Brookings
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brookings clear sky chart from 2005-11-06 to 2026-05-02.

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	17% 6% 2 5% 3 3 3 3 4 5% 49%	140856
	Night	22% 7% 2 5% 3 3 3 3 4 5% 44%	139077
Transparency	Day	38% 4 11% 22% 17% 9%	135653
	Night	44% 4 10% 19% 16% 8%	135277
Seeing	Day	21% 10% 25% 29% 13% 3	16909
	Night	24% 6% 17% 28% 21% 4	26039

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	40% 9% 3 6% 4 4 3 3 4 5% 19%	4764
	11th to 20th	Day	24% 5% 1 3 2 2 2 1 2 3 53%	5340
	21st to end	Day	27% 6% 2 6% 4 4 4 3 3 4 37%	5843
February	1st to 10th	Day	28% 8% 2 5% 4 4 4 2 5% 6% 32%	1890
	11th to 20th	Day	28% 9% 2 6% 4 4 3 3 4 5% 32%	1845
	21st to end	Day	31% 9% 3 6% 4 4 3 3 4 4 29%	1655
March	1st to 10th	Day	24% 17% 3 5% 3 3 2 3 4 6% 30%	5062
	11th to 20th	Day	33% 9% 2 5% 3 3 4 2 3 4 31%	5110
	21st to end	Day	38% 10% 2 5% 3 3 3 3 5% 6% 22%	5607
April	1st to 10th	Day	28% 8% 2 6% 4 5% 4 3 5% 6% 28%	2478
	11th to 20th	Day	23% 9% 2 5% 4 4 3 3 5% 6% 37%	2492
	21st to end	Day	18% 6% 2 7% 4 3 4 4 6% 7% 40%	2730
May	1st to 10th	Day	13% 5% 2 5% 4 3 4 3 4 5% 53%	5894
	11th to 20th	Day	12% 5% 1 4 3 3 3 3 5% 7% 55%	6095
	21st to end	Day	7% 4 1 3 2 3 4 3 6% 8% 59%	6902
June	1st to 10th	Day	16% 6% 2 5% 4 4 4 3 5% 7% 46%	2800
	11th to 20th	Day	8% 3 1 3 3 3 3 3 4 5% 64%	2800
	21st to end	Day	7% 3 1 3 3 2 4 3 5% 5% 63%	2800
July	1st to 10th	Day	2 2 1 3 2 2 2 2 4 3 78%	6319
	11th to 20th	Day	2 2 1 3 2 2 3 3 3 4 75%	6151
	21st to end	Day	1 1 2 5% 2 3 3 3 4 4 72%	6402
August	1st to 10th	Day	1 4 2 5% 2 3 4 3 5% 6% 65%	5777
	11th to 20th	Day	6% 4 2 4 3 3 4 3 5% 6% 61%	5796
	21st to end	Day	6% 3 1 4 3 2 3 3 5% 6% 63%	6172
September	1st to 10th	Day	3 3 1 3 2 2 3 3 4 6% 70%	2400
	11th to 20th	Day	10% 4 2 5% 2 3 3 3 5% 7% 54%	2280
	21st to end	Day	10% 4 1 3 3 3 3 2 3 6% 62%	2174
October	1st to 10th	Day	8% 2 1 4 2 2 2 2 3 5% 68%	3280
	11th to 20th	Day	21% 7% 2 6% 3 3 3 3 4 7% 43%	3261
	21st to end	Day	25% 6% 2 4 3 3 3 3 5% 7% 39%	3558
November	1st to 10th	Day	30% 8% 2 6% 4 3 4 3 4 7% 30%	1919
	11th to 20th	Day	33% 6% 2 6% 4 3 4 3 6% 6% 27%	1680
	21st to end	Day	25% 7% 2 6% 3 5% 4 3 4 6% 35%	1680
December	1st to 10th	Day	21% 4 1 3 3 3 5% 3 5% 6% 45%	2700
	11th to 20th	Day	45% 8% 3 6% 4 4 3 3 4 4 17%	3108
	21st to end	Day	44% 10% 3 7% 4 4 4 2 3 3 17%	4092
January	1st to 10th	Night	46% 8% 2 5% 4 3 3 3 4 4 19%	8116
	11th to 20th	Night	22% 5% 1 3 2 2 2 2 3 5% 52%	7650
	21st to end	Night	31% 7% 2 6% 3 3 3 3 3 4 35%	8371
February	1st to 10th	Night	28% 9% 2 5% 3 4 3 3 5% 6% 32%	2730
	11th to 20th	Night	28% 9% 2 6% 4 4 3 2 4 5% 32%	2662
	21st to end	Night	30% 7% 2 5% 3 4 4 3 4 6% 32%	2147
March	1st to 10th	Night	25% 19% 3 7% 4 3 3 2 4 6% 24%	5052
	11th to 20th	Night	31% 13% 2 4 3 3 3 3 3 4 32%	5110
	21st to end	Night	37% 13% 3 6% 3 3 3 3 4 5% 20%	5610
April	1st to 10th	Night	27% 9% 2 6% 4 3 4 3 5% 6% 32%	2142
	11th to 20th	Night	22% 8% 2 5% 3 2 3 2 5% 6% 40%	2073
	21st to end	Night	20% 7% 3 5% 4 3 3 3 4 6% 42%	1890
May	1st to 10th	Night	15% 4 2 5% 2 2 3 3 4 5% 56%	4109
	11th to 20th	Night	12% 4 2 3 2 2 2 2 4 5% 61%	3805
	21st to end	Night	6% 4 1 3 3 3 3 2 5% 6% 63%	3972
June	1st to 10th	Night	14% 6% 2 6% 3 4 4 3 5% 6% 48%	1600
	11th to 20th	Night	8% 3 1 1 2 2 2 2 4 6% 70%	1600
	21st to end	Night	6% 3 1 3 2 2 2 2 4 6% 69%	1600
July	1st to 10th	Night	2 3 1 3 1 2 2 2 3 4 78%	3640
	11th to 20th	Night	1 3 1 3 2 2 2 4 4 4 74%	3813
	21st to end	Night	2 5% 3 5% 3 2 2 3 4 4 66%	4472
August	1st to 10th	Night	1 5% 2 5% 3 3 3 4 4 6% 64%	4035
	11th to 20th	Night	6% 6% 2 7% 3 3 3 4 4 5% 56%	4052
	21st to end	Night	5% 4 2 5% 3 3 3 3 5% 6% 60%	4817
September	1st to 10th	Night	4 3 1 2 2 2 2 3 3 4 73%	2000
	11th to 20th	Night	8% 4 2 4 4 3 3 3 4 7% 58%	2116
	21st to end	Night	11% 5% 1 4 2 2 3 3 4 5% 60%	2225
October	1st to 10th	Night	7% 3 1 4 3 2 3 4 5% 4 65%	3920
	11th to 20th	Night	15% 9% 3 5% 3 4 4 3 4 6% 43%	3897
	21st to end	Night	29% 5% 2 5% 4 3 3 3 4 6% 36%	4253
November	1st to 10th	Night	27% 7% 2 5% 3 3 4 4 4 6% 33%	2596
	11th to 20th	Night	33% 8% 2 6% 4 3 3 3 4 5% 29%	2940
	21st to end	Night	26% 5% 2 5% 4 4 4 3 5% 6% 37%	2940
December	1st to 10th	Night	20% 5% 1 4 3 3 3 3 5% 5% 48%	4700
	11th to 20th	Night	43% 8% 3 6% 4 3 4 2 4 5% 18%	5316
	21st to end	Night	44% 9% 2 5% 3 3 3 2 3 4 22%	7106

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	70% 4 8% 11% 6% 1	4765
	11th to 20th	Day	39% 2 3 19% 20% 17%	5340
	21st to end	Day	52% 3 6% 11% 14% 14%	5843
February	1st to 10th	Day	54% 5% 10% 18% 9% 4	1890
	11th to 20th	Day	56% 5% 11% 17% 9% 2	1854
	21st to end	Day	60% 6% 9% 14% 10% 2	1655
March	1st to 10th	Day	56% 5% 8% 22% 8%	5062
	11th to 20th	Day	57% 4 11% 14% 12% 2	5110
	21st to end	Day	63% 7% 11% 14% 4	5607
April	1st to 10th	Day	58% 7% 11% 15% 8% 1	2478
	11th to 20th	Day	50% 7% 12% 16% 12% 3	2495
	21st to end	Day	43% 7% 15% 20% 13% 2	2730
May	1st to 10th	Day	34% 6% 9% 20% 25% 6%	5894
	11th to 20th	Day	31% 3 17% 20% 25% 4	6098
	21st to end	Day	25% 5% 24% 38% 8%	6904
June	1st to 10th	Day	38% 7% 13% 23% 16% 2	2800
	11th to 20th	Day	23% 7% 12% 37% 17% 4	2800
	21st to end	Day	20% 4 12% 34% 23% 7%	2800
July	1st to 10th	Day	10% 2 5% 31% 19% 33%	6251
	11th to 20th	Day	11% 2 6% 13% 29% 38%	6076
	21st to end	Day	12% 3 12% 35% 27% 11%	6217
August	1st to 10th	Day	19% 3 9% 29% 34% 5%	4206
	11th to 20th	Day	22% 3 14% 32% 21% 9%	4195
	21st to end	Day	18% 2 10% 34% 27% 8%	4451
September	1st to 10th	Day	16% 3 9% 26% 28% 18%	2400
	11th to 20th	Day	28% 4 16% 29% 16% 6%	2280
	21st to end	Day	24% 5% 12% 25% 22% 12%	2174
October	1st to 10th	Day	20% 3 10% 20% 23% 23%	3280
	11th to 20th	Day	42% 3 16% 21% 15% 3	3261
	21st to end	Day	46% 3 10% 19% 15% 6%	3558
November	1st to 10th	Day	56% 8% 10% 13% 10% 3	1919
	11th to 20th	Day	58% 7% 12% 11% 7% 6%	1680
	21st to end	Day	51% 6% 8% 12% 14% 9%	1680
December	1st to 10th	Day	40% 3 8% 20% 20% 9%	2700
	11th to 20th	Day	72% 4 10% 8% 5% 2	3108
	21st to end	Day	75% 3 5% 10% 5% 2	4092
January	1st to 10th	Night	71% 3 9% 10% 5% 1	8116
	11th to 20th	Night	38% 3 4 17% 22% 16%	7650
	21st to end	Night	54% 3 4 10% 16% 13%	8371
February	1st to 10th	Night	54% 5% 11% 16% 11% 3	2730
	11th to 20th	Night	56% 5% 8% 17% 11% 3	2664
	21st to end	Night	55% 7% 11% 14% 10% 3	2147
March	1st to 10th	Night	62% 4 8% 19% 7%	5052
	11th to 20th	Night	57% 3 12% 13% 10% 5%	5110
	21st to end	Night	67% 6% 10% 14% 3	5610
April	1st to 10th	Night	54% 7% 12% 19% 8% 1	2142
	11th to 20th	Night	45% 7% 15% 18% 11% 3	2073
	21st to end	Night	43% 8% 14% 23% 10% 2	1890
May	1st to 10th	Night	30% 6% 11% 22% 25% 6%	4106
	11th to 20th	Night	28% 3 12% 27% 28% 2	3805
	21st to end	Night	24% 5% 28% 35% 7% 1	3972
June	1st to 10th	Night	38% 5% 15% 25% 14% 2	1600
	11th to 20th	Night	19% 4 12% 42% 19% 4	1600
	21st to end	Night	16% 3 13% 36% 24% 7%	1600
July	1st to 10th	Night	13% 1 6% 25% 23% 32%	3600
	11th to 20th	Night	11% 2 5% 12% 38% 32%	3765
	21st to end	Night	21% 2 13% 26% 27% 12%	4345
August	1st to 10th	Night	19% 4 7% 28% 34% 7%	2929
	11th to 20th	Night	25% 4 10% 30% 24% 8%	2929
	21st to end	Night	18% 2 7% 32% 31% 10%	3462
September	1st to 10th	Night	14% 3 7% 24% 31% 22%	2000
	11th to 20th	Night	24% 5% 14% 31% 19% 7%	2116
	21st to end	Night	26% 4 12% 23% 21% 16%	2225
October	1st to 10th	Night	20% 3 10% 21% 18% 27%	3920
	11th to 20th	Night	42% 3 15% 23% 13% 5%	3897
	21st to end	Night	52% 3 9% 17% 14% 6%	4253
November	1st to 10th	Night	52% 6% 11% 16% 10% 5%	2596
	11th to 20th	Night	59% 7% 8% 14% 8% 5%	2940
	21st to end	Night	49% 6% 10% 13% 15% 8%	2940
December	1st to 10th	Night	39% 3 7% 20% 21% 9%	4700
	11th to 20th	Night	69% 3 9% 10% 7% 2	5316
	21st to end	Night	68% 4 8% 15% 4 2	7106

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	53% 7% 19% 15% 6%	236
	11th to 20th	Day	37% 9% 17% 21% 13% 3	430
	21st to end	Day	35% 10% 22% 20% 11% 2	473
February	1st to 10th	Day	30% 12% 27% 24% 7%	430
	11th to 20th	Day	39% 13% 27% 15% 5% 1	425
	21st to end	Day	34% 14% 28% 18% 5% 1	386
March	1st to 10th	Day	37% 19% 24% 15% 6%	540
	11th to 20th	Day	33% 12% 25% 20% 9% 1	540
	21st to end	Day	37% 15% 26% 17% 5%	594
April	1st to 10th	Day	31% 17% 24% 21% 6% 1	540
	11th to 20th	Day	26% 17% 27% 22% 7% 2	532
	21st to end	Day	21% 15% 31% 23% 8% 2	540
May	1st to 10th	Day	16% 20% 30% 23% 9% 2	508
	11th to 20th	Day	22% 12% 28% 23% 11% 3	528
	21st to end	Day	15% 14% 30% 29% 10% 3	573
June	1st to 10th	Day	15% 11% 28% 32% 13% 2	530
	11th to 20th	Day	11% 14% 38% 27% 8% 1	530
	21st to end	Day	5% 4 32% 41% 16% 2	530
July	1st to 10th	Day	6% 5% 27% 44% 14% 3	526
	11th to 20th	Day	2 2 31% 44% 16% 4	524
	21st to end	Day	1 2 21% 49% 21% 6%	575
August	1st to 10th	Day	3 2 20% 48% 23% 5%	527
	11th to 20th	Day	5% 3 20% 46% 21% 4	517
	21st to end	Day	6% 3 26% 40% 20% 5%	583
September	1st to 10th	Day	3 4 24% 40% 22% 5%	530
	11th to 20th	Day	14% 5% 26% 34% 19% 3	530
	21st to end	Day	13% 8% 26% 33% 18% 3	530
October	1st to 10th	Day	14% 9% 26% 28% 19% 4	530
	11th to 20th	Day	20% 7% 23% 27% 17% 5%	530
	21st to end	Day	25% 12% 23% 25% 13% 3	583
November	1st to 10th	Day	29% 11% 24% 22% 11% 3	437
	11th to 20th	Day	36% 12% 16% 20% 15% 2	220
	21st to end	Day	30% 7% 18% 30% 15% 1	220
December	1st to 10th	Day	31% 9% 19% 22% 13% 5%	220
	11th to 20th	Day	44% 5% 17% 25% 5% 3	220
	21st to end	Day	45% 9% 19% 17% 8% 2	242
January	1st to 10th	Night	46% 6% 18% 20% 8% 2	840
	11th to 20th	Night	34% 6% 19% 23% 15% 2	840
	21st to end	Night	35% 5% 17% 26% 16% 2	924
February	1st to 10th	Night	32% 7% 20% 27% 13%	840
	11th to 20th	Night	33% 8% 20% 26% 11% 2	824
	21st to end	Night	34% 9% 17% 29% 10%	692
March	1st to 10th	Night	37% 11% 19% 21% 10% 2	840
	11th to 20th	Night	35% 6% 15% 27% 15% 2	840
	21st to end	Night	37% 10% 20% 22% 11% 1	924
April	1st to 10th	Night	30% 11% 18% 24% 16% 2	840
	11th to 20th	Night	26% 10% 21% 25% 16% 3	825
	21st to end	Night	24% 5% 20% 30% 18% 2	630
May	1st to 10th	Night	17% 9% 24% 28% 20% 3	603
	11th to 20th	Night	17% 4 17% 28% 29% 5%	479
	21st to end	Night	16% 7% 17% 33% 23% 3	433
June	1st to 10th	Night	16% 4 23% 30% 24% 3	400
	11th to 20th	Night	9% 3 28% 39% 18% 3	400
	21st to end	Night	7% 3 14% 38% 36% 3	400
July	1st to 10th	Night	4 13% 37% 38% 8%	390
	11th to 20th	Night	5% 1 15% 39% 35% 6%	457
	21st to end	Night	2 1 8% 33% 44% 11%	657
August	1st to 10th	Night	3 1 7% 28% 49% 13%	597
	11th to 20th	Night	4 1 7% 34% 43% 11%	594
	21st to end	Night	5% 1 16% 32% 37% 9%	786
September	1st to 10th	Night	5% 1 13% 37% 39% 5%	800
	11th to 20th	Night	9% 4 11% 30% 35% 11%	800
	21st to end	Night	13% 4 15% 32% 29% 7%	800
October	1st to 10th	Night	13% 6% 20% 30% 28% 4	800
	11th to 20th	Night	20% 4 17% 32% 22% 6%	800
	21st to end	Night	24% 7% 18% 29% 18% 4	880
November	1st to 10th	Night	30% 6% 16% 29% 16% 3	820
	11th to 20th	Night	37% 6% 18% 26% 12% 1	840
	21st to end	Night	28% 7% 20% 31% 13% 1	840
December	1st to 10th	Night	31% 6% 21% 27% 13% 2	840
	11th to 20th	Night	42% 6% 17% 24% 10% 1	840
	21st to end	Night	45% 8% 19% 20% 7%	924

Page generated at 2026-04-28 23:21:44. Version ac07

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