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 2025-07-21.

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% 8% 3 8% 5% 5% 5% 4 6% 7% 40%	119967
	Night	9% 8% 3 8% 5% 5% 5% 4 6% 7% 41%	119387
Transparency	Day	41% 6% 13% 24% 13% 2	123556
	Night	40% 6% 13% 25% 14% 2	123934
Seeing	Day	15% 10% 31% 28% 13% 3	16967
	Night	13% 9% 35% 33% 10% 1	26857

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	9% 11% 4 13% 12% 8% 7% 5% 6% 6% 18%	3377
	11th to 20th	Day	4 10% 5% 13% 7% 7% 9% 5% 11% 9% 21%	3656
	21st to end	Day	4 8% 8% 16% 7% 6% 5% 4 4 6% 32%	4488
February	1st to 10th	Day	8% 12% 4 11% 8% 7% 5% 4 6% 6% 27%	1800
	11th to 20th	Day	7% 11% 4 10% 7% 7% 7% 5% 5% 7% 31%	1759
	21st to end	Day	8% 10% 4 10% 6% 6% 5% 4 6% 6% 33%	1650
March	1st to 10th	Day	11% 8% 3 7% 4 3 4 3 5% 6% 45%	4737
	11th to 20th	Day	8% 8% 3 7% 4 4 4 3 5% 9% 45%	4980
	21st to end	Day	20% 13% 4 11% 5% 5% 4 3 5% 7% 23%	5462
April	1st to 10th	Day	17% 7% 3 7% 4 4 5% 3 5% 7% 38%	2289
	11th to 20th	Day	18% 11% 4 8% 4 4 4 3 4 6% 35%	2572
	21st to end	Day	21% 10% 3 6% 4 3 4 3 5% 6% 36%	2600
May	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 6% 7% 50%	5862
	11th to 20th	Day	19% 8% 2 5% 3 3 3 2 4 5% 45%	5949
	21st to end	Day	19% 7% 2 6% 5% 5% 5% 4 5% 7% 35%	6935
June	1st to 10th	Day	11% 5% 2 5% 4 5% 4 4 6% 9% 45%	2979
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 4 6% 8% 42%	3000
	21st to end	Day	10% 5% 2 5% 4 4 4 4 7% 8% 46%	3000
July	1st to 10th	Day	10% 4 2 5% 5% 5% 5% 4 8% 10% 42%	4900
	11th to 20th	Day	5% 3 1 4 3 3 4 4 6% 8% 59%	4672
	21st to end	Day	5% 4 2 5% 4 4 5% 5% 8% 9% 49%	4502
August	1st to 10th	Day	10% 7% 2 6% 4 4 5% 4 7% 9% 42%	4104
	11th to 20th	Day	14% 7% 3 7% 4 4 5% 4 6% 9% 37%	4041
	21st to end	Day	9% 5% 2 6% 4 4 4 4 5% 8% 51%	4532
September	1st to 10th	Day	12% 5% 2 5% 3 3 4 3 5% 6% 53%	2234
	11th to 20th	Day	10% 6% 2 5% 4 3 4 3 5% 6% 52%	2090
	21st to end	Day	12% 6% 2 4 4 3 4 3 4 5% 53%	2090
October	1st to 10th	Day	15% 8% 2 6% 3 3 3 3 4 7% 47%	1954
	11th to 20th	Day	14% 6% 2 5% 3 4 3 3 5% 7% 47%	1900
	21st to end	Day	15% 9% 3 6% 3 4 4 2 4 5% 43%	1986
November	1st to 10th	Day	13% 9% 3 6% 5% 4 4 3 5% 7% 42%	1763
	11th to 20th	Day	8% 11% 4 10% 6% 5% 4 4 5% 6% 36%	1617
	21st to end	Day	11% 10% 2 8% 5% 5% 5% 4 6% 9% 36%	1600
December	1st to 10th	Day	5% 8% 6% 13% 8% 6% 6% 4 5% 7% 33%	2610
	11th to 20th	Day	7% 14% 6% 14% 8% 7% 5% 5% 5% 6% 23%	2779
	21st to end	Day	16% 12% 4 15% 7% 5% 4 4 5% 7% 23%	3498
January	1st to 10th	Night	6% 10% 4 14% 10% 9% 7% 5% 7% 7% 22%	6585
	11th to 20th	Night	3 8% 3 11% 7% 7% 10% 7% 9% 9% 25%	6322
	21st to end	Night	4 9% 6% 14% 5% 5% 5% 4 5% 6% 38%	6490
February	1st to 10th	Night	6% 11% 4 10% 7% 6% 6% 5% 5% 8% 33%	2600
	11th to 20th	Night	6% 9% 4 10% 6% 6% 6% 4 6% 6% 37%	2533
	21st to end	Night	8% 9% 3 9% 6% 5% 5% 4 6% 7% 38%	1994
March	1st to 10th	Night	9% 8% 3 6% 4 4 4 3 5% 7% 48%	5112
	11th to 20th	Night	9% 7% 2 6% 4 4 4 4 6% 8% 47%	4980
	21st to end	Night	19% 11% 3 8% 6% 4 4 3 4 5% 32%	5467
April	1st to 10th	Night	13% 10% 3 6% 5% 4 4 4 4 7% 39%	2105
	11th to 20th	Night	15% 9% 4 7% 4 3 4 3 4 6% 40%	1779
	21st to end	Night	17% 9% 4 7% 4 4 4 4 4 6% 37%	1800
May	1st to 10th	Night	12% 5% 1 3 3 2 2 2 4 6% 58%	4053
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 6% 50%	3962
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 7% 40%	3942
June	1st to 10th	Night	8% 4 2 4 3 4 4 4 7% 8% 52%	1417
	11th to 20th	Night	6% 5% 2 5% 4 5% 4 3 7% 9% 50%	1400
	21st to end	Night	8% 4 2 4 3 2 3 3 6% 8% 57%	1400
July	1st to 10th	Night	10% 6% 2 4 3 3 5% 3 6% 9% 48%	2290
	11th to 20th	Night	3 3 1 3 3 3 3 3 6% 8% 64%	2514
	21st to end	Night	3 3 1 4 4 4 5% 4 7% 12% 55%	3059
August	1st to 10th	Night	6% 8% 2 6% 3 4 4 4 6% 7% 49%	2836
	11th to 20th	Night	12% 5% 2 5% 3 3 4 4 6% 8% 48%	2798
	21st to end	Night	8% 4 2 5% 3 3 4 3 5% 7% 56%	3135
September	1st to 10th	Night	8% 5% 2 3 3 3 4 3 4 7% 59%	1942
	11th to 20th	Night	9% 5% 1 5% 3 4 3 3 4 7% 56%	2090
	21st to end	Night	11% 5% 2 4 3 2 3 3 3 5% 58%	2090
October	1st to 10th	Night	14% 6% 2 4 3 3 3 3 5% 6% 52%	2206
	11th to 20th	Night	14% 8% 2 5% 3 3 4 2 5% 6% 48%	2280
	21st to end	Night	13% 10% 2 6% 3 3 3 3 4 7% 45%	2608
November	1st to 10th	Night	9% 6% 3 7% 5% 4 4 3 5% 7% 47%	2535
	11th to 20th	Night	8% 10% 3 9% 4 4 4 4 5% 6% 42%	2780
	21st to end	Night	9% 8% 3 9% 5% 4 4 4 6% 7% 43%	2800
December	1st to 10th	Night	4 8% 4 13% 7% 7% 6% 4 6% 8% 34%	4580
	11th to 20th	Night	9% 10% 4 11% 7% 7% 5% 4 6% 7% 30%	5423
	21st to end	Night	14% 15% 5% 13% 6% 5% 4 3 5% 5% 23%	7480

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	63% 6% 13% 13% 5%	3378
	11th to 20th	Day	55% 4 10% 18% 12%	3656
	21st to end	Day	51% 6% 16% 15% 12%	4488
February	1st to 10th	Day	56% 6% 13% 15% 9%	1800
	11th to 20th	Day	51% 7% 12% 19% 10% 1	1768
	21st to end	Day	49% 6% 16% 17% 10% 2	1650
March	1st to 10th	Day	39% 5% 13% 31% 11% 2	4737
	11th to 20th	Day	35% 5% 10% 31% 16% 2	4980
	21st to end	Day	60% 6% 11% 17% 6% 1	5464
April	1st to 10th	Day	45% 7% 14% 22% 12% 1	2289
	11th to 20th	Day	49% 5% 10% 22% 11% 1	2572
	21st to end	Day	48% 6% 12% 24% 9% 1	2600
May	1st to 10th	Day	31% 7% 11% 27% 24%	5900
	11th to 20th	Day	41% 5% 9% 21% 15% 9%	5953
	21st to end	Day	45% 7% 11% 23% 12% 1	6949
June	1st to 10th	Day	35% 8% 16% 30% 11% 1	2979
	11th to 20th	Day	38% 9% 15% 24% 12% 2	3000
	21st to end	Day	33% 8% 16% 27% 15% 2	3000
July	1st to 10th	Day	33% 10% 20% 24% 12% 1	4826
	11th to 20th	Day	22% 7% 15% 34% 19% 3	4593
	21st to end	Day	26% 9% 18% 27% 18% 1	4360
August	1st to 10th	Day	36% 7% 14% 26% 14% 2	4090
	11th to 20th	Day	42% 9% 15% 26% 7% 1	4080
	21st to end	Day	31% 7% 19% 26% 14% 2	4531
September	1st to 10th	Day	32% 5% 12% 32% 17% 1	2234
	11th to 20th	Day	31% 7% 17% 30% 14% 1	2090
	21st to end	Day	33% 5% 11% 30% 19% 2	2090
October	1st to 10th	Day	27% 3 9% 29% 30% 1	3240
	11th to 20th	Day	34% 5% 12% 24% 21% 3	3140
	21st to end	Day	44% 5% 14% 21% 16% 1	3255
November	1st to 10th	Day	41% 6% 14% 23% 13% 3	1763
	11th to 20th	Day	47% 7% 13% 22% 11% 1	1617
	21st to end	Day	43% 6% 15% 23% 12% 1	1600
December	1st to 10th	Day	49% 6% 16% 21% 7% 1	2610
	11th to 20th	Day	60% 6% 16% 15% 4	2776
	21st to end	Day	60% 6% 11% 16% 7% 1	3498
January	1st to 10th	Night	58% 6% 11% 17% 8%	6585
	11th to 20th	Night	50% 6% 14% 16% 13% 1	6322
	21st to end	Night	45% 6% 14% 22% 13%	6490
February	1st to 10th	Night	48% 6% 11% 21% 11% 2	2600
	11th to 20th	Night	44% 7% 12% 22% 12% 3	2534
	21st to end	Night	43% 6% 13% 21% 12% 4	1994
March	1st to 10th	Night	35% 6% 11% 32% 15% 2	5112
	11th to 20th	Night	33% 5% 12% 32% 17% 1	4980
	21st to end	Night	53% 6% 12% 21% 6% 1	5467
April	1st to 10th	Night	43% 8% 12% 25% 11% 1	2105
	11th to 20th	Night	43% 6% 10% 25% 15% 2	1779
	21st to end	Night	46% 6% 11% 24% 12% 1	1800
May	1st to 10th	Night	27% 5% 12% 30% 23% 2	4080
	11th to 20th	Night	34% 6% 10% 21% 21% 7%	3963
	21st to end	Night	42% 7% 11% 29% 11% 1	3950
June	1st to 10th	Night	28% 7% 15% 31% 15% 3	1417
	11th to 20th	Night	29% 7% 18% 26% 17% 4	1400
	21st to end	Night	24% 7% 16% 30% 20% 2	1400
July	1st to 10th	Night	31% 8% 21% 24% 15% 2	2255
	11th to 20th	Night	17% 7% 13% 41% 20% 3	2470
	21st to end	Night	20% 9% 19% 29% 20% 3	2962
August	1st to 10th	Night	33% 8% 16% 25% 16% 3	2826
	11th to 20th	Night	32% 10% 16% 29% 12% 1	2825
	21st to end	Night	27% 7% 18% 27% 17% 3	3135
September	1st to 10th	Night	26% 7% 14% 31% 21% 2	1942
	11th to 20th	Night	29% 6% 14% 32% 18% 1	2090
	21st to end	Night	29% 5% 10% 31% 23% 2	2090
October	1st to 10th	Night	21% 3 7% 27% 38% 4	3687
	11th to 20th	Night	28% 4 14% 30% 22% 2	3779
	21st to end	Night	38% 4 14% 28% 15% 1	4295
November	1st to 10th	Night	37% 5% 13% 23% 19% 3	2535
	11th to 20th	Night	41% 6% 14% 23% 14% 2	2780
	21st to end	Night	40% 6% 16% 25% 13% 1	2800
December	1st to 10th	Night	45% 6% 18% 24% 7%	4580
	11th to 20th	Night	50% 7% 18% 19% 5%	5425
	21st to end	Night	61% 5% 10% 18% 6% 1	7480

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	16% 8% 30% 31% 12% 3	300
	11th to 20th	Day	14% 13% 36% 28% 7% 2	300
	21st to end	Day	16% 12% 36% 24% 12% 1	330
February	1st to 10th	Day	13% 14% 43% 27% 3	300
	11th to 20th	Day	11% 15% 37% 27% 9%	300
	21st to end	Day	15% 12% 40% 28% 4	249
March	1st to 10th	Day	19% 14% 42% 23% 2 1	300
	11th to 20th	Day	18% 11% 40% 21% 10% 1	300
	21st to end	Day	26% 12% 27% 24% 10% 1	330
April	1st to 10th	Day	26% 14% 35% 20% 4	389
	11th to 20th	Day	26% 14% 30% 22% 7% 1	690
	21st to end	Day	24% 15% 30% 21% 9% 1	700
May	1st to 10th	Day	19% 12% 30% 27% 10% 2	703
	11th to 20th	Day	22% 11% 28% 24% 12% 2	725
	21st to end	Day	20% 9% 28% 29% 12% 2	789
June	1st to 10th	Day	9% 10% 26% 32% 18% 5%	730
	11th to 20th	Day	15% 9% 34% 28% 13% 2	730
	21st to end	Day	12% 8% 33% 32% 14% 2	730
July	1st to 10th	Day	7% 3 24% 38% 21% 6%	721
	11th to 20th	Day	7% 4 33% 34% 17% 4	718
	21st to end	Day	6% 4 25% 39% 22% 5%	740
August	1st to 10th	Day	10% 3 25% 36% 22% 4	676
	11th to 20th	Day	12% 5% 25% 32% 21% 4	665
	21st to end	Day	8% 6% 28% 34% 20% 4	748
September	1st to 10th	Day	12% 8% 30% 31% 15% 4	444
	11th to 20th	Day	11% 7% 34% 31% 14% 2	300
	21st to end	Day	17% 14% 29% 24% 11% 4	300
October	1st to 10th	Day	20% 9% 29% 26% 11% 5%	300
	11th to 20th	Day	14% 19% 33% 23% 8% 2	300
	21st to end	Day	22% 17% 38% 15% 7% 1	330
November	1st to 10th	Day	23% 10% 34% 22% 9% 2	300
	11th to 20th	Day	14% 14% 42% 21% 7% 1	300
	21st to end	Day	19% 18% 31% 23% 8% 1	300
December	1st to 10th	Day	13% 12% 38% 26% 9% 2	300
	11th to 20th	Day	13% 15% 43% 26% 4	300
	21st to end	Day	24% 8% 38% 22% 7% 1	330
January	1st to 10th	Night	11% 10% 40% 30% 8% 1	1000
	11th to 20th	Night	11% 10% 40% 32% 7%	1000
	21st to end	Night	15% 10% 40% 28% 7%	1100
February	1st to 10th	Night	11% 10% 43% 29% 6% 1	1000
	11th to 20th	Night	12% 10% 40% 30% 7% 1	979
	21st to end	Night	12% 11% 36% 32% 7% 1	674
March	1st to 10th	Night	13% 10% 41% 30% 6%	694
	11th to 20th	Night	18% 11% 36% 30% 5%	600
	21st to end	Night	18% 10% 34% 28% 9% 1	660
April	1st to 10th	Night	18% 11% 38% 28% 5%	600
	11th to 20th	Night	21% 13% 32% 26% 8% 1	593
	21st to end	Night	20% 10% 34% 28% 7% 1	600
May	1st to 10th	Night	16% 10% 33% 31% 10% 1	600
	11th to 20th	Night	17% 7% 35% 31% 9% 1	591
	21st to end	Night	16% 5% 33% 34% 11% 1	650
June	1st to 10th	Night	10% 6% 26% 41% 18% 1	600
	11th to 20th	Night	8% 5% 37% 41% 10%	600
	21st to end	Night	10% 3 27% 45% 14% 1	600
July	1st to 10th	Night	7% 3 23% 45% 21% 2	585
	11th to 20th	Night	5% 2 30% 46% 17%	582
	21st to end	Night	3 2 25% 49% 19% 1	624
August	1st to 10th	Night	6% 5% 22% 49% 17% 1	567
	11th to 20th	Night	10% 3 23% 42% 21% 2	564
	21st to end	Night	7% 3 29% 41% 19% 2	627
September	1st to 10th	Night	9% 4 31% 40% 15% 2	570
	11th to 20th	Night	9% 6% 33% 37% 13% 3	570
	21st to end	Night	12% 8% 34% 33% 11% 1	570
October	1st to 10th	Night	18% 8% 33% 30% 10% 1	686
	11th to 20th	Night	18% 11% 35% 28% 8%	760
	21st to end	Night	18% 12% 34% 28% 7%	936
November	1st to 10th	Night	13% 10% 39% 30% 8% 1	975
	11th to 20th	Night	13% 14% 38% 26% 9% 1	1000
	21st to end	Night	12% 12% 38% 31% 7%	1000
December	1st to 10th	Night	12% 12% 39% 30% 7% 1	1000
	11th to 20th	Night	16% 10% 38% 29% 7% 1	1000
	21st to end	Night	22% 10% 36% 26% 6% 1	1100

Page generated at 2025-07-18 00:09:58. Version ac07

Copyright © 2025 Attilla Danko.