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 2024-01-17.

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% 5% 2 4 4 2 4 4 4 6% 47%	74728
	Night	22% 6% 2 4 4 2 3 3 4 5% 44%	75117
Transparency	Day	26% 38% 9% 12% 10% 4	74443
	Night	28% 38% 8% 12% 10% 4	74923
Seeing	Day	21% 10% 25% 30% 13% 2	13955
	Night	24% 6% 17% 29% 21% 4	23090

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% 8% 3 5% 4 3 4 4 4 5% 21%	1533
	11th to 20th	Day	35% 7% 2 3 4 2 4 3 3 5% 33%	1675
	21st to end	Day	29% 7% 2 4 4 2 3 3 3 6% 36%	1782
February	1st to 10th	Day	29% 6% 2 4 5% 2 5% 4 5% 7% 31%	1620
	11th to 20th	Day	26% 7% 2 5% 5% 3 3 4 4 5% 36%	1575
	21st to end	Day	31% 8% 3 6% 5% 3 3 3 4 4 30%	1415
March	1st to 10th	Day	33% 11% 2 5% 4 3 3 4 4 5% 25%	1980
	11th to 20th	Day	32% 8% 2 5% 4 3 4 4 5% 6% 27%	1980
	21st to end	Day	33% 7% 2 5% 4 4 5% 5% 5% 6% 25%	2178
April	1st to 10th	Day	30% 8% 3 5% 5% 3 5% 4 5% 6% 27%	2124
	11th to 20th	Day	25% 8% 3 5% 4 3 4 3 5% 6% 35%	2131
	21st to end	Day	18% 6% 2 5% 6% 2 4 4 5% 7% 40%	2340
May	1st to 10th	Day	15% 6% 2 4 4 2 3 4 5% 7% 49%	2340
	11th to 20th	Day	15% 7% 2 4 4 3 4 5% 5% 8% 42%	2399
	21st to end	Day	17% 6% 2 3 4 2 4 4 5% 8% 45%	2723
June	1st to 10th	Day	17% 6% 2 4 4 3 4 4 5% 7% 43%	2520
	11th to 20th	Day	9% 3 1 3 3 2 3 3 4 6% 64%	2520
	21st to end	Day	8% 2 1 2 4 1 3 4 4 5% 64%	2520
July	1st to 10th	Day	4 3 2 2 2 2 3 3 4 5% 70%	2520
	11th to 20th	Day	5% 2 1 2 2 1 3 4 3 4 73%	2455
	21st to end	Day	2 1 1 2 2 2 5% 4 4 5% 74%	2546
August	1st to 10th	Day	2 3 1 3 3 2 4 4 4 7% 67%	2328
	11th to 20th	Day	3 2 1 2 2 1 4 3 4 6% 73%	2298
	21st to end	Day	4 3 1 2 3 2 3 4 4 5% 68%	2460
September	1st to 10th	Day	3 3 1 2 2 2 3 4 3 5% 73%	2160
	11th to 20th	Day	11% 4 2 4 3 2 3 4 5% 8% 53%	2052
	21st to end	Day	10% 4 2 3 4 2 3 3 3 6% 61%	1958
October	1st to 10th	Day	13% 3 2 3 3 2 2 3 4 6% 59%	1800
	11th to 20th	Day	21% 5% 1 4 3 2 2 3 5% 6% 48%	1800
	21st to end	Day	22% 6% 2 4 3 2 3 3 4 5% 47%	1980
November	1st to 10th	Day	29% 8% 2 5% 5% 3 3 4 4 7% 30%	1734
	11th to 20th	Day	31% 6% 2 5% 4 3 4 4 6% 6% 29%	1520
	21st to end	Day	26% 6% 2 5% 4 4 4 4 4 6% 34%	1520
December	1st to 10th	Day	28% 5% 2 3 3 3 4 3 4 6% 38%	1520
	11th to 20th	Day	39% 7% 2 4 4 3 3 4 4 5% 24%	1928
	21st to end	Day	40% 10% 3 6% 5% 2 4 2 3 4 23%	2794
January	1st to 10th	Night	39% 9% 3 5% 5% 3 4 4 3 4 22%	2628
	11th to 20th	Night	34% 8% 2 4 4 2 4 3 3 5% 33%	2429
	21st to end	Night	30% 6% 2 4 4 2 3 3 4 5% 36%	2574
February	1st to 10th	Night	28% 7% 2 4 3 3 3 4 5% 7% 34%	2340
	11th to 20th	Night	26% 7% 2 4 5% 3 3 3 4 5% 36%	2272
	21st to end	Night	28% 6% 2 5% 4 3 4 4 5% 6% 33%	1838
March	1st to 10th	Night	33% 12% 3 5% 4 2 3 3 4 6% 27%	1980
	11th to 20th	Night	31% 9% 4 5% 4 3 3 4 3 6% 28%	1980
	21st to end	Night	34% 7% 3 4 5% 3 3 3 4 6% 29%	2178
April	1st to 10th	Night	29% 8% 3 5% 5% 2 4 3 5% 7% 31%	1836
	11th to 20th	Night	23% 8% 3 5% 4 1 3 3 5% 7% 39%	1775
	21st to end	Night	21% 6% 3 4 5% 2 4 3 4 7% 40%	1620
May	1st to 10th	Night	15% 6% 2 4 3 1 3 3 4 5% 54%	1620
	11th to 20th	Night	15% 7% 2 5% 4 3 4 4 5% 7% 44%	1493
	21st to end	Night	14% 7% 2 4 4 2 4 3 4 7% 49%	1552
June	1st to 10th	Night	15% 6% 3 5% 4 3 4 3 5% 6% 46%	1440
	11th to 20th	Night	9% 3 1 1 2 2 2 2 3 5% 70%	1440
	21st to end	Night	7% 2 1 2 3 2 2 3 3 6% 68%	1440
July	1st to 10th	Night	4 2 1 3 1 1 2 2 3 5% 75%	1440
	11th to 20th	Night	4 1 1 2 2 1 3 3 3 3 76%	1501
	21st to end	Night	2 1 1 2 2 1 2 4 3 4 79%	1773
August	1st to 10th	Night	2 3 1 3 3 2 2 4 4 6% 71%	1611
	11th to 20th	Night	2 1 3 2 2 2 3 4 4 77%	1602
	21st to end	Night	5% 1 1 3 3 2 3 3 4 4 72%	1894
September	1st to 10th	Night	4 3 1 2 2 1 1 4 3 4 76%	1800
	11th to 20th	Night	8% 4 3 3 4 2 3 4 4 6% 58%	1904
	21st to end	Night	12% 4 1 3 2 2 3 4 4 6% 58%	2002
October	1st to 10th	Night	12% 4 1 3 3 1 3 5% 6% 5% 56%	2160
	11th to 20th	Night	19% 5% 2 3 3 2 4 5% 4 5% 48%	2160
	21st to end	Night	22% 6% 2 3 4 2 4 3 3 5% 46%	2376
November	1st to 10th	Night	27% 7% 2 5% 4 3 4 4 4 6% 33%	2341
	11th to 20th	Night	32% 7% 2 6% 4 3 3 4 4 6% 29%	2660
	21st to end	Night	25% 5% 2 4 5% 2 4 4 4 7% 37%	2660
December	1st to 10th	Night	31% 6% 1 4 3 2 3 3 5% 5% 37%	2660
	11th to 20th	Night	38% 8% 3 5% 4 2 3 3 4 6% 24%	3276
	21st to end	Night	38% 9% 2 5% 4 2 3 3 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	43% 41% 6% 6% 3	1533
	11th to 20th	Day	31% 42% 4 9% 5% 9%	1675
	21st to end	Day	31% 39% 5% 11% 9% 5%	1782
February	1st to 10th	Day	31% 39% 8% 12% 8% 3	1620
	11th to 20th	Day	29% 38% 9% 15% 7% 1	1584
	21st to end	Day	39% 37% 8% 9% 7%	1415
March	1st to 10th	Day	40% 38% 6% 11% 4	1980
	11th to 20th	Day	39% 39% 8% 9% 4 1	1980
	21st to end	Day	38% 40% 10% 8% 2 1	2178
April	1st to 10th	Day	36% 40% 8% 9% 5% 1	2124
	11th to 20th	Day	33% 37% 13% 8% 6% 3	2134
	21st to end	Day	28% 41% 11% 12% 7% 1	2340
May	1st to 10th	Day	25% 39% 11% 15% 8% 2	2340
	11th to 20th	Day	23% 41% 12% 15% 7% 3	2402
	21st to end	Day	27% 41% 11% 17% 4	2724
June	1st to 10th	Day	31% 40% 7% 14% 7%	2520
	11th to 20th	Day	17% 38% 9% 24% 8% 3	2520
	21st to end	Day	15% 38% 10% 19% 13% 4	2520
July	1st to 10th	Day	10% 38% 9% 14% 18% 11%	2451
	11th to 20th	Day	12% 38% 5% 12% 18% 16%	2380
	21st to end	Day	13% 37% 7% 12% 24% 7%	2405
August	1st to 10th	Day	14% 39% 8% 19% 19% 2	2312
	11th to 20th	Day	12% 36% 10% 14% 23% 4	2298
	21st to end	Day	13% 38% 6% 13% 24% 6%	2460
September	1st to 10th	Day	9% 37% 8% 14% 18% 12%	2160
	11th to 20th	Day	21% 40% 11% 14% 11% 3	2052
	21st to end	Day	17% 38% 10% 13% 14% 8%	1958
October	1st to 10th	Day	19% 38% 13% 13% 14% 4	1800
	11th to 20th	Day	23% 38% 10% 15% 13% 2	1800
	21st to end	Day	24% 40% 9% 10% 12% 5%	1980
November	1st to 10th	Day	38% 43% 7% 5% 6% 2	1734
	11th to 20th	Day	33% 41% 9% 8% 5% 5%	1520
	21st to end	Day	30% 42% 5% 9% 8% 5%	1520
December	1st to 10th	Day	26% 42% 9% 11% 10% 2	1520
	11th to 20th	Day	44% 34% 9% 8% 4 1	1928
	21st to end	Day	57% 25% 5% 8% 3 1	2794
January	1st to 10th	Night	45% 40% 5% 7% 3 1	2628
	11th to 20th	Night	32% 42% 5% 7% 7% 8%	2429
	21st to end	Night	32% 37% 5% 10% 12% 4	2574
February	1st to 10th	Night	29% 37% 10% 11% 10% 2	2340
	11th to 20th	Night	30% 38% 9% 13% 8% 2	2274
	21st to end	Night	36% 38% 9% 10% 6% 1	1838
March	1st to 10th	Night	40% 36% 6% 10% 7%	1980
	11th to 20th	Night	41% 37% 8% 7% 6% 1	1980
	21st to end	Night	39% 39% 8% 10% 3 1	2178
April	1st to 10th	Night	36% 39% 8% 11% 4 1	1836
	11th to 20th	Night	32% 38% 11% 11% 6% 2	1775
	21st to end	Night	31% 38% 8% 15% 6% 2	1620
May	1st to 10th	Night	24% 38% 11% 18% 6% 3	1620
	11th to 20th	Night	24% 41% 12% 12% 8% 2	1493
	21st to end	Night	25% 40% 12% 18% 4	1552
June	1st to 10th	Night	30% 39% 11% 14% 7%	1440
	11th to 20th	Night	15% 36% 10% 26% 11% 2	1440
	21st to end	Night	12% 37% 9% 23% 14% 5%	1440
July	1st to 10th	Night	6% 36% 7% 19% 22% 10%	1400
	11th to 20th	Night	8% 37% 3 12% 24% 16%	1453
	21st to end	Night	7% 37% 6% 13% 27% 10%	1674
August	1st to 10th	Night	9% 38% 8% 20% 24% 2	1602
	11th to 20th	Night	8% 37% 5% 17% 26% 7%	1602
	21st to end	Night	10% 36% 7% 13% 26% 9%	1894
September	1st to 10th	Night	6% 36% 7% 13% 22% 16%	1800
	11th to 20th	Night	17% 39% 10% 18% 13% 4	1904
	21st to end	Night	18% 38% 8% 13% 13% 11%	2002
October	1st to 10th	Night	20% 39% 10% 13% 11% 6%	2160
	11th to 20th	Night	21% 38% 11% 13% 13% 4	2160
	21st to end	Night	24% 38% 8% 12% 11% 7%	2376
November	1st to 10th	Night	35% 43% 7% 6% 6% 3	2341
	11th to 20th	Night	35% 42% 6% 8% 5% 4	2660
	21st to end	Night	29% 41% 8% 9% 9% 4	2660
December	1st to 10th	Night	29% 41% 6% 11% 11% 3	2660
	11th to 20th	Night	45% 34% 6% 8% 7%	3276
	21st to end	Night	50% 27% 8% 12% 2 1	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% 16% 18% 11% 3	359
	21st to end	Day	36% 10% 21% 19% 12% 2	374
February	1st to 10th	Day	29% 11% 27% 25% 7%	340
	11th to 20th	Day	36% 13% 28% 16% 6% 1	335
	21st to end	Day	34% 16% 27% 18% 5%	302
March	1st to 10th	Day	38% 19% 21% 15% 6%	420
	11th to 20th	Day	34% 11% 24% 21% 9% 1	420
	21st to end	Day	35% 15% 26% 19% 6%	462
April	1st to 10th	Day	32% 18% 25% 21% 5%	420
	11th to 20th	Day	29% 16% 26% 20% 7% 2	412
	21st to end	Day	22% 16% 30% 23% 8% 1	420
May	1st to 10th	Day	15% 20% 29% 24% 10% 2	423
	11th to 20th	Day	23% 12% 25% 25% 12% 3	448
	21st to end	Day	17% 14% 26% 29% 10% 2	485
June	1st to 10th	Day	17% 10% 26% 32% 13% 2	450
	11th to 20th	Day	12% 13% 38% 28% 8% 1	450
	21st to end	Day	5% 4 30% 43% 17% 1	450
July	1st to 10th	Day	7% 5% 27% 44% 15% 2	446
	11th to 20th	Day	3 2 32% 44% 16% 3	444
	21st to end	Day	1 2 22% 49% 21% 5%	487
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	37% 7% 17% 23% 13% 2	748
	21st to end	Night	35% 3 16% 27% 17% 2	792
February	1st to 10th	Night	32% 6% 19% 27% 16% 1	720
	11th to 20th	Night	30% 8% 19% 29% 12% 3	704
	21st to end	Night	32% 10% 16% 31% 11%	592
March	1st to 10th	Night	38% 10% 18% 21% 11% 3	720
	11th to 20th	Night	35% 7% 13% 27% 17% 1	720
	21st to end	Night	37% 9% 20% 22% 11% 1	792
April	1st to 10th	Night	32% 11% 18% 21% 17% 3	720
	11th to 20th	Night	26% 10% 20% 24% 15% 4	707
	21st to end	Night	25% 5% 21% 30% 16% 3	540
May	1st to 10th	Night	18% 9% 24% 28% 19% 3	540
	11th to 20th	Night	18% 4 16% 27% 31% 5%	431
	21st to end	Night	18% 6% 16% 34% 22% 4	389
June	1st to 10th	Night	16% 4 21% 31% 24% 3	360
	11th to 20th	Night	10% 3 29% 37% 18% 3	360
	21st to end	Night	7% 3 15% 36% 35% 3	360
July	1st to 10th	Night	4 13% 37% 37% 8%	350
	11th to 20th	Night	5% 1 16% 38% 35% 4	409
	21st to end	Night	2 9% 35% 45% 9%	591
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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