BREIT IDEAS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BREIT IDEAS Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	9% 3 1 3 2 2 2 2 3 5% 67%	135543
	Night	12% 4 1 3 2 2 3 2 4 5% 61%	131888
Transparency	Day	22% 3 6% 18% 25% 25%	130458
	Night	27% 4 7% 18% 22% 22%	127960
Seeing	Day	12% 5% 20% 30% 25% 7%	16441
	Night	13% 2 12% 35% 32% 6%	25651

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	24% 6% 2 4 3 3 3 3 5% 7% 41%	3639
	11th to 20th	Day	19% 6% 2 4 3 3 2 1 2 3 54%	3985
	21st to end	Day	23% 5% 2 4 3 2 3 2 4 5% 47%	4510
February	1st to 10th	Day	22% 4 1 5% 2 3 4 2 5% 6% 46%	1800
	11th to 20th	Day	18% 6% 2 5% 3 3 3 3 5% 6% 46%	1842
	21st to end	Day	19% 4 1 3 2 3 3 2 6% 6% 49%	1650
March	1st to 10th	Day	20% 10% 3 6% 4 3 4 3 4 5% 38%	4711
	11th to 20th	Day	20% 6% 2 5% 3 3 4 3 4 6% 46%	5000
	21st to end	Day	18% 7% 2 6% 4 4 4 4 5% 7% 39%	5656
April	1st to 10th	Day	15% 5% 2 4 4 3 3 3 5% 7% 49%	2400
	11th to 20th	Day	11% 4 1 4 3 3 3 3 5% 5% 57%	2369
	21st to end	Day	7% 4 1 3 3 3 3 3 4 7% 63%	2400
May	1st to 10th	Day	5% 2 1 2 1 1 2 2 3 5% 75%	5709
	11th to 20th	Day	3 2 1 2 2 2 2 2 4 5% 77%	6277
	21st to end	Day	2 1 2 1 1 1 2 3 5% 80%	6902
June	1st to 10th	Day	5% 3 1 3 2 2 4 2 5% 7% 66%	2800
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 5% 80%	2800
	21st to end	Day	2 1 1 1 1 1 1 2 3 86%	2800
July	1st to 10th	Day	1 1 1 1 2 2 93%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 90%	6320
	21st to end	Day	1 1 1 1 1 2 2 3 4 85%	6430
August	1st to 10th	Day	1 1 1 2 1 1 2 1 3 4 83%	5777
	11th to 20th	Day	1 1 1 1 1 2 92%	5538
	21st to end	Day	1 1 2 1 1 1 1 2 3 86%	5984
September	1st to 10th	Day	1 1 1 1 1 1 1 2 4 87%	2400
	11th to 20th	Day	3 2 1 3 2 2 2 2 2 4 78%	2400
	21st to end	Day	4 2 1 2 1 1 1 2 2 5% 80%	2142
October	1st to 10th	Day	3 2 1 3 2 2 2 2 3 5% 76%	3280
	11th to 20th	Day	6% 3 1 2 1 2 2 2 4 6% 70%	3261
	21st to end	Day	7% 2 1 3 3 3 4 4 5% 7% 60%	3558
November	1st to 10th	Day	13% 3 1 3 3 3 3 3 5% 7% 55%	1832
	11th to 20th	Day	16% 3 1 5% 3 3 4 4 5% 7% 50%	1800
	21st to end	Day	14% 4 2 4 4 2 3 3 5% 5% 55%	1600
December	1st to 10th	Day	11% 3 1 3 3 3 2 3 5% 6% 59%	2620
	11th to 20th	Day	31% 6% 2 5% 3 3 4 4 5% 9% 27%	3028
	21st to end	Day	30% 7% 2 6% 3 3 4 3 5% 7% 29%	4004
January	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 6% 8% 42%	6306
	11th to 20th	Night	19% 4 2 4 3 3 2 2 3 4 54%	5987
	21st to end	Night	21% 6% 2 5% 3 3 3 2 3 5% 47%	6468
February	1st to 10th	Night	21% 5% 2 3 3 3 2 3 4 6% 48%	2600
	11th to 20th	Night	16% 5% 2 5% 3 4 3 3 5% 6% 48%	2443
	21st to end	Night	18% 5% 2 4 3 3 3 3 4 5% 51%	1980
March	1st to 10th	Night	21% 10% 3 6% 4 3 3 3 4 6% 37%	5166
	11th to 20th	Night	23% 6% 2 4 3 2 3 2 3 5% 47%	5000
	21st to end	Night	20% 8% 3 7% 4 4 3 3 5% 6% 37%	5318
April	1st to 10th	Night	10% 7% 2 5% 3 2 4 4 5% 7% 50%	2000
	11th to 20th	Night	11% 4 2 3 3 2 3 3 4 7% 58%	1979
	21st to end	Night	7% 4 1 3 3 2 2 2 4 6% 65%	2000
May	1st to 10th	Night	4 2 1 3 2 2 2 2 4 5% 73%	4261
	11th to 20th	Night	2 2 1 3 2 2 3 3 4 6% 72%	3616
	21st to end	Night	2 1 1 1 1 1 2 2 3 4 82%	3972
June	1st to 10th	Night	5% 3 1 3 2 2 3 4 5% 8% 65%	1600
	11th to 20th	Night	1 1 1 2 2 2 3 4 6% 78%	1600
	21st to end	Night	2 1 1 2 1 2 2 2 3 4 81%	1600
July	1st to 10th	Night	1 1 1 1 1 2 2 92%	3640
	11th to 20th	Night	1 2 1 1 1 1 2 2 89%	3640
	21st to end	Night	1 2 1 1 2 2 3 4 82%	4425
August	1st to 10th	Night	2 2 1 2 1 1 1 2 3 3 82%	4035
	11th to 20th	Night	1 1 1 1 1 2 2 90%	4321
	21st to end	Night	2 1 2 1 2 1 1 2 3 83%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 3 87%	2000
	11th to 20th	Night	3 2 1 3 1 2 2 2 3 5% 76%	2000
	21st to end	Night	3 1 2 1 1 2 2 3 4 80%	2252
October	1st to 10th	Night	4 2 1 3 2 2 2 2 3 3 78%	3920
	11th to 20th	Night	4 2 1 2 2 2 3 2 4 4 76%	3897
	21st to end	Night	9% 2 1 2 2 3 3 3 5% 7% 64%	4253
November	1st to 10th	Night	14% 4 1 3 2 2 3 3 5% 7% 55%	2511
	11th to 20th	Night	13% 4 1 3 3 3 3 3 6% 7% 54%	2600
	21st to end	Night	14% 4 2 3 3 3 3 2 4 6% 55%	2795
December	1st to 10th	Night	12% 3 1 3 2 2 3 2 5% 7% 61%	4560
	11th to 20th	Night	31% 6% 2 5% 3 3 4 3 6% 8% 27%	5176
	21st to end	Night	30% 7% 2 6% 4 4 3 3 4 5% 32%	6952

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	44% 6% 10% 23% 9% 8%	3639
	11th to 20th	Day	39% 3 5% 17% 22% 15%	3985
	21st to end	Day	41% 4 7% 13% 20% 16%	4510
February	1st to 10th	Day	40% 3 9% 19% 19% 9%	1800
	11th to 20th	Day	39% 5% 9% 19% 21% 7%	1851
	21st to end	Day	37% 3 9% 25% 18% 8%	1650
March	1st to 10th	Day	48% 5% 6% 20% 17% 3	4711
	11th to 20th	Day	40% 5% 9% 24% 14% 8%	5000
	21st to end	Day	43% 7% 12% 25% 11% 2	5656
April	1st to 10th	Day	35% 5% 10% 24% 20% 5%	2400
	11th to 20th	Day	29% 4 12% 23% 23% 9%	2372
	21st to end	Day	22% 5% 12% 30% 26% 6%	2400
May	1st to 10th	Day	13% 2 7% 23% 34% 21%	5709
	11th to 20th	Day	12% 3 6% 26% 39% 14%	6280
	21st to end	Day	9% 2 8% 30% 39% 11%	6904
June	1st to 10th	Day	18% 4 10% 25% 27% 14%	2800
	11th to 20th	Day	9% 3 7% 19% 33% 29%	2800
	21st to end	Day	7% 2 3 13% 36% 40%	2800
July	1st to 10th	Day	3 1 1 7% 21% 67%	6251
	11th to 20th	Day	4 1 2 5% 14% 74%	6245
	21st to end	Day	6% 2 3 17% 28% 44%	6244
August	1st to 10th	Day	10% 1 3 7% 27% 52%	4206
	11th to 20th	Day	4 1 2 7% 41% 45%	4009
	21st to end	Day	6% 1 2 10% 40% 42%	4311
September	1st to 10th	Day	6% 1 2 9% 27% 55%	2400
	11th to 20th	Day	13% 2 5% 20% 38% 22%	2400
	21st to end	Day	11% 2 5% 18% 32% 33%	2142
October	1st to 10th	Day	13% 2 5% 11% 25% 44%	3280
	11th to 20th	Day	16% 4 4 22% 32% 22%	3261
	21st to end	Day	23% 4 6% 22% 29% 16%	3558
November	1st to 10th	Day	28% 4 8% 26% 20% 14%	1832
	11th to 20th	Day	35% 4 11% 23% 19% 9%	1800
	21st to end	Day	32% 5% 9% 20% 22% 13%	1600
December	1st to 10th	Day	27% 3 7% 24% 28% 12%	2620
	11th to 20th	Day	54% 5% 10% 15% 13% 3	3028
	21st to end	Day	53% 6% 13% 17% 8% 2	4004
January	1st to 10th	Night	38% 7% 16% 19% 12% 7%	6306
	11th to 20th	Night	35% 4 8% 15% 22% 16%	5987
	21st to end	Night	42% 3 8% 9% 18% 19%	6468
February	1st to 10th	Night	38% 4 8% 19% 19% 12%	2600
	11th to 20th	Night	36% 4 9% 21% 22% 9%	2445
	21st to end	Night	36% 4 7% 22% 21% 9%	1980
March	1st to 10th	Night	48% 6% 7% 18% 15% 6%	5166
	11th to 20th	Night	40% 5% 8% 21% 17% 9%	5000
	21st to end	Night	47% 6% 10% 22% 12% 3	5318
April	1st to 10th	Night	33% 6% 12% 22% 20% 8%	2000
	11th to 20th	Night	27% 5% 12% 24% 19% 13%	1979
	21st to end	Night	21% 4 13% 28% 25% 8%	2000
May	1st to 10th	Night	15% 3 10% 22% 28% 22%	4258
	11th to 20th	Night	14% 3 6% 24% 36% 17%	3616
	21st to end	Night	9% 2 8% 28% 39% 14%	3972
June	1st to 10th	Night	18% 6% 11% 26% 28% 12%	1600
	11th to 20th	Night	8% 4 8% 18% 32% 31%	1600
	21st to end	Night	10% 2 3 12% 31% 42%	1600
July	1st to 10th	Night	3 1 1 7% 20% 68%	3600
	11th to 20th	Night	6% 2 5% 10% 77%	3592
	21st to end	Night	7% 2 3 18% 26% 44%	4300
August	1st to 10th	Night	13% 1 2 7% 25% 51%	2929
	11th to 20th	Night	4 1 2 8% 35% 50%	3120
	21st to end	Night	7% 1 2 11% 33% 47%	3608
September	1st to 10th	Night	6% 1 2 7% 28% 56%	2000
	11th to 20th	Night	13% 2 5% 17% 38% 25%	2000
	21st to end	Night	9% 2 5% 16% 30% 38%	2252
October	1st to 10th	Night	14% 2 4 10% 18% 53%	3920
	11th to 20th	Night	14% 3 5% 24% 31% 24%	3897
	21st to end	Night	21% 3 5% 21% 31% 20%	4253
November	1st to 10th	Night	29% 4 9% 23% 18% 16%	2511
	11th to 20th	Night	28% 5% 13% 25% 19% 11%	2600
	21st to end	Night	33% 4 6% 21% 23% 14%	2795
December	1st to 10th	Night	25% 3 8% 23% 24% 18%	4560
	11th to 20th	Night	53% 4 8% 16% 15% 4	5176
	21st to end	Night	55% 6% 11% 15% 10% 3	6952

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	44% 3 16% 22% 14% 3	200
	11th to 20th	Day	25% 5% 19% 29% 15% 6%	350
	21st to end	Day	22% 6% 18% 29% 20% 5%	440
February	1st to 10th	Day	27% 6% 18% 28% 19% 3	400
	11th to 20th	Day	25% 8% 26% 24% 15% 2	440
	21st to end	Day	20% 8% 24% 33% 12% 3	414
March	1st to 10th	Day	29% 10% 28% 18% 13% 2	500
	11th to 20th	Day	28% 8% 24% 21% 15% 4	500
	21st to end	Day	23% 10% 26% 24% 14% 2	550
April	1st to 10th	Day	19% 9% 28% 24% 16% 4	500
	11th to 20th	Day	16% 10% 29% 23% 16% 5%	492
	21st to end	Day	8% 10% 24% 32% 20% 6%	500
May	1st to 10th	Day	9% 11% 28% 30% 17% 5%	503
	11th to 20th	Day	9% 9% 22% 30% 23% 7%	528
	21st to end	Day	5% 6% 22% 30% 27% 9%	573
June	1st to 10th	Day	6% 7% 20% 29% 29% 9%	530
	11th to 20th	Day	4 6% 25% 37% 21% 7%	530
	21st to end	Day	2 4 18% 38% 30% 9%	530
July	1st to 10th	Day	2 2 16% 42% 32% 6%	526
	11th to 20th	Day	1 1 12% 39% 38% 9%	524
	21st to end	Day	1 1 11% 38% 38% 10%	575
August	1st to 10th	Day	2 1 13% 39% 34% 11%	527
	11th to 20th	Day	1 1 17% 34% 33% 13%	517
	21st to end	Day	1 2 12% 35% 37% 13%	583
September	1st to 10th	Day	2 2 13% 42% 30% 10%	530
	11th to 20th	Day	6% 3 20% 28% 33% 9%	530
	21st to end	Day	4 5% 21% 30% 32% 8%	530
October	1st to 10th	Day	6% 6% 22% 24% 32% 11%	530
	11th to 20th	Day	8% 6% 17% 27% 33% 9%	530
	21st to end	Day	13% 4 18% 29% 26% 9%	583
November	1st to 10th	Day	18% 3 17% 28% 27% 8%	356
	11th to 20th	Day	16% 2 16% 32% 22% 12%	300
	21st to end	Day	16% 6% 19% 33% 18% 9%	200
December	1st to 10th	Day	26% 3 15% 26% 20% 11%	200
	11th to 20th	Day	28% 4 18% 22% 21% 9%	200
	21st to end	Day	27% 5% 22% 24% 20% 4	220
January	1st to 10th	Night	26% 2 12% 33% 22% 5%	800
	11th to 20th	Night	21% 3 14% 36% 22% 5%	800
	21st to end	Night	21% 2 14% 35% 24% 5%	880
February	1st to 10th	Night	23% 3 12% 38% 22% 3	800
	11th to 20th	Night	19% 4 14% 35% 23% 5%	784
	21st to end	Night	21% 5% 15% 39% 17% 3	660
March	1st to 10th	Night	26% 6% 15% 31% 19% 3	800
	11th to 20th	Night	24% 4 14% 30% 26% 3	800
	21st to end	Night	22% 5% 18% 34% 20% 2	880
April	1st to 10th	Night	14% 5% 18% 40% 22% 2	800
	11th to 20th	Night	13% 4 17% 38% 23% 6%	791
	21st to end	Night	10% 4 15% 38% 28% 5%	800
May	1st to 10th	Night	8% 4 21% 36% 26% 4	683
	11th to 20th	Night	6% 3 18% 38% 32% 3	394
	21st to end	Night	6% 3 14% 33% 42% 3	433
June	1st to 10th	Night	5% 3 14% 35% 38% 6%	400
	11th to 20th	Night	2 2 17% 41% 35% 4	400
	21st to end	Night	3 1 11% 38% 42% 5%	400
July	1st to 10th	Night	1 1 6% 38% 50% 5%	390
	11th to 20th	Night	1 5% 41% 50% 3	388
	21st to end	Night	1 2 33% 57% 7%	636
August	1st to 10th	Night	3 1 3 35% 53% 7%	597
	11th to 20th	Night	6% 35% 50% 9%	707
	21st to end	Night	1 1 4 32% 55% 7%	880
September	1st to 10th	Night	2 1 7% 34% 50% 7%	800
	11th to 20th	Night	5% 1 7% 34% 43% 11%	800
	21st to end	Night	4 2 11% 36% 37% 11%	800
October	1st to 10th	Night	7% 3 10% 40% 33% 8%	800
	11th to 20th	Night	8% 2 11% 34% 38% 8%	800
	21st to end	Night	11% 2 10% 35% 31% 11%	880
November	1st to 10th	Night	17% 3 10% 31% 31% 9%	788
	11th to 20th	Night	16% 2 11% 38% 29% 5%	800
	21st to end	Night	16% 2 12% 35% 29% 6%	800
December	1st to 10th	Night	22% 2 10% 35% 23% 8%	800
	11th to 20th	Night	30% 2 10% 33% 22% 3	800
	21st to end	Night	27% 3 13% 34% 20% 2	880

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