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 2024-04-03.

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	12% 7% 3 7% 6% 3 5% 5% 5% 7% 39%	78693
	Night	10% 8% 3 7% 6% 3 5% 5% 5% 7% 41%	81476
Transparency	Day	43% 7% 14% 24% 12% 1	78394
	Night	41% 6% 14% 24% 13% 2	81295
Seeing	Day	15% 10% 31% 29% 13% 2	15039
	Night	13% 9% 35% 33% 10% 1	25165

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	12% 11% 4 9% 13% 6% 7% 7% 6% 6% 20%	2347
	11th to 20th	Day	5% 10% 6% 14% 8% 4 7% 8% 9% 9% 21%	2543
	21st to end	Day	5% 10% 10% 17% 8% 5% 6% 5% 5% 6% 23%	3135
February	1st to 10th	Day	8% 9% 5% 10% 10% 6% 6% 6% 6% 7% 27%	1710
	11th to 20th	Day	7% 10% 5% 10% 7% 6% 6% 6% 5% 6% 31%	1668
	21st to end	Day	9% 10% 4 10% 7% 5% 6% 5% 5% 7% 32%	1570
March	1st to 10th	Day	13% 11% 6% 10% 6% 3 4 4 4 7% 33%	2000
	11th to 20th	Day	13% 9% 5% 8% 7% 4 4 4 5% 6% 35%	2090
	21st to end	Day	17% 12% 5% 9% 6% 4 5% 4 5% 6% 26%	2299
April	1st to 10th	Day	17% 7% 3 6% 5% 3 5% 4 5% 7% 38%	2081
	11th to 20th	Day	19% 10% 4 7% 5% 3 4 4 4 6% 34%	2312
	21st to end	Day	20% 9% 3 5% 4 3 4 4 4 7% 37%	2340
May	1st to 10th	Day	18% 8% 3 5% 6% 4 5% 5% 6% 7% 35%	2340
	11th to 20th	Day	17% 8% 4 6% 6% 3 5% 4 5% 8% 36%	2333
	21st to end	Day	18% 6% 3 6% 5% 3 4 4 5% 8% 38%	2725
June	1st to 10th	Day	11% 5% 2 4 5% 3 5% 5% 5% 10% 45%	2681
	11th to 20th	Day	14% 5% 2 4 5% 3 5% 5% 5% 8% 44%	2700
	21st to end	Day	9% 4 2 4 5% 3 4 5% 7% 9% 47%	2700
July	1st to 10th	Day	6% 3 1 3 4 3 5% 4 7% 11% 53%	2700
	11th to 20th	Day	6% 3 1 3 3 2 4 4 6% 9% 59%	2577
	21st to end	Day	6% 3 2 4 4 3 5% 6% 7% 10% 51%	2569
August	1st to 10th	Day	9% 6% 2 5% 6% 4 6% 6% 8% 9% 41%	2328
	11th to 20th	Day	11% 5% 2 5% 3 3 4 5% 6% 9% 47%	2301
	21st to end	Day	7% 3 1 4 4 2 4 5% 5% 8% 56%	2574
September	1st to 10th	Day	13% 5% 2 4 4 2 4 4 5% 6% 52%	2117
	11th to 20th	Day	11% 6% 2 4 4 2 3 5% 5% 7% 51%	1980
	21st to end	Day	13% 6% 2 4 4 3 4 5% 4 5% 51%	1980
October	1st to 10th	Day	16% 7% 2 5% 4 2 3 3 4 6% 45%	1852
	11th to 20th	Day	15% 6% 3 3 4 3 3 3 5% 7% 47%	1800
	21st to end	Day	16% 9% 3 6% 4 3 4 3 4 6% 43%	1882
November	1st to 10th	Day	12% 8% 3 5% 5% 3 4 4 5% 7% 44%	1673
	11th to 20th	Day	8% 10% 4 8% 7% 4 6% 5% 5% 7% 36%	1537
	21st to end	Day	12% 10% 3 6% 5% 3 5% 5% 5% 9% 37%	1520
December	1st to 10th	Day	8% 8% 3 10% 9% 4 8% 6% 5% 7% 30%	1520
	11th to 20th	Day	9% 11% 5% 11% 9% 5% 4 5% 5% 8% 28%	1767
	21st to end	Day	18% 9% 4 10% 9% 3 4 5% 4 6% 25%	2442
January	1st to 10th	Night	8% 11% 5% 11% 10% 6% 8% 7% 7% 6% 22%	4605
	11th to 20th	Night	4 8% 5% 10% 10% 6% 9% 8% 9% 9% 23%	4425
	21st to end	Night	5% 10% 7% 17% 7% 4 6% 5% 5% 6% 28%	4532
February	1st to 10th	Night	6% 10% 5% 8% 8% 5% 6% 6% 5% 7% 34%	2470
	11th to 20th	Night	7% 8% 4 9% 7% 5% 5% 6% 6% 6% 38%	2403
	21st to end	Night	9% 9% 3 8% 7% 4 5% 5% 6% 7% 37%	1898
March	1st to 10th	Night	10% 8% 4 7% 6% 3 6% 5% 5% 8% 39%	2180
	11th to 20th	Night	11% 10% 4 8% 5% 3 4 4 6% 5% 39%	2090
	21st to end	Night	15% 10% 4 7% 5% 4 4 4 5% 7% 35%	2299
April	1st to 10th	Night	14% 9% 3 6% 5% 3 5% 5% 4 7% 39%	1918
	11th to 20th	Night	17% 9% 4 7% 5% 2 4 4 4 6% 39%	1599
	21st to end	Night	15% 9% 4 7% 4 3 4 5% 4 6% 38%	1620
May	1st to 10th	Night	15% 6% 2 4 4 2 5% 4 4 8% 45%	1620
	11th to 20th	Night	14% 6% 2 5% 3 3 4 4 5% 6% 48%	1558
	21st to end	Night	14% 6% 2 5% 5% 4 5% 4 6% 9% 41%	1551
June	1st to 10th	Night	9% 4 1 3 4 3 4 6% 6% 9% 52%	1276
	11th to 20th	Night	6% 5% 2 5% 4 3 4 4 6% 9% 52%	1260
	21st to end	Night	7% 3 1 3 3 2 3 4 6% 8% 60%	1260
July	1st to 10th	Night	5% 4 2 3 3 2 3 4 5% 9% 60%	1260
	11th to 20th	Night	3 2 1 2 3 2 4 4 6% 9% 64%	1379
	21st to end	Night	4 3 2 3 4 3 4 5% 5% 10% 59%	1746
August	1st to 10th	Night	6% 4 2 5% 4 3 5% 5% 7% 9% 50%	1611
	11th to 20th	Night	9% 3 1 3 3 2 3 5% 5% 8% 59%	1600
	21st to end	Night	5% 2 1 3 3 1 3 4 4 6% 66%	1782
September	1st to 10th	Night	8% 5% 2 3 3 2 4 4 5% 7% 57%	1839
	11th to 20th	Night	9% 5% 2 4 4 3 3 3 4 7% 56%	1980
	21st to end	Night	11% 5% 2 4 4 2 3 4 4 5% 56%	1980
October	1st to 10th	Night	15% 6% 3 4 3 2 3 4 5% 6% 50%	2089
	11th to 20th	Night	14% 8% 2 5% 4 3 4 3 4 6% 48%	2160
	21st to end	Night	14% 9% 3 6% 3 3 3 3 4 7% 45%	2470
November	1st to 10th	Night	9% 6% 3 6% 6% 3 4 4 5% 7% 47%	2405
	11th to 20th	Night	8% 9% 4 9% 6% 3 5% 4 5% 6% 41%	2641
	21st to end	Night	10% 7% 3 7% 5% 2 4 5% 5% 7% 44%	2660
December	1st to 10th	Night	7% 10% 3 11% 8% 5% 6% 5% 6% 7% 32%	2660
	11th to 20th	Night	10% 9% 4 8% 7% 3 5% 5% 6% 8% 33%	3425
	21st to end	Night	18% 12% 5% 12% 6% 4 4 4 5% 5% 25%	5225

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	61% 6% 11% 15% 5% 1	2348
	11th to 20th	Day	56% 4 10% 23% 7%	2543
	21st to end	Day	60% 6% 12% 15% 7%	3135
February	1st to 10th	Day	55% 6% 14% 16% 9%	1710
	11th to 20th	Day	51% 7% 12% 19% 9% 2	1677
	21st to end	Day	51% 6% 15% 17% 10% 2	1570
March	1st to 10th	Day	50% 6% 14% 22% 7%	2000
	11th to 20th	Day	49% 6% 12% 24% 9% 1	2090
	21st to end	Day	57% 7% 14% 16% 5% 1	2299
April	1st to 10th	Day	45% 7% 14% 22% 11% 1	2081
	11th to 20th	Day	50% 5% 10% 22% 11% 1	2312
	21st to end	Day	48% 6% 12% 25% 8%	2340
May	1st to 10th	Day	48% 8% 11% 21% 11%	2340
	11th to 20th	Day	47% 7% 14% 21% 11%	2337
	21st to end	Day	44% 7% 14% 23% 11% 1	2725
June	1st to 10th	Day	35% 8% 17% 29% 11% 1	2681
	11th to 20th	Day	37% 8% 15% 25% 13% 2	2700
	21st to end	Day	31% 9% 17% 28% 14% 2	2700
July	1st to 10th	Day	24% 9% 19% 32% 16% 1	2626
	11th to 20th	Day	22% 7% 15% 29% 22% 4	2498
	21st to end	Day	26% 9% 18% 27% 18% 2	2426
August	1st to 10th	Day	36% 9% 16% 23% 13% 3	2314
	11th to 20th	Day	32% 9% 16% 30% 11% 2	2301
	21st to end	Day	25% 7% 14% 32% 18% 4	2574
September	1st to 10th	Day	33% 6% 12% 31% 17% 1	2117
	11th to 20th	Day	32% 7% 17% 29% 14% 1	1980
	21st to end	Day	35% 5% 11% 30% 17% 1	1980
October	1st to 10th	Day	41% 4 13% 26% 15% 1	1852
	11th to 20th	Day	35% 6% 13% 27% 17% 2	1800
	21st to end	Day	43% 5% 13% 24% 14% 1	1882
November	1st to 10th	Day	39% 6% 14% 24% 13% 3	1673
	11th to 20th	Day	47% 7% 13% 21% 11% 1	1537
	21st to end	Day	42% 6% 15% 23% 12% 1	1520
December	1st to 10th	Day	51% 7% 16% 16% 8% 2	1520
	11th to 20th	Day	54% 6% 16% 18% 5%	1764
	21st to end	Day	57% 6% 11% 17% 7% 1	2442
January	1st to 10th	Night	58% 7% 11% 17% 7%	4605
	11th to 20th	Night	53% 6% 13% 19% 9% 1	4425
	21st to end	Night	54% 8% 13% 16% 9% 1	4532
February	1st to 10th	Night	48% 6% 11% 22% 11% 2	2470
	11th to 20th	Night	43% 7% 12% 23% 12% 3	2404
	21st to end	Night	44% 6% 13% 20% 12% 4	1898
March	1st to 10th	Night	42% 8% 16% 22% 10% 1	2180
	11th to 20th	Night	45% 4 11% 25% 13% 1	2090
	21st to end	Night	49% 7% 15% 19% 8% 1	2299
April	1st to 10th	Night	43% 8% 12% 25% 11% 1	1918
	11th to 20th	Night	45% 5% 11% 23% 14% 2	1599
	21st to end	Night	45% 6% 11% 26% 11% 1	1620
May	1st to 10th	Night	37% 7% 15% 25% 15% 1	1620
	11th to 20th	Night	37% 7% 15% 23% 17% 1	1559
	21st to end	Night	40% 8% 15% 22% 13% 2	1552
June	1st to 10th	Night	28% 7% 16% 31% 16% 2	1276
	11th to 20th	Night	28% 7% 18% 26% 17% 4	1260
	21st to end	Night	22% 7% 17% 31% 21% 3	1260
July	1st to 10th	Night	22% 6% 16% 35% 19% 3	1225
	11th to 20th	Night	16% 7% 14% 34% 23% 6%	1335
	21st to end	Night	20% 8% 17% 30% 21% 4	1649
August	1st to 10th	Night	29% 9% 19% 26% 15% 3	1601
	11th to 20th	Night	23% 8% 14% 38% 16% 1	1600
	21st to end	Night	20% 5% 16% 32% 22% 5%	1782
September	1st to 10th	Night	26% 7% 14% 28% 21% 3	1839
	11th to 20th	Night	29% 6% 13% 32% 18% 1	1980
	21st to end	Night	30% 5% 10% 31% 22% 1	1980
October	1st to 10th	Night	35% 5% 13% 27% 19% 1	2089
	11th to 20th	Night	37% 5% 11% 27% 17% 3	2160
	21st to end	Night	40% 5% 14% 25% 16% 1	2470
November	1st to 10th	Night	37% 5% 13% 23% 19% 4	2405
	11th to 20th	Night	41% 6% 14% 22% 14% 2	2641
	21st to end	Night	39% 6% 17% 25% 14% 1	2660
December	1st to 10th	Night	48% 7% 17% 19% 8%	2660
	11th to 20th	Night	46% 7% 21% 19% 6%	3427
	21st to end	Night	59% 5% 10% 18% 7% 1	5225

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	17% 8% 29% 32% 12% 1	270
	11th to 20th	Day	14% 12% 36% 29% 7% 2	270
	21st to end	Day	18% 11% 35% 24% 12% 1	297
February	1st to 10th	Day	12% 12% 44% 28% 3	270
	11th to 20th	Day	12% 15% 38% 27% 8%	270
	21st to end	Day	15% 13% 40% 28% 4	225
March	1st to 10th	Day	19% 15% 41% 23% 2 1	270
	11th to 20th	Day	17% 10% 41% 21% 10% 1	270
	21st to end	Day	27% 12% 25% 24% 11% 1	297
April	1st to 10th	Day	26% 14% 34% 20% 5%	325
	11th to 20th	Day	27% 12% 29% 23% 7% 1	590
	21st to end	Day	22% 15% 30% 22% 10% 2	600
May	1st to 10th	Day	20% 12% 31% 27% 8% 1	603
	11th to 20th	Day	21% 12% 30% 24% 10% 3	625
	21st to end	Day	19% 9% 28% 31% 11% 2	679
June	1st to 10th	Day	9% 8% 25% 34% 19% 5%	630
	11th to 20th	Day	13% 9% 33% 30% 13% 3	630
	21st to end	Day	9% 8% 31% 34% 16% 2	630
July	1st to 10th	Day	6% 3 24% 38% 22% 6%	621
	11th to 20th	Day	7% 4 34% 33% 17% 4	618
	21st to end	Day	6% 4 25% 39% 21% 5%	684
August	1st to 10th	Day	9% 3 24% 37% 23% 4	626
	11th to 20th	Day	12% 5% 24% 33% 22% 4	615
	21st to end	Day	7% 5% 29% 34% 21% 4	693
September	1st to 10th	Day	13% 9% 29% 30% 15% 3	407
	11th to 20th	Day	11% 7% 33% 31% 15% 2	270
	21st to end	Day	19% 14% 30% 24% 10% 3	270
October	1st to 10th	Day	21% 9% 30% 26% 10% 4	270
	11th to 20th	Day	15% 19% 33% 23% 8% 3	270
	21st to end	Day	21% 18% 39% 14% 7% 1	297
November	1st to 10th	Day	22% 10% 34% 23% 10% 2	270
	11th to 20th	Day	12% 15% 43% 22% 7% 1	270
	21st to end	Day	19% 19% 29% 25% 7% 1	270
December	1st to 10th	Day	12% 13% 40% 25% 9% 2	270
	11th to 20th	Day	12% 14% 43% 26% 4	270
	21st to end	Day	26% 9% 37% 22% 6% 1	297
January	1st to 10th	Night	11% 10% 41% 29% 8% 1	950
	11th to 20th	Night	11% 10% 39% 32% 7%	950
	21st to end	Night	16% 10% 39% 28% 7%	1045
February	1st to 10th	Night	11% 10% 43% 30% 6% 1	950
	11th to 20th	Night	13% 10% 40% 30% 7% 1	929
	21st to end	Night	13% 11% 35% 32% 7% 2	642
March	1st to 10th	Night	13% 10% 42% 29% 7%	660
	11th to 20th	Night	18% 10% 35% 31% 5%	570
	21st to end	Night	19% 10% 33% 29% 10% 1	627
April	1st to 10th	Night	18% 11% 38% 28% 5%	546
	11th to 20th	Night	21% 12% 30% 27% 9% 1	533
	21st to end	Night	19% 10% 35% 27% 8% 1	540
May	1st to 10th	Night	16% 10% 34% 31% 9% 1	540
	11th to 20th	Night	17% 7% 36% 30% 9% 1	531
	21st to end	Night	16% 5% 35% 33% 11% 1	584
June	1st to 10th	Night	10% 5% 26% 41% 18% 1	540
	11th to 20th	Night	8% 5% 36% 41% 10%	540
	21st to end	Night	8% 3 26% 47% 15% 1	540
July	1st to 10th	Night	6% 3 23% 46% 21% 2	525
	11th to 20th	Night	5% 2 30% 46% 17%	522
	21st to end	Night	4 2 25% 50% 18% 1	591
August	1st to 10th	Night	6% 5% 21% 50% 17% 1	537
	11th to 20th	Night	9% 3 23% 41% 22% 2	534
	21st to end	Night	6% 3 29% 41% 19% 2	594
September	1st to 10th	Night	9% 4 31% 39% 14% 2	540
	11th to 20th	Night	9% 6% 32% 38% 13% 2	540
	21st to end	Night	13% 8% 35% 33% 10% 1	540
October	1st to 10th	Night	19% 8% 34% 30% 9% 1	649
	11th to 20th	Night	18% 12% 34% 27% 8%	720
	21st to end	Night	18% 12% 33% 29% 8%	886
November	1st to 10th	Night	12% 9% 39% 30% 8% 1	925
	11th to 20th	Night	13% 13% 38% 27% 9% 1	950
	21st to end	Night	12% 12% 38% 31% 7%	950
December	1st to 10th	Night	13% 12% 39% 29% 7% 1	950
	11th to 20th	Night	16% 9% 38% 29% 7% 1	950
	21st to end	Night	22% 11% 35% 26% 5% 1	1045

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