Blue Dome
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Dome clear sky chart from 2005-11-06 to 2024-03-01.

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	4 6% 3 6% 6% 4 5% 6% 6% 8% 46%	78280
	Night	3 6% 3 7% 6% 3 5% 5% 6% 8% 47%	81134
Transparency	Day	34% 9% 15% 21% 15% 6%	77986
	Night	33% 10% 16% 22% 15% 6%	80949
Seeing	Day	7% 11% 29% 32% 16% 4	14149
	Night	6% 6% 25% 41% 19% 3	23865

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	1 5% 4 14% 9% 6% 8% 9% 8% 8% 28%	2511
	11th to 20th	Day	2 6% 3 7% 7% 5% 8% 8% 9% 9% 37%	2540
	21st to end	Day	2 15% 7% 11% 6% 3 4 5% 5% 7% 34%	2992
February	1st to 10th	Day	2 6% 3 8% 8% 3 5% 7% 7% 10% 40%	1710
	11th to 20th	Day	2 6% 5% 8% 8% 4 6% 7% 9% 11% 34%	1715
	21st to end	Day	2 6% 5% 10% 7% 5% 8% 7% 8% 10% 34%	1570
March	1st to 10th	Day	2 7% 4 7% 6% 4 6% 6% 8% 9% 40%	1852
	11th to 20th	Day	3 7% 4 6% 7% 4 6% 7% 7% 9% 40%	1980
	21st to end	Day	4 8% 4 9% 8% 4 6% 6% 7% 9% 34%	2178
April	1st to 10th	Day	4 8% 4 7% 6% 3 6% 6% 7% 9% 39%	2076
	11th to 20th	Day	4 7% 4 6% 6% 4 6% 5% 6% 9% 43%	2307
	21st to end	Day	6% 7% 3 6% 6% 3 5% 5% 5% 9% 44%	2340
May	1st to 10th	Day	5% 8% 3 7% 6% 3 4 5% 5% 8% 44%	2340
	11th to 20th	Day	8% 7% 3 6% 6% 4 6% 6% 7% 8% 37%	2301
	21st to end	Day	12% 10% 4 7% 7% 4 5% 6% 7% 8% 30%	2720
June	1st to 10th	Day	8% 6% 2 6% 6% 3 6% 6% 8% 11% 38%	2700
	11th to 20th	Day	8% 6% 3 6% 6% 3 5% 5% 6% 8% 44%	2700
	21st to end	Day	2 3 2 4 5% 3 4 5% 6% 8% 59%	2700
July	1st to 10th	Day	3 3 1 4 4 3 5% 6% 6% 8% 59%	2642
	11th to 20th	Day	3 2 1 3 3 2 4 4 5% 6% 65%	2520
	21st to end	Day	5% 3 1 4 5% 3 5% 6% 7% 7% 55%	2641
August	1st to 10th	Day	6% 5% 2 4 5% 4 5% 6% 6% 7% 50%	2328
	11th to 20th	Day	3 2 1 3 3 2 4 5% 6% 8% 63%	2299
	21st to end	Day	3 3 1 4 4 3 4 5% 6% 7% 59%	2574
September	1st to 10th	Day	5% 3 1 3 4 2 4 4 4 6% 65%	2085
	11th to 20th	Day	8% 4 2 4 5% 3 4 5% 5% 6% 54%	1980
	21st to end	Day	6% 4 2 4 4 3 3 4 5% 8% 57%	1980
October	1st to 10th	Day	10% 7% 3 6% 5% 3 4 5% 5% 6% 47%	1915
	11th to 20th	Day	4 5% 2 4 5% 3 4 5% 5% 6% 57%	1800
	21st to end	Day	4 4 1 4 5% 3 6% 7% 7% 9% 50%	1824
November	1st to 10th	Day	4 6% 3 8% 7% 4 6% 5% 7% 7% 42%	1674
	11th to 20th	Day	1 6% 3 6% 7% 4 7% 6% 8% 9% 44%	1628
	21st to end	Day	3 7% 3 7% 6% 3 4 5% 6% 8% 48%	1520
December	1st to 10th	Day	2 5% 3 7% 9% 5% 7% 7% 8% 8% 40%	1511
	11th to 20th	Day	2 10% 6% 11% 8% 5% 6% 6% 7% 7% 34%	1685
	21st to end	Day	2 8% 5% 10% 7% 4 5% 6% 6% 7% 41%	2442
January	1st to 10th	Night	1 5% 4 10% 8% 5% 6% 6% 6% 9% 40%	4462
	11th to 20th	Night	1 5% 4 9% 9% 5% 6% 7% 7% 8% 37%	4420
	21st to end	Night	2 12% 6% 11% 7% 4 5% 5% 5% 6% 36%	4667
February	1st to 10th	Night	2 6% 4 10% 7% 4 6% 6% 7% 9% 41%	2470
	11th to 20th	Night	2 7% 5% 9% 7% 3 6% 6% 6% 7% 42%	2350
	21st to end	Night	2 4 4 7% 10% 6% 8% 6% 7% 9% 38%	1884
March	1st to 10th	Night	2 7% 4 9% 7% 4 4 6% 6% 10% 41%	2114
	11th to 20th	Night	3 6% 3 6% 5% 3 5% 6% 8% 10% 45%	1980
	21st to end	Night	6% 7% 3 7% 7% 4 5% 6% 6% 8% 40%	2178
April	1st to 10th	Night	6% 9% 2 5% 7% 4 5% 6% 6% 8% 42%	1880
	11th to 20th	Night	5% 5% 3 6% 5% 4 6% 6% 7% 8% 46%	1601
	21st to end	Night	6% 6% 3 6% 7% 4 5% 6% 5% 8% 45%	1620
May	1st to 10th	Night	4 5% 3 6% 4 3 6% 5% 6% 8% 51%	1620
	11th to 20th	Night	9% 6% 2 6% 5% 4 5% 5% 7% 9% 42%	1594
	21st to end	Night	14% 7% 3 7% 6% 3 5% 4 5% 9% 37%	1548
June	1st to 10th	Night	7% 4 2 6% 6% 3 6% 7% 7% 8% 44%	1260
	11th to 20th	Night	9% 3 2 5% 5% 2 5% 5% 6% 6% 51%	1260
	21st to end	Night	2 3 1 4 4 2 4 4 4 7% 64%	1260
July	1st to 10th	Night	2 2 1 3 4 2 5% 4 6% 8% 64%	1314
	11th to 20th	Night	2 1 1 2 3 2 4 3 5% 5% 72%	1440
	21st to end	Night	3 2 1 3 4 2 4 4 5% 8% 65%	1674
August	1st to 10th	Night	5% 3 2 3 5% 2 5% 6% 5% 7% 56%	1611
	11th to 20th	Night	2 2 1 2 3 2 4 4 6% 7% 68%	1602
	21st to end	Night	4 3 1 3 4 2 3 4 5% 6% 64%	1782
September	1st to 10th	Night	4 3 2 3 3 2 3 3 4 5% 67%	1871
	11th to 20th	Night	5% 3 2 4 4 2 5% 4 4 7% 61%	1980
	21st to end	Night	7% 4 1 4 4 3 3 3 3 6% 61%	1980
October	1st to 10th	Night	8% 5% 2 4 4 3 3 4 6% 9% 53%	2044
	11th to 20th	Night	4 5% 2 4 4 3 3 4 5% 7% 59%	2160
	21st to end	Night	4 4 2 5% 5% 3 5% 6% 6% 9% 52%	2528
November	1st to 10th	Night	5% 6% 2 6% 6% 3 6% 6% 7% 8% 45%	2405
	11th to 20th	Night	2 6% 3 6% 5% 3 6% 6% 7% 9% 47%	2531
	21st to end	Night	2 5% 3 7% 6% 4 5% 5% 7% 8% 49%	2660
December	1st to 10th	Night	2 6% 3 7% 7% 4 6% 6% 7% 9% 42%	2664
	11th to 20th	Night	2 9% 5% 9% 8% 5% 6% 6% 7% 9% 35%	3506
	21st to end	Night	2 9% 5% 9% 8% 4 5% 6% 6% 7% 40%	5214

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	47% 15% 15% 16% 6% 1	2511
	11th to 20th	Day	38% 12% 21% 16% 10% 2	2540
	21st to end	Day	50% 9% 16% 18% 6% 2	2992
February	1st to 10th	Day	39% 15% 20% 19% 6% 1	1710
	11th to 20th	Day	40% 13% 19% 17% 9% 1	1724
	21st to end	Day	42% 14% 17% 17% 8% 3	1570
March	1st to 10th	Day	37% 12% 19% 19% 10% 2	1852
	11th to 20th	Day	37% 12% 19% 22% 9% 1	1980
	21st to end	Day	44% 12% 21% 18% 5% 1	2178
April	1st to 10th	Day	39% 10% 20% 20% 10% 1	2076
	11th to 20th	Day	36% 8% 19% 24% 12% 2	2310
	21st to end	Day	36% 8% 16% 28% 12%	2340
May	1st to 10th	Day	35% 9% 15% 27% 13% 1	2340
	11th to 20th	Day	40% 8% 16% 23% 12% 1	2304
	21st to end	Day	48% 12% 15% 19% 6% 1	2720
June	1st to 10th	Day	36% 10% 19% 26% 9% 1	2700
	11th to 20th	Day	35% 8% 12% 20% 18% 7%	2700
	21st to end	Day	21% 6% 11% 21% 31% 10%	2700
July	1st to 10th	Day	19% 6% 10% 21% 27% 17%	2571
	11th to 20th	Day	16% 4 8% 16% 26% 30%	2445
	21st to end	Day	23% 6% 8% 12% 27% 23%	2494
August	1st to 10th	Day	28% 7% 11% 16% 25% 13%	2313
	11th to 20th	Day	16% 5% 11% 23% 31% 14%	2299
	21st to end	Day	20% 6% 11% 21% 27% 14%	2574
September	1st to 10th	Day	20% 4 9% 18% 31% 18%	2085
	11th to 20th	Day	28% 6% 11% 21% 25% 10%	1980
	21st to end	Day	24% 6% 12% 27% 24% 6%	1980
October	1st to 10th	Day	37% 8% 14% 24% 14% 4	1915
	11th to 20th	Day	26% 8% 17% 30% 16% 2	1800
	21st to end	Day	28% 11% 20% 23% 16% 2	1824
November	1st to 10th	Day	37% 11% 17% 19% 15% 1	1674
	11th to 20th	Day	35% 11% 21% 22% 8% 2	1628
	21st to end	Day	35% 9% 21% 22% 10% 3	1520
December	1st to 10th	Day	38% 15% 16% 18% 10% 3	1511
	11th to 20th	Day	46% 13% 18% 18% 5%	1684
	21st to end	Day	38% 13% 16% 19% 10% 5%	2442
January	1st to 10th	Night	39% 11% 15% 22% 12% 1	4462
	11th to 20th	Night	40% 15% 15% 17% 9% 4	4420
	21st to end	Night	47% 12% 17% 16% 7% 1	4667
February	1st to 10th	Night	40% 12% 18% 20% 8% 1	2470
	11th to 20th	Night	39% 10% 20% 21% 9% 2	2351
	21st to end	Night	41% 12% 17% 19% 8% 3	1884
March	1st to 10th	Night	38% 11% 18% 22% 8% 2	2114
	11th to 20th	Night	31% 10% 22% 21% 14% 2	1980
	21st to end	Night	37% 12% 22% 21% 7% 1	2178
April	1st to 10th	Night	37% 9% 18% 21% 13% 2	1880
	11th to 20th	Night	32% 9% 19% 25% 14% 2	1601
	21st to end	Night	35% 8% 16% 29% 12%	1620
May	1st to 10th	Night	28% 7% 18% 30% 16% 1	1620
	11th to 20th	Night	34% 10% 16% 26% 14% 1	1595
	21st to end	Night	43% 7% 17% 22% 10% 1	1548
June	1st to 10th	Night	32% 10% 15% 28% 14% 1	1260
	11th to 20th	Night	30% 8% 12% 23% 18% 10%	1260
	21st to end	Night	19% 5% 10% 23% 31% 12%	1260
July	1st to 10th	Night	16% 5% 10% 18% 33% 18%	1276
	11th to 20th	Night	11% 4 7% 16% 28% 34%	1392
	21st to end	Night	15% 3 8% 16% 32% 26%	1581
August	1st to 10th	Night	23% 7% 11% 19% 22% 19%	1602
	11th to 20th	Night	15% 5% 8% 22% 33% 17%	1602
	21st to end	Night	19% 6% 7% 21% 28% 19%	1782
September	1st to 10th	Night	19% 5% 7% 16% 31% 22%	1871
	11th to 20th	Night	22% 6% 9% 25% 26% 13%	1980
	21st to end	Night	25% 5% 9% 25% 31% 6%	1980
October	1st to 10th	Night	27% 8% 15% 29% 19% 4	2044
	11th to 20th	Night	24% 6% 15% 34% 18% 3	2160
	21st to end	Night	25% 11% 18% 27% 17% 1	2528
November	1st to 10th	Night	34% 9% 18% 24% 13% 2	2405
	11th to 20th	Night	31% 11% 20% 24% 12% 2	2531
	21st to end	Night	32% 10% 18% 24% 13% 3	2660
December	1st to 10th	Night	36% 14% 20% 17% 11% 2	2664
	11th to 20th	Night	43% 13% 17% 19% 6% 1	3507
	21st to end	Night	39% 11% 16% 18% 11% 5%	5214

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	4 7% 25% 36% 19% 9%	180
	11th to 20th	Day	2 3 24% 44% 17% 8%	180
	21st to end	Day	6% 11% 23% 37% 19% 4	256
February	1st to 10th	Day	3 13% 43% 29% 10% 1	270
	11th to 20th	Day	3 10% 37% 30% 15% 4	320
	21st to end	Day	3 8% 37% 38% 12% 3	382
March	1st to 10th	Day	5% 7% 37% 37% 13%	421
	11th to 20th	Day	5% 10% 27% 41% 15% 2	420
	21st to end	Day	8% 11% 34% 31% 16% 1	462
April	1st to 10th	Day	6% 14% 28% 33% 15% 4	420
	11th to 20th	Day	5% 17% 40% 23% 12% 3	412
	21st to end	Day	7% 14% 27% 30% 18% 4	420
May	1st to 10th	Day	6% 14% 28% 30% 19% 4	423
	11th to 20th	Day	15% 11% 23% 29% 18% 4	448
	21st to end	Day	13% 10% 20% 27% 23% 7%	613
June	1st to 10th	Day	8% 11% 24% 30% 20% 7%	630
	11th to 20th	Day	13% 17% 28% 27% 12% 2	630
	21st to end	Day	4 11% 27% 36% 19% 3	630
July	1st to 10th	Day	5% 9% 28% 33% 22% 4	566
	11th to 20th	Day	2 12% 37% 36% 10% 2	444
	21st to end	Day	5% 9% 36% 30% 16% 4	487
August	1st to 10th	Day	8% 9% 30% 33% 16% 4	447
	11th to 20th	Day	5% 8% 27% 34% 20% 7%	437
	21st to end	Day	4 12% 31% 33% 16% 5%	495
September	1st to 10th	Day	5% 13% 30% 30% 16% 6%	450
	11th to 20th	Day	12% 12% 28% 28% 16% 5%	450
	21st to end	Day	10% 13% 28% 30% 16% 3	450
October	1st to 10th	Day	13% 11% 28% 29% 15% 5%	385
	11th to 20th	Day	5% 16% 25% 28% 19% 7%	270
	21st to end	Day	7% 14% 33% 34% 10% 2	297
November	1st to 10th	Day	6% 13% 34% 29% 16% 3	270
	11th to 20th	Day	3 10% 30% 40% 11% 4	270
	21st to end	Day	4 10% 26% 39% 17% 3	270
December	1st to 10th	Day	5% 6% 28% 36% 18% 8%	266
	11th to 20th	Day	8% 8% 26% 39% 16% 2	180
	21st to end	Day	5% 9% 30% 37% 16% 4	198
January	1st to 10th	Night	4 5% 28% 44% 17% 3	790
	11th to 20th	Night	4 8% 28% 41% 15% 4	760
	21st to end	Night	4 7% 28% 43% 15% 2	836
February	1st to 10th	Night	3 8% 37% 38% 11% 3	760
	11th to 20th	Night	4 6% 34% 41% 14% 2	744
	21st to end	Night	3 7% 31% 42% 14% 3	628
March	1st to 10th	Night	4 6% 29% 45% 14% 2	722
	11th to 20th	Night	5% 4 31% 38% 18% 3	720
	21st to end	Night	9% 10% 25% 38% 17% 1	792
April	1st to 10th	Night	8% 10% 27% 35% 16% 4	710
	11th to 20th	Night	6% 9% 33% 36% 14% 2	533
	21st to end	Night	9% 7% 29% 35% 18% 2	540
May	1st to 10th	Night	4 8% 29% 36% 19% 3	540
	11th to 20th	Night	10% 4 23% 32% 27% 4	531
	21st to end	Night	16% 5% 22% 29% 24% 4	517
June	1st to 10th	Night	8% 5% 19% 38% 26% 4	360
	11th to 20th	Night	9% 8% 22% 43% 16% 2	360
	21st to end	Night	3 3 16% 50% 26% 3	360
July	1st to 10th	Night	3 3 13% 47% 31% 4	350
	11th to 20th	Night	2 2 14% 56% 24% 1	348
	21st to end	Night	3 1 11% 48% 33% 4	492
August	1st to 10th	Night	6% 2 15% 45% 29% 3	537
	11th to 20th	Night	3 2 16% 45% 31% 3	534
	21st to end	Night	5% 3 16% 42% 29% 4	594
September	1st to 10th	Night	7% 3 16% 40% 28% 7%	679
	11th to 20th	Night	6% 2 15% 37% 32% 8%	720
	21st to end	Night	8% 6% 18% 35% 25% 8%	720
October	1st to 10th	Night	9% 4 23% 39% 21% 4	720
	11th to 20th	Night	6% 7% 22% 40% 19% 6%	720
	21st to end	Night	6% 9% 26% 39% 17% 5%	792
November	1st to 10th	Night	7% 5% 24% 43% 17% 4	740
	11th to 20th	Night	4 8% 31% 41% 14% 1	821
	21st to end	Night	3 8% 30% 39% 16% 3	950
December	1st to 10th	Night	4 8% 30% 43% 13% 2	950
	11th to 20th	Night	5% 6% 27% 49% 12% 2	950
	21st to end	Night	6% 9% 31% 40% 12% 2	1045

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