Brooks
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brooks clear sky chart from 2005-11-06 to 2025-02-25.

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	8% 8% 3 7% 5% 5% 5% 4 6% 8% 42%	112067
	Night	7% 9% 3 9% 6% 5% 5% 4 6% 8% 38%	115211
Transparency	Day	38% 7% 16% 26% 11% 2	111808
	Night	41% 7% 16% 23% 11% 2	115116
Seeing	Day	11% 7% 24% 31% 21% 6%	15752
	Night	12% 7% 29% 35% 16% 2	25137

By Month

Percentage Cloud Cover NW (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	5% 15% 6% 14% 8% 5% 5% 4 7% 7% 24%	3179
	11th to 20th	Day	4 10% 4 10% 7% 6% 7% 7% 7% 8% 31%	3159
	21st to end	Day	3 9% 4 8% 6% 5% 5% 3 5% 8% 43%	3894
February	1st to 10th	Day	8% 13% 4 13% 8% 7% 5% 4 6% 6% 26%	1702
	11th to 20th	Day	7% 12% 4 12% 7% 8% 6% 5% 7% 8% 25%	1755
	21st to end	Day	4 13% 6% 14% 7% 6% 5% 4 4 8% 30%	1482
March	1st to 10th	Day	5% 10% 4 12% 7% 6% 6% 5% 5% 7% 34%	3222
	11th to 20th	Day	8% 9% 3 7% 4 4 4 4 6% 9% 43%	3489
	21st to end	Day	10% 16% 6% 12% 5% 5% 5% 3 5% 7% 25%	3825
April	1st to 10th	Day	9% 11% 4 9% 7% 5% 6% 4 6% 9% 31%	2328
	11th to 20th	Day	9% 11% 4 7% 6% 6% 5% 4 6% 8% 34%	2440
	21st to end	Day	10% 9% 3 8% 6% 6% 6% 4 6% 9% 34%	2470
May	1st to 10th	Day	19% 11% 4 7% 5% 4 4 3 6% 6% 32%	4316
	11th to 20th	Day	8% 7% 2 7% 5% 5% 6% 5% 7% 9% 36%	4632
	21st to end	Day	14% 7% 3 10% 6% 6% 6% 5% 7% 7% 30%	5161
June	1st to 10th	Day	13% 7% 2 6% 5% 5% 5% 4 6% 8% 38%	2850
	11th to 20th	Day	16% 7% 3 6% 5% 5% 5% 5% 7% 9% 32%	2850
	21st to end	Day	7% 5% 1 5% 4 4 5% 5% 7% 9% 47%	2850
July	1st to 10th	Day	5% 3 1 4 4 4 5% 4 6% 7% 56%	4740
	11th to 20th	Day	5% 2 1 4 4 3 4 4 6% 8% 59%	4740
	21st to end	Day	3 2 1 2 2 2 3 3 4 8% 69%	5138
August	1st to 10th	Day	6% 4 2 6% 4 4 4 4 6% 8% 53%	4295
	11th to 20th	Day	6% 3 2 5% 4 4 4 4 7% 9% 54%	4028
	21st to end	Day	8% 3 1 3 3 3 3 3 5% 8% 60%	4521
September	1st to 10th	Day	10% 5% 2 4 2 3 4 3 5% 7% 54%	2376
	11th to 20th	Day	15% 5% 2 5% 4 3 4 3 4 8% 47%	2090
	21st to end	Day	8% 7% 2 5% 4 4 3 4 5% 8% 49%	2090
October	1st to 10th	Day	6% 5% 3 8% 5% 4 6% 5% 6% 7% 44%	3238
	11th to 20th	Day	5% 6% 2 5% 4 5% 5% 4 7% 9% 48%	3076
	21st to end	Day	8% 6% 2 6% 4 4 4 4 6% 7% 49%	3083
November	1st to 10th	Day	11% 13% 6% 8% 7% 5% 4 4 5% 8% 29%	1568
	11th to 20th	Day	5% 12% 5% 9% 6% 7% 4 4 6% 8% 35%	1600
	21st to end	Day	5% 12% 5% 9% 7% 5% 5% 3 6% 10% 31%	1450
December	1st to 10th	Day	8% 14% 4 9% 5% 6% 6% 4 7% 7% 27%	2290
	11th to 20th	Day	8% 11% 4 11% 8% 6% 5% 4 6% 7% 30%	2642
	21st to end	Day	3 8% 4 10% 5% 5% 5% 4 6% 7% 43%	3498
January	1st to 10th	Night	5% 13% 6% 13% 8% 6% 5% 4 6% 8% 26%	6766
	11th to 20th	Night	4 9% 4 12% 7% 6% 6% 4 6% 9% 34%	6739
	21st to end	Night	3 10% 5% 10% 5% 4 4 3 6% 7% 42%	7094
February	1st to 10th	Night	6% 12% 4 11% 7% 5% 6% 5% 6% 7% 30%	2697
	11th to 20th	Night	5% 12% 4 10% 6% 6% 7% 5% 7% 9% 29%	2532
	21st to end	Night	5% 9% 5% 10% 6% 6% 6% 4 7% 10% 32%	2063
March	1st to 10th	Night	4 10% 4 13% 8% 5% 6% 5% 6% 7% 32%	3651
	11th to 20th	Night	9% 7% 3 7% 5% 4 5% 5% 7% 8% 40%	3490
	21st to end	Night	8% 17% 5% 12% 7% 7% 6% 5% 7% 7% 20%	3828
April	1st to 10th	Night	8% 11% 3 10% 6% 6% 5% 5% 6% 9% 31%	1848
	11th to 20th	Night	9% 11% 2 9% 7% 5% 6% 5% 7% 8% 33%	1691
	21st to end	Night	10% 9% 3 8% 5% 5% 5% 5% 6% 8% 36%	1710
May	1st to 10th	Night	20% 11% 4 8% 5% 4 4 4 4 6% 31%	2652
	11th to 20th	Night	7% 5% 2 7% 5% 6% 6% 6% 9% 12% 36%	2246
	21st to end	Night	13% 8% 3 10% 7% 6% 6% 5% 7% 8% 26%	2414
June	1st to 10th	Night	14% 6% 3 7% 5% 5% 6% 4 7% 9% 34%	1330
	11th to 20th	Night	13% 6% 2 7% 5% 6% 5% 6% 8% 9% 33%	1330
	21st to end	Night	7% 4 1 5% 5% 4 5% 4 7% 8% 50%	1330
July	1st to 10th	Night	5% 3 2 5% 5% 5% 6% 4 7% 7% 51%	2220
	11th to 20th	Night	5% 3 2 4 4 5% 4 5% 8% 11% 48%	2220
	21st to end	Night	4 3 1 4 3 3 3 4 5% 9% 60%	2414
August	1st to 10th	Night	7% 4 2 5% 4 4 4 4 6% 9% 48%	2622
	11th to 20th	Night	6% 2 1 6% 5% 5% 5% 4 7% 11% 46%	2811
	21st to end	Night	8% 3 1 4 3 3 3 3 5% 8% 60%	3146
September	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 6% 8% 48%	1800
	11th to 20th	Night	12% 5% 1 4 4 4 3 3 6% 6% 51%	2090
	21st to end	Night	7% 6% 2 6% 4 4 4 4 6% 8% 50%	2090
October	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 5% 8% 50%	3733
	11th to 20th	Night	5% 5% 2 6% 4 5% 5% 4 6% 8% 48%	3838
	21st to end	Night	5% 7% 3 8% 4 4 5% 4 6% 8% 44%	4457
November	1st to 10th	Night	10% 12% 4 7% 5% 5% 4 5% 6% 8% 34%	2716
	11th to 20th	Night	5% 10% 4 8% 6% 6% 6% 4 7% 9% 37%	2800
	21st to end	Night	7% 11% 4 10% 7% 5% 5% 4 7% 8% 33%	2945
December	1st to 10th	Night	5% 12% 4 11% 7% 6% 7% 5% 7% 8% 28%	4890
	11th to 20th	Night	7% 11% 5% 12% 7% 6% 6% 5% 6% 8% 28%	5550
	21st to end	Night	4 8% 4 9% 4 5% 5% 4 6% 7% 43%	7458

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	55% 7% 12% 19% 5% 1	3179
	11th to 20th	Day	48% 7% 14% 15% 14% 1	3180
	21st to end	Day	37% 7% 21% 27% 8%	3894
February	1st to 10th	Day	56% 7% 15% 14% 7% 1	1702
	11th to 20th	Day	52% 10% 14% 16% 7% 1	1764
	21st to end	Day	52% 7% 12% 19% 10%	1482
March	1st to 10th	Day	44% 9% 14% 21% 11%	3222
	11th to 20th	Day	36% 8% 23% 23% 10%	3490
	21st to end	Day	55% 11% 15% 15% 4	3825
April	1st to 10th	Day	47% 9% 15% 20% 8%	2328
	11th to 20th	Day	46% 7% 14% 24% 8% 1	2440
	21st to end	Day	44% 9% 15% 23% 9% 1	2470
May	1st to 10th	Day	50% 8% 11% 24% 6%	4316
	11th to 20th	Day	35% 11% 17% 26% 10%	4636
	21st to end	Day	44% 13% 15% 22% 6% 1	5162
June	1st to 10th	Day	41% 9% 16% 21% 11% 1	2850
	11th to 20th	Day	45% 9% 15% 20% 8% 2	2850
	21st to end	Day	29% 9% 20% 30% 11% 2	2850
July	1st to 10th	Day	25% 6% 18% 28% 18% 4	4666
	11th to 20th	Day	22% 6% 17% 33% 19% 2	4659
	21st to end	Day	14% 5% 11% 30% 25% 14%	4976
August	1st to 10th	Day	28% 6% 13% 37% 15% 2	4277
	11th to 20th	Day	25% 6% 20% 38% 9% 1	4067
	21st to end	Day	23% 4 17% 40% 15% 2	4520
September	1st to 10th	Day	29% 5% 16% 27% 16% 8%	2376
	11th to 20th	Day	36% 7% 15% 27% 13% 2	2090
	21st to end	Day	32% 6% 16% 29% 13% 3	2090
October	1st to 10th	Day	36% 6% 19% 29% 9% 1	3238
	11th to 20th	Day	32% 5% 18% 31% 13% 1	3076
	21st to end	Day	33% 6% 11% 37% 12% 1	3083
November	1st to 10th	Day	53% 7% 13% 19% 7% 1	1568
	11th to 20th	Day	46% 7% 17% 21% 8%	1600
	21st to end	Day	47% 7% 17% 20% 8% 1	1450
December	1st to 10th	Day	52% 8% 18% 18% 3 1	2290
	11th to 20th	Day	52% 6% 18% 19% 4 1	2644
	21st to end	Day	39% 5% 14% 25% 15% 1	3498
January	1st to 10th	Night	53% 8% 16% 17% 5% 1	6766
	11th to 20th	Night	47% 6% 14% 16% 15% 1	6780
	21st to end	Night	41% 6% 17% 29% 7% 1	7094
February	1st to 10th	Night	50% 7% 12% 20% 10% 1	2697
	11th to 20th	Night	48% 9% 14% 18% 10% 1	2533
	21st to end	Night	43% 9% 17% 21% 9% 1	2063
March	1st to 10th	Night	45% 10% 16% 21% 7%	3651
	11th to 20th	Night	37% 8% 21% 25% 9% 1	3490
	21st to end	Night	57% 13% 14% 12% 4 1	3828
April	1st to 10th	Night	47% 8% 15% 21% 9%	1848
	11th to 20th	Night	46% 8% 13% 21% 11% 1	1691
	21st to end	Night	41% 8% 14% 23% 11% 2	1710
May	1st to 10th	Night	53% 7% 11% 20% 9% 1	2652
	11th to 20th	Night	31% 13% 18% 27% 11% 1	2246
	21st to end	Night	47% 14% 16% 16% 5% 1	2414
June	1st to 10th	Night	42% 8% 18% 22% 10% 1	1330
	11th to 20th	Night	41% 10% 16% 21% 10% 1	1330
	21st to end	Night	29% 7% 20% 31% 12% 2	1330
July	1st to 10th	Night	29% 6% 20% 23% 20% 2	2185
	11th to 20th	Night	27% 7% 15% 35% 13% 3	2178
	21st to end	Night	20% 5% 10% 31% 23% 11%	2337
August	1st to 10th	Night	31% 7% 16% 27% 17% 2	2615
	11th to 20th	Night	30% 6% 18% 31% 14% 2	2838
	21st to end	Night	23% 4 14% 36% 19% 3	3146
September	1st to 10th	Night	31% 7% 13% 24% 18% 7%	1800
	11th to 20th	Night	32% 7% 14% 27% 14% 5%	2090
	21st to end	Night	29% 7% 16% 30% 16% 3	2090
October	1st to 10th	Night	32% 5% 13% 34% 14% 2	3733
	11th to 20th	Night	32% 6% 16% 29% 15% 2	3838
	21st to end	Night	37% 6% 12% 34% 11% 1	4457
November	1st to 10th	Night	47% 8% 15% 21% 8% 1	2716
	11th to 20th	Night	41% 8% 19% 20% 10% 1	2800
	21st to end	Night	47% 9% 18% 17% 8% 1	2945
December	1st to 10th	Night	53% 7% 18% 17% 5% 1	4890
	11th to 20th	Night	52% 7% 25% 13% 3	5547
	21st to end	Night	39% 5% 13% 24% 17% 2	7458

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	7% 8% 27% 32% 18% 10%	200
	11th to 20th	Day	8% 9% 31% 34% 14% 5%	200
	21st to end	Day	7% 9% 32% 32% 15% 5%	220
February	1st to 10th	Day	16% 9% 33% 29% 12% 2	252
	11th to 20th	Day	9% 5% 30% 36% 15% 4	300
	21st to end	Day	6% 9% 27% 33% 19% 6%	270
March	1st to 10th	Day	10% 13% 30% 30% 13% 4	460
	11th to 20th	Day	9% 8% 31% 32% 15% 6%	460
	21st to end	Day	11% 9% 30% 25% 18% 6%	506
April	1st to 10th	Day	12% 13% 28% 30% 15% 2	460
	11th to 20th	Day	13% 9% 29% 31% 17% 2	452
	21st to end	Day	11% 9% 27% 32% 16% 5%	460
May	1st to 10th	Day	14% 8% 19% 30% 22% 7%	578
	11th to 20th	Day	15% 5% 17% 33% 24% 7%	675
	21st to end	Day	15% 6% 17% 27% 25% 9%	734
June	1st to 10th	Day	13% 9% 23% 29% 22% 4	680
	11th to 20th	Day	15% 8% 28% 30% 16% 3	680
	21st to end	Day	10% 3 23% 34% 21% 9%	680
July	1st to 10th	Day	6% 3 17% 35% 30% 9%	671
	11th to 20th	Day	8% 4 20% 34% 26% 7%	668
	21st to end	Day	5% 4 20% 32% 30% 10%	739
August	1st to 10th	Day	6% 4 16% 35% 26% 13%	504
	11th to 20th	Day	8% 3 15% 31% 31% 12%	477
	21st to end	Day	9% 6% 20% 30% 24% 12%	539
September	1st to 10th	Day	12% 6% 23% 29% 24% 6%	490
	11th to 20th	Day	14% 6% 25% 27% 22% 6%	490
	21st to end	Day	13% 11% 27% 28% 15% 5%	490
October	1st to 10th	Day	16% 5% 23% 28% 22% 7%	342
	11th to 20th	Day	12% 8% 29% 29% 17% 5%	300
	21st to end	Day	15% 12% 28% 27% 15% 3	330
November	1st to 10th	Day	19% 10% 27% 29% 13% 2	300
	11th to 20th	Day	9% 8% 42% 26% 12% 2	300
	21st to end	Day	10% 9% 33% 29% 12% 8%	225
December	1st to 10th	Day	13% 9% 31% 26% 19% 3	200
	11th to 20th	Day	13% 12% 35% 28% 10% 3	200
	21st to end	Day	14% 5% 23% 31% 22% 6%	220
January	1st to 10th	Night	11% 9% 29% 34% 14% 2	1000
	11th to 20th	Night	13% 10% 30% 34% 13% 2	1000
	21st to end	Night	12% 8% 32% 33% 14% 1	944
February	1st to 10th	Night	12% 6% 27% 36% 16% 3	800
	11th to 20th	Night	10% 7% 32% 37% 15% 1	784
	21st to end	Night	8% 5% 26% 36% 22% 3	646
March	1st to 10th	Night	8% 7% 32% 36% 14% 2	760
	11th to 20th	Night	10% 8% 31% 34% 14% 2	760
	21st to end	Night	12% 8% 27% 35% 16% 1	836
April	1st to 10th	Night	13% 7% 31% 33% 14% 2	641
	11th to 20th	Night	15% 5% 31% 35% 13% 1	563
	21st to end	Night	13% 7% 29% 38% 13% 1	570
May	1st to 10th	Night	16% 6% 23% 33% 21% 1	570
	11th to 20th	Night	10% 3 28% 38% 20% 1	408
	21st to end	Night	19% 4 24% 29% 22% 2	411
June	1st to 10th	Night	15% 8% 24% 34% 17% 2	380
	11th to 20th	Night	15% 7% 29% 33% 15% 1	380
	21st to end	Night	9% 4 27% 35% 22% 3	380
July	1st to 10th	Night	7% 5% 20% 47% 21% 1	370
	11th to 20th	Night	8% 6% 21% 47% 17% 1	368
	21st to end	Night	5% 4 22% 41% 24% 3	416
August	1st to 10th	Night	8% 3 23% 38% 25% 2	452
	11th to 20th	Night	8% 3 20% 42% 26% 2	564
	21st to end	Night	8% 5% 24% 38% 24% 2	627
September	1st to 10th	Night	12% 3 22% 42% 18% 2	570
	11th to 20th	Night	15% 4 20% 39% 21% 1	760
	21st to end	Night	10% 7% 30% 34% 17% 3	760
October	1st to 10th	Night	14% 3 27% 39% 14% 2	760
	11th to 20th	Night	13% 5% 32% 34% 15% 2	760
	21st to end	Night	11% 9% 33% 37% 11%	836
November	1st to 10th	Night	17% 8% 33% 33% 9% 1	961
	11th to 20th	Night	11% 10% 39% 29% 10% 1	1000
	21st to end	Night	12% 8% 33% 34% 12% 1	1000
December	1st to 10th	Night	10% 8% 31% 33% 16% 2	1000
	11th to 20th	Night	10% 9% 34% 34% 12% 1	1000
	21st to end	Night	13% 6% 32% 35% 13% 2	1100

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