Brooks Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brooks Astronomical Observatory clear sky chart from 2005-11-06 to 2024-11-25.

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	13% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	101077
	Night	14% 10% 4 8% 5% 5% 4 4 5% 7% 34%	98421
Transparency	Day	47% 7% 12% 21% 12% 1	104649
	Night	47% 7% 12% 21% 12% 1	102976
Seeing	Day	18% 16% 30% 20% 11% 4	15745
	Night	19% 12% 31% 27% 10% 1	25775

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	14% 20% 7% 12% 7% 5% 5% 3 4 5% 18%	2527
	11th to 20th	Day	13% 18% 7% 17% 13% 10% 5% 3 4 4 7%	2530
	21st to end	Day	21% 21% 8% 13% 6% 5% 5% 3 4 5% 10%	2788
February	1st to 10th	Day	11% 13% 4 13% 9% 6% 5% 5% 5% 6% 22%	1710
	11th to 20th	Day	11% 14% 5% 13% 9% 7% 5% 3 4 6% 23%	1843
	21st to end	Day	12% 13% 5% 12% 8% 7% 5% 4 5% 6% 24%	1570
March	1st to 10th	Day	14% 12% 4 8% 6% 5% 4 3 5% 7% 33%	3120
	11th to 20th	Day	11% 9% 4 11% 7% 6% 5% 4 5% 6% 31%	3442
	21st to end	Day	18% 11% 5% 8% 6% 5% 4 3 4 5% 31%	3869
April	1st to 10th	Day	17% 14% 3 8% 6% 4 4 3 5% 6% 30%	2280
	11th to 20th	Day	16% 10% 4 7% 4 4 4 4 5% 7% 36%	2361
	21st to end	Day	16% 9% 4 8% 6% 5% 4 4 5% 6% 34%	2470
May	1st to 10th	Day	16% 8% 2 6% 5% 5% 5% 4 6% 7% 37%	4082
	11th to 20th	Day	14% 6% 2 7% 4 4 5% 4 6% 9% 39%	4081
	21st to end	Day	10% 6% 2 6% 4 4 4 4 6% 9% 45%	4781
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 10% 42%	2660
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 5% 9% 43%	2660
	21st to end	Day	11% 6% 2 6% 4 5% 5% 5% 7% 8% 40%	2660
July	1st to 10th	Day	10% 7% 3 8% 5% 6% 6% 5% 7% 8% 36%	4440
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 6% 10% 51%	4440
	21st to end	Day	7% 4 2 6% 5% 5% 5% 4 7% 10% 45%	4796
August	1st to 10th	Day	11% 5% 2 7% 5% 5% 5% 5% 6% 7% 41%	4104
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 5% 7% 47%	4044
	21st to end	Day	10% 5% 2 7% 5% 5% 6% 4 6% 8% 43%	4357
September	1st to 10th	Day	10% 5% 2 7% 6% 6% 4 4 6% 8% 43%	2218
	11th to 20th	Day	10% 5% 2 6% 4 4 4 3 6% 8% 47%	2090
	21st to end	Day	12% 6% 3 8% 4 5% 4 4 5% 8% 39%	2090
October	1st to 10th	Day	15% 7% 2 7% 5% 4 5% 3 5% 6% 42%	2089
	11th to 20th	Day	19% 10% 3 8% 5% 4 5% 4 5% 7% 29%	1848
	21st to end	Day	20% 9% 5% 9% 5% 6% 6% 4 5% 7% 25%	1881
November	1st to 10th	Day	14% 9% 4 9% 5% 4 4 3 6% 7% 36%	1762
	11th to 20th	Day	13% 15% 5% 9% 6% 6% 5% 4 6% 6% 24%	1800
	21st to end	Day	19% 15% 5% 10% 5% 5% 4 3 5% 5% 23%	1742
December	1st to 10th	Day	13% 17% 6% 17% 7% 5% 4 3 4 5% 19%	1406
	11th to 20th	Day	14% 15% 6% 11% 6% 5% 5% 5% 6% 6% 22%	1753
	21st to end	Day	26% 19% 6% 12% 6% 5% 5% 4 4 4 10%	2783
January	1st to 10th	Night	15% 18% 8% 12% 6% 5% 4 3 5% 5% 19%	4428
	11th to 20th	Night	13% 15% 7% 16% 9% 9% 5% 4 5% 5% 13%	4440
	21st to end	Night	20% 19% 7% 12% 6% 4 4 3 5% 5% 15%	4876
February	1st to 10th	Night	13% 14% 4 12% 8% 6% 6% 3 5% 6% 23%	2470
	11th to 20th	Night	12% 14% 5% 10% 7% 6% 4 4 5% 6% 27%	2220
	21st to end	Night	15% 13% 4 10% 6% 5% 5% 5% 5% 5% 26%	1884
March	1st to 10th	Night	14% 8% 4 8% 5% 4 4 3 5% 6% 39%	3733
	11th to 20th	Night	10% 7% 2 6% 6% 5% 5% 4 6% 8% 41%	3517
	21st to end	Night	15% 11% 4 8% 5% 4 4 3 5% 7% 34%	3791
April	1st to 10th	Night	15% 11% 3 6% 5% 5% 5% 3 5% 6% 36%	1900
	11th to 20th	Night	18% 9% 2 7% 4 4 4 3 5% 6% 38%	1768
	21st to end	Night	16% 9% 3 5% 5% 4 3 4 6% 7% 38%	1710
May	1st to 10th	Night	12% 8% 3 7% 5% 4 4 4 5% 8% 39%	2813
	11th to 20th	Night	16% 6% 2 7% 3 4 4 3 5% 7% 42%	2812
	21st to end	Night	13% 6% 2 6% 4 4 3 4 5% 7% 46%	2782
June	1st to 10th	Night	10% 7% 2 6% 5% 4 4 4 6% 9% 43%	1520
	11th to 20th	Night	10% 5% 3 7% 4 4 4 5% 6% 8% 46%	1520
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 4 5% 9% 48%	1520
July	1st to 10th	Night	7% 7% 2 7% 5% 4 6% 4 6% 8% 42%	2530
	11th to 20th	Night	6% 4 2 5% 3 4 4 3 5% 7% 57%	2530
	21st to end	Night	7% 4 2 6% 4 4 5% 4 7% 8% 49%	2763
August	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 6% 8% 50%	2825
	11th to 20th	Night	12% 5% 2 4 3 3 4 3 6% 7% 51%	2785
	21st to end	Night	9% 5% 2 6% 4 4 4 4 5% 8% 48%	3283
September	1st to 10th	Night	9% 4 2 5% 4 3 4 3 5% 7% 52%	1943
	11th to 20th	Night	10% 4 2 4 3 4 4 3 5% 7% 56%	2090
	21st to end	Night	12% 7% 2 6% 4 5% 5% 4 5% 7% 43%	2090
October	1st to 10th	Night	17% 5% 2 4 4 3 3 3 5% 6% 48%	2090
	11th to 20th	Night	18% 10% 3 7% 5% 4 5% 3 4 5% 36%	2329
	21st to end	Night	21% 10% 3 8% 4 4 4 3 5% 7% 30%	2717
November	1st to 10th	Night	13% 10% 3 7% 5% 4 4 4 5% 6% 41%	2536
	11th to 20th	Night	15% 13% 4 8% 6% 5% 5% 4 7% 7% 27%	2600
	21st to end	Night	17% 13% 4 9% 5% 5% 5% 4 5% 7% 26%	2529
December	1st to 10th	Night	14% 17% 5% 11% 6% 6% 5% 4 5% 5% 20%	2770
	11th to 20th	Night	14% 15% 5% 11% 7% 5% 5% 4 6% 6% 20%	3423
	21st to end	Night	27% 21% 5% 9% 5% 4 3 3 3 4 15%	4884

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	65% 8% 14% 9% 2	2528
	11th to 20th	Day	81% 7% 6% 5% 1	2530
	21st to end	Day	75% 9% 8% 6% 3	2788
February	1st to 10th	Day	61% 7% 11% 15% 5%	1710
	11th to 20th	Day	62% 6% 10% 14% 8% 1	1853
	21st to end	Day	61% 6% 11% 15% 6% 1	1570
March	1st to 10th	Day	51% 7% 12% 21% 8% 1	3120
	11th to 20th	Day	49% 9% 13% 19% 9% 1	3442
	21st to end	Day	56% 6% 11% 16% 10% 2	3869
April	1st to 10th	Day	54% 5% 11% 18% 11%	2280
	11th to 20th	Day	48% 6% 12% 20% 12% 2	2361
	21st to end	Day	50% 6% 12% 18% 13% 2	2470
May	1st to 10th	Day	42% 9% 12% 19% 16% 2	4120
	11th to 20th	Day	38% 10% 14% 25% 12% 1	4085
	21st to end	Day	35% 8% 12% 22% 22% 2	4794
June	1st to 10th	Day	37% 7% 13% 27% 14% 1	2660
	11th to 20th	Day	37% 6% 14% 23% 18% 1	2660
	21st to end	Day	37% 9% 15% 24% 14% 2	2660
July	1st to 10th	Day	43% 6% 14% 20% 15% 2	4371
	11th to 20th	Day	28% 6% 15% 34% 17% 1	4365
	21st to end	Day	34% 6% 15% 22% 22% 2	4645
August	1st to 10th	Day	41% 5% 11% 27% 15% 1	4091
	11th to 20th	Day	36% 6% 12% 30% 15% 1	4083
	21st to end	Day	38% 5% 14% 28% 14% 1	4356
September	1st to 10th	Day	37% 8% 15% 25% 13% 1	2218
	11th to 20th	Day	34% 7% 15% 29% 15% 1	2090
	21st to end	Day	41% 8% 13% 25% 12%	2090
October	1st to 10th	Day	31% 4 8% 33% 21% 3	3466
	11th to 20th	Day	41% 4 12% 29% 13%	3051
	21st to end	Day	53% 5% 12% 22% 7% 1	3081
November	1st to 10th	Day	46% 7% 14% 22% 11%	1762
	11th to 20th	Day	58% 7% 13% 15% 6%	1800
	21st to end	Day	62% 6% 14% 13% 4	1742
December	1st to 10th	Day	68% 7% 10% 12% 2	1406
	11th to 20th	Day	61% 8% 14% 14% 3	1749
	21st to end	Day	74% 8% 8% 7% 3	2783
January	1st to 10th	Night	64% 8% 11% 12% 5% 1	4428
	11th to 20th	Night	72% 9% 8% 8% 3	4440
	21st to end	Night	69% 8% 11% 8% 3	4876
February	1st to 10th	Night	60% 7% 10% 16% 6% 1	2470
	11th to 20th	Night	57% 6% 11% 17% 8% 1	2221
	21st to end	Night	56% 7% 13% 15% 8% 1	1884
March	1st to 10th	Night	45% 7% 15% 25% 8% 1	3733
	11th to 20th	Night	36% 8% 16% 24% 15% 2	3517
	21st to end	Night	49% 8% 14% 20% 9% 1	3791
April	1st to 10th	Night	48% 5% 11% 20% 15% 1	1900
	11th to 20th	Night	46% 6% 9% 21% 16% 3	1768
	21st to end	Night	44% 7% 11% 21% 14% 3	1710
May	1st to 10th	Night	41% 9% 12% 19% 16% 3	2840
	11th to 20th	Night	39% 7% 11% 25% 16% 2	2813
	21st to end	Night	37% 7% 11% 21% 20% 4	2790
June	1st to 10th	Night	36% 7% 12% 26% 17% 2	1520
	11th to 20th	Night	35% 7% 14% 22% 21% 1	1520
	21st to end	Night	32% 7% 13% 29% 17% 2	1520
July	1st to 10th	Night	38% 5% 11% 26% 14% 7%	2490
	11th to 20th	Night	26% 5% 13% 33% 22% 1	2482
	21st to end	Night	32% 6% 18% 22% 20% 2	2675
August	1st to 10th	Night	32% 6% 14% 30% 18% 1	2814
	11th to 20th	Night	32% 5% 13% 33% 16% 1	2812
	21st to end	Night	35% 4 12% 31% 16% 1	3283
September	1st to 10th	Night	30% 7% 13% 30% 19% 1	1943
	11th to 20th	Night	27% 6% 14% 34% 18% 1	2090
	21st to end	Night	39% 7% 15% 26% 13% 1	2090
October	1st to 10th	Night	27% 4 11% 35% 22% 1	3480
	11th to 20th	Night	43% 4 11% 24% 17% 1	3865
	21st to end	Night	45% 4 10% 29% 11%	4467
November	1st to 10th	Night	42% 6% 14% 24% 14% 1	2536
	11th to 20th	Night	53% 8% 15% 17% 8% 1	2600
	21st to end	Night	56% 7% 16% 16% 5% 1	2529
December	1st to 10th	Night	64% 8% 12% 12% 4	2770
	11th to 20th	Night	61% 8% 14% 11% 6% 1	3425
	21st to end	Night	73% 6% 10% 7% 4	4884

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	24% 19% 37% 14% 4 1	270
	11th to 20th	Day	25% 22% 38% 11% 4	270
	21st to end	Day	27% 19% 36% 16% 3	297
February	1st to 10th	Day	24% 28% 31% 13% 1 1	270
	11th to 20th	Day	23% 29% 37% 9% 1	270
	21st to end	Day	34% 32% 28% 6%	225
March	1st to 10th	Day	26% 30% 36% 7%	270
	11th to 20th	Day	25% 19% 31% 20% 5%	435
	21st to end	Day	28% 18% 30% 18% 5% 1	506
April	1st to 10th	Day	24% 20% 32% 17% 5% 1	460
	11th to 20th	Day	30% 21% 29% 11% 6% 3	452
	21st to end	Day	21% 19% 30% 20% 8% 2	460
May	1st to 10th	Day	24% 19% 30% 16% 7% 3	463
	11th to 20th	Day	18% 15% 30% 21% 11% 5%	487
	21st to end	Day	14% 13% 29% 25% 14% 5%	704
June	1st to 10th	Day	11% 14% 36% 26% 9% 4	680
	11th to 20th	Day	14% 13% 35% 23% 11% 5%	680
	21st to end	Day	14% 11% 34% 25% 13% 3	680
July	1st to 10th	Day	8% 7% 26% 29% 21% 8%	671
	11th to 20th	Day	9% 10% 28% 28% 17% 8%	668
	21st to end	Day	8% 8% 30% 29% 18% 7%	734
August	1st to 10th	Day	9% 7% 26% 24% 20% 13%	486
	11th to 20th	Day	10% 5% 21% 31% 21% 11%	479
	21st to end	Day	13% 8% 23% 28% 21% 7%	539
September	1st to 10th	Day	11% 11% 28% 23% 16% 11%	490
	11th to 20th	Day	9% 8% 20% 27% 29% 8%	490
	21st to end	Day	19% 13% 22% 23% 18% 6%	490
October	1st to 10th	Day	19% 16% 26% 21% 14% 4	490
	11th to 20th	Day	24% 18% 34% 18% 6% 1	438
	21st to end	Day	27% 21% 36% 12% 3 1	330
November	1st to 10th	Day	23% 21% 36% 14% 5%	300
	11th to 20th	Day	23% 25% 38% 9% 4 1	300
	21st to end	Day	25% 27% 33% 13% 3	280
December	1st to 10th	Day	29% 28% 27% 9% 6% 1	180
	11th to 20th	Day	24% 29% 36% 10% 1	204
	21st to end	Day	30% 27% 32% 10% 1 1	297
January	1st to 10th	Night	22% 20% 34% 18% 5% 1	950
	11th to 20th	Night	23% 19% 31% 22% 4 1	950
	21st to end	Night	19% 14% 37% 24% 7%	1037
February	1st to 10th	Night	21% 16% 35% 24% 4	760
	11th to 20th	Night	21% 18% 34% 24% 4	743
	21st to end	Night	24% 22% 34% 18% 3	628
March	1st to 10th	Night	19% 16% 33% 25% 6%	760
	11th to 20th	Night	18% 17% 35% 23% 6% 1	760
	21st to end	Night	22% 15% 33% 24% 5% 1	822
April	1st to 10th	Night	21% 15% 36% 23% 5% 1	570
	11th to 20th	Night	22% 16% 31% 23% 7% 1	563
	21st to end	Night	21% 12% 33% 26% 7% 1	570
May	1st to 10th	Night	17% 9% 36% 30% 7% 1	570
	11th to 20th	Night	19% 7% 33% 30% 10% 1	561
	21st to end	Night	14% 6% 30% 32% 17% 1	617
June	1st to 10th	Night	11% 8% 31% 36% 13% 1	570
	11th to 20th	Night	13% 6% 29% 35% 15% 1	570
	21st to end	Night	9% 5% 30% 38% 17% 1	570
July	1st to 10th	Night	7% 4 21% 41% 23% 4	555
	11th to 20th	Night	9% 4 22% 40% 22% 3	552
	21st to end	Night	8% 2 29% 40% 20% 1	624
August	1st to 10th	Night	9% 3 22% 44% 20% 2	567
	11th to 20th	Night	9% 3 21% 39% 25% 3	564
	21st to end	Night	12% 5% 21% 35% 24% 2	627
September	1st to 10th	Night	11% 5% 27% 34% 21% 4	613
	11th to 20th	Night	11% 5% 22% 35% 24% 3	760
	21st to end	Night	15% 8% 27% 29% 18% 3	760
October	1st to 10th	Night	20% 8% 28% 28% 14% 1	760
	11th to 20th	Night	25% 14% 30% 23% 7% 1	760
	21st to end	Night	26% 14% 33% 21% 5%	836
November	1st to 10th	Night	20% 15% 33% 22% 9%	780
	11th to 20th	Night	23% 19% 32% 20% 6%	800
	21st to end	Night	24% 17% 32% 22% 4	779
December	1st to 10th	Night	24% 17% 33% 21% 4	872
	11th to 20th	Night	24% 18% 37% 18% 3	950
	21st to end	Night	32% 16% 33% 18% 2	1045

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