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 2024-11-30.

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	7% 6% 2 5% 4 4 4 3 5% 6% 54%	105358
	Night	9% 7% 2 6% 4 4 4 3 5% 6% 50%	102898
Transparency	Day	28% 7% 11% 19% 20% 15%	105102
	Night	32% 8% 12% 19% 17% 12%	102743
Seeing	Day	9% 8% 23% 29% 22% 9%	14562
	Night	12% 5% 20% 38% 22% 3	23895

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	13% 15% 3 9% 7% 5% 6% 4 6% 8% 24%	2539
	11th to 20th	Day	14% 15% 4 12% 7% 5% 5% 4 5% 5% 21%	2540
	21st to end	Day	16% 12% 5% 12% 7% 5% 4 3 5% 6% 23%	2794
February	1st to 10th	Day	11% 11% 4 9% 6% 6% 5% 4 4 7% 34%	1687
	11th to 20th	Day	10% 12% 3 8% 6% 4 3 5% 4 7% 38%	1850
	21st to end	Day	8% 13% 3 6% 6% 5% 6% 4 6% 7% 36%	1570
March	1st to 10th	Day	8% 12% 4 10% 5% 5% 5% 4 7% 7% 31%	3138
	11th to 20th	Day	6% 6% 2 5% 3 3 4 3 5% 5% 58%	3352
	21st to end	Day	9% 8% 3 8% 6% 6% 7% 5% 7% 10% 32%	4009
April	1st to 10th	Day	10% 8% 3 5% 4 4 5% 4 7% 8% 41%	2280
	11th to 20th	Day	7% 8% 2 6% 5% 3 4 3 6% 8% 46%	2282
	21st to end	Day	8% 6% 2 5% 4 4 4 4 5% 8% 50%	2470
May	1st to 10th	Day	9% 6% 2 5% 4 4 4 3 6% 6% 50%	4110
	11th to 20th	Day	6% 3 1 3 2 3 3 3 4 6% 68%	4205
	21st to end	Day	6% 4 1 4 4 4 4 4 6% 8% 56%	4813
June	1st to 10th	Day	8% 5% 2 5% 4 4 4 5% 7% 9% 46%	2660
	11th to 20th	Day	6% 3 1 4 3 3 3 3 5% 7% 61%	2660
	21st to end	Day	3 2 1 2 2 2 3 2 5% 5% 73%	2660
July	1st to 10th	Day	1 1 1 2 2 2 2 2 3 4 82%	4420
	11th to 20th	Day	2 2 1 2 2 2 3 2 3 4 79%	4420
	21st to end	Day	3 2 1 3 2 2 2 2 4 5% 75%	4790
August	1st to 10th	Day	3 3 1 5% 3 3 4 3 5% 7% 64%	4225
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 5% 80%	4015
	21st to end	Day	2 2 1 3 2 2 2 2 3 4 77%	4180
September	1st to 10th	Day	2 2 1 2 2 2 2 2 4 5% 76%	2280
	11th to 20th	Day	4 3 1 4 3 3 2 3 4 6% 66%	2109
	21st to end	Day	5% 3 1 4 3 2 3 2 4 6% 67%	2090
October	1st to 10th	Day	5% 2 1 4 2 3 3 2 3 4 70%	3490
	11th to 20th	Day	6% 4 2 4 3 4 3 3 5% 7% 57%	3048
	21st to end	Day	6% 6% 2 7% 4 4 4 4 6% 7% 50%	3101
November	1st to 10th	Day	11% 8% 2 7% 5% 3 5% 4 5% 7% 43%	1764
	11th to 20th	Day	12% 9% 3 7% 6% 5% 5% 4 6% 8% 37%	1800
	21st to end	Day	11% 8% 2 10% 6% 5% 6% 4 6% 8% 34%	1765
December	1st to 10th	Day	14% 9% 4 12% 4 4 5% 4 5% 7% 33%	1520
	11th to 20th	Day	15% 10% 5% 11% 7% 5% 6% 4 6% 7% 25%	1928
	21st to end	Day	11% 10% 5% 11% 7% 6% 5% 4 6% 7% 28%	2794
January	1st to 10th	Night	14% 14% 5% 10% 7% 6% 6% 4 5% 6% 23%	4406
	11th to 20th	Night	19% 16% 4 11% 7% 6% 4 3 4 5% 22%	4420
	21st to end	Night	17% 12% 4 11% 6% 4 4 4 5% 6% 28%	4862
February	1st to 10th	Night	13% 10% 3 8% 5% 4 5% 4 5% 7% 35%	2488
	11th to 20th	Night	11% 9% 3 6% 4 5% 4 4 6% 8% 39%	2218
	21st to end	Night	11% 11% 3 8% 5% 6% 6% 5% 6% 6% 35%	1884
March	1st to 10th	Night	10% 11% 3 8% 5% 5% 4 3 6% 8% 36%	3736
	11th to 20th	Night	9% 8% 2 5% 3 3 3 3 3 6% 53%	3628
	21st to end	Night	11% 9% 2 8% 5% 5% 6% 5% 7% 9% 34%	3630
April	1st to 10th	Night	10% 7% 2 5% 4 4 6% 4 6% 6% 45%	1900
	11th to 20th	Night	10% 6% 2 5% 4 5% 5% 4 6% 6% 48%	1844
	21st to end	Night	7% 5% 2 5% 4 3 5% 3 7% 7% 52%	1710
May	1st to 10th	Night	10% 6% 2 5% 2 4 5% 4 6% 8% 50%	2860
	11th to 20th	Night	5% 3 1 3 2 3 3 3 4 5% 67%	2689
	21st to end	Night	6% 5% 2 5% 4 4 4 3 5% 7% 55%	2762
June	1st to 10th	Night	9% 5% 2 5% 5% 4 5% 4 6% 8% 48%	1520
	11th to 20th	Night	7% 4 2 4 2 3 3 3 5% 6% 61%	1520
	21st to end	Night	3 3 1 4 2 3 3 3 4 6% 69%	1520
July	1st to 10th	Night	1 1 1 1 1 2 2 2 3 4 82%	2540
	11th to 20th	Night	1 1 1 2 2 3 2 4 4 79%	2540
	21st to end	Night	2 2 1 3 3 3 4 3 5% 6% 69%	2761
August	1st to 10th	Night	3 4 2 5% 3 3 4 3 5% 6% 63%	2708
	11th to 20th	Night	1 1 2 2 2 2 2 4 5% 78%	2811
	21st to end	Night	3 2 1 3 2 2 2 2 3 4 77%	3495
September	1st to 10th	Night	3 2 1 3 2 2 2 2 3 4 76%	1900
	11th to 20th	Night	5% 3 1 4 3 3 3 3 4 6% 66%	2071
	21st to end	Night	6% 3 1 4 2 3 3 2 4 5% 68%	2090
October	1st to 10th	Night	4 3 1 3 2 3 3 3 3 5% 71%	3490
	11th to 20th	Night	6% 4 2 4 2 3 4 2 4 6% 63%	3880
	21st to end	Night	9% 6% 2 6% 3 4 4 4 6% 8% 49%	4457
November	1st to 10th	Night	12% 7% 2 6% 5% 4 4 3 5% 7% 45%	2535
	11th to 20th	Night	15% 7% 3 6% 4 4 4 4 5% 7% 39%	2600
	21st to end	Night	13% 9% 2 6% 4 3 4 3 5% 7% 45%	2625
December	1st to 10th	Night	14% 11% 3 9% 5% 5% 4 3 5% 6% 35%	2660
	11th to 20th	Night	16% 11% 4 9% 6% 6% 5% 4 5% 6% 28%	3276
	21st to end	Night	13% 11% 4 9% 6% 5% 5% 4 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	50% 17% 19% 13% 1	2539
	11th to 20th	Day	60% 14% 16% 10% 1	2540
	21st to end	Day	58% 14% 17% 10% 1	2794
February	1st to 10th	Day	48% 10% 20% 18% 3 1	1687
	11th to 20th	Day	44% 9% 19% 20% 8%	1860
	21st to end	Day	43% 10% 18% 20% 9% 1	1570
March	1st to 10th	Day	40% 15% 16% 21% 7% 1	3138
	11th to 20th	Day	25% 8% 12% 39% 14% 1	3352
	21st to end	Day	37% 13% 21% 19% 7% 2	4009
April	1st to 10th	Day	37% 8% 15% 23% 14% 3	2280
	11th to 20th	Day	33% 7% 14% 25% 17% 4	2285
	21st to end	Day	30% 7% 14% 27% 18% 4	2470
May	1st to 10th	Day	27% 8% 11% 25% 25% 5%	4110
	11th to 20th	Day	17% 5% 8% 23% 34% 13%	4208
	21st to end	Day	21% 9% 14% 31% 22% 4	4814
June	1st to 10th	Day	31% 9% 13% 24% 20% 3	2660
	11th to 20th	Day	21% 5% 9% 23% 30% 12%	2660
	21st to end	Day	12% 3 8% 18% 34% 25%	2660
July	1st to 10th	Day	8% 2 4 9% 19% 58%	4351
	11th to 20th	Day	11% 2 2 6% 21% 59%	4345
	21st to end	Day	13% 2 3 7% 31% 44%	4639
August	1st to 10th	Day	20% 3 4 11% 26% 36%	4209
	11th to 20th	Day	9% 2 4 15% 36% 34%	4054
	21st to end	Day	13% 2 4 15% 36% 30%	4179
September	1st to 10th	Day	11% 3 5% 16% 31% 34%	2280
	11th to 20th	Day	18% 4 10% 24% 27% 16%	2109
	21st to end	Day	20% 3 6% 25% 34% 11%	2090
October	1st to 10th	Day	19% 3 7% 17% 39% 13%	3490
	11th to 20th	Day	27% 4 12% 28% 25% 4	3048
	21st to end	Day	31% 5% 13% 30% 20% 1	3101
November	1st to 10th	Day	39% 7% 17% 26% 12%	1764
	11th to 20th	Day	43% 10% 20% 22% 5%	1800
	21st to end	Day	43% 12% 23% 18% 4	1765
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	53% 16% 20% 9% 1	4406
	11th to 20th	Night	60% 14% 13% 11% 2	4420
	21st to end	Night	54% 13% 17% 13% 2	4862
February	1st to 10th	Night	46% 10% 19% 21% 3 1	2488
	11th to 20th	Night	39% 10% 20% 21% 10% 1	2219
	21st to end	Night	43% 9% 17% 21% 10% 1	1884
March	1st to 10th	Night	37% 12% 15% 24% 11% 1	3736
	11th to 20th	Night	31% 6% 9% 31% 22% 2	3628
	21st to end	Night	37% 14% 19% 24% 7%	3630
April	1st to 10th	Night	35% 7% 13% 25% 17% 3	1900
	11th to 20th	Night	32% 7% 11% 26% 20% 3	1844
	21st to end	Night	29% 6% 12% 27% 23% 3	1710
May	1st to 10th	Night	25% 9% 12% 24% 24% 6%	2860
	11th to 20th	Night	17% 5% 8% 21% 39% 10%	2689
	21st to end	Night	25% 8% 12% 27% 24% 4	2762
June	1st to 10th	Night	31% 9% 13% 22% 22% 2	1520
	11th to 20th	Night	23% 5% 8% 20% 33% 10%	1520
	21st to end	Night	15% 4 6% 17% 31% 26%	1520
July	1st to 10th	Night	8% 2 3 8% 20% 59%	2500
	11th to 20th	Night	9% 2 3 6% 18% 62%	2492
	21st to end	Night	17% 3 3 6% 28% 43%	2674
August	1st to 10th	Night	21% 3 5% 9% 22% 40%	2700
	11th to 20th	Night	10% 2 4 9% 27% 48%	2838
	21st to end	Night	14% 2 3 12% 33% 36%	3495
September	1st to 10th	Night	13% 3 4 13% 29% 39%	1900
	11th to 20th	Night	19% 4 8% 19% 25% 25%	2071
	21st to end	Night	20% 3 5% 20% 32% 20%	2090
October	1st to 10th	Night	17% 3 6% 16% 31% 26%	3490
	11th to 20th	Night	23% 3 8% 28% 31% 6%	3880
	21st to end	Night	32% 5% 10% 28% 21% 3	4457
November	1st to 10th	Night	38% 6% 13% 24% 18% 1	2535
	11th to 20th	Night	41% 8% 15% 27% 10%	2600
	21st to end	Night	39% 7% 19% 27% 9%	2625
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	17% 9% 21% 28% 19% 6%	180
	21st to end	Day	13% 6% 22% 33% 20% 6%	198
February	1st to 10th	Day	15% 15% 27% 23% 15% 4	295
	11th to 20th	Day	17% 13% 27% 25% 15% 4	455
	21st to end	Day	13% 14% 30% 26% 14% 3	382
March	1st to 10th	Day	13% 12% 32% 30% 12% 2	460
	11th to 20th	Day	12% 10% 28% 31% 15% 5%	460
	21st to end	Day	13% 12% 31% 24% 17% 3	506
April	1st to 10th	Day	15% 16% 25% 20% 18% 7%	460
	11th to 20th	Day	9% 15% 32% 25% 15% 5%	452
	21st to end	Day	9% 15% 32% 25% 15% 4	460
May	1st to 10th	Day	5% 13% 30% 29% 16% 6%	463
	11th to 20th	Day	16% 7% 20% 29% 18% 9%	488
	21st to end	Day	9% 5% 17% 31% 22% 15%	529
June	1st to 10th	Day	8% 7% 21% 31% 22% 12%	490
	11th to 20th	Day	6% 10% 25% 34% 18% 6%	490
	21st to end	Day	4 4 22% 33% 24% 11%	490
July	1st to 10th	Day	2 2 20% 38% 28% 9%	486
	11th to 20th	Day	2 2 21% 38% 28% 9%	484
	21st to end	Day	3 2 15% 34% 36% 11%	531
August	1st to 10th	Day	4 3 19% 31% 34% 9%	487
	11th to 20th	Day	2 1 16% 32% 32% 16%	477
	21st to end	Day	3 3 19% 29% 27% 19%	539
September	1st to 10th	Day	4 2 14% 27% 34% 18%	490
	11th to 20th	Day	5% 4 16% 30% 28% 16%	490
	21st to end	Day	4 6% 19% 31% 28% 13%	490
October	1st to 10th	Day	7% 8% 16% 25% 29% 16%	490
	11th to 20th	Day	11% 6% 21% 24% 24% 14%	388
	21st to end	Day	10% 9% 22% 32% 21% 5%	330
November	1st to 10th	Day	14% 8% 22% 30% 20% 5%	300
	11th to 20th	Day	16% 6% 24% 30% 19% 6%	300
	21st to end	Day	17% 6% 15% 31% 25% 6%	284
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	760
	21st to end	Night	16% 5% 20% 32% 25% 3	836
February	1st to 10th	Night	17% 7% 25% 35% 15% 2	760
	11th to 20th	Night	14% 5% 25% 38% 17% 1	744
	21st to end	Night	14% 7% 30% 36% 13% 1	628
March	1st to 10th	Night	14% 9% 29% 33% 13% 1	760
	11th to 20th	Night	13% 8% 22% 39% 15% 2	760
	21st to end	Night	14% 9% 28% 34% 13% 1	836
April	1st to 10th	Night	12% 9% 22% 38% 18% 2	760
	11th to 20th	Night	11% 8% 27% 36% 15% 2	716
	21st to end	Night	8% 8% 24% 44% 14% 2	570
May	1st to 10th	Night	9% 7% 27% 38% 18% 1	570
	11th to 20th	Night	8% 6% 23% 34% 25% 3	482
	21st to end	Night	8% 7% 19% 36% 26% 5%	411
June	1st to 10th	Night	10% 5% 20% 38% 23% 4	380
	11th to 20th	Night	9% 7% 24% 37% 21% 2	380
	21st to end	Night	4 3 20% 43% 27% 2	380
July	1st to 10th	Night	2 1 17% 52% 27% 2	370
	11th to 20th	Night	1 1 16% 59% 22%	368
	21st to end	Night	3 14% 50% 30% 2	416
August	1st to 10th	Night	6% 1 13% 46% 32% 1	479
	11th to 20th	Night	1 2 13% 50% 33% 1	564
	21st to end	Night	3 3 14% 44% 32% 4	833
September	1st to 10th	Night	4 1 12% 43% 36% 5%	760
	11th to 20th	Night	6% 2 12% 39% 36% 5%	760
	21st to end	Night	8% 3 16% 35% 29% 9%	760
October	1st to 10th	Night	8% 3 18% 39% 27% 4	760
	11th to 20th	Night	9% 4 15% 41% 26% 6%	760
	21st to end	Night	11% 5% 20% 39% 22% 4	836
November	1st to 10th	Night	11% 6% 19% 38% 23% 3	780
	11th to 20th	Night	18% 6% 22% 35% 17% 2	800
	21st to end	Night	16% 3 19% 37% 21% 4	800
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

Page generated at 2024-11-27 15:30:51. Version ac07

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