Brown's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brown's Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	16% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	127163
	Night	16% 10% 3 8% 5% 5% 5% 4 5% 6% 33%	124210
Transparency	Day	47% 7% 12% 21% 12% 2	130657
	Night	47% 7% 12% 20% 12% 2	128803
Seeing	Day	24% 20% 29% 17% 7% 2	16292
	Night	22% 19% 28% 23% 7% 1	25277

By Month

Percentage Cloud Cover NE (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	10% 9% 5% 17% 9% 5% 4 4 5% 5% 27%	3605
	11th to 20th	Day	11% 10% 4 12% 6% 5% 5% 4 6% 6% 31%	3570
	21st to end	Day	11% 13% 5% 10% 5% 5% 4 4 5% 5% 35%	3960
February	1st to 10th	Day	15% 11% 3 9% 6% 6% 5% 4 6% 7% 29%	1794
	11th to 20th	Day	11% 11% 5% 10% 7% 6% 4 4 5% 7% 31%	1948
	21st to end	Day	16% 12% 3 9% 6% 5% 4 3 6% 5% 31%	1650
March	1st to 10th	Day	17% 14% 4 12% 5% 5% 4 3 4 4 27%	4476
	11th to 20th	Day	15% 10% 4 8% 5% 5% 5% 5% 6% 6% 33%	4691
	21st to end	Day	21% 10% 3 8% 5% 5% 5% 3 4 7% 30%	5914
April	1st to 10th	Day	18% 12% 3 8% 4 4 4 3 5% 6% 33%	2400
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 6% 34%	2374
	21st to end	Day	20% 10% 3 8% 5% 4 4 4 4 5% 32%	2591
May	1st to 10th	Day	29% 12% 4 8% 5% 4 4 3 4 5% 21%	5886
	11th to 20th	Day	18% 12% 4 9% 5% 5% 5% 4 5% 7% 27%	6221
	21st to end	Day	18% 8% 3 7% 5% 5% 5% 4 6% 8% 32%	6949
June	1st to 10th	Day	18% 8% 3 8% 6% 5% 6% 4 6% 7% 30%	2800
	11th to 20th	Day	17% 7% 2 7% 5% 5% 5% 3 6% 9% 35%	2800
	21st to end	Day	18% 8% 3 8% 5% 5% 5% 4 6% 7% 31%	2800
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 5% 7% 10% 33%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 6% 6% 5% 8% 11% 36%	6360
	21st to end	Day	15% 6% 2 6% 4 5% 5% 4 6% 8% 40%	6926
August	1st to 10th	Day	15% 6% 3 7% 6% 5% 6% 5% 6% 8% 33%	4433
	11th to 20th	Day	14% 7% 3 7% 5% 5% 6% 4 7% 8% 33%	4076
	21st to end	Day	9% 5% 2 7% 5% 4 5% 5% 7% 9% 41%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 6% 8% 45%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 3 4 4 6% 51%	2238
	21st to end	Day	15% 6% 2 6% 4 4 4 3 6% 8% 42%	2200
October	1st to 10th	Day	20% 9% 3 7% 5% 3 3 3 4 5% 38%	2193
	11th to 20th	Day	14% 9% 3 6% 4 4 4 4 5% 8% 38%	1915
	21st to end	Day	21% 11% 3 8% 5% 4 4 3 4 6% 31%	1980
November	1st to 10th	Day	11% 8% 3 7% 5% 3 4 4 6% 7% 41%	1851
	11th to 20th	Day	15% 8% 3 7% 5% 4 4 4 5% 5% 39%	1890
	21st to end	Day	18% 11% 3 10% 4 4 4 4 5% 5% 32%	1806
December	1st to 10th	Day	12% 12% 5% 12% 6% 5% 5% 4 6% 6% 27%	2680
	11th to 20th	Day	17% 14% 4 7% 5% 4 5% 3 5% 5% 32%	3066
	21st to end	Day	18% 13% 4 10% 5% 4 4 3 4 5% 31%	4048
January	1st to 10th	Night	12% 8% 5% 16% 8% 5% 4 3 5% 6% 29%	6364
	11th to 20th	Night	10% 10% 3 13% 9% 7% 6% 5% 6% 6% 25%	6315
	21st to end	Night	13% 10% 4 9% 6% 6% 5% 4 5% 7% 31%	6996
February	1st to 10th	Night	14% 10% 4 8% 6% 5% 6% 5% 6% 7% 29%	2601
	11th to 20th	Night	11% 9% 4 8% 8% 5% 6% 6% 6% 8% 29%	2338
	21st to end	Night	13% 10% 4 10% 5% 5% 5% 3 7% 6% 33%	1980
March	1st to 10th	Night	19% 11% 3 8% 5% 5% 5% 4 6% 6% 28%	5364
	11th to 20th	Night	14% 11% 3 9% 6% 5% 5% 3 5% 6% 33%	5267
	21st to end	Night	23% 11% 3 7% 5% 5% 4 3 4 6% 29%	5014
April	1st to 10th	Night	19% 9% 3 8% 5% 5% 4 4 5% 6% 32%	2000
	11th to 20th	Night	17% 11% 3 7% 4 4 3 3 6% 7% 35%	1975
	21st to end	Night	18% 9% 2 8% 6% 5% 4 3 6% 5% 35%	1807
May	1st to 10th	Night	31% 12% 3 6% 4 4 5% 3 5% 5% 22%	4054
	11th to 20th	Night	16% 12% 3 10% 6% 5% 5% 3 5% 7% 28%	3713
	21st to end	Night	19% 8% 3 8% 6% 5% 5% 4 5% 6% 31%	3926
June	1st to 10th	Night	19% 8% 2 7% 5% 5% 5% 3 6% 7% 34%	1600
	11th to 20th	Night	19% 8% 2 5% 4 4 3 3 7% 7% 37%	1600
	21st to end	Night	16% 6% 2 7% 6% 5% 5% 4 6% 7% 35%	1600
July	1st to 10th	Night	11% 7% 2 6% 4 4 4 4 7% 9% 40%	3600
	11th to 20th	Night	12% 8% 3 7% 4 5% 5% 4 5% 7% 38%	3600
	21st to end	Night	11% 6% 2 6% 4 4 4 3 5% 6% 47%	3926
August	1st to 10th	Night	20% 8% 2 8% 4 4 4 4 5% 6% 35%	2683
	11th to 20th	Night	14% 7% 3 8% 4 4 5% 3 6% 8% 38%	2967
	21st to end	Night	8% 6% 2 4 3 4 5% 4 7% 9% 48%	3586
September	1st to 10th	Night	12% 6% 2 5% 3 4 3 4 5% 7% 50%	2000
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 5% 6% 51%	2160
	21st to end	Night	13% 7% 2 5% 3 4 4 4 5% 7% 45%	2200
October	1st to 10th	Night	20% 8% 3 6% 5% 4 3 3 5% 6% 39%	2205
	11th to 20th	Night	15% 9% 2 7% 4 5% 4 4 6% 6% 39%	2485
	21st to end	Night	20% 11% 2 7% 5% 4 4 3 5% 6% 32%	2860
November	1st to 10th	Night	13% 9% 3 8% 5% 5% 3 3 5% 6% 41%	2667
	11th to 20th	Night	14% 9% 4 7% 5% 5% 5% 4 4 5% 38%	2730
	21st to end	Night	14% 11% 4 8% 5% 4 5% 4 6% 7% 33%	2809
December	1st to 10th	Night	13% 12% 5% 13% 6% 6% 4 4 5% 6% 26%	4720
	11th to 20th	Night	21% 11% 3 9% 5% 4 4 3 4 5% 31%	5348
	21st to end	Night	18% 13% 5% 10% 5% 5% 4 4 5% 5% 27%	7150

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	53% 8% 9% 22% 8%	3606
	11th to 20th	Day	47% 9% 11% 19% 11% 3	3600
	21st to end	Day	48% 6% 9% 19% 15% 3	3960
February	1st to 10th	Day	52% 7% 12% 18% 9% 2	1794
	11th to 20th	Day	50% 8% 11% 19% 11% 2	1957
	21st to end	Day	53% 6% 9% 18% 12% 1	1650
March	1st to 10th	Day	58% 5% 9% 12% 10% 7%	4479
	11th to 20th	Day	46% 9% 15% 19% 9% 2	4691
	21st to end	Day	51% 6% 7% 14% 18% 4	5914
April	1st to 10th	Day	52% 5% 8% 19% 13% 3	2400
	11th to 20th	Day	47% 8% 9% 18% 16% 2	2374
	21st to end	Day	53% 6% 7% 17% 14% 3	2591
May	1st to 10th	Day	64% 5% 9% 14% 6% 1	5897
	11th to 20th	Day	55% 7% 13% 16% 8% 1	6225
	21st to end	Day	47% 8% 12% 23% 10% 1	6949
June	1st to 10th	Day	49% 7% 12% 21% 10% 1	2800
	11th to 20th	Day	45% 8% 11% 19% 15% 2	2800
	21st to end	Day	50% 7% 12% 18% 12% 1	2800
July	1st to 10th	Day	45% 6% 14% 21% 13%	6290
	11th to 20th	Day	38% 8% 16% 23% 14% 1	6285
	21st to end	Day	40% 6% 13% 27% 13% 1	6728
August	1st to 10th	Day	46% 7% 14% 22% 11%	4419
	11th to 20th	Day	46% 7% 16% 25% 7% 1	4115
	21st to end	Day	34% 7% 15% 28% 14% 1	4312
September	1st to 10th	Day	36% 6% 13% 28% 16% 1	2400
	11th to 20th	Day	35% 6% 8% 28% 21% 2	2238
	21st to end	Day	39% 6% 11% 26% 16% 2	2200
October	1st to 10th	Day	44% 6% 9% 23% 17% 1	3570
	11th to 20th	Day	41% 5% 12% 25% 14% 3	3092
	21st to end	Day	38% 5% 9% 31% 14% 3	3180
November	1st to 10th	Day	41% 6% 14% 23% 15% 1	1851
	11th to 20th	Day	43% 8% 11% 19% 17% 2	1890
	21st to end	Day	50% 7% 15% 20% 8% 1	1806
December	1st to 10th	Day	50% 8% 14% 21% 6%	2680
	11th to 20th	Day	49% 7% 10% 17% 15% 2	3066
	21st to end	Day	55% 5% 7% 16% 13% 5%	4048
January	1st to 10th	Night	51% 8% 11% 20% 9% 1	6364
	11th to 20th	Night	50% 12% 11% 16% 9% 2	6360
	21st to end	Night	49% 6% 11% 15% 15% 4	6996
February	1st to 10th	Night	50% 9% 11% 18% 11% 1	2601
	11th to 20th	Night	46% 9% 12% 18% 12% 2	2340
	21st to end	Night	47% 8% 12% 21% 9% 3	1980
March	1st to 10th	Night	51% 6% 10% 15% 14% 3	5374
	11th to 20th	Night	49% 7% 16% 16% 10% 2	5268
	21st to end	Night	51% 7% 7% 12% 20% 3	5014
April	1st to 10th	Night	50% 6% 11% 18% 14% 1	2000
	11th to 20th	Night	47% 7% 9% 21% 15% 1	1975
	21st to end	Night	48% 6% 11% 18% 15% 3	1807
May	1st to 10th	Night	63% 7% 10% 12% 8% 1	4060
	11th to 20th	Night	54% 8% 12% 16% 8% 1	3714
	21st to end	Night	52% 7% 13% 17% 10% 1	3927
June	1st to 10th	Night	48% 6% 12% 21% 12%	1600
	11th to 20th	Night	45% 8% 15% 19% 11% 2	1600
	21st to end	Night	44% 9% 13% 22% 11% 1	1600
July	1st to 10th	Night	39% 6% 17% 26% 11%	3560
	11th to 20th	Night	41% 9% 14% 21% 14%	3552
	21st to end	Night	35% 5% 14% 33% 12% 1	3809
August	1st to 10th	Night	49% 6% 14% 20% 10%	2673
	11th to 20th	Night	44% 7% 17% 21% 11%	2994
	21st to end	Night	29% 7% 18% 32% 14% 1	3586
September	1st to 10th	Night	33% 7% 14% 28% 18% 1	2000
	11th to 20th	Night	34% 5% 10% 28% 21% 1	2160
	21st to end	Night	37% 6% 11% 28% 16% 2	2200
October	1st to 10th	Night	42% 6% 16% 23% 12% 1	3608
	11th to 20th	Night	37% 5% 13% 24% 18% 3	4047
	21st to end	Night	42% 5% 10% 30% 12% 1	4610
November	1st to 10th	Night	43% 6% 11% 23% 15% 2	2667
	11th to 20th	Night	44% 8% 11% 21% 15% 2	2730
	21st to end	Night	45% 10% 15% 21% 9% 1	2809
December	1st to 10th	Night	53% 8% 12% 22% 5%	4720
	11th to 20th	Night	51% 8% 11% 13% 17% 1	5348
	21st to end	Night	53% 8% 9% 16% 11% 4	7150

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	29% 29% 26% 12% 4	202
	11th to 20th	Day	31% 27% 29% 11% 1 1	200
	21st to end	Day	25% 32% 30% 10% 4	220
February	1st to 10th	Day	32% 33% 26% 8% 1	324
	11th to 20th	Day	33% 30% 25% 11% 1	496
	21st to end	Day	29% 30% 27% 12% 2	414
March	1st to 10th	Day	32% 23% 32% 11% 2	500
	11th to 20th	Day	30% 30% 25% 11% 4 1	500
	21st to end	Day	32% 23% 27% 10% 5% 2	550
April	1st to 10th	Day	39% 20% 23% 11% 5% 1	500
	11th to 20th	Day	33% 24% 25% 10% 6% 1	492
	21st to end	Day	29% 24% 29% 11% 5% 2	500
May	1st to 10th	Day	33% 21% 25% 14% 5% 1	503
	11th to 20th	Day	19% 22% 37% 16% 5% 1	527
	21st to end	Day	22% 16% 32% 23% 7% 1	573
June	1st to 10th	Day	20% 17% 36% 16% 10% 1	530
	11th to 20th	Day	20% 18% 33% 20% 8% 2	530
	21st to end	Day	23% 18% 33% 17% 8% 2	530
July	1st to 10th	Day	17% 12% 34% 24% 11% 2	525
	11th to 20th	Day	14% 9% 36% 30% 9% 2	524
	21st to end	Day	13% 12% 38% 29% 6% 2	576
August	1st to 10th	Day	12% 12% 32% 27% 14% 3	526
	11th to 20th	Day	15% 12% 32% 23% 13% 6%	519
	21st to end	Day	12% 13% 33% 25% 12% 4	583
September	1st to 10th	Day	15% 12% 30% 24% 14% 5%	530
	11th to 20th	Day	15% 14% 26% 19% 17% 8%	530
	21st to end	Day	18% 15% 28% 22% 12% 5%	530
October	1st to 10th	Day	25% 15% 25% 19% 11% 5%	530
	11th to 20th	Day	22% 20% 29% 21% 6% 2	483
	21st to end	Day	32% 18% 27% 17% 5% 1	462
November	1st to 10th	Day	25% 24% 24% 16% 10%	425
	11th to 20th	Day	28% 25% 24% 17% 5% 1	430
	21st to end	Day	36% 22% 25% 13% 3	346
December	1st to 10th	Day	28% 30% 24% 15% 3	220
	11th to 20th	Day	36% 27% 26% 8% 3	220
	21st to end	Day	34% 24% 27% 10% 5%	242
January	1st to 10th	Night	22% 29% 29% 18% 2	805
	11th to 20th	Night	26% 25% 30% 16% 3 1	800
	21st to end	Night	19% 27% 32% 19% 3	880
February	1st to 10th	Night	25% 25% 33% 16% 2	800
	11th to 20th	Night	24% 26% 33% 15% 2	783
	21st to end	Night	26% 24% 31% 17% 2	660
March	1st to 10th	Night	28% 22% 31% 18% 2	800
	11th to 20th	Night	26% 23% 28% 19% 4	800
	21st to end	Night	29% 21% 26% 18% 5%	880
April	1st to 10th	Night	27% 23% 27% 20% 3	800
	11th to 20th	Night	23% 26% 28% 17% 6%	791
	21st to end	Night	27% 18% 29% 22% 4	607
May	1st to 10th	Night	29% 16% 29% 21% 5%	600
	11th to 20th	Night	18% 17% 32% 28% 4	454
	21st to end	Night	21% 8% 31% 31% 9%	433
June	1st to 10th	Night	21% 10% 28% 30% 11%	400
	11th to 20th	Night	20% 11% 29% 32% 8% 1	400
	21st to end	Night	17% 10% 27% 38% 8% 1	400
July	1st to 10th	Night	16% 6% 29% 37% 11% 1	390
	11th to 20th	Night	13% 6% 26% 39% 15% 2	388
	21st to end	Night	13% 6% 28% 40% 12% 1	438
August	1st to 10th	Night	15% 5% 21% 42% 17%	456
	11th to 20th	Night	12% 8% 31% 31% 16% 2	648
	21st to end	Night	14% 8% 24% 34% 17% 2	880
September	1st to 10th	Night	15% 10% 28% 28% 19% 1	800
	11th to 20th	Night	14% 10% 25% 28% 19% 4	800
	21st to end	Night	17% 11% 26% 30% 15% 2	800
October	1st to 10th	Night	23% 14% 22% 27% 13% 2	800
	11th to 20th	Night	19% 18% 27% 25% 10% 1	800
	21st to end	Night	27% 20% 25% 21% 6%	880
November	1st to 10th	Night	20% 22% 28% 22% 7% 1	820
	11th to 20th	Night	22% 23% 28% 20% 6% 1	840
	21st to end	Night	23% 24% 28% 19% 5%	840
December	1st to 10th	Night	24% 26% 31% 16% 4	840
	11th to 20th	Night	28% 24% 29% 16% 3	840
	21st to end	Night	29% 25% 29% 15% 2	924

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