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 2026-07-01.

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% 7% 3 7% 5% 5% 5% 4 6% 8% 42%	142005
	Night	7% 8% 3 9% 5% 5% 5% 4 6% 8% 38%	139041
Transparency	Day	38% 7% 15% 26% 12% 2	141702
	Night	41% 7% 16% 24% 11% 1	138927
Seeing	Day	11% 7% 25% 30% 20% 6%	17639
	Night	12% 7% 29% 36% 15% 2	26801

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	4 14% 7% 14% 8% 6% 5% 5% 7% 8% 23%	4125
	11th to 20th	Day	3 8% 5% 10% 7% 6% 7% 7% 7% 7% 34%	4119
	21st to end	Day	2 8% 4 9% 6% 5% 5% 3 5% 8% 43%	5048
February	1st to 10th	Day	8% 13% 4 12% 7% 7% 5% 4 6% 7% 27%	1787
	11th to 20th	Day	7% 12% 4 11% 7% 8% 5% 5% 7% 8% 26%	1845
	21st to end	Day	4 13% 6% 13% 6% 6% 5% 4 5% 8% 31%	1594
March	1st to 10th	Day	5% 8% 3 10% 6% 5% 5% 4 6% 8% 39%	4744
	11th to 20th	Day	6% 8% 3 9% 5% 5% 5% 5% 7% 8% 38%	5109
	21st to end	Day	9% 14% 5% 12% 5% 5% 5% 4 7% 8% 26%	5607
April	1st to 10th	Day	9% 11% 4 8% 7% 5% 6% 4 6% 9% 32%	2573
	11th to 20th	Day	9% 11% 4 8% 6% 6% 5% 4 6% 8% 34%	2700
	21st to end	Day	10% 9% 3 8% 5% 6% 6% 4 6% 8% 35%	2730
May	1st to 10th	Day	12% 7% 3 5% 4 4 4 4 7% 8% 43%	7998
	11th to 20th	Day	10% 7% 3 8% 6% 6% 6% 5% 8% 9% 32%	8643
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 8% 41%	9649
June	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 8% 38%	3150
	11th to 20th	Day	15% 7% 3 6% 5% 5% 5% 5% 7% 9% 32%	3150
	21st to end	Day	7% 5% 1 5% 4 4 5% 5% 8% 9% 46%	3150
July	1st to 10th	Day	7% 3 2 4 3 3 4 4 6% 7% 56%	6781
	11th to 20th	Day	7% 3 2 5% 4 4 5% 4 6% 8% 52%	6780
	21st to end	Day	6% 4 2 4 2 3 3 3 5% 8% 62%	7376
August	1st to 10th	Day	6% 4 2 6% 4 4 4 4 6% 8% 52%	4432
	11th to 20th	Day	6% 3 2 5% 4 4 4 4 6% 9% 54%	4158
	21st to end	Day	8% 3 1 3 3 3 3 3 5% 8% 60%	4664
September	1st to 10th	Day	10% 5% 2 4 2 3 4 3 5% 7% 55%	2501
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 4 7% 48%	2200
	21st to end	Day	8% 7% 2 5% 4 4 4 4 6% 8% 50%	2200
October	1st to 10th	Day	6% 5% 3 8% 4 4 5% 5% 6% 7% 45%	3340
	11th to 20th	Day	5% 6% 2 5% 4 5% 5% 4 7% 9% 48%	3174
	21st to end	Day	7% 6% 2 6% 4 4 4 4 6% 7% 49%	3181
November	1st to 10th	Day	11% 13% 5% 9% 7% 5% 4 4 6% 8% 29%	1648
	11th to 20th	Day	5% 12% 5% 9% 6% 6% 4 3 6% 8% 35%	1680
	21st to end	Day	5% 12% 5% 9% 7% 5% 5% 3 6% 10% 31%	1522
December	1st to 10th	Day	8% 14% 4 9% 6% 7% 6% 4 7% 7% 27%	2360
	11th to 20th	Day	8% 11% 4 11% 8% 6% 5% 4 6% 7% 30%	2712
	21st to end	Day	3 8% 4 10% 6% 5% 5% 4 6% 7% 43%	3575
January	1st to 10th	Night	5% 12% 5% 13% 8% 6% 5% 4 7% 8% 27%	8768
	11th to 20th	Night	3 8% 4 11% 6% 5% 6% 4 6% 9% 39%	8779
	21st to end	Night	3 8% 4 10% 6% 6% 6% 4 7% 8% 40%	9178
February	1st to 10th	Night	6% 11% 4 11% 7% 6% 6% 5% 7% 8% 31%	2832
	11th to 20th	Night	5% 12% 4 10% 6% 6% 7% 5% 7% 9% 30%	2662
	21st to end	Night	5% 10% 4 10% 6% 6% 5% 4 7% 10% 33%	2209
March	1st to 10th	Night	3 7% 4 11% 7% 5% 5% 5% 6% 10% 36%	5369
	11th to 20th	Night	7% 8% 3 8% 5% 5% 6% 5% 8% 9% 35%	5110
	21st to end	Night	8% 15% 5% 11% 6% 6% 6% 5% 7% 8% 22%	5610
April	1st to 10th	Night	8% 11% 3 10% 6% 5% 5% 5% 6% 9% 32%	2042
	11th to 20th	Night	9% 11% 3 9% 7% 5% 6% 5% 6% 8% 33%	1871
	21st to end	Night	9% 9% 4 8% 5% 4 5% 5% 6% 8% 37%	1890
May	1st to 10th	Night	14% 8% 3 7% 4 4 4 4 5% 7% 38%	4948
	11th to 20th	Night	11% 6% 3 9% 5% 6% 6% 5% 7% 9% 34%	4206
	21st to end	Night	11% 6% 3 8% 6% 5% 6% 5% 7% 8% 35%	4522
June	1st to 10th	Night	15% 6% 3 6% 4 5% 6% 4 7% 9% 34%	1470
	11th to 20th	Night	12% 6% 2 7% 5% 6% 5% 6% 8% 9% 34%	1470
	21st to end	Night	8% 4 1 5% 5% 4 4 4 7% 9% 49%	1470
July	1st to 10th	Night	6% 3 1 5% 4 4 5% 4 6% 7% 54%	3180
	11th to 20th	Night	7% 4 2 5% 5% 5% 5% 5% 8% 10% 45%	3180
	21st to end	Night	7% 4 2 5% 3 4 4 4 5% 9% 54%	3467
August	1st to 10th	Night	7% 4 2 5% 4 4 4 4 6% 9% 47%	2705
	11th to 20th	Night	6% 2 1 6% 5% 5% 5% 4 7% 11% 46%	2901
	21st to end	Night	8% 2 1 4 2 2 3 3 5% 8% 61%	3245
September	1st to 10th	Night	10% 4 2 5% 4 4 4 4 6% 8% 48%	1895
	11th to 20th	Night	12% 5% 1 4 4 4 3 3 6% 6% 52%	2200
	21st to end	Night	7% 6% 2 5% 4 4 4 4 6% 9% 50%	2200
October	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 6% 8% 50%	3851
	11th to 20th	Night	5% 5% 2 6% 4 5% 5% 5% 6% 8% 48%	3960
	21st to end	Night	5% 7% 3 8% 4 4 5% 4 6% 8% 45%	4600
November	1st to 10th	Night	10% 12% 4 8% 5% 5% 4 5% 6% 8% 34%	2856
	11th to 20th	Night	5% 9% 4 8% 6% 6% 6% 4 7% 9% 36%	2940
	21st to end	Night	7% 11% 4 10% 7% 5% 5% 4 7% 8% 33%	3092
December	1st to 10th	Night	5% 13% 5% 11% 7% 6% 7% 5% 7% 8% 27%	5040
	11th to 20th	Night	7% 12% 5% 12% 7% 6% 6% 5% 6% 8% 28%	5700
	21st to end	Night	4 8% 4 9% 4 5% 5% 4 6% 7% 43%	7623

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	56% 7% 12% 18% 6%	4125
	11th to 20th	Day	46% 6% 16% 16% 13% 2	4140
	21st to end	Day	39% 6% 20% 27% 7%	5049
February	1st to 10th	Day	54% 6% 15% 14% 8% 1	1787
	11th to 20th	Day	51% 10% 13% 17% 9% 1	1854
	21st to end	Day	51% 7% 12% 21% 9%	1594
March	1st to 10th	Day	40% 7% 13% 25% 15%	4744
	11th to 20th	Day	41% 6% 17% 23% 13%	5110
	21st to end	Day	52% 9% 17% 17% 6%	5607
April	1st to 10th	Day	47% 8% 15% 21% 8%	2573
	11th to 20th	Day	46% 7% 14% 24% 8% 1	2700
	21st to end	Day	43% 8% 14% 24% 10% 1	2730
May	1st to 10th	Day	38% 5% 10% 28% 14% 4	7998
	11th to 20th	Day	42% 8% 13% 26% 10% 1	8647
	21st to end	Day	36% 9% 14% 30% 11%	9650
June	1st to 10th	Day	42% 9% 15% 21% 12% 1	3150
	11th to 20th	Day	45% 9% 15% 21% 9% 2	3150
	21st to end	Day	30% 8% 19% 30% 10% 2	3150
July	1st to 10th	Day	27% 5% 16% 27% 20% 5%	6707
	11th to 20th	Day	30% 5% 19% 30% 15% 2	6699
	21st to end	Day	22% 5% 16% 29% 18% 10%	7169
August	1st to 10th	Day	28% 6% 14% 36% 14% 2	4414
	11th to 20th	Day	25% 6% 20% 38% 9% 1	4197
	21st to end	Day	22% 4 16% 40% 16% 2	4663
September	1st to 10th	Day	28% 5% 15% 28% 16% 7%	2501
	11th to 20th	Day	35% 7% 15% 27% 14% 2	2200
	21st to end	Day	31% 6% 16% 30% 14% 3	2200
October	1st to 10th	Day	35% 6% 19% 29% 10% 1	3340
	11th to 20th	Day	33% 5% 18% 31% 13% 1	3174
	21st to end	Day	33% 6% 10% 37% 13% 1	3181
November	1st to 10th	Day	52% 7% 13% 18% 8% 1	1648
	11th to 20th	Day	46% 7% 18% 21% 8%	1680
	21st to end	Day	47% 7% 17% 20% 8% 1	1522
December	1st to 10th	Day	53% 8% 18% 17% 3 1	2360
	11th to 20th	Day	52% 6% 18% 19% 4 1	2714
	21st to end	Day	40% 5% 14% 25% 15% 1	3575
January	1st to 10th	Night	52% 7% 15% 18% 8% 1	8769
	11th to 20th	Night	42% 5% 18% 20% 13% 1	8820
	21st to end	Night	41% 5% 19% 27% 8%	9178
February	1st to 10th	Night	49% 7% 12% 20% 10% 1	2832
	11th to 20th	Night	48% 9% 13% 18% 11% 1	2663
	21st to end	Night	43% 8% 17% 22% 9% 1	2209
March	1st to 10th	Night	40% 8% 19% 23% 10%	5369
	11th to 20th	Night	39% 7% 18% 27% 8% 1	5110
	21st to end	Night	55% 10% 15% 15% 5% 1	5610
April	1st to 10th	Night	46% 8% 15% 21% 10%	2042
	11th to 20th	Night	46% 8% 13% 22% 10% 2	1871
	21st to end	Night	42% 8% 13% 24% 11% 1	1890
May	1st to 10th	Night	44% 5% 7% 25% 15% 3	4945
	11th to 20th	Night	41% 9% 15% 24% 11% 1	4206
	21st to end	Night	42% 9% 14% 26% 9% 1	4526
June	1st to 10th	Night	42% 7% 17% 22% 11% 1	1470
	11th to 20th	Night	41% 10% 16% 22% 10% 1	1470
	21st to end	Night	30% 6% 19% 32% 11% 1	1470
July	1st to 10th	Night	28% 5% 19% 23% 22% 2	3145
	11th to 20th	Night	32% 6% 17% 31% 11% 2	3138
	21st to end	Night	28% 5% 13% 30% 16% 8%	3369
August	1st to 10th	Night	31% 6% 17% 27% 17% 2	2698
	11th to 20th	Night	30% 6% 18% 30% 14% 2	2928
	21st to end	Night	22% 4 14% 36% 20% 3	3245
September	1st to 10th	Night	31% 7% 13% 25% 18% 7%	1895
	11th to 20th	Night	31% 7% 14% 28% 15% 4	2200
	21st to end	Night	29% 7% 16% 30% 16% 2	2200
October	1st to 10th	Night	32% 5% 13% 35% 14% 2	3851
	11th to 20th	Night	32% 6% 16% 29% 15% 2	3960
	21st to end	Night	37% 6% 12% 35% 11% 1	4600
November	1st to 10th	Night	46% 7% 15% 21% 9% 1	2856
	11th to 20th	Night	42% 8% 20% 20% 10% 1	2940
	21st to end	Night	47% 9% 18% 18% 8% 1	3092
December	1st to 10th	Night	53% 7% 18% 17% 5% 1	5040
	11th to 20th	Night	52% 7% 24% 13% 3	5697
	21st to end	Night	39% 5% 13% 24% 17% 2	7623

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% 7% 30% 31% 16% 9%	220
	11th to 20th	Day	8% 10% 32% 32% 12% 5%	220
	21st to end	Day	7% 9% 32% 33% 14% 5%	242
February	1st to 10th	Day	15% 8% 34% 30% 11% 3	277
	11th to 20th	Day	9% 5% 29% 37% 15% 4	330
	21st to end	Day	8% 10% 26% 33% 18% 5%	310
March	1st to 10th	Day	10% 13% 31% 29% 13% 5%	540
	11th to 20th	Day	11% 9% 32% 30% 13% 5%	540
	21st to end	Day	11% 10% 34% 25% 16% 5%	594
April	1st to 10th	Day	13% 12% 28% 29% 16% 3	540
	11th to 20th	Day	13% 10% 30% 29% 16% 2	532
	21st to end	Day	12% 9% 26% 31% 17% 5%	540
May	1st to 10th	Day	14% 8% 21% 30% 21% 6%	670
	11th to 20th	Day	15% 6% 19% 32% 22% 7%	775
	21st to end	Day	14% 6% 18% 28% 25% 9%	844
June	1st to 10th	Day	14% 10% 22% 29% 20% 4	780
	11th to 20th	Day	14% 8% 30% 29% 17% 3	780
	21st to end	Day	11% 3 21% 35% 22% 8%	780
July	1st to 10th	Day	7% 2 18% 34% 30% 9%	722
	11th to 20th	Day	9% 3 20% 35% 25% 7%	718
	21st to end	Day	6% 3 20% 31% 30% 10%	794
August	1st to 10th	Day	6% 4 17% 34% 26% 13%	545
	11th to 20th	Day	7% 3 17% 31% 30% 11%	517
	21st to end	Day	8% 5% 19% 30% 26% 11%	583
September	1st to 10th	Day	11% 6% 22% 29% 24% 7%	530
	11th to 20th	Day	13% 5% 23% 28% 24% 6%	530
	21st to end	Day	12% 11% 28% 29% 15% 4	530
October	1st to 10th	Day	15% 5% 23% 29% 21% 7%	374
	11th to 20th	Day	12% 9% 30% 28% 16% 5%	330
	21st to end	Day	15% 13% 29% 27% 14% 3	363
November	1st to 10th	Day	18% 11% 29% 28% 12% 2	330
	11th to 20th	Day	10% 8% 41% 26% 12% 2	330
	21st to end	Day	10% 9% 35% 28% 11% 7%	247
December	1st to 10th	Day	12% 10% 31% 26% 18% 3	220
	11th to 20th	Day	13% 14% 36% 25% 9% 3	220
	21st to end	Day	12% 6% 25% 31% 20% 6%	242
January	1st to 10th	Night	11% 9% 30% 34% 14% 2	1050
	11th to 20th	Night	12% 11% 30% 33% 12% 1	1050
	21st to end	Night	12% 8% 32% 34% 14% 1	991
February	1st to 10th	Night	12% 7% 27% 36% 16% 3	840
	11th to 20th	Night	9% 7% 31% 37% 14% 1	824
	21st to end	Night	9% 5% 27% 36% 21% 3	692
March	1st to 10th	Night	8% 7% 32% 37% 14% 2	840
	11th to 20th	Night	10% 10% 31% 34% 13% 2	840
	21st to end	Night	12% 8% 29% 34% 15% 1	924
April	1st to 10th	Night	14% 7% 30% 33% 13% 2	708
	11th to 20th	Night	15% 5% 31% 35% 13%	623
	21st to end	Night	15% 6% 28% 37% 14% 1	630
May	1st to 10th	Night	16% 6% 24% 33% 20% 1	630
	11th to 20th	Night	11% 4 28% 37% 19% 1	452
	21st to end	Night	18% 4 24% 31% 22% 2	455
June	1st to 10th	Night	16% 8% 25% 34% 16% 2	420
	11th to 20th	Night	14% 7% 31% 34% 14% 1	420
	21st to end	Night	10% 4 26% 36% 21% 3	420
July	1st to 10th	Night	7% 5% 20% 46% 21% 1	390
	11th to 20th	Night	8% 5% 21% 47% 17% 1	388
	21st to end	Night	5% 4 22% 42% 24% 3	438
August	1st to 10th	Night	8% 4 22% 38% 25% 2	476
	11th to 20th	Night	8% 3 20% 42% 25% 2	594
	21st to end	Night	8% 5% 23% 39% 23% 2	660
September	1st to 10th	Night	12% 3 21% 44% 18% 2	600
	11th to 20th	Night	14% 4 19% 40% 22% 1	800
	21st to end	Night	10% 7% 30% 35% 17% 2	800
October	1st to 10th	Night	13% 4 28% 39% 14% 2	800
	11th to 20th	Night	13% 5% 32% 34% 14% 2	800
	21st to end	Night	10% 10% 34% 36% 10%	880
November	1st to 10th	Night	17% 9% 33% 32% 8% 1	1011
	11th to 20th	Night	11% 10% 38% 29% 10% 1	1050
	21st to end	Night	12% 9% 32% 35% 11% 1	1050
December	1st to 10th	Night	11% 8% 31% 33% 15% 2	1050
	11th to 20th	Night	11% 10% 34% 33% 12% 1	1050
	21st to end	Night	13% 6% 32% 35% 12% 2	1155

Page generated at 2026-06-28 00:10:47. Version ac07

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