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 2024-07-31.

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	19% 6% 2 4 3 3 3 3 4 5% 46%	90088
	Night	24% 7% 2 4 3 3 3 3 4 5% 42%	89876
Transparency	Day	40% 6% 11% 21% 17% 6%	89803
	Night	45% 5% 10% 19% 15% 6%	89682
Seeing	Day	20% 10% 25% 29% 13% 2	14698
	Night	24% 6% 17% 28% 21% 4	23708

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	37% 10% 3 7% 4 4 3 3 3 5% 21%	2553
	11th to 20th	Day	44% 9% 2 5% 3 2 3 1 3 3 23%	2860
	21st to end	Day	42% 7% 2 5% 3 3 4 3 3 5% 23%	3146
February	1st to 10th	Day	29% 8% 2 5% 4 4 4 3 5% 6% 31%	1710
	11th to 20th	Day	28% 8% 2 6% 4 3 3 3 4 5% 34%	1665
	21st to end	Day	30% 9% 3 6% 4 4 3 2 4 4 29%	1501
March	1st to 10th	Day	28% 22% 2 5% 3 2 2 3 3 4 26%	3442
	11th to 20th	Day	23% 9% 2 5% 3 3 5% 2 3 4 42%	3490
	21st to end	Day	32% 11% 2 6% 4 4 4 4 4 5% 23%	3825
April	1st to 10th	Day	28% 9% 2 6% 4 5% 4 3 5% 6% 28%	2242
	11th to 20th	Day	24% 9% 2 5% 4 4 3 3 5% 6% 35%	2252
	21st to end	Day	18% 6% 2 7% 5% 3 4 4 6% 7% 38%	2470
May	1st to 10th	Day	17% 6% 2 5% 3 3 3 3 4 5% 49%	4110
	11th to 20th	Day	8% 4 1 3 2 2 2 3 4 6% 64%	4247
	21st to end	Day	9% 4 1 3 2 3 3 3 5% 7% 59%	4813
June	1st to 10th	Day	17% 6% 2 5% 4 4 4 3 5% 7% 44%	2660
	11th to 20th	Day	8% 3 1 3 3 3 3 3 4 5% 64%	2660
	21st to end	Day	8% 3 1 3 3 2 4 3 5% 5% 64%	2660
July	1st to 10th	Day	3 3 1 3 2 3 3 2 4 4 71%	2660
	11th to 20th	Day	5% 2 1 2 2 2 3 3 3 4 73%	2591
	21st to end	Day	2 1 1 2 2 3 5% 3 4 5% 74%	2679
August	1st to 10th	Day	2 3 1 3 3 3 4 3 4 7% 67%	2328
	11th to 20th	Day	3 2 1 2 1 2 4 2 4 6% 73%	2298
	21st to end	Day	4 3 1 3 2 3 3 3 4 5% 68%	2460
September	1st to 10th	Day	3 3 1 3 2 2 3 3 4 5% 73%	2160
	11th to 20th	Day	11% 4 2 5% 2 3 4 3 5% 7% 53%	2052
	21st to end	Day	10% 4 1 3 3 3 3 2 3 6% 61%	1958
October	1st to 10th	Day	13% 3 1 4 2 3 2 3 4 6% 59%	1800
	11th to 20th	Day	21% 5% 1 4 2 3 3 3 5% 6% 48%	1800
	21st to end	Day	22% 6% 2 5% 3 2 3 3 4 5% 46%	1980
November	1st to 10th	Day	29% 8% 2 6% 4 4 4 3 4 7% 30%	1734
	11th to 20th	Day	31% 6% 2 6% 4 4 4 3 6% 6% 29%	1520
	21st to end	Day	25% 7% 2 6% 3 5% 4 3 4 6% 34%	1520
December	1st to 10th	Day	27% 6% 2 4 3 4 4 3 4 6% 38%	1520
	11th to 20th	Day	38% 8% 2 5% 3 3 4 3 4 5% 24%	1928
	21st to end	Day	39% 10% 2 7% 4 3 4 1 3 3 23%	2794
January	1st to 10th	Night	40% 10% 3 6% 5% 4 4 3 4 4 18%	4376
	11th to 20th	Night	41% 9% 2 5% 4 3 3 2 3 4 24%	4110
	21st to end	Night	48% 8% 2 4 3 3 2 2 3 4 21%	4521
February	1st to 10th	Night	28% 9% 2 5% 3 4 3 3 5% 6% 32%	2470
	11th to 20th	Night	28% 9% 2 5% 4 4 3 2 4 5% 34%	2402
	21st to end	Night	28% 7% 2 5% 3 4 4 3 5% 6% 33%	1949
March	1st to 10th	Night	32% 22% 2 5% 3 2 2 2 3 4 24%	3432
	11th to 20th	Night	23% 13% 2 4 3 2 2 3 3 4 42%	3490
	21st to end	Night	34% 15% 2 6% 4 4 3 2 4 5% 22%	3828
April	1st to 10th	Night	27% 9% 2 6% 4 3 3 3 5% 6% 33%	1938
	11th to 20th	Night	22% 9% 2 5% 3 2 3 2 5% 7% 39%	1874
	21st to end	Night	21% 7% 3 5% 4 3 3 3 4 7% 40%	1710
May	1st to 10th	Night	19% 5% 1 4 2 2 3 2 3 4 54%	2860
	11th to 20th	Night	8% 4 1 3 2 2 2 2 4 4 67%	2649
	21st to end	Night	8% 5% 1 3 2 3 2 2 4 6% 63%	2762
June	1st to 10th	Night	15% 6% 2 6% 3 4 4 3 5% 6% 46%	1520
	11th to 20th	Night	8% 3 1 1 2 2 2 2 4 5% 70%	1520
	21st to end	Night	6% 3 1 3 2 2 2 2 4 6% 69%	1520
July	1st to 10th	Night	4 2 1 3 1 1 2 1 3 5% 76%	1520
	11th to 20th	Night	3 2 1 3 2 2 3 3 3 3 75%	1585
	21st to end	Night	2 2 1 2 2 1 3 3 3 4 78%	1872
August	1st to 10th	Night	2 3 1 3 2 2 3 3 5% 5% 71%	1611
	11th to 20th	Night	2 1 3 2 2 2 2 4 4 77%	1602
	21st to end	Night	5% 1 1 3 2 3 3 2 4 4 72%	1894
September	1st to 10th	Night	4 3 1 2 2 2 1 3 3 4 76%	1800
	11th to 20th	Night	8% 4 2 4 4 3 3 3 5% 7% 58%	1904
	21st to end	Night	12% 4 1 4 2 2 3 3 4 6% 58%	2002
October	1st to 10th	Night	12% 4 1 4 3 2 4 5% 6% 5% 56%	2160
	11th to 20th	Night	19% 5% 2 4 2 3 4 4 4 5% 48%	2160
	21st to end	Night	22% 6% 2 4 3 3 3 3 3 5% 46%	2376
November	1st to 10th	Night	27% 7% 2 6% 4 4 4 4 5% 6% 33%	2341
	11th to 20th	Night	32% 8% 2 6% 4 3 3 3 4 6% 29%	2660
	21st to end	Night	25% 5% 2 5% 4 4 4 3 5% 7% 37%	2660
December	1st to 10th	Night	30% 6% 1 4 3 3 3 2 5% 5% 36%	2660
	11th to 20th	Night	38% 8% 2 5% 4 3 3 2 4 6% 24%	3276
	21st to end	Night	38% 9% 2 5% 3 3 3 2 3 4 27%	4862

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	68% 6% 10% 10% 4 2	2553
	11th to 20th	Day	69% 4 4 11% 6% 6%	2860
	21st to end	Day	65% 5% 8% 9% 8% 4	3146
February	1st to 10th	Day	54% 5% 10% 18% 9% 3	1710
	11th to 20th	Day	54% 5% 12% 17% 10% 2	1674
	21st to end	Day	59% 6% 10% 14% 9% 2	1501
March	1st to 10th	Day	62% 8% 6% 18% 6%	3442
	11th to 20th	Day	45% 5% 15% 18% 16% 3	3490
	21st to end	Day	63% 9% 11% 11% 5%	3825
April	1st to 10th	Day	58% 8% 12% 14% 8% 1	2242
	11th to 20th	Day	52% 8% 12% 15% 10% 3	2255
	21st to end	Day	45% 8% 15% 18% 11% 3	2470
May	1st to 10th	Day	37% 8% 11% 21% 18% 4	4110
	11th to 20th	Day	23% 4 10% 21% 36% 6%	4250
	21st to end	Day	25% 7% 23% 35% 9%	4814
June	1st to 10th	Day	40% 7% 13% 22% 16% 2	2660
	11th to 20th	Day	23% 7% 12% 37% 16% 5%	2660
	21st to end	Day	20% 4 11% 35% 23% 8%	2660
July	1st to 10th	Day	14% 4 11% 27% 25% 19%	2591
	11th to 20th	Day	14% 3 8% 21% 32% 23%	2516
	21st to end	Day	9% 2 6% 28% 40% 14%	2538
August	1st to 10th	Day	14% 4 15% 35% 25% 7%	2312
	11th to 20th	Day	11% 3 12% 30% 33% 11%	2298
	21st to end	Day	15% 4 11% 28% 33% 9%	2460
September	1st to 10th	Day	13% 3 8% 26% 29% 19%	2160
	11th to 20th	Day	28% 5% 17% 28% 16% 6%	2052
	21st to end	Day	25% 5% 13% 25% 20% 12%	1958
October	1st to 10th	Day	28% 6% 11% 23% 23% 10%	1800
	11th to 20th	Day	37% 5% 11% 20% 21% 6%	1800
	21st to end	Day	42% 5% 10% 18% 16% 10%	1980
November	1st to 10th	Day	55% 9% 10% 12% 10% 4	1734
	11th to 20th	Day	55% 8% 13% 11% 7% 6%	1520
	21st to end	Day	52% 6% 8% 12% 13% 9%	1520
December	1st to 10th	Day	48% 5% 12% 16% 15% 5%	1520
	11th to 20th	Day	63% 6% 13% 9% 7% 2	1928
	21st to end	Day	68% 4 6% 13% 7% 2	2794
January	1st to 10th	Night	71% 6% 8% 8% 4 2	4376
	11th to 20th	Night	66% 5% 6% 10% 6% 6%	4110
	21st to end	Night	68% 4 5% 9% 9% 4	4521
February	1st to 10th	Night	53% 6% 12% 16% 10% 3	2470
	11th to 20th	Night	54% 5% 8% 18% 12% 2	2404
	21st to end	Night	53% 8% 12% 15% 10% 3	1949
March	1st to 10th	Night	65% 5% 6% 17% 6%	3432
	11th to 20th	Night	46% 4 13% 15% 14% 7%	3490
	21st to end	Night	66% 7% 10% 13% 4 1	3828
April	1st to 10th	Night	54% 7% 12% 19% 7% 1	1938
	11th to 20th	Night	46% 8% 16% 18% 9% 3	1874
	21st to end	Night	44% 9% 14% 21% 10% 2	1710
May	1st to 10th	Night	33% 7% 13% 24% 16% 6%	2860
	11th to 20th	Night	22% 4 9% 22% 40% 3	2649
	21st to end	Night	25% 6% 25% 35% 8%	2762
June	1st to 10th	Night	40% 5% 16% 24% 14% 2	1520
	11th to 20th	Night	19% 4 13% 41% 19% 5%	1520
	21st to end	Night	16% 3 12% 37% 24% 8%	1520
July	1st to 10th	Night	13% 2 6% 28% 33% 18%	1480
	11th to 20th	Night	13% 2 8% 19% 37% 22%	1537
	21st to end	Night	8% 2 4 27% 40% 18%	1773
August	1st to 10th	Night	13% 3 12% 35% 30% 8%	1602
	11th to 20th	Night	10% 3 9% 27% 37% 13%	1602
	21st to end	Night	13% 3 8% 27% 35% 14%	1894
September	1st to 10th	Night	12% 3 7% 23% 32% 24%	1800
	11th to 20th	Night	23% 5% 14% 29% 20% 8%	1904
	21st to end	Night	27% 4 12% 22% 19% 16%	2002
October	1st to 10th	Night	26% 5% 14% 21% 21% 13%	2160
	11th to 20th	Night	37% 4 10% 21% 20% 8%	2160
	21st to end	Night	41% 5% 9% 19% 15% 11%	2376
November	1st to 10th	Night	52% 7% 10% 16% 10% 5%	2341
	11th to 20th	Night	58% 7% 8% 13% 8% 6%	2660
	21st to end	Night	49% 6% 10% 13% 14% 8%	2660
December	1st to 10th	Night	51% 4 9% 16% 15% 5%	2660
	11th to 20th	Night	63% 4 8% 12% 12% 2	3276
	21st to end	Night	62% 6% 9% 17% 5% 2	4862

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% 9% 19% 13% 6% 1	194
	11th to 20th	Day	43% 9% 17% 18% 11% 2	370
	21st to end	Day	37% 11% 22% 18% 11% 1	407
February	1st to 10th	Day	29% 13% 27% 25% 7%	370
	11th to 20th	Day	38% 13% 27% 16% 6% 1	365
	21st to end	Day	33% 16% 27% 19% 5%	332
March	1st to 10th	Day	38% 19% 22% 14% 6%	460
	11th to 20th	Day	32% 11% 24% 21% 10% 2	460
	21st to end	Day	34% 16% 27% 17% 5%	506
April	1st to 10th	Day	30% 18% 25% 22% 6%	460
	11th to 20th	Day	28% 17% 26% 21% 6% 2	452
	21st to end	Day	22% 17% 30% 22% 8% 2	460
May	1st to 10th	Day	16% 21% 29% 23% 10% 2	463
	11th to 20th	Day	21% 11% 28% 25% 12% 3	488
	21st to end	Day	16% 14% 29% 29% 10% 2	529
June	1st to 10th	Day	16% 10% 27% 33% 12% 2	490
	11th to 20th	Day	12% 14% 39% 27% 8% 1	490
	21st to end	Day	5% 4 31% 42% 17% 1	490
July	1st to 10th	Day	6% 5% 27% 43% 15% 3	486
	11th to 20th	Day	3 2 32% 44% 16% 4	484
	21st to end	Day	1 2 23% 49% 20% 5%	528
August	1st to 10th	Day	3 2 18% 48% 25% 4	447
	11th to 20th	Day	3 2 20% 49% 22% 4	437
	21st to end	Day	6% 3 27% 41% 20% 4	495
September	1st to 10th	Day	3 4 25% 41% 23% 5%	450
	11th to 20th	Day	14% 5% 25% 34% 20% 2	450
	21st to end	Day	13% 8% 26% 32% 18% 3	450
October	1st to 10th	Day	14% 10% 26% 27% 20% 4	450
	11th to 20th	Day	20% 7% 21% 27% 18% 6%	450
	21st to end	Day	24% 12% 21% 27% 14% 3	495
November	1st to 10th	Day	27% 12% 23% 24% 11% 3	372
	11th to 20th	Day	31% 14% 16% 21% 17% 2	180
	21st to end	Day	29% 9% 18% 32% 11% 1	180
December	1st to 10th	Day	33% 11% 17% 23% 13% 4	180
	11th to 20th	Day	42% 6% 16% 28% 4 3	180
	21st to end	Day	41% 9% 20% 19% 9% 1	198
January	1st to 10th	Night	44% 6% 18% 21% 9% 2	760
	11th to 20th	Night	38% 7% 18% 23% 13% 2	760
	21st to end	Night	36% 4 16% 26% 16% 2	836
February	1st to 10th	Night	32% 7% 19% 26% 15% 1	760
	11th to 20th	Night	31% 9% 19% 27% 11% 3	744
	21st to end	Night	32% 10% 16% 30% 11%	628
March	1st to 10th	Night	38% 11% 18% 20% 11% 2	760
	11th to 20th	Night	35% 6% 14% 26% 17% 2	760
	21st to end	Night	37% 10% 20% 22% 11% 1	836
April	1st to 10th	Night	30% 11% 18% 21% 16% 2	760
	11th to 20th	Night	26% 10% 21% 24% 15% 4	746
	21st to end	Night	25% 6% 21% 30% 16% 2	570
May	1st to 10th	Night	18% 9% 25% 27% 19% 3	570
	11th to 20th	Night	17% 4 18% 27% 30% 5%	455
	21st to end	Night	17% 7% 18% 33% 21% 4	411
June	1st to 10th	Night	16% 4 22% 32% 23% 3	380
	11th to 20th	Night	9% 3 29% 38% 18% 3	380
	21st to end	Night	7% 3 14% 37% 36% 3	380
July	1st to 10th	Night	4 13% 36% 38% 8%	370
	11th to 20th	Night	5% 1 15% 39% 36% 4	433
	21st to end	Night	2 9% 35% 45% 10%	624
August	1st to 10th	Night	3 1 7% 28% 48% 13%	537
	11th to 20th	Night	2 1 7% 35% 43% 12%	534
	21st to end	Night	5% 2 16% 33% 36% 8%	706
September	1st to 10th	Night	5% 1 14% 37% 38% 5%	720
	11th to 20th	Night	9% 4 10% 30% 36% 12%	720
	21st to end	Night	13% 4 14% 32% 29% 7%	720
October	1st to 10th	Night	13% 6% 21% 28% 28% 4	720
	11th to 20th	Night	20% 3 15% 32% 22% 6%	720
	21st to end	Night	23% 7% 16% 29% 20% 5%	792
November	1st to 10th	Night	30% 6% 15% 29% 17% 3	740
	11th to 20th	Night	35% 6% 17% 28% 13% 1	760
	21st to end	Night	26% 8% 20% 31% 14% 1	760
December	1st to 10th	Night	32% 6% 20% 28% 12% 2	760
	11th to 20th	Night	41% 7% 16% 25% 10% 1	760
	21st to end	Night	44% 8% 18% 22% 7%	836

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