Brown Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brown Observing Site clear sky chart from 2005-11-06 to 2024-01-15.

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% 6% 2 5% 5% 3 4 4 5% 7% 51%	74854
	Night	10% 7% 3 5% 5% 3 4 4 5% 7% 48%	74933
Transparency	Day	31% 7% 13% 20% 18% 11%	74558
	Night	34% 8% 13% 20% 16% 9%	74750
Seeing	Day	10% 7% 23% 29% 22% 9%	13374
	Night	12% 5% 20% 38% 22% 3	22816

By Month

Percentage Cloud Cover SW (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	15% 11% 4 8% 8% 4 5% 7% 6% 7% 26%	1519
	11th to 20th	Day	13% 8% 4 11% 9% 4 7% 6% 6% 7% 26%	1472
	21st to end	Day	12% 11% 4 11% 8% 4 6% 5% 6% 8% 25%	1584
February	1st to 10th	Day	11% 10% 5% 8% 7% 4 5% 5% 4 7% 34%	1599
	11th to 20th	Day	10% 12% 3 6% 6% 3 4 5% 4 7% 39%	1750
	21st to end	Day	9% 12% 4 5% 6% 4 6% 5% 6% 7% 35%	1480
March	1st to 10th	Day	10% 9% 5% 6% 6% 4 6% 6% 6% 7% 35%	1800
	11th to 20th	Day	10% 8% 4 7% 5% 3 5% 6% 6% 7% 39%	1894
	21st to end	Day	10% 8% 3 7% 7% 4 6% 5% 6% 9% 34%	2285
April	1st to 10th	Day	10% 8% 3 4 4 3 5% 6% 7% 9% 41%	2160
	11th to 20th	Day	7% 8% 3 6% 6% 2 4 4 6% 8% 46%	2160
	21st to end	Day	9% 6% 2 4 4 3 5% 5% 5% 7% 51%	2340
May	1st to 10th	Day	6% 5% 2 4 4 4 4 4 6% 7% 55%	2340
	11th to 20th	Day	10% 5% 2 5% 4 3 4 5% 5% 8% 50%	2370
	21st to end	Day	8% 4 2 4 5% 3 5% 5% 7% 9% 47%	2723
June	1st to 10th	Day	8% 5% 2 4 5% 3 5% 5% 6% 10% 46%	2520
	11th to 20th	Day	7% 3 2 3 4 3 3 4 5% 7% 60%	2520
	21st to end	Day	3 2 1 2 2 1 3 3 4 5% 73%	2520
July	1st to 10th	Day	2 2 1 2 3 2 3 4 4 5% 73%	2520
	11th to 20th	Day	2 2 1 1 1 2 2 3 3 4 80%	2520
	21st to end	Day	4 2 1 2 3 2 2 3 4 4 74%	2741
August	1st to 10th	Day	4 2 1 3 3 2 4 3 4 6% 66%	2410
	11th to 20th	Day	2 1 1 1 3 1 2 3 3 6% 77%	2299
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 76%	2376
September	1st to 10th	Day	2 2 1 2 2 2 2 3 3 4 77%	2160
	11th to 20th	Day	4 3 1 3 3 2 2 3 4 5% 68%	1998
	21st to end	Day	6% 3 2 3 3 2 3 3 4 6% 65%	1980
October	1st to 10th	Day	8% 4 2 5% 3 2 5% 4 4 6% 58%	1980
	11th to 20th	Day	7% 4 1 4 4 3 3 4 6% 7% 57%	1744
	21st to end	Day	8% 5% 2 6% 4 3 4 4 6% 8% 50%	1782
November	1st to 10th	Day	12% 7% 3 5% 6% 2 5% 5% 5% 8% 43%	1674
	11th to 20th	Day	12% 8% 3 5% 7% 4 6% 5% 6% 8% 37%	1710
	21st to end	Day	10% 8% 2 8% 8% 3 6% 5% 6% 9% 34%	1682
December	1st to 10th	Day	14% 9% 4 10% 5% 3 4 5% 5% 7% 33%	1520
	11th to 20th	Day	15% 10% 5% 10% 8% 4 5% 5% 6% 7% 25%	1928
	21st to end	Day	11% 10% 5% 9% 9% 4 5% 6% 6% 7% 28%	2794
January	1st to 10th	Night	15% 11% 5% 8% 7% 4 6% 6% 5% 7% 26%	2646
	11th to 20th	Night	15% 9% 4 10% 7% 5% 5% 5% 6% 7% 29%	2576
	21st to end	Night	12% 11% 5% 9% 6% 4 5% 6% 5% 6% 32%	2772
February	1st to 10th	Night	13% 9% 3 7% 6% 3 5% 5% 5% 7% 36%	2356
	11th to 20th	Night	10% 8% 3 6% 5% 3 6% 5% 6% 9% 40%	2098
	21st to end	Night	11% 10% 3 7% 6% 4 6% 6% 6% 6% 34%	1776
March	1st to 10th	Night	12% 9% 3 7% 6% 3 5% 4 6% 8% 36%	2160
	11th to 20th	Night	12% 8% 3 6% 5% 3 5% 5% 5% 9% 40%	2066
	21st to end	Night	12% 9% 3 6% 6% 3 6% 6% 6% 8% 36%	2070
April	1st to 10th	Night	10% 6% 3 5% 5% 3 6% 4 7% 6% 45%	1800
	11th to 20th	Night	10% 6% 2 5% 5% 4 5% 5% 5% 7% 47%	1746
	21st to end	Night	7% 5% 2 4 5% 3 4 3 6% 7% 53%	1620
May	1st to 10th	Night	6% 5% 1 4 3 2 4 5% 5% 8% 56%	1620
	11th to 20th	Night	8% 5% 2 4 4 3 5% 7% 7% 8% 48%	1519
	21st to end	Night	8% 5% 3 5% 7% 3 7% 5% 6% 9% 43%	1552
June	1st to 10th	Night	9% 4 3 4 6% 4 5% 5% 5% 8% 48%	1440
	11th to 20th	Night	7% 4 2 3 3 2 4 3 5% 7% 60%	1440
	21st to end	Night	3 3 1 3 3 2 3 4 4 6% 69%	1440
July	1st to 10th	Night	2 2 1 1 2 2 3 4 4 5% 72%	1440
	11th to 20th	Night	1 1 1 2 1 1 3 4 6% 79%	1440
	21st to end	Night	3 2 1 2 3 2 3 4 4 5% 72%	1576
August	1st to 10th	Night	4 3 1 3 4 3 2 3 5% 6% 66%	1527
	11th to 20th	Night	1 1 1 2 1 2 3 5% 5% 76%	1602
	21st to end	Night	3 1 1 2 3 2 2 3 4 5% 74%	1977
September	1st to 10th	Night	3 2 1 2 2 2 2 2 3 4 76%	1800
	11th to 20th	Night	4 2 1 3 3 2 3 4 4 6% 67%	1962
	21st to end	Night	6% 3 1 3 3 2 4 3 4 5% 67%	1980
October	1st to 10th	Night	6% 4 1 4 4 3 4 4 4 6% 60%	1980
	11th to 20th	Night	8% 3 2 3 4 2 4 4 5% 6% 59%	2212
	21st to end	Night	9% 5% 2 4 4 2 4 5% 5% 8% 51%	2574
November	1st to 10th	Night	13% 7% 3 5% 5% 3 4 4 5% 7% 45%	2405
	11th to 20th	Night	16% 6% 3 5% 5% 3 4 5% 5% 7% 40%	2470
	21st to end	Night	13% 8% 3 5% 5% 2 4 4 5% 6% 44%	2493
December	1st to 10th	Night	14% 10% 3 8% 5% 4 4 4 5% 6% 36%	2660
	11th to 20th	Night	16% 11% 4 8% 7% 4 5% 5% 5% 6% 28%	3276
	21st to end	Night	14% 10% 4 8% 7% 4 5% 5% 5% 6% 31%	4862

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	54% 15% 18% 13% 1	1519
	11th to 20th	Day	55% 14% 18% 13% 1	1472
	21st to end	Day	55% 14% 19% 10% 2	1584
February	1st to 10th	Day	47% 10% 21% 18% 4 1	1599
	11th to 20th	Day	43% 9% 19% 20% 8%	1760
	21st to end	Day	44% 9% 17% 20% 10% 1	1480
March	1st to 10th	Day	44% 11% 16% 22% 7% 1	1800
	11th to 20th	Day	40% 10% 16% 23% 10% 1	1894
	21st to end	Day	44% 9% 19% 19% 8% 1	2285
April	1st to 10th	Day	37% 8% 15% 22% 14% 3	2160
	11th to 20th	Day	34% 7% 14% 24% 17% 4	2163
	21st to end	Day	31% 7% 13% 27% 19% 4	2340
May	1st to 10th	Day	24% 7% 12% 27% 25% 5%	2340
	11th to 20th	Day	30% 8% 12% 26% 20% 5%	2373
	21st to end	Day	30% 9% 16% 24% 17% 4	2724
June	1st to 10th	Day	32% 8% 14% 24% 20% 3	2520
	11th to 20th	Day	22% 5% 9% 23% 29% 10%	2520
	21st to end	Day	13% 3 8% 18% 34% 24%	2520
July	1st to 10th	Day	13% 4 6% 16% 28% 33%	2451
	11th to 20th	Day	10% 2 3 9% 28% 48%	2445
	21st to end	Day	13% 3 4 9% 27% 44%	2589
August	1st to 10th	Day	18% 4 8% 16% 25% 29%	2393
	11th to 20th	Day	10% 3 6% 16% 32% 32%	2299
	21st to end	Day	12% 2 6% 15% 37% 27%	2376
September	1st to 10th	Day	11% 3 5% 16% 30% 34%	2160
	11th to 20th	Day	17% 4 10% 24% 28% 17%	1998
	21st to end	Day	22% 4 6% 25% 32% 11%	1980
October	1st to 10th	Day	27% 5% 12% 26% 24% 6%	1980
	11th to 20th	Day	24% 6% 13% 33% 21% 3	1744
	21st to end	Day	30% 7% 17% 29% 15% 2	1782
November	1st to 10th	Day	39% 7% 17% 25% 12%	1674
	11th to 20th	Day	42% 10% 21% 22% 5%	1710
	21st to end	Day	43% 12% 23% 18% 5%	1682
December	1st to 10th	Day	48% 13% 20% 14% 5%	1520
	11th to 20th	Day	55% 13% 20% 10% 2	1928
	21st to end	Day	48% 14% 20% 15% 4	2794
January	1st to 10th	Night	54% 12% 20% 11% 2	2646
	11th to 20th	Night	53% 11% 15% 17% 4	2576
	21st to end	Night	50% 10% 21% 15% 4	2772
February	1st to 10th	Night	45% 11% 20% 21% 3 1	2356
	11th to 20th	Night	38% 10% 19% 22% 10% 1	2099
	21st to end	Night	44% 9% 17% 20% 10% 1	1776
March	1st to 10th	Night	44% 8% 13% 21% 11% 2	2160
	11th to 20th	Night	39% 7% 12% 26% 14% 1	2066
	21st to end	Night	43% 9% 15% 23% 9% 1	2070
April	1st to 10th	Night	35% 7% 13% 24% 18% 3	1800
	11th to 20th	Night	34% 8% 11% 25% 20% 3	1746
	21st to end	Night	29% 5% 12% 27% 23% 3	1620
May	1st to 10th	Night	24% 6% 10% 22% 33% 5%	1620
	11th to 20th	Night	29% 8% 13% 24% 22% 5%	1519
	21st to end	Night	34% 10% 12% 26% 14% 5%	1552
June	1st to 10th	Night	32% 9% 13% 22% 22% 2	1440
	11th to 20th	Night	24% 6% 9% 20% 33% 9%	1440
	21st to end	Night	16% 4 7% 17% 30% 26%	1440
July	1st to 10th	Night	13% 3 5% 14% 30% 35%	1400
	11th to 20th	Night	7% 3 5% 10% 26% 49%	1392
	21st to end	Night	14% 3 5% 8% 23% 47%	1488
August	1st to 10th	Night	18% 3 8% 14% 22% 35%	1519
	11th to 20th	Night	10% 3 5% 12% 30% 40%	1602
	21st to end	Night	13% 4 5% 14% 30% 35%	1977
September	1st to 10th	Night	13% 3 4 13% 28% 39%	1800
	11th to 20th	Night	18% 4 8% 18% 26% 27%	1962
	21st to end	Night	21% 4 5% 20% 32% 19%	1980
October	1st to 10th	Night	25% 5% 9% 23% 26% 11%	1980
	11th to 20th	Night	24% 5% 10% 29% 26% 5%	2212
	21st to end	Night	30% 6% 14% 26% 21% 3	2574
November	1st to 10th	Night	38% 6% 13% 24% 18% 1	2405
	11th to 20th	Night	40% 8% 15% 27% 10%	2470
	21st to end	Night	39% 7% 18% 27% 9%	2493
December	1st to 10th	Night	48% 10% 19% 16% 6%	2660
	11th to 20th	Night	53% 10% 18% 17% 2	3276
	21st to end	Night	45% 14% 20% 15% 5%	4862

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	21% 9% 22% 26% 17% 5%	180
	11th to 20th	Day	16% 8% 22% 29% 18% 7%	168
	21st to end	Day	14% 5% 20% 35% 20% 6%	176
February	1st to 10th	Day	15% 13% 28% 24% 15% 4	267
	11th to 20th	Day	18% 11% 27% 25% 15% 4	415
	21st to end	Day	13% 14% 32% 25% 14% 3	346
March	1st to 10th	Day	13% 12% 31% 30% 11% 2	420
	11th to 20th	Day	13% 10% 29% 31% 14% 3	420
	21st to end	Day	13% 12% 32% 24% 15% 3	462
April	1st to 10th	Day	15% 16% 25% 20% 18% 6%	420
	11th to 20th	Day	9% 15% 31% 25% 14% 5%	412
	21st to end	Day	10% 15% 32% 24% 15% 4	420
May	1st to 10th	Day	5% 13% 29% 30% 16% 7%	423
	11th to 20th	Day	18% 6% 20% 29% 18% 9%	448
	21st to end	Day	10% 5% 17% 31% 22% 16%	485
June	1st to 10th	Day	8% 6% 22% 30% 21% 12%	450
	11th to 20th	Day	6% 10% 26% 34% 18% 6%	450
	21st to end	Day	5% 4 22% 33% 24% 12%	450
July	1st to 10th	Day	2 2 19% 39% 28% 9%	446
	11th to 20th	Day	2 2 22% 38% 28% 7%	444
	21st to end	Day	3 2 15% 35% 35% 10%	487
August	1st to 10th	Day	4 3 19% 30% 34% 9%	447
	11th to 20th	Day	2 2 15% 31% 33% 17%	437
	21st to end	Day	3 3 17% 29% 29% 18%	495
September	1st to 10th	Day	5% 3 14% 28% 33% 18%	450
	11th to 20th	Day	5% 4 17% 30% 28% 16%	450
	21st to end	Day	4 6% 19% 31% 27% 13%	450
October	1st to 10th	Day	7% 8% 17% 25% 28% 14%	450
	11th to 20th	Day	11% 6% 21% 25% 24% 13%	354
	21st to end	Day	11% 9% 22% 29% 22% 6%	297
November	1st to 10th	Day	16% 7% 23% 31% 20% 4	270
	11th to 20th	Day	13% 6% 22% 32% 21% 6%	270
	21st to end	Day	16% 6% 15% 33% 25% 5%	257
December	1st to 10th	Day	22% 4 22% 22% 18% 12%	180
	11th to 20th	Day	17% 8% 30% 25% 15% 6%	180
	21st to end	Day	19% 8% 27% 24% 18% 5%	198
January	1st to 10th	Night	17% 9% 22% 31% 19% 3	760
	11th to 20th	Night	18% 7% 21% 32% 20% 3	736
	21st to end	Night	16% 5% 20% 31% 25% 3	792
February	1st to 10th	Night	17% 5% 25% 36% 15% 2	720
	11th to 20th	Night	14% 4 25% 38% 18% 1	704
	21st to end	Night	14% 6% 31% 34% 14% 1	592
March	1st to 10th	Night	15% 8% 29% 33% 13% 2	720
	11th to 20th	Night	14% 8% 22% 40% 14% 2	720
	21st to end	Night	15% 9% 28% 34% 13% 1	792
April	1st to 10th	Night	11% 9% 22% 38% 18% 2	720
	11th to 20th	Night	12% 8% 26% 36% 16% 2	678
	21st to end	Night	9% 8% 24% 43% 15% 2	540
May	1st to 10th	Night	9% 7% 26% 38% 19% 1	540
	11th to 20th	Night	9% 5% 23% 34% 25% 4	457
	21st to end	Night	8% 6% 19% 35% 27% 5%	389
June	1st to 10th	Night	10% 5% 20% 37% 24% 4	360
	11th to 20th	Night	9% 7% 24% 35% 22% 2	360
	21st to end	Night	4 3 20% 43% 28% 3	360
July	1st to 10th	Night	2 1 17% 50% 28% 2	350
	11th to 20th	Night	1 1 17% 59% 21%	348
	21st to end	Night	3 1 14% 51% 30% 2	394
August	1st to 10th	Night	6% 2 13% 46% 32% 2	453
	11th to 20th	Night	1 1 11% 51% 35% 1	534
	21st to end	Night	3 3 14% 44% 33% 4	789
September	1st to 10th	Night	4 1 12% 43% 35% 5%	720
	11th to 20th	Night	5% 2 12% 38% 37% 6%	720
	21st to end	Night	8% 3 16% 35% 29% 9%	720
October	1st to 10th	Night	9% 3 18% 39% 27% 4	720
	11th to 20th	Night	9% 4 16% 40% 26% 6%	720
	21st to end	Night	11% 4 20% 38% 23% 4	792
November	1st to 10th	Night	12% 6% 19% 37% 24% 3	740
	11th to 20th	Night	18% 6% 21% 36% 18% 2	760
	21st to end	Night	16% 3 19% 37% 22% 4	760
December	1st to 10th	Night	18% 6% 19% 37% 17% 2	760
	11th to 20th	Night	20% 6% 19% 36% 16% 2	760
	21st to end	Night	24% 7% 25% 32% 11% 1	836

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