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 2024-01-12.

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	13% 8% 4 7% 6% 3 5% 5% 5% 7% 37%	74771
	Night	14% 9% 4 6% 5% 3 4 4 5% 7% 39%	74966
Transparency	Day	46% 4 13% 23% 12% 1	74471
	Night	45% 4 12% 23% 14% 2	74786
Seeing	Day	16% 10% 28% 26% 14% 5%	14679
	Night	17% 9% 31% 30% 12% 1	24661

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	11% 11% 5% 10% 6% 3 5% 6% 6% 7% 30%	1517
	11th to 20th	Day	13% 13% 7% 12% 6% 4 4 5% 6% 7% 24%	1448
	21st to end	Day	12% 15% 7% 11% 7% 3 5% 4 5% 6% 24%	1701
February	1st to 10th	Day	12% 13% 7% 8% 7% 3 5% 5% 5% 6% 29%	1620
	11th to 20th	Day	14% 10% 4 7% 7% 4 6% 4 4 7% 34%	1690
	21st to end	Day	13% 13% 6% 9% 7% 4 4 3 5% 6% 32%	1480
March	1st to 10th	Day	13% 11% 5% 7% 7% 4 5% 4 5% 7% 33%	1800
	11th to 20th	Day	12% 9% 3 7% 5% 2 5% 4 5% 8% 39%	1930
	21st to end	Day	19% 10% 5% 7% 6% 4 6% 5% 5% 6% 27%	2333
April	1st to 10th	Day	18% 11% 4 7% 6% 3 5% 4 5% 7% 31%	2160
	11th to 20th	Day	17% 8% 3 7% 5% 4 5% 6% 4 7% 33%	2134
	21st to end	Day	21% 8% 4 7% 5% 3 4 4 4 7% 33%	2186
May	1st to 10th	Day	17% 9% 4 8% 6% 3 4 6% 6% 8% 29%	2405
	11th to 20th	Day	19% 8% 3 5% 6% 4 5% 4 6% 7% 33%	2479
	21st to end	Day	15% 7% 3 6% 6% 4 6% 6% 5% 8% 36%	2725
June	1st to 10th	Day	10% 5% 2 5% 6% 3 6% 6% 5% 9% 42%	2520
	11th to 20th	Day	11% 5% 2 5% 5% 3 4 6% 6% 9% 45%	2520
	21st to end	Day	11% 7% 3 6% 6% 4 5% 6% 7% 9% 38%	2520
July	1st to 10th	Day	7% 4 2 4 5% 3 5% 5% 7% 10% 47%	2520
	11th to 20th	Day	7% 4 2 5% 5% 3 6% 6% 7% 10% 46%	2520
	21st to end	Day	7% 5% 2 6% 5% 4 6% 6% 7% 10% 44%	2743
August	1st to 10th	Day	6% 4 2 5% 6% 4 6% 7% 8% 11% 40%	2427
	11th to 20th	Day	10% 5% 3 5% 5% 3 5% 5% 6% 8% 46%	2172
	21st to end	Day	10% 5% 2 6% 4 4 4 5% 5% 7% 49%	2376
September	1st to 10th	Day	13% 5% 3 5% 5% 3 5% 5% 5% 7% 46%	2160
	11th to 20th	Day	12% 4 2 4 3 2 4 4 5% 8% 52%	2111
	21st to end	Day	12% 6% 3 6% 5% 2 5% 4 5% 8% 46%	1980
October	1st to 10th	Day	13% 6% 3 5% 4 3 4 4 4 6% 48%	1827
	11th to 20th	Day	18% 8% 4 7% 5% 4 4 4 5% 6% 36%	1800
	21st to end	Day	19% 8% 3 6% 5% 2 4 3 3 6% 40%	1839
November	1st to 10th	Day	11% 7% 4 5% 5% 3 5% 4 5% 6% 44%	1672
	11th to 20th	Day	15% 11% 4 6% 6% 4 5% 4 5% 6% 34%	1710
	21st to end	Day	14% 12% 5% 7% 4 3 3 4 5% 5% 37%	1525
December	1st to 10th	Day	13% 14% 5% 8% 6% 4 4 4 4 5% 33%	1520
	11th to 20th	Day	13% 13% 4 9% 7% 3 6% 4 5% 6% 30%	1918
	21st to end	Day	22% 19% 9% 11% 6% 3 4 4 4 4 14%	2783
January	1st to 10th	Night	12% 12% 6% 8% 6% 3 4 4 4 5% 35%	2648
	11th to 20th	Night	13% 13% 5% 10% 8% 3 4 3 5% 6% 28%	2541
	21st to end	Night	14% 12% 6% 9% 7% 3 5% 5% 6% 7% 26%	2650
February	1st to 10th	Night	13% 13% 5% 8% 8% 4 5% 3 5% 5% 32%	2340
	11th to 20th	Night	13% 9% 4 8% 6% 4 4 4 4 6% 39%	2155
	21st to end	Night	17% 11% 4 7% 4 3 4 4 5% 6% 35%	1776
March	1st to 10th	Night	15% 11% 4 5% 6% 3 4 4 5% 6% 36%	2160
	11th to 20th	Night	12% 7% 4 6% 5% 4 4 5% 5% 7% 41%	2046
	21st to end	Night	20% 10% 4 7% 6% 4 5% 4 4 5% 30%	2022
April	1st to 10th	Night	16% 9% 5% 5% 4 3 5% 4 5% 6% 38%	1800
	11th to 20th	Night	18% 8% 4 6% 5% 3 4 4 3 6% 39%	1775
	21st to end	Night	17% 7% 2 6% 5% 4 5% 5% 6% 8% 35%	1774
May	1st to 10th	Night	15% 7% 3 6% 6% 4 6% 4 7% 8% 36%	1551
	11th to 20th	Night	17% 8% 2 5% 5% 3 5% 5% 6% 6% 38%	1416
	21st to end	Night	10% 5% 3 6% 6% 4 4 6% 5% 11% 41%	1551
June	1st to 10th	Night	8% 4 2 5% 5% 2 6% 6% 6% 9% 47%	1440
	11th to 20th	Night	11% 4 2 3 6% 3 4 4 6% 10% 46%	1440
	21st to end	Night	9% 5% 3 7% 4 3 4 5% 7% 8% 45%	1440
July	1st to 10th	Night	6% 3 2 4 4 2 3 5% 6% 12% 53%	1440
	11th to 20th	Night	7% 4 2 4 4 2 4 4 6% 8% 57%	1440
	21st to end	Night	6% 3 2 5% 4 2 4 5% 7% 9% 52%	1575
August	1st to 10th	Night	5% 4 2 4 5% 3 5% 4 6% 9% 53%	1509
	11th to 20th	Night	8% 4 2 4 5% 2 4 4 4 7% 54%	1729
	21st to end	Night	9% 4 2 4 4 2 4 4 5% 8% 54%	1980
September	1st to 10th	Night	10% 6% 2 4 4 2 4 4 4 5% 56%	1800
	11th to 20th	Night	10% 4 2 5% 4 3 4 4 4 8% 52%	1831
	21st to end	Night	11% 5% 2 4 4 3 4 5% 5% 7% 50%	1980
October	1st to 10th	Night	12% 6% 3 4 4 3 3 4 4 6% 51%	2131
	11th to 20th	Night	17% 7% 3 5% 4 3 3 4 4 6% 44%	2160
	21st to end	Night	20% 9% 2 5% 5% 3 4 4 4 6% 38%	2515
November	1st to 10th	Night	12% 7% 3 5% 4 2 4 3 4 5% 51%	2406
	11th to 20th	Night	15% 10% 3 7% 6% 3 4 5% 4 6% 37%	2470
	21st to end	Night	17% 9% 3 6% 6% 2 4 4 4 6% 39%	2652
December	1st to 10th	Night	15% 11% 4 9% 6% 4 5% 5% 5% 6% 31%	2660
	11th to 20th	Night	14% 11% 5% 8% 6% 3 5% 4 5% 7% 31%	3279
	21st to end	Night	21% 16% 6% 10% 6% 3 5% 4 4 4 19%	4884

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	54% 4 13% 17% 11% 1	1518
	11th to 20th	Day	60% 4 13% 15% 7% 1	1448
	21st to end	Day	62% 3 12% 15% 8% 1	1701
February	1st to 10th	Day	57% 3 13% 18% 9% 1	1620
	11th to 20th	Day	50% 4 11% 23% 10% 2	1699
	21st to end	Day	53% 5% 9% 21% 10% 2	1480
March	1st to 10th	Day	50% 3 16% 21% 9% 1	1800
	11th to 20th	Day	45% 4 14% 24% 12% 1	1930
	21st to end	Day	57% 5% 11% 20% 6% 1	2333
April	1st to 10th	Day	53% 4 11% 22% 10%	2160
	11th to 20th	Day	50% 3 10% 23% 11% 2	2134
	21st to end	Day	52% 4 12% 18% 12% 2	2186
May	1st to 10th	Day	53% 5% 13% 22% 7% 1	2405
	11th to 20th	Day	50% 5% 11% 22% 11%	2483
	21st to end	Day	46% 6% 12% 20% 14% 2	2725
June	1st to 10th	Day	37% 4 14% 28% 15% 1	2520
	11th to 20th	Day	36% 5% 13% 25% 19% 2	2520
	21st to end	Day	42% 6% 14% 24% 12% 1	2520
July	1st to 10th	Day	32% 6% 16% 28% 15% 3	2451
	11th to 20th	Day	32% 6% 16% 28% 15% 2	2445
	21st to end	Day	34% 5% 17% 26% 15% 3	2591
August	1st to 10th	Day	35% 6% 17% 28% 13% 1	2412
	11th to 20th	Day	34% 6% 15% 28% 17% 1	2172
	21st to end	Day	35% 5% 11% 29% 18% 3	2376
September	1st to 10th	Day	38% 4 9% 28% 19% 2	2160
	11th to 20th	Day	32% 4 11% 32% 18% 3	2111
	21st to end	Day	38% 3 9% 29% 19% 1	1980
October	1st to 10th	Day	39% 4 10% 29% 17%	1827
	11th to 20th	Day	50% 3 10% 22% 13% 1	1800
	21st to end	Day	48% 3 12% 24% 12% 2	1839
November	1st to 10th	Day	40% 3 15% 25% 14% 2	1672
	11th to 20th	Day	52% 3 13% 21% 10% 1	1710
	21st to end	Day	50% 4 15% 22% 8% 1	1525
December	1st to 10th	Day	53% 3 15% 19% 8% 2	1520
	11th to 20th	Day	57% 3 14% 16% 9% 2	1915
	21st to end	Day	75% 3 8% 9% 5%	2783
January	1st to 10th	Night	52% 3 11% 20% 13% 1	2648
	11th to 20th	Night	58% 4 14% 15% 9% 1	2541
	21st to end	Night	58% 4 12% 15% 9% 2	2650
February	1st to 10th	Night	56% 2 10% 19% 12% 1	2340
	11th to 20th	Night	48% 3 11% 22% 14% 1	2156
	21st to end	Night	50% 3 12% 21% 11% 3	1776
March	1st to 10th	Night	49% 3 14% 22% 10% 1	2160
	11th to 20th	Night	43% 4 12% 22% 17% 2	2046
	21st to end	Night	57% 4 10% 19% 9% 2	2022
April	1st to 10th	Night	48% 3 11% 22% 14% 2	1800
	11th to 20th	Night	48% 3 6% 22% 18% 4	1775
	21st to end	Night	47% 4 11% 23% 13% 2	1774
May	1st to 10th	Night	46% 5% 13% 23% 12%	1551
	11th to 20th	Night	46% 5% 12% 22% 14% 1	1417
	21st to end	Night	38% 6% 14% 23% 17% 2	1552
June	1st to 10th	Night	33% 6% 14% 25% 21% 1	1440
	11th to 20th	Night	34% 5% 10% 24% 24% 3	1440
	21st to end	Night	36% 7% 15% 23% 17% 3	1440
July	1st to 10th	Night	26% 5% 16% 33% 16% 4	1400
	11th to 20th	Night	25% 6% 12% 32% 21% 3	1392
	21st to end	Night	26% 6% 18% 27% 18% 5%	1487
August	1st to 10th	Night	30% 4 15% 34% 14% 3	1500
	11th to 20th	Night	30% 4 13% 30% 21% 1	1729
	21st to end	Night	28% 4 11% 29% 22% 5%	1980
September	1st to 10th	Night	31% 4 12% 27% 24% 3	1800
	11th to 20th	Night	32% 4 11% 31% 19% 3	1831
	21st to end	Night	34% 4 10% 30% 20% 2	1980
October	1st to 10th	Night	35% 4 9% 27% 23% 2	2131
	11th to 20th	Night	42% 2 10% 25% 19% 2	2160
	21st to end	Night	49% 3 11% 24% 12% 2	2515
November	1st to 10th	Night	36% 3 10% 27% 20% 3	2406
	11th to 20th	Night	49% 2 12% 22% 13% 1	2470
	21st to end	Night	48% 4 11% 24% 11% 1	2652
December	1st to 10th	Night	55% 4 11% 22% 8% 1	2660
	11th to 20th	Night	52% 3 14% 20% 9% 1	3281
	21st to end	Night	69% 3 8% 15% 5%	4884

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	16% 13% 30% 29% 10% 1	270
	11th to 20th	Day	19% 12% 42% 21% 6% 1	243
	21st to end	Day	17% 11% 39% 25% 6% 1	264
February	1st to 10th	Day	21% 14% 39% 19% 6%	240
	11th to 20th	Day	22% 19% 28% 24% 7%	240
	21st to end	Day	24% 16% 41% 18% 2	198
March	1st to 10th	Day	14% 22% 39% 18% 7%	240
	11th to 20th	Day	20% 16% 36% 20% 8% 1	240
	21st to end	Day	29% 15% 32% 17% 6% 1	419
April	1st to 10th	Day	24% 15% 34% 20% 6% 1	420
	11th to 20th	Day	22% 15% 30% 21% 10% 1	412
	21st to end	Day	25% 12% 29% 23% 10% 2	420
May	1st to 10th	Day	20% 13% 32% 24% 9% 2	488
	11th to 20th	Day	24% 10% 26% 26% 12% 3	625
	21st to end	Day	18% 9% 25% 23% 18% 7%	679
June	1st to 10th	Day	10% 6% 27% 32% 19% 6%	630
	11th to 20th	Day	12% 7% 31% 30% 15% 6%	630
	21st to end	Day	13% 8% 27% 32% 16% 4	630
July	1st to 10th	Day	8% 3 16% 32% 26% 15%	621
	11th to 20th	Day	10% 4 25% 30% 23% 9%	618
	21st to end	Day	8% 4 19% 37% 24% 7%	684
August	1st to 10th	Day	6% 4 19% 34% 26% 11%	626
	11th to 20th	Day	13% 4 15% 32% 22% 14%	486
	21st to end	Day	13% 3 19% 33% 23% 10%	495
September	1st to 10th	Day	16% 6% 19% 32% 21% 7%	450
	11th to 20th	Day	11% 4 20% 33% 21% 10%	450
	21st to end	Day	15% 11% 26% 24% 17% 7%	450
October	1st to 10th	Day	16% 10% 26% 27% 16% 5%	297
	11th to 20th	Day	26% 20% 30% 16% 8%	270
	21st to end	Day	26% 17% 29% 21% 5% 1	297
November	1st to 10th	Day	15% 15% 36% 21% 10% 3	270
	11th to 20th	Day	21% 17% 34% 18% 8% 1	270
	21st to end	Day	24% 13% 36% 22% 4	270
December	1st to 10th	Day	17% 15% 41% 19% 8% 1	270
	11th to 20th	Day	16% 12% 38% 26% 7% 1	270
	21st to end	Day	23% 18% 40% 17% 2	297
January	1st to 10th	Night	18% 12% 39% 24% 5% 1	950
	11th to 20th	Night	19% 10% 39% 25% 6%	908
	21st to end	Night	20% 10% 36% 28% 5%	990
February	1st to 10th	Night	21% 12% 38% 25% 5%	900
	11th to 20th	Night	17% 13% 39% 26% 5%	760
	21st to end	Night	23% 13% 39% 20% 5% 1	592
March	1st to 10th	Night	22% 10% 36% 25% 7%	720
	11th to 20th	Night	16% 10% 37% 30% 7%	606
	21st to end	Night	24% 11% 33% 25% 7% 1	594
April	1st to 10th	Night	20% 12% 37% 25% 5%	540
	11th to 20th	Night	22% 12% 30% 28% 7%	533
	21st to end	Night	20% 9% 33% 29% 8% 1	540
May	1st to 10th	Night	18% 8% 36% 29% 9% 1	540
	11th to 20th	Night	22% 6% 29% 33% 10% 1	531
	21st to end	Night	11% 5% 31% 35% 16% 1	584
June	1st to 10th	Night	10% 4 25% 44% 16% 1	540
	11th to 20th	Night	13% 4 27% 42% 13% 1	540
	21st to end	Night	10% 4 27% 43% 16% 1	540
July	1st to 10th	Night	8% 2 11% 42% 33% 4	525
	11th to 20th	Night	9% 2 18% 44% 25% 2	522
	21st to end	Night	8% 2 18% 45% 26% 1	591
August	1st to 10th	Night	6% 2 14% 44% 30% 4	537
	11th to 20th	Night	9% 2 11% 40% 32% 6%	534
	21st to end	Night	10% 1 16% 44% 25% 3	594
September	1st to 10th	Night	12% 4 18% 39% 22% 6%	540
	11th to 20th	Night	10% 5% 22% 34% 25% 5%	571
	21st to end	Night	14% 8% 25% 33% 19% 1	720
October	1st to 10th	Night	15% 6% 29% 31% 18% 2	720
	11th to 20th	Night	21% 12% 30% 28% 9%	720
	21st to end	Night	24% 11% 32% 25% 7% 1	792
November	1st to 10th	Night	16% 9% 40% 24% 10% 1	740
	11th to 20th	Night	19% 15% 37% 24% 6%	760
	21st to end	Night	20% 12% 35% 26% 7%	942
December	1st to 10th	Night	19% 12% 40% 24% 5%	950
	11th to 20th	Night	22% 9% 38% 25% 6%	950
	21st to end	Night	27% 13% 35% 22% 3	1045

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