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 2024-10-25.

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	4 6% 3 7% 5% 5% 5% 4 6% 8% 48%	101171
	Night	3 6% 3 7% 5% 4 5% 4 6% 8% 49%	97795
Transparency	Day	30% 9% 14% 21% 17% 8%	100917
	Night	30% 9% 15% 22% 16% 7%	97638
Seeing	Day	7% 11% 30% 32% 16% 4	15128
	Night	6% 6% 25% 41% 19% 3	24614

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 6% 4 16% 8% 8% 8% 6% 8% 8% 28%	2511
	11th to 20th	Day	2 6% 3 8% 6% 6% 8% 6% 9% 9% 37%	2540
	21st to end	Day	2 15% 7% 12% 5% 4 4 4 5% 6% 34%	2992
February	1st to 10th	Day	2 7% 3 9% 6% 5% 5% 5% 8% 9% 40%	1710
	11th to 20th	Day	2 8% 4 9% 7% 6% 6% 5% 9% 11% 34%	1715
	21st to end	Day	2 7% 4 10% 6% 7% 7% 5% 8% 10% 34%	1570
March	1st to 10th	Day	1 6% 3 9% 6% 6% 6% 5% 7% 8% 43%	3228
	11th to 20th	Day	2 7% 4 8% 4 4 4 4 5% 6% 54%	3490
	21st to end	Day	3 11% 5% 12% 8% 7% 6% 5% 7% 8% 29%	3825
April	1st to 10th	Day	4 9% 3 8% 6% 5% 5% 5% 7% 9% 40%	2192
	11th to 20th	Day	4 7% 3 7% 5% 5% 5% 4 6% 9% 44%	2437
	21st to end	Day	6% 8% 3 7% 5% 5% 4 4 6% 9% 43%	2470
May	1st to 10th	Day	5% 9% 3 6% 5% 5% 5% 4 6% 9% 44%	4110
	11th to 20th	Day	5% 6% 2 6% 4 4 5% 3 6% 7% 51%	4071
	21st to end	Day	7% 9% 3 7% 5% 5% 4 4 6% 7% 44%	4831
June	1st to 10th	Day	7% 7% 2 7% 5% 4 6% 5% 8% 10% 38%	2850
	11th to 20th	Day	7% 6% 3 7% 6% 4 5% 4 6% 7% 45%	2850
	21st to end	Day	2 3 1 5% 4 4 4 4 6% 7% 59%	2850
July	1st to 10th	Day	2 2 1 3 3 3 4 3 5% 6% 69%	4626
	11th to 20th	Day	2 2 1 4 4 5% 6% 5% 7% 8% 56%	4420
	21st to end	Day	3 3 1 5% 4 4 6% 5% 8% 8% 53%	4615
August	1st to 10th	Day	4 3 1 5% 4 6% 7% 5% 7% 8% 49%	4092
	11th to 20th	Day	3 2 1 2 3 3 4 4 6% 8% 64%	4028
	21st to end	Day	2 3 1 4 3 3 3 3 5% 6% 64%	4521
September	1st to 10th	Day	4 3 1 4 3 3 4 3 4 6% 65%	2200
	11th to 20th	Day	8% 4 2 5% 4 4 5% 4 6% 7% 52%	2090
	21st to end	Day	6% 4 2 4 4 3 3 3 5% 7% 60%	2090
October	1st to 10th	Day	9% 7% 3 6% 4 4 5% 3 5% 6% 49%	2021
	11th to 20th	Day	4 5% 2 5% 4 4 4 4 5% 6% 58%	1900
	21st to end	Day	4 4 1 5% 4 5% 6% 5% 7% 9% 49%	1866
November	1st to 10th	Day	4 7% 2 10% 6% 5% 5% 4 7% 8% 42%	1674
	11th to 20th	Day	1 6% 2 7% 5% 6% 6% 5% 8% 8% 44%	1628
	21st to end	Day	2 8% 3 8% 6% 4 5% 4 6% 7% 48%	1520
December	1st to 10th	Day	2 5% 3 8% 7% 7% 6% 5% 8% 8% 40%	1511
	11th to 20th	Day	2 10% 6% 12% 6% 6% 5% 5% 7% 7% 34%	1685
	21st to end	Day	2 9% 4 11% 6% 5% 5% 4 6% 7% 41%	2442
January	1st to 10th	Night	1 5% 4 12% 7% 6% 6% 5% 6% 9% 39%	4462
	11th to 20th	Night	1 6% 3 11% 7% 7% 6% 5% 7% 8% 37%	4420
	21st to end	Night	1 13% 6% 12% 6% 5% 5% 4 5% 6% 36%	4667
February	1st to 10th	Night	1 7% 3 11% 6% 5% 6% 5% 7% 8% 41%	2470
	11th to 20th	Night	1 8% 4 10% 6% 4 6% 5% 6% 7% 42%	2350
	21st to end	Night	2 5% 3 8% 9% 8% 6% 4 8% 8% 38%	1884
March	1st to 10th	Night	1 9% 4 9% 5% 4 4 4 6% 9% 46%	3646
	11th to 20th	Night	2 6% 2 6% 5% 4 3 4 6% 7% 57%	3490
	21st to end	Night	4 9% 3 8% 5% 5% 5% 5% 9% 11% 36%	3828
April	1st to 10th	Night	5% 10% 2 6% 6% 5% 5% 5% 6% 8% 42%	1984
	11th to 20th	Night	4 6% 2 6% 5% 4 6% 5% 7% 8% 46%	1691
	21st to end	Night	6% 7% 2 7% 6% 5% 4 5% 5% 7% 45%	1710
May	1st to 10th	Night	7% 8% 2 6% 3 4 4 3 5% 7% 50%	2860
	11th to 20th	Night	5% 6% 2 6% 4 4 5% 4 5% 7% 53%	2834
	21st to end	Night	8% 6% 3 8% 5% 5% 5% 4 6% 8% 44%	2744
June	1st to 10th	Night	6% 5% 2 7% 5% 5% 5% 6% 7% 8% 44%	1330
	11th to 20th	Night	9% 3 2 6% 4 4 5% 4 6% 6% 51%	1330
	21st to end	Night	2 3 1 4 4 3 4 3 4 7% 64%	1330
July	1st to 10th	Night	1 1 2 3 2 3 3 5% 7% 74%	2316
	11th to 20th	Night	1 1 1 3 3 3 4 3 5% 7% 68%	2540
	21st to end	Night	2 2 1 3 3 3 4 4 6% 7% 65%	2940
August	1st to 10th	Night	3 2 1 3 4 3 5% 4 7% 9% 59%	2847
	11th to 20th	Night	1 2 1 3 3 3 4 3 6% 6% 68%	2811
	21st to end	Night	2 3 1 3 3 3 3 3 5% 5% 69%	3146
September	1st to 10th	Night	4 3 2 4 3 3 3 3 4 5% 68%	1976
	11th to 20th	Night	5% 4 1 5% 3 3 4 3 5% 7% 59%	2090
	21st to end	Night	6% 4 1 5% 3 3 3 3 3 6% 63%	2090
October	1st to 10th	Night	7% 5% 2 4 3 3 3 3 7% 8% 55%	2158
	11th to 20th	Night	3 5% 2 4 3 3 4 4 5% 7% 59%	2280
	21st to end	Night	4 4 2 5% 4 4 5% 5% 7% 9% 52%	2591
November	1st to 10th	Night	5% 7% 2 7% 5% 4 6% 5% 7% 8% 45%	2405
	11th to 20th	Night	2 7% 2 7% 5% 5% 5% 5% 7% 9% 47%	2531
	21st to end	Night	2 6% 3 7% 6% 5% 4 4 7% 7% 49%	2660
December	1st to 10th	Night	2 6% 3 8% 6% 6% 6% 5% 7% 8% 42%	2664
	11th to 20th	Night	2 10% 5% 10% 6% 6% 6% 5% 7% 9% 35%	3506
	21st to end	Night	2 9% 4 11% 7% 5% 5% 4 6% 6% 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	32% 12% 19% 24% 12% 1	3228
	11th to 20th	Day	29% 9% 17% 31% 12% 1	3490
	21st to end	Day	47% 15% 19% 16% 3	3825
April	1st to 10th	Day	38% 10% 21% 20% 10% 1	2192
	11th to 20th	Day	34% 8% 19% 24% 13% 2	2440
	21st to end	Day	36% 8% 16% 28% 12%	2470
May	1st to 10th	Day	32% 11% 15% 25% 14% 3	4110
	11th to 20th	Day	26% 9% 15% 30% 14% 7%	4074
	21st to end	Day	34% 11% 14% 31% 9% 1	4831
June	1st to 10th	Day	35% 10% 18% 26% 10% 1	2850
	11th to 20th	Day	34% 8% 12% 20% 19% 8%	2850
	21st to end	Day	20% 6% 11% 21% 32% 11%	2850
July	1st to 10th	Day	13% 5% 7% 14% 25% 36%	4555
	11th to 20th	Day	18% 7% 9% 13% 22% 31%	4345
	21st to end	Day	20% 7% 9% 14% 29% 21%	4469
August	1st to 10th	Day	25% 7% 10% 15% 30% 13%	4078
	11th to 20th	Day	14% 5% 9% 22% 36% 14%	4067
	21st to end	Day	18% 6% 9% 21% 31% 16%	4520
September	1st to 10th	Day	19% 4 9% 19% 32% 17%	2200
	11th to 20th	Day	28% 7% 11% 21% 23% 10%	2090
	21st to end	Day	23% 6% 12% 27% 24% 7%	2090
October	1st to 10th	Day	35% 7% 13% 24% 16% 4	2021
	11th to 20th	Day	25% 8% 17% 30% 16% 3	1900
	21st to end	Day	28% 11% 20% 24% 16% 2	1866
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	40% 12% 17% 19% 8% 3	1884
March	1st to 10th	Night	31% 11% 18% 23% 12% 4	3646
	11th to 20th	Night	24% 8% 20% 31% 15% 1	3490
	21st to end	Night	33% 14% 23% 25% 5% 1	3828
April	1st to 10th	Night	36% 9% 18% 22% 12% 2	1984
	11th to 20th	Night	31% 9% 19% 25% 14% 2	1691
	21st to end	Night	34% 8% 16% 29% 12%	1710
May	1st to 10th	Night	30% 7% 17% 27% 17% 3	2860
	11th to 20th	Night	23% 10% 13% 30% 20% 3	2835
	21st to end	Night	32% 10% 15% 28% 15% 1	2744
June	1st to 10th	Night	32% 10% 14% 29% 14% 1	1330
	11th to 20th	Night	30% 8% 11% 22% 19% 10%	1330
	21st to end	Night	19% 4 10% 22% 32% 13%	1330
July	1st to 10th	Night	10% 4 7% 12% 26% 41%	2278
	11th to 20th	Night	12% 4 6% 14% 23% 41%	2492
	21st to end	Night	15% 4 7% 18% 31% 26%	2848
August	1st to 10th	Night	18% 6% 8% 18% 33% 17%	2838
	11th to 20th	Night	14% 5% 7% 24% 30% 20%	2838
	21st to end	Night	16% 5% 6% 22% 30% 21%	3146
September	1st to 10th	Night	19% 5% 7% 18% 31% 21%	1976
	11th to 20th	Night	23% 6% 10% 25% 25% 12%	2090
	21st to end	Night	23% 5% 9% 25% 31% 7%	2090
October	1st to 10th	Night	25% 7% 14% 28% 21% 5%	2158
	11th to 20th	Night	24% 6% 15% 33% 19% 3	2280
	21st to end	Night	25% 11% 19% 27% 16% 1	2591
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% 36% 38% 12% 3	382
March	1st to 10th	Day	6% 8% 35% 38% 13%	460
	11th to 20th	Day	5% 11% 27% 39% 16% 3	460
	21st to end	Day	7% 11% 34% 30% 16% 1	506
April	1st to 10th	Day	6% 13% 30% 32% 15% 4	460
	11th to 20th	Day	5% 16% 38% 25% 12% 3	452
	21st to end	Day	7% 14% 25% 32% 18% 5%	460
May	1st to 10th	Day	6% 14% 29% 29% 19% 3	463
	11th to 20th	Day	15% 11% 23% 29% 18% 4	488
	21st to end	Day	12% 10% 21% 28% 22% 7%	665
June	1st to 10th	Day	8% 11% 25% 31% 19% 6%	680
	11th to 20th	Day	14% 18% 29% 26% 12% 2	680
	21st to end	Day	4 11% 29% 35% 18% 3	680
July	1st to 10th	Day	4 10% 30% 31% 20% 4	612
	11th to 20th	Day	2 12% 37% 36% 12% 2	484
	21st to end	Day	5% 9% 36% 30% 16% 4	531
August	1st to 10th	Day	7% 9% 32% 32% 15% 4	487
	11th to 20th	Day	5% 10% 27% 33% 18% 6%	477
	21st to end	Day	5% 13% 31% 31% 15% 5%	539
September	1st to 10th	Day	5% 13% 30% 30% 16% 6%	490
	11th to 20th	Day	12% 12% 30% 27% 16% 4	490
	21st to end	Day	9% 14% 28% 30% 16% 3	490
October	1st to 10th	Day	12% 10% 29% 29% 15% 4	421
	11th to 20th	Day	6% 16% 24% 29% 19% 7%	300
	21st to end	Day	7% 14% 34% 34% 10% 2	311
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 7% 30% 44% 14% 2	760
	11th to 20th	Night	5% 5% 31% 38% 19% 3	760
	21st to end	Night	9% 11% 24% 39% 17% 1	836
April	1st to 10th	Night	7% 11% 27% 36% 15% 3	749
	11th to 20th	Night	5% 9% 35% 36% 14% 2	563
	21st to end	Night	9% 6% 29% 36% 18% 2	570
May	1st to 10th	Night	5% 9% 29% 36% 18% 3	570
	11th to 20th	Night	10% 4 23% 33% 27% 4	561
	21st to end	Night	15% 5% 23% 29% 24% 4	546
June	1st to 10th	Night	7% 5% 19% 40% 25% 4	380
	11th to 20th	Night	8% 9% 23% 42% 16% 2	380
	21st to end	Night	2 3 18% 49% 25% 3	380
July	1st to 10th	Night	3 4 14% 46% 30% 4	370
	11th to 20th	Night	2 2 14% 57% 23% 1	368
	21st to end	Night	3 1 12% 48% 32% 4	520
August	1st to 10th	Night	5% 2 17% 45% 28% 3	567
	11th to 20th	Night	3 2 18% 45% 30% 3	564
	21st to end	Night	4 4 18% 41% 29% 4	627
September	1st to 10th	Night	7% 3 16% 40% 28% 6%	717
	11th to 20th	Night	6% 2 16% 38% 30% 7%	760
	21st to end	Night	8% 6% 19% 36% 24% 8%	760
October	1st to 10th	Night	9% 4 23% 39% 21% 4	760
	11th to 20th	Night	7% 7% 21% 40% 20% 6%	760
	21st to end	Night	6% 8% 26% 38% 17% 5%	812
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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