29% 21% 8%	1879
	21st to end	Night	18% 8% 16% 34% 22% 3	1900
May	1st to 10th	Night	12% 6% 10% 31% 36% 6%	2993
	11th to 20th	Night	13% 4 9% 22% 41% 12%	2627
	21st to end	Night	15% 5% 10% 28% 38% 3	2762
June	1st to 10th	Night	16% 7% 12% 23% 32% 10%	1520
	11th to 20th	Night	7% 4 8% 21% 36% 24%	1520
	21st to end	Night	10% 2 5% 13% 34% 36%	1520
July	1st to 10th	Night	4 1 2 5% 19% 69%	2500
	11th to 20th	Night	16% 3 4 7% 16% 54%	2492
	21st to end	Night	10% 3 4 9% 22% 52%	2787
August	1st to 10th	Night	12% 3 3 10% 29% 43%	2924
	11th to 20th	Night	6% 1 3 6% 19% 64%	3156
	21st to end	Night	6% 1 3 10% 40% 40%	3498
September	1st to 10th	Night	5% 1 4 9% 37% 42%	1900
	11th to 20th	Night	9% 3 7% 19% 36% 25%	1938
	21st to end	Night	11% 2 7% 17% 37% 26%	2090
October	1st to 10th	Night	11% 3 5% 12% 38% 32%	3725
	11th to 20th	Night	10% 3 4 19% 49% 15%	3777
	21st to end	Night	15% 4 6% 23% 39% 14%	4201
November	1st to 10th	Night	15% 6% 13% 27% 27% 12%	2535
	11th to 20th	Night	14% 7% 13% 27% 31% 7%	2600
	21st to end	Night	17% 6% 13% 27% 28% 9%	2628
December	1st to 10th	Night	16% 6% 14% 24% 28% 12%	4560
	11th to 20th	Night	23% 11% 18% 23% 21% 4	5176
	21st to end	Night	31% 10% 17% 20% 17% 5%	6952

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	19% 29% 26% 17% 10% 2	200
	11th to 20th	Day	14% 23% 27% 21% 11% 3	252
	21st to end	Day	8% 25% 30% 23% 10% 3	440
February	1st to 10th	Day	20% 27% 26% 18% 7% 2	413
	11th to 20th	Day	15% 27% 32% 21% 6%	495
	21st to end	Day	15% 25% 34% 21% 5% 1	399
March	1st to 10th	Day	17% 32% 30% 15% 4 1	460
	11th to 20th	Day	15% 31% 26% 17% 9% 1	460
	21st to end	Day	14% 35% 29% 17% 5%	506
April	1st to 10th	Day	13% 30% 32% 16% 8% 1	460
	11th to 20th	Day	18% 28% 30% 15% 6% 2	452
	21st to end	Day	12% 25% 31% 19% 9% 3	460
May	1st to 10th	Day	13% 30% 29% 18% 7% 2	463
	11th to 20th	Day	12% 24% 30% 22% 8% 4	488
	21st to end	Day	13% 18% 29% 22% 13% 5%	529
June	1st to 10th	Day	9% 22% 30% 24% 10% 4	490
	11th to 20th	Day	8% 24% 35% 21% 8% 4	490
	21st to end	Day	3 15% 38% 27% 11% 5%	490
July	1st to 10th	Day	2 14% 38% 27% 14% 3	486
	11th to 20th	Day	3 12% 36% 30% 14% 5%	484
	21st to end	Day	2 10% 37% 28% 18% 5%	531
August	1st to 10th	Day	4 15% 32% 30% 15% 5%	487
	11th to 20th	Day	3 15% 36% 24% 16% 6%	477
	21st to end	Day	3 13% 37% 29% 14% 4	539
September	1st to 10th	Day	3 10% 38% 29% 16% 3	490
	11th to 20th	Day	6% 19% 34% 28% 12% 3	490
	21st to end	Day	5% 16% 33% 34% 10% 2	490
October	1st to 10th	Day	5% 20% 26% 28% 18% 2	490
	11th to 20th	Day	7% 17% 26% 31% 14% 4	490
	21st to end	Day	8% 19% 29% 25% 16% 3	492
November	1st to 10th	Day	11% 18% 25% 18% 23% 5%	300
	11th to 20th	Day	9% 19% 25% 27% 14% 6%	300
	21st to end	Day	8% 22% 26% 25% 15% 4	284
December	1st to 10th	Day	12% 22% 23% 21% 16% 8%	200
	11th to 20th	Day	11% 27% 27% 18% 12% 6%	200
	21st to end	Day	21% 17% 29% 20% 10% 2	220
January	1st to 10th	Night	7% 18% 29% 23% 15% 8%	800
	11th to 20th	Night	4 17% 29% 32% 13% 6%	800
	21st to end	Night	5% 13% 33% 28% 15% 5%	880
February	1st to 10th	Night	9% 15% 29% 29% 16% 3	800
	11th to 20th	Night	7% 18% 30% 26% 15% 5%	784
	21st to end	Night	6% 23% 26% 31% 11% 3	646
March	1st to 10th	Night	7% 25% 28% 24% 12% 3	760
	11th to 20th	Night	6% 21% 27% 24% 18% 4	760
	21st to end	Night	6% 25% 31% 26% 11% 1	836
April	1st to 10th	Night	7% 18% 30% 29% 15% 1	760
	11th to 20th	Night	8% 19% 28% 27% 14% 4	751
	21st to end	Night	6% 15% 29% 29% 16% 4	760
May	1st to 10th	Night	6% 18% 27% 29% 17% 3	651
	11th to 20th	Night	4 12% 26% 38% 19% 2	446
	21st to end	Night	7% 7% 25% 34% 27% 2	411
June	1st to 10th	Night	5% 11% 22% 37% 23% 2	380
	11th to 20th	Night	2 11% 25% 35% 26% 1	380
	21st to end	Night	2 6% 25% 43% 22% 2	380
July	1st to 10th	Night	1 3 22% 46% 27% 1	370
	11th to 20th	Night	2 4 25% 44% 25%	368
	21st to end	Night	3 3 17% 40% 33% 5%	483
August	1st to 10th	Night	3 3 19% 42% 30% 3	613
	11th to 20th	Night	2 4 25% 39% 26% 5%	752
	21st to end	Night	3 4 23% 42% 25% 3	836
September	1st to 10th	Night	1 4 23% 37% 29% 4	760
	11th to 20th	Night	3 9% 22% 33% 28% 6%	760
	21st to end	Night	2 9% 23% 32% 27% 7%	760
October	1st to 10th	Night	5% 9% 22% 36% 22% 6%	760
	11th to 20th	Night	6% 9% 18% 39% 24% 6%	760
	21st to end	Night	4 12% 22% 27% 23% 11%	836
November	1st to 10th	Night	5% 15% 22% 29% 23% 7%	780
	11th to 20th	Night	5% 15% 29% 31% 15% 6%	800
	21st to end	Night	8% 15% 24% 29% 18% 6%	800
December	1st to 10th	Night	6% 16% 28% 30% 15% 5%	800
	11th to 20th	Night	7% 19% 27% 31% 11% 6%	800
	21st to end	Night	9% 19% 33% 27% 11% 2	880

Page generated at 2025-02-22 03:51:35. Version ac07

Copyright © 2025 Attilla Danko.