Bixby Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bixby Creek Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	11% 6% 2 5% 4 4 4 3 5% 7% 48%	126416
	Night	11% 6% 2 5% 3 3 3 3 5% 7% 53%	122910
Transparency	Day	34% 5% 10% 25% 20% 5%	130054
	Night	31% 4 10% 24% 23% 7%	127335
Seeing	Day	13% 7% 22% 28% 22% 7%	16613
	Night	13% 7% 26% 35% 17% 2	26361

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	12% 11% 4 6% 3 3 3 2 4 6% 45%	4077
	11th to 20th	Day	7% 5% 2 6% 4 3 4 3 4 5% 58%	4080
	21st to end	Day	21% 8% 2 4 2 2 2 2 3 5% 48%	4488
February	1st to 10th	Day	19% 9% 2 7% 4 3 3 3 4 4 42%	1800
	11th to 20th	Day	16% 7% 3 6% 4 3 3 2 5% 6% 43%	1873
	21st to end	Day	17% 8% 2 5% 3 4 4 3 4 6% 43%	1738
March	1st to 10th	Day	15% 7% 2 5% 3 3 4 2 3 5% 51%	4936
	11th to 20th	Day	8% 5% 2 6% 4 4 4 3 5% 6% 53%	4980
	21st to end	Day	14% 8% 3 7% 5% 3 5% 3 5% 8% 40%	5462
April	1st to 10th	Day	13% 5% 2 6% 4 4 4 3 5% 6% 47%	2232
	11th to 20th	Day	15% 7% 2 7% 4 3 3 3 5% 6% 46%	2374
	21st to end	Day	18% 8% 3 7% 4 4 4 3 5% 5% 41%	2480
May	1st to 10th	Day	16% 8% 3 7% 4 4 4 4 4 6% 40%	5862
	11th to 20th	Day	11% 6% 2 7% 4 4 4 4 5% 7% 43%	5862
	21st to end	Day	15% 8% 3 8% 5% 5% 5% 4 5% 7% 35%	6434
June	1st to 10th	Day	8% 5% 2 6% 5% 4 4 4 7% 9% 46%	2600
	11th to 20th	Day	7% 4 1 5% 5% 5% 5% 4 6% 9% 49%	2600
	21st to end	Day	3 4 1 4 5% 4 4 3 6% 8% 58%	2600
July	1st to 10th	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 11% 41%	5900
	11th to 20th	Day	5% 4 2 5% 4 4 4 4 6% 8% 55%	5900
	21st to end	Day	4 3 1 4 4 4 4 5% 7% 10% 55%	6422
August	1st to 10th	Day	3 3 2 4 4 3 4 4 6% 8% 59%	4234
	11th to 20th	Day	8% 4 2 4 4 3 4 4 6% 9% 52%	4171
	21st to end	Day	5% 3 2 4 4 4 5% 5% 7% 8% 55%	4352
September	1st to 10th	Day	10% 5% 2 4 3 3 4 3 5% 7% 53%	2295
	11th to 20th	Day	10% 5% 2 6% 3 4 5% 4 6% 8% 48%	2178
	21st to end	Day	8% 3 2 4 4 4 5% 4 6% 5% 56%	2090
October	1st to 10th	Day	8% 4 2 5% 3 3 4 3 4 5% 59%	1938
	11th to 20th	Day	11% 4 2 4 3 3 2 2 3 5% 62%	1900
	21st to end	Day	17% 5% 2 5% 3 3 3 2 4 5% 52%	2090
November	1st to 10th	Day	11% 5% 1 4 3 3 2 3 5% 7% 56%	1958
	11th to 20th	Day	11% 5% 2 4 2 2 3 2 3 6% 60%	1891
	21st to end	Day	14% 6% 2 4 3 3 2 3 5% 6% 53%	1800
December	1st to 10th	Day	14% 7% 2 4 3 3 3 3 3 6% 52%	2940
	11th to 20th	Day	17% 8% 2 5% 3 3 3 2 4 6% 47%	3391
	21st to end	Day	22% 11% 3 7% 3 3 3 2 4 5% 36%	4488
January	1st to 10th	Night	14% 7% 2 8% 4 3 3 3 4 6% 45%	5888
	11th to 20th	Night	7% 5% 2 4 3 3 3 3 4 6% 60%	5900
	21st to end	Night	21% 8% 2 4 2 2 2 2 3 5% 47%	6490
February	1st to 10th	Night	16% 9% 3 5% 3 3 3 2 4 6% 46%	2600
	11th to 20th	Night	16% 8% 2 6% 3 3 2 2 4 6% 47%	2430
	21st to end	Night	14% 7% 2 6% 4 3 3 3 5% 5% 49%	1890
March	1st to 10th	Night	18% 7% 3 6% 4 3 3 2 3 4 48%	4919
	11th to 20th	Night	9% 5% 2 4 3 3 3 3 5% 6% 58%	4980
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 8% 46%	5467
April	1st to 10th	Night	13% 5% 2 5% 4 3 3 3 5% 6% 50%	2166
	11th to 20th	Night	12% 6% 2 4 4 3 3 3 4 7% 52%	1975
	21st to end	Night	12% 6% 2 5% 5% 4 3 3 5% 7% 48%	1916
May	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 45%	4053
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 4 6% 8% 46%	4053
	21st to end	Night	12% 6% 2 6% 4 4 4 3 6% 9% 43%	4443
June	1st to 10th	Night	5% 3 2 5% 3 4 4 4 6% 9% 56%	1800
	11th to 20th	Night	5% 4 1 4 3 3 4 4 7% 8% 58%	1800
	21st to end	Night	3 2 1 3 3 3 3 4 5% 8% 66%	1800
July	1st to 10th	Night	6% 5% 2 5% 5% 4 4 4 7% 10% 48%	4078
	11th to 20th	Night	3 3 1 4 2 3 3 3 5% 9% 64%	4077
	21st to end	Night	3 3 1 3 3 3 4 3 7% 10% 61%	4449
August	1st to 10th	Night	2 2 1 4 3 3 4 4 5% 8% 64%	2926
	11th to 20th	Night	4 3 1 3 3 3 4 4 7% 9% 59%	2888
	21st to end	Night	3 3 1 4 3 3 4 3 6% 9% 61%	3544
September	1st to 10th	Night	8% 5% 2 4 2 3 4 3 5% 6% 58%	2100
	11th to 20th	Night	9% 4 2 5% 3 3 5% 4 6% 7% 52%	2178
	21st to end	Night	7% 4 1 5% 3 3 3 3 5% 6% 60%	2090
October	1st to 10th	Night	7% 4 1 3 2 3 3 3 4 5% 64%	2221
	11th to 20th	Night	12% 4 2 3 2 3 3 2 3 4 63%	2280
	21st to end	Night	16% 5% 1 4 3 3 2 2 3 5% 55%	2508
November	1st to 10th	Night	11% 5% 1 3 3 3 2 2 4 4 61%	2340
	11th to 20th	Night	8% 5% 2 4 3 2 3 3 5% 5% 59%	2505
	21st to end	Night	15% 6% 2 4 3 3 2 2 5% 5% 55%	2600
December	1st to 10th	Night	11% 7% 2 5% 3 3 3 2 4 5% 54%	4250
	11th to 20th	Night	16% 6% 3 5% 3 3 3 2 3 4 52%	4816
	21st to end	Night	20% 9% 3 6% 3 3 3 2 3 4 44%	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	43% 3 9% 24% 19% 2	4078
	11th to 20th	Day	30% 3 10% 25% 28% 3	4080
	21st to end	Day	42% 3 5% 21% 25% 6%	4488
February	1st to 10th	Day	47% 3 10% 20% 17% 4	1800
	11th to 20th	Day	44% 3 8% 22% 18% 4	1882
	21st to end	Day	43% 3 6% 18% 22% 8%	1738
March	1st to 10th	Day	38% 3 5% 18% 18% 18%	4936
	11th to 20th	Day	32% 4 9% 19% 20% 16%	4980
	21st to end	Day	43% 5% 10% 30% 12% 1	5464
April	1st to 10th	Day	37% 6% 11% 21% 19% 6%	2232
	11th to 20th	Day	40% 5% 10% 22% 18% 5%	2374
	21st to end	Day	46% 6% 8% 22% 17% 2	2480
May	1st to 10th	Day	45% 6% 9% 23% 16% 1	5900
	11th to 20th	Day	39% 7% 11% 22% 20% 3	5866
	21st to end	Day	46% 8% 10% 18% 16% 3	6447
June	1st to 10th	Day	33% 8% 14% 27% 16% 2	2600
	11th to 20th	Day	31% 7% 14% 26% 19% 3	2600
	21st to end	Day	24% 7% 13% 30% 22% 4	2600
July	1st to 10th	Day	33% 8% 16% 29% 12% 1	5836
	11th to 20th	Day	26% 6% 13% 33% 22% 1	5831
	21st to end	Day	23% 7% 16% 37% 17% 1	6240
August	1st to 10th	Day	22% 7% 12% 28% 27% 5%	4220
	11th to 20th	Day	29% 5% 16% 31% 16% 2	4210
	21st to end	Day	25% 8% 13% 38% 13% 2	4351
September	1st to 10th	Day	30% 6% 9% 24% 22% 9%	2295
	11th to 20th	Day	33% 6% 10% 24% 21% 6%	2178
	21st to end	Day	28% 6% 9% 21% 28% 8%	2090
October	1st to 10th	Day	17% 4 6% 13% 40% 21%	3224
	11th to 20th	Day	17% 2 4 18% 42% 17%	3140
	21st to end	Day	28% 3 7% 29% 26% 7%	3428
November	1st to 10th	Day	28% 4 9% 25% 27% 7%	1958
	11th to 20th	Day	28% 3 8% 27% 24% 10%	1891
	21st to end	Day	33% 4 9% 22% 27% 6%	1800
December	1st to 10th	Day	35% 3 7% 34% 18% 2	2940
	11th to 20th	Day	40% 3 5% 24% 24% 5%	3389
	21st to end	Day	51% 4 6% 22% 14% 3	4488
January	1st to 10th	Night	40% 3 9% 26% 18% 2	5888
	11th to 20th	Night	27% 4 11% 18% 33% 6%	5900
	21st to end	Night	42% 3 6% 16% 28% 5%	6490
February	1st to 10th	Night	41% 3 10% 23% 18% 5%	2600
	11th to 20th	Night	41% 3 7% 24% 19% 6%	2431
	21st to end	Night	38% 3 8% 19% 23% 10%	1890
March	1st to 10th	Night	41% 3 5% 16% 21% 13%	4919
	11th to 20th	Night	27% 4 7% 24% 20% 18%	4980
	21st to end	Night	36% 5% 11% 26% 18% 3	5467
April	1st to 10th	Night	34% 5% 9% 20% 21% 10%	2166
	11th to 20th	Night	32% 6% 8% 24% 19% 10%	1975
	21st to end	Night	35% 6% 10% 25% 22% 3	1916
May	1st to 10th	Night	38% 7% 12% 22% 17% 4	4080
	11th to 20th	Night	34% 7% 11% 23% 20% 5%	4054
	21st to end	Night	36% 8% 15% 21% 16% 3	4452
June	1st to 10th	Night	24% 7% 14% 31% 21% 4	1800
	11th to 20th	Night	23% 6% 13% 31% 24% 4	1800
	21st to end	Night	16% 5% 14% 31% 28% 6%	1800
July	1st to 10th	Night	30% 7% 17% 27% 15% 3	4035
	11th to 20th	Night	18% 5% 14% 35% 26% 3	4026
	21st to end	Night	18% 6% 18% 36% 21% 2	4320
August	1st to 10th	Night	18% 5% 13% 28% 28% 8%	2916
	11th to 20th	Night	21% 6% 18% 31% 21% 3	2915
	21st to end	Night	20% 6% 12% 37% 22% 4	3544
September	1st to 10th	Night	26% 7% 8% 25% 22% 12%	2100
	11th to 20th	Night	30% 5% 10% 22% 24% 7%	2178
	21st to end	Night	24% 5% 8% 24% 30% 9%	2090
October	1st to 10th	Night	14% 3 6% 13% 36% 28%	3702
	11th to 20th	Night	19% 2 4 14% 41% 21%	3779
	21st to end	Night	28% 3 5% 29% 26% 9%	4120
November	1st to 10th	Night	28% 3 8% 23% 28% 10%	2340
	11th to 20th	Night	27% 3 8% 24% 27% 11%	2505
	21st to end	Night	33% 3 7% 22% 28% 7%	2600
December	1st to 10th	Night	33% 4 7% 32% 20% 4	4250
	11th to 20th	Night	38% 2 5% 21% 23% 10%	4817
	21st to end	Night	44% 4 8% 23% 16% 5%	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	19% 15% 33% 27% 6% 1	300
	11th to 20th	Day	17% 17% 37% 24% 4	300
	21st to end	Day	18% 14% 38% 22% 8%	330
February	1st to 10th	Day	28% 15% 31% 21% 5%	300
	11th to 20th	Day	24% 18% 38% 16% 4	300
	21st to end	Day	26% 16% 36% 18% 4	249
March	1st to 10th	Day	21% 15% 34% 22% 7%	300
	11th to 20th	Day	21% 21% 34% 19% 5%	300
	21st to end	Day	25% 15% 38% 16% 5% 1	330
April	1st to 10th	Day	17% 15% 39% 20% 7% 2	332
	11th to 20th	Day	20% 15% 34% 25% 5% 1	492
	21st to end	Day	19% 13% 32% 24% 9% 2	580
May	1st to 10th	Day	14% 8% 33% 33% 12%	703
	11th to 20th	Day	15% 6% 28% 32% 15% 4	725
	21st to end	Day	14% 5% 26% 31% 19% 5%	789
June	1st to 10th	Day	9% 4 14% 31% 32% 9%	730
	11th to 20th	Day	8% 3 13% 36% 33% 7%	730
	21st to end	Day	3 2 15% 38% 32% 9%	730
July	1st to 10th	Day	8% 1 11% 29% 35% 16%	721
	11th to 20th	Day	3 2 7% 33% 42% 14%	718
	21st to end	Day	5% 1 6% 28% 42% 18%	794
August	1st to 10th	Day	6% 1 8% 34% 43% 8%	726
	11th to 20th	Day	9% 2 6% 30% 42% 11%	715
	21st to end	Day	7% 2 7% 22% 44% 19%	610
September	1st to 10th	Day	8% 2 7% 24% 38% 21%	425
	11th to 20th	Day	4 1 9% 35% 32% 19%	324
	21st to end	Day	10% 3 16% 27% 28% 15%	300
October	1st to 10th	Day	5% 5% 23% 33% 23% 12%	300
	11th to 20th	Day	9% 8% 28% 35% 18% 1	300
	21st to end	Day	22% 11% 25% 28% 11% 2	330
November	1st to 10th	Day	16% 7% 32% 30% 13% 2	300
	11th to 20th	Day	15% 10% 34% 26% 14% 1	300
	21st to end	Day	20% 13% 30% 27% 8% 1	300
December	1st to 10th	Day	17% 12% 34% 25% 10% 2	300
	11th to 20th	Day	23% 12% 36% 23% 5%	300
	21st to end	Day	20% 17% 37% 21% 5%	330
January	1st to 10th	Night	17% 10% 35% 31% 6% 1	1000
	11th to 20th	Night	17% 13% 37% 29% 5%	1000
	21st to end	Night	16% 9% 37% 34% 4	1100
February	1st to 10th	Night	21% 12% 34% 30% 4	1000
	11th to 20th	Night	21% 11% 36% 29% 3	876
	21st to end	Night	18% 12% 39% 28% 2	570
March	1st to 10th	Night	18% 10% 34% 34% 4	600
	11th to 20th	Night	20% 10% 32% 34% 4	600
	21st to end	Night	16% 9% 39% 28% 7%	660
April	1st to 10th	Night	16% 10% 37% 32% 6%	600
	11th to 20th	Night	14% 11% 34% 32% 9%	593
	21st to end	Night	15% 9% 33% 33% 10% 1	600
May	1st to 10th	Night	12% 7% 28% 44% 9%	600
	11th to 20th	Night	13% 6% 23% 41% 15% 2	591
	21st to end	Night	10% 3 24% 39% 22% 2	650
June	1st to 10th	Night	6% 3 13% 42% 34% 3	600
	11th to 20th	Night	6% 2 14% 51% 25% 2	600
	21st to end	Night	2 2 13% 44% 36% 4	600
July	1st to 10th	Night	4 1 6% 40% 43% 6%	585
	11th to 20th	Night	3 1 6% 39% 47% 5%	582
	21st to end	Night	4 1 3 40% 45% 6%	657
August	1st to 10th	Night	3 1 6% 41% 45% 4	597
	11th to 20th	Night	6% 1 7% 38% 45% 5%	594
	21st to end	Night	5% 2 7% 35% 46% 6%	660
September	1st to 10th	Night	10% 1 8% 37% 42% 3	600
	11th to 20th	Night	11% 2 9% 38% 36% 5%	594
	21st to end	Night	9% 2 16% 36% 32% 4	570
October	1st to 10th	Night	8% 2 16% 41% 30% 3	701
	11th to 20th	Night	15% 4 26% 40% 14% 1	760
	21st to end	Night	17% 7% 26% 33% 15% 1	836
November	1st to 10th	Night	14% 5% 29% 39% 13% 1	780
	11th to 20th	Night	10% 11% 32% 35% 11%	905
	21st to end	Night	18% 9% 30% 34% 9%	1000
December	1st to 10th	Night	17% 10% 34% 33% 7%	1000
	11th to 20th	Night	22% 8% 33% 32% 5%	1000
	21st to end	Night	18% 11% 34% 30% 6%	1100

Page generated at 2025-09-16 00:49:14. Version ac07

Copyright © 2025 Attilla Danko.