Brogan
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brogan clear sky chart from 2005-11-06 to 2026-09-29.

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	6% 6% 2 6% 4 4 4 4 5% 7% 53%	158070
	Night	7% 7% 3 6% 4 4 4 3 5% 6% 51%	150703
Transparency	Day	31% 4 9% 21% 21% 14%	152834
	Night	34% 4 11% 22% 19% 10%	146940
Seeing	Day	10% 7% 25% 32% 21% 6%	16885
	Night	11% 6% 25% 37% 18% 3	26027

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	11% 18% 7% 15% 6% 6% 5% 4 4 4 21%	4722
	11th to 20th	Day	9% 11% 3 10% 9% 5% 5% 6% 6% 6% 30%	4740
	21st to end	Day	13% 12% 4 11% 6% 7% 5% 5% 6% 5% 24%	5187
February	1st to 10th	Day	10% 14% 4 11% 6% 4 4 3 6% 6% 30%	1809
	11th to 20th	Day	8% 11% 5% 10% 7% 5% 6% 4 6% 7% 30%	2050
	21st to end	Day	6% 11% 3 10% 5% 5% 6% 4 7% 8% 35%	1730
March	1st to 10th	Day	5% 13% 4 11% 6% 6% 5% 4 6% 9% 31%	4618
	11th to 20th	Day	5% 7% 2 6% 4 4 5% 4 6% 8% 49%	4787
	21st to end	Day	6% 9% 3 9% 6% 7% 7% 5% 7% 9% 33%	6055
April	1st to 10th	Day	7% 8% 3 6% 5% 5% 6% 6% 6% 9% 39%	2520
	11th to 20th	Day	5% 7% 2 7% 6% 4 5% 4 6% 8% 45%	2489
	21st to end	Day	7% 8% 2 6% 5% 4 4 4 5% 7% 48%	2725
May	1st to 10th	Day	5% 4 1 5% 3 3 4 3 5% 6% 60%	7657
	11th to 20th	Day	5% 3 1 4 3 3 4 3 6% 8% 60%	8160
	21st to end	Day	4 4 2 5% 4 4 4 4 6% 8% 57%	8992
June	1st to 10th	Day	8% 5% 2 5% 6% 5% 4 4 7% 9% 45%	2940
	11th to 20th	Day	6% 4 1 4 4 4 4 4 6% 7% 55%	2940
	21st to end	Day	3 2 1 4 3 3 3 3 4 6% 67%	2940
July	1st to 10th	Day	2 2 1 2 1 2 2 2 4 5% 78%	8219
	11th to 20th	Day	2 2 1 3 2 3 3 2 4 5% 74%	8220
	21st to end	Day	2 1 1 2 2 2 3 3 5% 6% 73%	8922
August	1st to 10th	Day	3 2 1 3 3 3 3 3 4 7% 68%	8064
	11th to 20th	Day	2 2 1 2 2 2 3 2 4 6% 74%	7395
	21st to end	Day	3 3 1 5% 3 3 4 3 5% 7% 63%	7750
September	1st to 10th	Day	2 2 1 3 2 3 3 3 4 6% 71%	2520
	11th to 20th	Day	4 3 1 4 3 3 4 3 6% 7% 61%	2389
	21st to end	Day	4 4 2 5% 3 3 3 3 4 6% 64%	2289
October	1st to 10th	Day	4 3 1 4 3 3 2 3 4 5% 68%	3519
	11th to 20th	Day	5% 6% 2 5% 3 4 5% 3 4 8% 55%	3146
	21st to end	Day	8% 6% 2 6% 4 5% 5% 5% 5% 7% 46%	3200
November	1st to 10th	Day	12% 9% 3 5% 4 4 4 4 5% 8% 41%	1854
	11th to 20th	Day	11% 11% 3 9% 5% 4 5% 4 6% 6% 35%	1890
	21st to end	Day	9% 10% 4 10% 7% 5% 5% 4 7% 7% 32%	1732
December	1st to 10th	Day	10% 11% 4 11% 8% 4 4 7% 6% 6% 29%	2700
	11th to 20th	Day	17% 17% 8% 17% 7% 4 4 4 3 4 15%	3108
	21st to end	Day	12% 15% 7% 13% 7% 5% 5% 4 4 6% 21%	4092
January	1st to 10th	Night	15% 16% 7% 11% 6% 5% 5% 4 5% 6% 22%	8172
	11th to 20th	Night	10% 8% 3 6% 4 4 4 4 4 5% 48%	8220
	21st to end	Night	13% 11% 4 8% 4 4 4 3 5% 6% 39%	8994
February	1st to 10th	Night	10% 11% 4 8% 5% 4 4 4 6% 7% 37%	2808
	11th to 20th	Night	9% 10% 4 8% 5% 5% 5% 4 6% 7% 37%	2458
	21st to end	Night	7% 10% 4 7% 5% 5% 5% 5% 6% 8% 38%	2076
March	1st to 10th	Night	5% 10% 3 9% 6% 5% 5% 4 5% 8% 41%	5496
	11th to 20th	Night	7% 10% 3 9% 5% 4 4 3 4 5% 47%	5432
	21st to end	Night	6% 9% 3 8% 6% 6% 6% 5% 7% 9% 35%	5148
April	1st to 10th	Night	8% 8% 3 7% 5% 4 5% 3 6% 8% 45%	2100
	11th to 20th	Night	7% 7% 2 7% 5% 5% 4 3 5% 6% 49%	2079
	21st to end	Night	8% 5% 2 4 4 4 4 4 7% 7% 51%	1895
May	1st to 10th	Night	6% 4 1 4 3 3 3 3 4 6% 64%	5285
	11th to 20th	Night	4 3 2 5% 3 3 3 3 4 7% 63%	4700
	21st to end	Night	5% 4 2 5% 4 3 4 3 5% 7% 57%	5181
June	1st to 10th	Night	6% 4 2 5% 5% 5% 5% 4 6% 8% 49%	1680
	11th to 20th	Night	7% 4 1 3 4 3 3 3 4 8% 59%	1680
	21st to end	Night	4 2 1 4 3 3 3 3 4 6% 67%	1680
July	1st to 10th	Night	1 2 1 3 2 2 3 3 4 5% 75%	4740
	11th to 20th	Night	1 1 1 2 1 2 2 3 5% 5% 77%	4740
	21st to end	Night	2 2 1 3 2 3 3 3 5% 7% 68%	5153
August	1st to 10th	Night	2 2 1 3 2 3 3 3 4 6% 71%	4717
	11th to 20th	Night	2 2 1 3 2 2 3 2 4 6% 74%	5466
	21st to end	Night	4 4 2 4 3 3 3 3 5% 6% 63%	6499
September	1st to 10th	Night	4 2 1 3 2 2 3 3 4 5% 72%	2100
	11th to 20th	Night	3 2 1 4 3 3 3 4 5% 5% 66%	2230
	21st to end	Night	7% 3 1 3 2 3 3 2 4 6% 65%	2288
October	1st to 10th	Night	4 3 1 3 3 3 3 2 3 5% 70%	3679
	11th to 20th	Night	4 4 2 4 3 3 4 3 4 7% 61%	4006
	21st to end	Night	7% 5% 2 6% 4 4 4 4 6% 7% 51%	4600
November	1st to 10th	Night	11% 7% 2 7% 4 5% 5% 4 5% 6% 43%	2665
	11th to 20th	Night	12% 9% 2 7% 5% 4 5% 4 5% 7% 39%	2730
	21st to end	Night	9% 10% 3 7% 4 4 4 4 4 6% 45%	2884
December	1st to 10th	Night	8% 7% 3 8% 5% 3 3 3 4 5% 50%	4700
	11th to 20th	Night	16% 16% 5% 12% 6% 5% 5% 4 5% 5% 22%	5316
	21st to end	Night	15% 15% 5% 9% 5% 5% 5% 4 5% 6% 28%	7106

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	64% 5% 13% 13% 4	4723
	11th to 20th	Day	49% 8% 14% 20% 8%	4740
	21st to end	Day	57% 6% 12% 16% 7% 2	5187
February	1st to 10th	Day	53% 6% 18% 17% 6%	1809
	11th to 20th	Day	51% 8% 13% 20% 7% 1	2060
	21st to end	Day	44% 8% 15% 20% 12% 1	1730
March	1st to 10th	Day	46% 8% 14% 18% 13% 1	4618
	11th to 20th	Day	30% 4 15% 36% 15%	4787
	21st to end	Day	42% 8% 17% 22% 9% 1	6055
April	1st to 10th	Day	39% 6% 15% 26% 12% 2	2520
	11th to 20th	Day	35% 5% 13% 26% 19% 3	2492
	21st to end	Day	34% 5% 11% 28% 19% 3	2725
May	1st to 10th	Day	23% 3 6% 27% 36% 5%	7657
	11th to 20th	Day	21% 4 8% 31% 29% 8%	8163
	21st to end	Day	23% 5% 9% 30% 26% 6%	8994
June	1st to 10th	Day	34% 5% 10% 27% 21% 3	2940
	11th to 20th	Day	27% 4 8% 24% 27% 10%	2940
	21st to end	Day	19% 2 5% 21% 34% 18%	2940
July	1st to 10th	Day	11% 2 2 11% 32% 43%	8151
	11th to 20th	Day	15% 1 2 7% 22% 54%	8145
	21st to end	Day	12% 2 2 13% 32% 39%	8723
August	1st to 10th	Day	16% 2 2 8% 25% 47%	6389
	11th to 20th	Day	13% 2 4 21% 36% 24%	5830
	21st to end	Day	18% 2 4 24% 31% 21%	6077
September	1st to 10th	Day	15% 3 3 23% 33% 23%	2520
	11th to 20th	Day	21% 4 8% 26% 29% 14%	2389
	21st to end	Day	22% 3 7% 30% 29% 9%	2289
October	1st to 10th	Day	20% 3 7% 24% 37% 10%	3519
	11th to 20th	Day	29% 3 15% 30% 19% 4	3146
	21st to end	Day	35% 5% 14% 29% 15% 1	3200
November	1st to 10th	Day	41% 6% 18% 25% 10% 1	1854
	11th to 20th	Day	47% 7% 19% 20% 6% 1	1890
	21st to end	Day	48% 5% 19% 18% 8% 1	1732
December	1st to 10th	Day	49% 6% 24% 17% 5%	2700
	11th to 20th	Day	72% 7% 9% 9% 3	3108
	21st to end	Day	63% 5% 13% 13% 5%	4092
January	1st to 10th	Night	62% 6% 16% 12% 5%	8172
	11th to 20th	Night	38% 5% 18% 27% 11%	8220
	21st to end	Night	47% 5% 14% 21% 10% 3	8994
February	1st to 10th	Night	46% 7% 20% 20% 7% 1	2808
	11th to 20th	Night	45% 6% 19% 20% 11% 1	2459
	21st to end	Night	41% 9% 15% 21% 11% 3	2076
March	1st to 10th	Night	40% 5% 11% 27% 16% 1	5496
	11th to 20th	Night	39% 4 9% 31% 17% 1	5432
	21st to end	Night	42% 7% 17% 27% 7% 1	5148
April	1st to 10th	Night	37% 5% 13% 29% 14% 2	2100
	11th to 20th	Night	36% 5% 10% 30% 15% 4	2079
	21st to end	Night	31% 4 12% 30% 22% 2	1895
May	1st to 10th	Night	22% 2 5% 28% 36% 6%	5283
	11th to 20th	Night	21% 3 7% 34% 32% 3	4700
	21st to end	Night	25% 4 10% 35% 23% 3	5182
June	1st to 10th	Night	32% 6% 12% 27% 21% 2	1680
	11th to 20th	Night	25% 3 9% 28% 28% 7%	1680
	21st to end	Night	19% 3 5% 25% 35% 14%	1680
July	1st to 10th	Night	13% 2 2 13% 29% 41%	4700
	11th to 20th	Night	11% 1 2 9% 25% 53%	4692
	21st to end	Night	16% 2 2 15% 28% 36%	5040
August	1st to 10th	Night	16% 2 2 9% 23% 48%	3724
	11th to 20th	Night	12% 2 4 19% 34% 28%	4304
	21st to end	Night	19% 2 3 21% 32% 23%	5092
September	1st to 10th	Night	15% 3 3 21% 30% 28%	2100
	11th to 20th	Night	20% 3 6% 25% 27% 19%	2230
	21st to end	Night	21% 3 4 26% 31% 15%	2288
October	1st to 10th	Night	19% 2 6% 19% 33% 20%	3679
	11th to 20th	Night	23% 3 13% 29% 28% 4	4006
	21st to end	Night	31% 3 12% 33% 18% 2	4600
November	1st to 10th	Night	40% 6% 14% 24% 15% 1	2665
	11th to 20th	Night	44% 6% 18% 24% 8%	2730
	21st to end	Night	41% 5% 18% 24% 11% 1	2884
December	1st to 10th	Night	36% 6% 21% 30% 8%	4700
	11th to 20th	Night	63% 6% 16% 11% 4	5316
	21st to end	Night	56% 5% 16% 14% 8%	7106

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	23% 2 20% 30% 20% 6%	220
	11th to 20th	Day	22% 3 13% 35% 20% 7%	220
	21st to end	Day	17% 1 18% 35% 20% 8%	242
February	1st to 10th	Day	13% 4 24% 35% 19% 4	347
	11th to 20th	Day	12% 7% 26% 36% 17% 2	535
	21st to end	Day	10% 8% 30% 33% 18% 3	446
March	1st to 10th	Day	14% 7% 24% 36% 18% 1	540
	11th to 20th	Day	13% 8% 26% 29% 19% 4	540
	21st to end	Day	13% 9% 29% 32% 15% 2	594
April	1st to 10th	Day	11% 12% 26% 28% 20% 3	540
	11th to 20th	Day	10% 10% 32% 31% 12% 5%	532
	21st to end	Day	11% 11% 31% 29% 16% 4	540
May	1st to 10th	Day	8% 13% 27% 30% 16% 5%	543
	11th to 20th	Day	17% 9% 27% 25% 16% 6%	568
	21st to end	Day	10% 7% 22% 30% 20% 12%	617
June	1st to 10th	Day	8% 8% 25% 28% 24% 8%	570
	11th to 20th	Day	9% 14% 27% 29% 17% 4	570
	21st to end	Day	3 8% 29% 31% 23% 7%	570
July	1st to 10th	Day	3 4 28% 35% 25% 4	566
	11th to 20th	Day	3 5% 27% 39% 20% 6%	564
	21st to end	Day	3 2 23% 39% 26% 7%	619
August	1st to 10th	Day	6% 2 22% 37% 23% 9%	567
	11th to 20th	Day	3 3 23% 34% 29% 8%	557
	21st to end	Day	5% 4 23% 33% 25% 10%	627
September	1st to 10th	Day	4 5% 20% 34% 27% 10%	570
	11th to 20th	Day	6% 4 23% 32% 26% 8%	570
	21st to end	Day	6% 6% 24% 31% 26% 7%	563
October	1st to 10th	Day	8% 6% 25% 26% 28% 7%	530
	11th to 20th	Day	9% 5% 23% 32% 23% 8%	466
	21st to end	Day	11% 8% 26% 36% 13% 6%	363
November	1st to 10th	Day	16% 7% 23% 33% 19% 2	330
	11th to 20th	Day	14% 5% 22% 35% 21% 4	330
	21st to end	Day	13% 5% 23% 35% 21% 3	247
December	1st to 10th	Day	22% 4 15% 30% 18% 10%	220
	11th to 20th	Day	25% 5% 19% 35% 15% 2	220
	21st to end	Day	16% 6% 17% 38% 20% 3	242
January	1st to 10th	Night	19% 3 18% 35% 20% 5%	840
	11th to 20th	Night	17% 5% 19% 31% 23% 5%	840
	21st to end	Night	16% 3 19% 35% 23% 5%	924
February	1st to 10th	Night	13% 5% 28% 36% 15% 3	840
	11th to 20th	Night	14% 4 27% 37% 16% 3	824
	21st to end	Night	11% 6% 29% 38% 14% 2	692
March	1st to 10th	Night	9% 6% 27% 39% 16% 2	840
	11th to 20th	Night	11% 4 28% 37% 18% 3	840
	21st to end	Night	12% 10% 27% 38% 12% 2	924
April	1st to 10th	Night	11% 8% 26% 38% 15% 3	840
	11th to 20th	Night	11% 11% 28% 32% 15% 3	831
	21st to end	Night	11% 9% 31% 35% 12% 2	635
May	1st to 10th	Night	8% 10% 31% 34% 15% 1	620
	11th to 20th	Night	9% 9% 29% 31% 18% 3	414
	21st to end	Night	10% 8% 27% 35% 16% 4	455
June	1st to 10th	Night	8% 10% 31% 31% 19% 1	420
	11th to 20th	Night	9% 12% 35% 29% 12% 2	420
	21st to end	Night	6% 7% 33% 40% 13% 2	420
July	1st to 10th	Night	3 8% 37% 39% 12% 2	410
	11th to 20th	Night	7% 46% 36% 10%	408
	21st to end	Night	2 5% 37% 40% 14% 1	460
August	1st to 10th	Night	4 4 35% 36% 18% 3	428
	11th to 20th	Night	2 4 30% 40% 21% 3	682
	21st to end	Night	4 5% 25% 42% 21% 4	924
September	1st to 10th	Night	5% 4 23% 42% 23% 4	840
	11th to 20th	Night	4 5% 23% 39% 23% 7%	840
	21st to end	Night	8% 5% 21% 36% 22% 8%	832
October	1st to 10th	Night	9% 6% 21% 41% 19% 4	800
	11th to 20th	Night	9% 6% 21% 38% 20% 6%	800
	21st to end	Night	10% 4 23% 39% 19% 5%	880
November	1st to 10th	Night	14% 5% 21% 37% 21% 2	820
	11th to 20th	Night	15% 4 21% 41% 15% 3	840
	21st to end	Night	13% 4 23% 38% 17% 5%	840
December	1st to 10th	Night	16% 3 18% 40% 20% 4	840
	11th to 20th	Night	23% 3 16% 38% 17% 3	840
	21st to end	Night	22% 6% 22% 35% 14% 2	924

Page generated at 2026-09-26 01:18:43. Version ac07

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