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 2025-05-26.

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% 7% 3 7% 4 4 4 4 5% 6% 49%	117820
	Night	8% 8% 3 6% 4 4 4 3 5% 6% 48%	120247
Transparency	Day	34% 5% 10% 21% 18% 12%	117564
	Night	37% 4 12% 22% 17% 8%	120092
Seeing	Day	10% 6% 24% 33% 21% 6%	15110
	Night	11% 6% 24% 37% 18% 4	24435

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	12% 18% 7% 15% 6% 5% 5% 4 4 4 21%	3639
	11th to 20th	Day	12% 14% 4 11% 9% 5% 5% 5% 5% 6% 25%	3640
	21st to end	Day	17% 15% 5% 9% 5% 7% 5% 5% 6% 5% 21%	4004
February	1st to 10th	Day	10% 15% 5% 12% 6% 4 4 3 6% 6% 29%	1722
	11th to 20th	Day	9% 11% 5% 10% 7% 5% 6% 4 6% 7% 30%	1950
	21st to end	Day	5% 11% 3 10% 5% 5% 6% 5% 7% 8% 35%	1650
March	1st to 10th	Day	5% 13% 4 11% 6% 6% 5% 4 6% 9% 31%	4518
	11th to 20th	Day	5% 7% 2 6% 4 4 5% 4 6% 8% 49%	4684
	21st to end	Day	6% 9% 3 9% 6% 7% 6% 5% 7% 9% 33%	5924
April	1st to 10th	Day	7% 8% 3 7% 5% 5% 6% 6% 6% 9% 39%	2400
	11th to 20th	Day	5% 7% 2 7% 5% 5% 5% 4 6% 7% 46%	2369
	21st to end	Day	7% 8% 3 6% 5% 5% 4 4 5% 7% 48%	2595
May	1st to 10th	Day	7% 6% 2 7% 4 4 4 3 6% 6% 50%	4259
	11th to 20th	Day	6% 4 1 4 3 3 3 3 5% 7% 63%	4517
	21st to end	Day	5% 5% 2 6% 5% 4 5% 4 7% 8% 49%	4886
June	1st to 10th	Day	8% 5% 2 6% 6% 5% 5% 4 7% 9% 42%	2660
	11th to 20th	Day	7% 5% 2 4 4 4 4 4 6% 7% 54%	2660
	21st to end	Day	3 2 1 3 3 3 3 3 4 6% 68%	2660
July	1st to 10th	Day	2 1 2 1 2 2 2 4 4 80%	4420
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 5% 79%	4420
	21st to end	Day	1 1 1 2 2 2 2 3 4 5% 78%	4790
August	1st to 10th	Day	4 3 2 4 4 3 4 3 4 7% 61%	4379
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 5% 76%	3936
	21st to end	Day	2 2 1 3 2 2 3 2 3 4 75%	4180
September	1st to 10th	Day	2 2 1 2 2 3 3 2 4 6% 73%	2280
	11th to 20th	Day	4 3 1 4 3 3 4 3 6% 7% 61%	2161
	21st to end	Day	4 4 2 5% 3 3 3 3 4 6% 64%	2090
October	1st to 10th	Day	4 3 1 3 3 3 3 3 4 5% 68%	3411
	11th to 20th	Day	5% 6% 2 5% 3 4 5% 3 4 8% 55%	3049
	21st to end	Day	8% 6% 2 6% 4 5% 5% 5% 5% 7% 46%	3101
November	1st to 10th	Day	13% 9% 3 5% 4 4 4 4 5% 8% 40%	1764
	11th to 20th	Day	11% 11% 4 9% 5% 4 5% 4 6% 6% 36%	1800
	21st to end	Day	10% 10% 3 10% 7% 5% 5% 4 8% 7% 32%	1650
December	1st to 10th	Day	9% 11% 4 11% 8% 4 5% 8% 6% 6% 30%	2620
	11th to 20th	Day	17% 17% 8% 17% 7% 4 4 4 3 4 15%	3028
	21st to end	Day	12% 15% 7% 13% 7% 5% 5% 4 4 6% 20%	4004
January	1st to 10th	Night	16% 16% 7% 12% 6% 4 4 3 4 5% 22%	6306
	11th to 20th	Night	12% 11% 4 7% 5% 5% 4 3 4 5% 41%	6320
	21st to end	Night	16% 13% 4 8% 4 3 4 2 4 4 38%	6952
February	1st to 10th	Night	10% 11% 4 8% 5% 4 4 4 6% 7% 36%	2675
	11th to 20th	Night	9% 10% 4 8% 5% 5% 5% 4 6% 7% 38%	2338
	21st to end	Night	7% 10% 4 7% 5% 5% 5% 5% 7% 8% 37%	1980
March	1st to 10th	Night	5% 10% 3 9% 6% 5% 5% 4 5% 8% 41%	5376
	11th to 20th	Night	7% 10% 3 8% 5% 4 4 3 4 5% 47%	5315
	21st to end	Night	6% 9% 3 8% 6% 6% 6% 5% 7% 9% 35%	5037
April	1st to 10th	Night	8% 8% 3 7% 5% 4 5% 3 6% 8% 44%	2000
	11th to 20th	Night	7% 7% 2 6% 5% 5% 4 3 5% 6% 50%	1979
	21st to end	Night	8% 5% 2 4 4 4 4 4 7% 8% 50%	1805
May	1st to 10th	Night	8% 6% 2 5% 4 4 4 3 5% 6% 52%	2927
	11th to 20th	Night	4 3 1 4 3 2 4 3 4 7% 65%	2596
	21st to end	Night	7% 6% 2 6% 4 4 4 4 6% 7% 51%	2805
June	1st to 10th	Night	7% 5% 2 5% 6% 5% 5% 4 6% 8% 46%	1520
	11th to 20th	Night	7% 5% 1 3 3 3 3 3 5% 8% 58%	1520
	21st to end	Night	4 2 1 3 3 3 3 3 4 6% 68%	1520
July	1st to 10th	Night	1 1 1 2 2 2 2 3 4 81%	2540
	11th to 20th	Night	1 1 2 2 2 4 5% 82%	2540
	21st to end	Night	2 2 1 3 2 2 3 2 5% 5% 73%	2761
August	1st to 10th	Night	3 3 2 5% 3 4 4 3 5% 5% 62%	2551
	11th to 20th	Night	1 1 1 2 2 2 2 2 4 6% 77%	2902
	21st to end	Night	2 3 1 2 2 2 2 2 3 4 76%	3498
September	1st to 10th	Night	3 2 1 3 2 2 2 3 4 4 75%	1900
	11th to 20th	Night	4 2 1 4 3 3 3 3 4 5% 67%	2018
	21st to end	Night	7% 3 1 3 2 3 3 2 4 6% 65%	2090
October	1st to 10th	Night	4 3 1 3 3 3 3 2 3 5% 70%	3567
	11th to 20th	Night	4 4 2 4 3 3 4 3 4 7% 61%	3883
	21st to end	Night	7% 5% 2 6% 4 4 5% 4 6% 7% 51%	4457
November	1st to 10th	Night	12% 7% 2 6% 4 5% 5% 4 5% 6% 43%	2535
	11th to 20th	Night	12% 9% 2 6% 5% 4 5% 4 5% 7% 40%	2600
	21st to end	Night	9% 10% 3 7% 4 4 4 4 4 6% 45%	2746
December	1st to 10th	Night	8% 7% 3 7% 5% 3 3 3 4 5% 51%	4560
	11th to 20th	Night	17% 16% 6% 12% 6% 5% 5% 4 5% 5% 21%	5176
	21st to end	Night	15% 15% 5% 9% 5% 5% 5% 4 5% 5% 28%	6952

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% 5% 11% 14% 5%	3639
	11th to 20th	Day	58% 7% 10% 19% 6%	3640
	21st to end	Day	61% 5% 11% 15% 5% 3	4004
February	1st to 10th	Day	54% 6% 18% 16% 6% 1	1722
	11th to 20th	Day	51% 8% 13% 20% 7% 1	1960
	21st to end	Day	44% 9% 16% 20% 11% 1	1650
March	1st to 10th	Day	46% 8% 14% 18% 13% 1	4518
	11th to 20th	Day	30% 4 15% 36% 15%	4684
	21st to end	Day	42% 8% 18% 22% 9% 1	5924
April	1st to 10th	Day	40% 6% 15% 25% 13% 2	2400
	11th to 20th	Day	35% 5% 12% 26% 19% 3	2372
	21st to end	Day	35% 5% 11% 28% 18% 2	2595
May	1st to 10th	Day	31% 5% 8% 27% 25% 4	4259
	11th to 20th	Day	21% 4 6% 31% 27% 10%	4520
	21st to end	Day	28% 7% 13% 33% 16% 3	4887
June	1st to 10th	Day	36% 6% 11% 27% 19% 1	2660
	11th to 20th	Day	27% 5% 8% 25% 25% 10%	2660
	21st to end	Day	18% 3 5% 21% 35% 18%	2660
July	1st to 10th	Day	9% 2 2 10% 21% 56%	4351
	11th to 20th	Day	11% 1 2 8% 21% 57%	4345
	21st to end	Day	10% 2 1 10% 30% 48%	4639
August	1st to 10th	Day	22% 3 3 10% 31% 32%	4363
	11th to 20th	Day	12% 2 2 16% 38% 30%	3975
	21st to end	Day	14% 2 3 24% 31% 25%	4179
September	1st to 10th	Day	14% 3 3 22% 33% 25%	2280
	11th to 20th	Day	21% 4 8% 25% 28% 14%	2161
	21st to end	Day	22% 3 7% 30% 29% 9%	2090
October	1st to 10th	Day	20% 3 7% 23% 37% 10%	3411
	11th to 20th	Day	29% 3 16% 30% 18% 4	3049
	21st to end	Day	35% 5% 14% 29% 15% 1	3101
November	1st to 10th	Day	42% 7% 17% 24% 10% 1	1764
	11th to 20th	Day	47% 7% 18% 20% 6% 1	1800
	21st to end	Day	48% 5% 20% 17% 8% 1	1650
December	1st to 10th	Day	48% 6% 25% 17% 5%	2620
	11th to 20th	Day	73% 7% 9% 8% 3	3028
	21st to end	Day	63% 5% 13% 13% 5%	4004
January	1st to 10th	Night	64% 5% 12% 13% 5%	6306
	11th to 20th	Night	47% 5% 16% 23% 9%	6320
	21st to end	Night	51% 4 13% 19% 10% 3	6952
February	1st to 10th	Night	47% 7% 20% 19% 7% 1	2675
	11th to 20th	Night	45% 6% 19% 19% 11% 1	2339
	21st to end	Night	41% 9% 16% 21% 11% 2	1980
March	1st to 10th	Night	40% 5% 11% 27% 16% 1	5376
	11th to 20th	Night	39% 3 9% 32% 16%	5315
	21st to end	Night	42% 7% 17% 27% 7%	5037
April	1st to 10th	Night	38% 5% 13% 28% 15% 2	2000
	11th to 20th	Night	35% 5% 10% 30% 16% 4	1979
	21st to end	Night	31% 4 12% 30% 21% 2	1805
May	1st to 10th	Night	32% 4 8% 31% 22% 3	2927
	11th to 20th	Night	19% 3 7% 31% 35% 4	2596
	21st to end	Night	31% 5% 15% 34% 14% 1	2805
June	1st to 10th	Night	34% 6% 13% 27% 19% 1	1520
	11th to 20th	Night	25% 4 9% 29% 26% 6%	1520
	21st to end	Night	18% 3 5% 25% 35% 13%	1520
July	1st to 10th	Night	9% 2 1 11% 22% 55%	2500
	11th to 20th	Night	7% 1 2 11% 23% 57%	2492
	21st to end	Night	15% 1 2 8% 31% 43%	2674
August	1st to 10th	Night	23% 2 2 12% 29% 31%	2543
	11th to 20th	Night	10% 2 3 15% 36% 34%	2929
	21st to end	Night	14% 2 2 20% 32% 29%	3498
September	1st to 10th	Night	14% 3 3 20% 30% 31%	1900
	11th to 20th	Night	20% 3 7% 24% 27% 20%	2018
	21st to end	Night	21% 3 4 27% 31% 14%	2090
October	1st to 10th	Night	18% 2 6% 19% 34% 21%	3567
	11th to 20th	Night	23% 3 13% 29% 28% 4	3883
	21st to end	Night	31% 3 12% 33% 18% 2	4457
November	1st to 10th	Night	40% 6% 13% 24% 16% 1	2535
	11th to 20th	Night	43% 6% 18% 24% 9%	2600
	21st to end	Night	40% 5% 18% 25% 11% 1	2746
December	1st to 10th	Night	35% 6% 21% 30% 8%	4560
	11th to 20th	Night	64% 6% 16% 11% 3	5176
	21st to end	Night	57% 5% 16% 14% 8%	6952

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% 1 18% 31% 22% 7%	200
	11th to 20th	Day	24% 4 14% 32% 20% 7%	200
	21st to end	Day	19% 1 16% 35% 20% 8%	220
February	1st to 10th	Day	14% 4 23% 37% 18% 3	320
	11th to 20th	Day	12% 6% 25% 37% 18% 2	495
	21st to end	Day	9% 5% 29% 34% 19% 3	414
March	1st to 10th	Day	14% 5% 23% 37% 19% 2	500
	11th to 20th	Day	13% 7% 24% 31% 20% 5%	500
	21st to end	Day	13% 8% 28% 33% 16% 2	550
April	1st to 10th	Day	11% 13% 26% 29% 19% 3	500
	11th to 20th	Day	9% 11% 32% 31% 12% 6%	492
	21st to end	Day	11% 11% 30% 29% 16% 4	500
May	1st to 10th	Day	9% 13% 28% 28% 16% 6%	503
	11th to 20th	Day	18% 9% 27% 24% 17% 6%	528
	21st to end	Day	10% 7% 21% 30% 21% 12%	550
June	1st to 10th	Day	9% 6% 24% 28% 24% 8%	490
	11th to 20th	Day	10% 14% 27% 27% 17% 4	490
	21st to end	Day	2 8% 28% 31% 24% 7%	490
July	1st to 10th	Day	3 4 28% 35% 25% 4	486
	11th to 20th	Day	3 5% 24% 40% 21% 6%	484
	21st to end	Day	2 2 22% 39% 27% 7%	531
August	1st to 10th	Day	6% 2 20% 38% 23% 10%	487
	11th to 20th	Day	3 3 22% 32% 30% 9%	477
	21st to end	Day	4 4 22% 35% 24% 11%	539
September	1st to 10th	Day	4 4 18% 34% 29% 11%	490
	11th to 20th	Day	7% 4 22% 33% 26% 8%	490
	21st to end	Day	7% 6% 24% 32% 24% 7%	490
October	1st to 10th	Day	9% 6% 23% 26% 29% 7%	490
	11th to 20th	Day	9% 5% 21% 33% 24% 8%	429
	21st to end	Day	12% 8% 26% 35% 13% 6%	330
November	1st to 10th	Day	17% 6% 23% 32% 20% 2	300
	11th to 20th	Day	13% 5% 22% 34% 21% 4	300
	21st to end	Day	13% 3 22% 36% 23% 3	225
December	1st to 10th	Day	22% 3 14% 32% 19% 12%	200
	11th to 20th	Day	25% 4 20% 37% 14% 2	200
	21st to end	Day	15% 5% 16% 39% 20% 4	220
January	1st to 10th	Night	20% 3 17% 35% 21% 5%	800
	11th to 20th	Night	18% 5% 19% 31% 23% 5%	800
	21st to end	Night	16% 3 18% 34% 23% 5%	880
February	1st to 10th	Night	14% 5% 28% 36% 15% 3	800
	11th to 20th	Night	14% 4 26% 37% 16% 3	784
	21st to end	Night	11% 5% 28% 39% 15% 3	660
March	1st to 10th	Night	10% 6% 25% 40% 17% 2	800
	11th to 20th	Night	12% 4 25% 38% 19% 3	800
	21st to end	Night	13% 9% 25% 39% 12% 2	880
April	1st to 10th	Night	11% 8% 26% 37% 16% 3	800
	11th to 20th	Night	11% 11% 27% 31% 16% 3	791
	21st to end	Night	11% 9% 30% 35% 13% 2	605
May	1st to 10th	Night	9% 11% 31% 33% 16% 2	590
	11th to 20th	Night	9% 9% 27% 31% 19% 4	394
	21st to end	Night	10% 7% 26% 35% 18% 4	422
June	1st to 10th	Night	8% 9% 31% 30% 21% 1	380
	11th to 20th	Night	9% 12% 32% 30% 14% 3	380
	21st to end	Night	6% 7% 32% 40% 14% 2	380
July	1st to 10th	Night	3 7% 35% 39% 13% 2	370
	11th to 20th	Night	1 6% 45% 36% 11% 1	368
	21st to end	Night	2 4 36% 41% 15% 1	416
August	1st to 10th	Night	4 4 33% 36% 20% 3	387
	11th to 20th	Night	2 4 28% 41% 23% 3	616
	21st to end	Night	3 4 24% 42% 23% 4	836
September	1st to 10th	Night	4 4 21% 42% 24% 4	760
	11th to 20th	Night	4 4 21% 39% 25% 7%	760
	21st to end	Night	9% 5% 20% 35% 23% 9%	760
October	1st to 10th	Night	8% 6% 21% 41% 20% 4	760
	11th to 20th	Night	9% 5% 19% 39% 21% 6%	760
	21st to end	Night	10% 4 22% 39% 20% 6%	836
November	1st to 10th	Night	14% 5% 20% 37% 22% 2	780
	11th to 20th	Night	15% 4 21% 41% 16% 3	800
	21st to end	Night	13% 4 22% 39% 17% 5%	800
December	1st to 10th	Night	16% 3 17% 40% 21% 4	800
	11th to 20th	Night	24% 3 16% 37% 18% 4	800
	21st to end	Night	22% 6% 22% 35% 14% 2	880

Page generated at 2025-05-23 06:21:03. Version ac07

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