Doug Firebaugh Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Doug Firebaugh Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	12% 8% 3 8% 5% 5% 5% 4 5% 7% 38%	126756
	Night	13% 9% 4 8% 5% 4 4 3 5% 6% 39%	122890
Transparency	Day	45% 3 12% 25% 14% 1	130250
	Night	46% 3 11% 24% 15% 2	127469
Seeing	Day	16% 10% 29% 26% 14% 5%	17154
	Night	17% 9% 31% 30% 11% 1	26924

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	8% 11% 8% 15% 7% 6% 6% 4 5% 5% 25%	3617
	11th to 20th	Day	8% 9% 7% 12% 5% 5% 5% 7% 8% 7% 29%	3620
	21st to end	Day	12% 15% 7% 9% 4 4 3 3 5% 6% 33%	4281
February	1st to 10th	Day	11% 14% 5% 9% 6% 5% 5% 4 5% 6% 30%	1800
	11th to 20th	Day	12% 11% 4 8% 6% 5% 5% 3 5% 6% 36%	1883
	21st to end	Day	11% 14% 4 10% 5% 4 4 3 5% 6% 33%	1650
March	1st to 10th	Day	16% 8% 3 6% 4 3 3 2 4 6% 45%	4480
	11th to 20th	Day	10% 6% 3 7% 4 4 4 4 5% 8% 44%	4860
	21st to end	Day	19% 11% 4 11% 6% 5% 5% 3 4 5% 26%	5868
April	1st to 10th	Day	17% 13% 3 8% 5% 4 5% 3 5% 7% 31%	2400
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 5% 4 6% 33%	2374
	21st to end	Day	20% 8% 3 8% 4 4 4 3 5% 7% 34%	2430
May	1st to 10th	Day	15% 8% 3 7% 5% 4 4 4 5% 7% 37%	6039
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 6% 7% 42%	6319
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 7% 9% 38%	6935
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 5% 9% 43%	2800
	11th to 20th	Day	10% 6% 2 6% 4 4 4 4 6% 9% 44%	2800
	21st to end	Day	11% 7% 2 7% 5% 4 5% 5% 7% 9% 39%	2800
July	1st to 10th	Day	9% 7% 2 8% 5% 5% 5% 5% 7% 9% 38%	6360
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 46%	6360
	21st to end	Day	10% 7% 3 8% 5% 5% 6% 4 6% 9% 38%	6924
August	1st to 10th	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 9% 41%	4408
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 9% 46%	3924
	21st to end	Day	9% 6% 2 6% 3 4 4 4 5% 8% 48%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 5% 4 6% 7% 47%	2400
	11th to 20th	Day	11% 4 2 4 3 3 4 3 6% 8% 52%	2345
	21st to end	Day	11% 6% 2 7% 4 3 4 3 5% 7% 47%	2200
October	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 4 6% 49%	1959
	11th to 20th	Day	17% 9% 3 7% 4 5% 4 3 5% 6% 38%	1900
	21st to end	Day	18% 9% 2 6% 5% 3 4 3 4 6% 40%	1941
November	1st to 10th	Day	12% 8% 3 7% 4 4 4 3 5% 6% 44%	1762
	11th to 20th	Day	15% 12% 3 8% 6% 6% 4 3 5% 6% 33%	1800
	21st to end	Day	14% 14% 4 8% 3 5% 3 3 5% 5% 35%	1605
December	1st to 10th	Day	8% 10% 4 8% 4 4 4 3 4 7% 44%	2610
	11th to 20th	Day	13% 14% 5% 11% 7% 6% 6% 4 5% 5% 23%	3008
	21st to end	Day	21% 22% 9% 13% 5% 5% 4 3 4 4 12%	3982
January	1st to 10th	Night	10% 11% 7% 15% 6% 5% 4 3 4 4 32%	6348
	11th to 20th	Night	10% 13% 6% 13% 6% 6% 5% 5% 7% 6% 23%	6360
	21st to end	Night	14% 14% 6% 9% 6% 4 4 3 4 5% 31%	6692
February	1st to 10th	Night	12% 13% 5% 9% 6% 5% 4 3 5% 5% 33%	2600
	11th to 20th	Night	12% 10% 3 9% 5% 4 4 3 4 5% 39%	2402
	21st to end	Night	16% 11% 3 8% 3 4 4 3 5% 6% 37%	1980
March	1st to 10th	Night	17% 9% 4 6% 4 4 3 3 4 4 43%	5373
	11th to 20th	Night	9% 6% 2 6% 4 5% 4 3 5% 7% 49%	5130
	21st to end	Night	20% 12% 5% 10% 5% 5% 4 3 4 5% 26%	5060
April	1st to 10th	Night	16% 10% 3 6% 4 4 4 3 5% 6% 39%	2000
	11th to 20th	Night	17% 8% 3 7% 5% 5% 3 3 3 6% 39%	1975
	21st to end	Night	17% 8% 2 7% 4 5% 4 4 6% 8% 35%	1970
May	1st to 10th	Night	18% 7% 3 7% 5% 4 4 3 6% 6% 37%	3860
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 5% 6% 7% 45%	3596
	21st to end	Night	13% 6% 2 7% 4 5% 4 4 6% 8% 40%	3942
June	1st to 10th	Night	8% 4 2 5% 4 4 6% 5% 6% 9% 47%	1600
	11th to 20th	Night	10% 4 2 4 5% 5% 4 4 7% 10% 44%	1600
	21st to end	Night	9% 6% 3 8% 4 4 4 5% 7% 8% 44%	1600
July	1st to 10th	Night	12% 7% 2 6% 5% 4 4 3 7% 9% 41%	3618
	11th to 20th	Night	7% 5% 2 5% 4 3 5% 3 6% 8% 53%	3617
	21st to end	Night	8% 5% 2 7% 4 5% 4 4 6% 7% 47%	3947
August	1st to 10th	Night	6% 4 2 5% 4 4 4 3 7% 8% 52%	2749
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 4 5% 7% 51%	3126
	21st to end	Night	8% 5% 2 5% 3 4 4 3 6% 9% 51%	3586
September	1st to 10th	Night	9% 6% 1 4 3 3 4 3 4 6% 57%	2000
	11th to 20th	Night	9% 4 2 6% 3 4 4 4 5% 9% 51%	2035
	21st to end	Night	10% 5% 2 5% 4 4 4 4 5% 7% 50%	2200
October	1st to 10th	Night	11% 6% 3 5% 3 4 4 3 4 6% 51%	2286
	11th to 20th	Night	16% 7% 2 5% 4 3 3 3 4 6% 45%	2280
	21st to end	Night	19% 9% 2 6% 4 5% 4 4 4 6% 37%	2655
November	1st to 10th	Night	12% 8% 3 6% 3 3 3 3 5% 5% 50%	2536
	11th to 20th	Night	14% 11% 3 8% 5% 4 4 4 4 6% 36%	2600
	21st to end	Night	16% 11% 3 8% 5% 3 3 3 4 6% 38%	2792
December	1st to 10th	Night	9% 8% 3 7% 5% 5% 5% 4 5% 7% 42%	4580
	11th to 20th	Night	16% 15% 4 10% 5% 4 4 3 4 6% 29%	5199
	21st to end	Night	21% 19% 7% 12% 5% 4 4 3 4 4 17%	6996

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	61% 3 11% 17% 7% 1	3618
	11th to 20th	Day	51% 2 13% 11% 19% 3	3620
	21st to end	Day	54% 3 15% 19% 9%	4281
February	1st to 10th	Day	56% 3 12% 17% 11% 1	1800
	11th to 20th	Day	49% 3 11% 25% 10% 2	1892
	21st to end	Day	51% 4 10% 22% 11% 2	1650
March	1st to 10th	Day	43% 2 11% 30% 10% 4	4480
	11th to 20th	Day	40% 2 12% 32% 13% 1	4860
	21st to end	Day	62% 3 12% 15% 8% 1	5870
April	1st to 10th	Day	54% 4 10% 22% 10%	2400
	11th to 20th	Day	51% 3 10% 23% 11% 2	2374
	21st to end	Day	52% 4 12% 19% 12% 1	2430
May	1st to 10th	Day	47% 3 9% 22% 18%	6074
	11th to 20th	Day	42% 3 10% 28% 15% 1	6323
	21st to end	Day	42% 4 11% 28% 12% 3	6949
June	1st to 10th	Day	37% 4 14% 28% 15% 1	2800
	11th to 20th	Day	37% 5% 13% 25% 19% 2	2800
	21st to end	Day	41% 5% 14% 25% 13% 2	2800
July	1st to 10th	Day	41% 4 14% 28% 11% 1	6291
	11th to 20th	Day	36% 3 15% 31% 15% 1	6285
	21st to end	Day	42% 4 16% 21% 15% 2	6728
August	1st to 10th	Day	37% 5% 16% 27% 13% 1	4393
	11th to 20th	Day	34% 4 14% 32% 15% 1	3961
	21st to end	Day	35% 4 12% 32% 16% 2	4311
September	1st to 10th	Day	36% 4 9% 30% 19% 2	2400
	11th to 20th	Day	31% 4 12% 32% 20% 2	2345
	21st to end	Day	38% 3 9% 29% 21% 1	2200
October	1st to 10th	Day	24% 3 6% 37% 29% 1	3232
	11th to 20th	Day	34% 2 9% 26% 26% 2	3140
	21st to end	Day	43% 2 9% 29% 14% 3	3179
November	1st to 10th	Day	41% 3 15% 25% 13% 2	1762
	11th to 20th	Day	53% 3 12% 21% 10% 1	1800
	21st to end	Day	51% 4 15% 22% 8% 1	1605
December	1st to 10th	Day	40% 3 19% 30% 7% 1	2610
	11th to 20th	Day	63% 3 12% 15% 7% 1	3005
	21st to end	Day	77% 3 7% 8% 5%	3982
January	1st to 10th	Night	57% 2 10% 20% 10% 1	6348
	11th to 20th	Night	61% 3 10% 11% 15% 1	6360
	21st to end	Night	57% 2 11% 18% 11% 1	6692
February	1st to 10th	Night	55% 2 10% 19% 12% 2	2600
	11th to 20th	Night	48% 3 10% 24% 14% 1	2403
	21st to end	Night	48% 3 13% 21% 12% 3	1980
March	1st to 10th	Night	46% 2 11% 23% 12% 5%	5373
	11th to 20th	Night	36% 2 10% 34% 14% 3	5130
	21st to end	Night	62% 3 9% 18% 8% 1	5060
April	1st to 10th	Night	47% 3 11% 23% 14% 2	2000
	11th to 20th	Night	47% 3 7% 22% 18% 3	1975
	21st to end	Night	47% 3 11% 24% 13% 2	1970
May	1st to 10th	Night	48% 3 11% 18% 18% 2	3887
	11th to 20th	Night	38% 4 14% 25% 17% 3	3597
	21st to end	Night	42% 4 10% 28% 15% 2	3950
June	1st to 10th	Night	33% 6% 13% 27% 20% 1	1600
	11th to 20th	Night	34% 5% 11% 24% 23% 3	1600
	21st to end	Night	37% 6% 14% 22% 18% 3	1600
July	1st to 10th	Night	40% 3 15% 25% 14% 2	3580
	11th to 20th	Night	29% 4 13% 32% 20% 2	3572
	21st to end	Night	35% 5% 17% 23% 17% 3	3832
August	1st to 10th	Night	30% 3 15% 32% 18% 1	2740
	11th to 20th	Night	33% 3 13% 32% 18% 1	3155
	21st to end	Night	31% 4 12% 32% 17% 4	3586
September	1st to 10th	Night	29% 4 11% 29% 24% 3	2000
	11th to 20th	Night	32% 4 11% 32% 18% 3	2035
	21st to end	Night	34% 4 9% 30% 21% 2	2200
October	1st to 10th	Night	25% 2 6% 35% 30% 2	3793
	11th to 20th	Night	33% 2 7% 25% 29% 3	3779
	21st to end	Night	45% 2 8% 26% 16% 3	4367
November	1st to 10th	Night	38% 3 10% 28% 19% 3	2536
	11th to 20th	Night	50% 2 12% 22% 13% 1	2600
	21st to end	Night	49% 4 11% 24% 11% 1	2792
December	1st to 10th	Night	43% 3 15% 29% 10% 1	4580
	11th to 20th	Night	57% 3 13% 18% 9%	5201
	21st to end	Night	72% 2 8% 13% 5%	6996

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	15% 15% 31% 29% 10% 1	300
	11th to 20th	Day	17% 15% 42% 20% 5% 1	300
	21st to end	Day	18% 13% 38% 24% 6% 1	330
February	1st to 10th	Day	21% 15% 37% 20% 7%	300
	11th to 20th	Day	19% 20% 31% 24% 6%	300
	21st to end	Day	22% 14% 45% 15% 3	249
March	1st to 10th	Day	16% 21% 41% 17% 5%	300
	11th to 20th	Day	22% 17% 36% 18% 6% 1	300
	21st to end	Day	28% 15% 33% 17% 5% 1	503
April	1st to 10th	Day	25% 15% 34% 19% 6% 1	500
	11th to 20th	Day	23% 18% 29% 21% 9% 1	492
	21st to end	Day	25% 12% 30% 23% 8% 2	500
May	1st to 10th	Day	19% 12% 33% 24% 9% 3	576
	11th to 20th	Day	22% 11% 26% 25% 12% 3	725
	21st to end	Day	18% 10% 27% 24% 16% 6%	789
June	1st to 10th	Day	10% 8% 28% 31% 18% 5%	730
	11th to 20th	Day	11% 8% 33% 29% 14% 5%	730
	21st to end	Day	13% 8% 29% 31% 15% 4	730
July	1st to 10th	Day	9% 3 19% 31% 24% 13%	721
	11th to 20th	Day	10% 4 25% 29% 23% 8%	718
	21st to end	Day	9% 4 20% 37% 24% 7%	794
August	1st to 10th	Day	7% 4 20% 33% 25% 10%	726
	11th to 20th	Day	14% 4 17% 30% 22% 13%	570
	21st to end	Day	11% 3 18% 34% 24% 9%	583
September	1st to 10th	Day	14% 7% 20% 32% 21% 7%	530
	11th to 20th	Day	10% 4 19% 33% 25% 10%	530
	21st to end	Day	14% 10% 27% 25% 18% 7%	530
October	1st to 10th	Day	15% 10% 25% 27% 18% 4	338
	11th to 20th	Day	24% 19% 30% 19% 8%	300
	21st to end	Day	25% 17% 30% 21% 5% 1	330
November	1st to 10th	Day	18% 15% 36% 20% 9% 2	300
	11th to 20th	Day	22% 19% 33% 17% 8% 1	300
	21st to end	Day	25% 14% 36% 21% 4	300
December	1st to 10th	Day	16% 17% 41% 18% 7% 1	300
	11th to 20th	Day	17% 13% 39% 23% 7% 1	300
	21st to end	Day	24% 17% 39% 18% 3	330
January	1st to 10th	Night	18% 12% 39% 24% 5% 1	1000
	11th to 20th	Night	19% 11% 41% 24% 5%	1000
	21st to end	Night	20% 11% 36% 28% 5%	1100
February	1st to 10th	Night	20% 12% 39% 24% 5%	1000
	11th to 20th	Night	17% 14% 40% 25% 4	847
	21st to end	Night	22% 13% 41% 20% 4	660
March	1st to 10th	Night	21% 10% 38% 24% 6%	800
	11th to 20th	Night	16% 11% 37% 29% 7%	672
	21st to end	Night	24% 12% 34% 23% 6% 1	660
April	1st to 10th	Night	21% 13% 38% 24% 4	600
	11th to 20th	Night	21% 13% 31% 27% 7%	593
	21st to end	Night	21% 9% 33% 30% 8% 1	600
May	1st to 10th	Night	17% 8% 36% 30% 8% 1	600
	11th to 20th	Night	20% 6% 30% 33% 10% 1	591
	21st to end	Night	12% 5% 31% 36% 14% 1	650
June	1st to 10th	Night	10% 4 27% 44% 15% 1	600
	11th to 20th	Night	13% 4 30% 40% 12% 1	600
	21st to end	Night	10% 5% 28% 42% 15% 1	600
July	1st to 10th	Night	9% 2 12% 42% 31% 4	585
	11th to 20th	Night	9% 2 18% 43% 25% 2	582
	21st to end	Night	8% 2 17% 45% 26% 2	657
August	1st to 10th	Night	6% 2 15% 45% 28% 3	597
	11th to 20th	Night	9% 3 12% 40% 31% 6%	594
	21st to end	Night	10% 1 15% 45% 26% 3	660
September	1st to 10th	Night	11% 4 18% 38% 23% 5%	600
	11th to 20th	Night	10% 5% 20% 35% 26% 4	635
	21st to end	Night	13% 7% 25% 34% 20% 1	800
October	1st to 10th	Night	14% 6% 29% 32% 17% 1	773
	11th to 20th	Night	20% 11% 30% 28% 9%	760
	21st to end	Night	23% 11% 32% 26% 7%	836
November	1st to 10th	Night	18% 9% 39% 23% 10% 1	780
	11th to 20th	Night	20% 15% 36% 23% 6%	800
	21st to end	Night	20% 13% 35% 25% 6%	992
December	1st to 10th	Night	19% 12% 40% 24% 5%	1000
	11th to 20th	Night	22% 10% 37% 25% 6%	1000
	21st to end	Night	27% 13% 34% 22% 3	1100

Page generated at 2025-10-01 00:39:06. Version ac07

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