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 2024-11-02.

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	7% 7% 3 6% 4 4 4 3 5% 6% 51%	101382
	Night	8% 8% 3 6% 4 4 4 3 5% 6% 48%	97739
Transparency	Day	33% 5% 9% 21% 19% 14%	101126
	Night	37% 5% 11% 21% 17% 10%	97584
Seeing	Day	10% 6% 23% 33% 21% 6%	14463
	Night	11% 5% 24% 37% 19% 4	23665

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% 8% 15% 6% 5% 4 4 4 5% 21%	2539
	11th to 20th	Day	17% 20% 6% 12% 6% 5% 4 3 4 4 20%	2540
	21st to end	Day	22% 20% 7% 12% 5% 4 4 4 4 5% 14%	2794
February	1st to 10th	Day	10% 15% 5% 12% 6% 4 4 3 6% 6% 30%	1635
	11th to 20th	Day	9% 11% 4 10% 7% 5% 6% 4 6% 7% 29%	1850
	21st to end	Day	5% 11% 3 10% 5% 5% 6% 5% 7% 8% 35%	1570
March	1st to 10th	Day	6% 15% 4 8% 6% 7% 5% 4 6% 9% 31%	3138
	11th to 20th	Day	6% 7% 2 5% 3 3 3 3 4 6% 57%	3265
	21st to end	Day	7% 8% 3 9% 7% 7% 7% 6% 7% 8% 31%	4133
April	1st to 10th	Day	7% 9% 3 7% 5% 5% 6% 6% 6% 8% 37%	2280
	11th to 20th	Day	5% 8% 2 7% 6% 5% 5% 4 6% 8% 45%	2249
	21st to end	Day	7% 8% 3 6% 5% 4 4 4 5% 7% 47%	2465
May	1st to 10th	Day	7% 6% 2 7% 4 4 5% 3 6% 6% 50%	4128
	11th to 20th	Day	6% 4 1 4 3 3 3 3 4 6% 64%	4377
	21st to end	Day	5% 5% 2 6% 5% 4 5% 4 7% 8% 49%	4813
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	7% 5% 2 4 4 3 4 4 5% 6% 57%	2047
	11th to 20th	Day	6% 5% 2 5% 3 3 5% 3 5% 8% 55%	1839
	21st to end	Day	9% 6% 3 7% 5% 5% 4 4 5% 7% 47%	1881
November	1st to 10th	Day	13% 9% 3 5% 4 5% 4 4 5% 8% 39%	1683
	11th to 20th	Day	11% 10% 4 9% 5% 4 5% 4 6% 6% 36%	1710
	21st to end	Day	10% 10% 3 10% 6% 5% 5% 4 8% 7% 33%	1568
December	1st to 10th	Day	15% 15% 5% 11% 7% 3 4 3 5% 5% 27%	1520
	11th to 20th	Day	17% 15% 7% 16% 8% 5% 4 4 3 4 17%	1928
	21st to end	Day	11% 15% 8% 13% 7% 5% 4 4 4 6% 23%	2794
January	1st to 10th	Night	13% 15% 7% 12% 7% 5% 5% 4 5% 6% 21%	4406
	11th to 20th	Night	18% 15% 5% 10% 6% 6% 5% 3 4 5% 23%	4420
	21st to end	Night	22% 17% 5% 10% 5% 4 4 3 3 4 23%	4862
February	1st to 10th	Night	10% 11% 4 8% 5% 5% 4 5% 6% 7% 36%	2542
	11th to 20th	Night	9% 10% 3 8% 5% 5% 5% 4 6% 7% 38%	2218
	21st to end	Night	7% 11% 4 8% 5% 5% 5% 5% 7% 8% 36%	1884
March	1st to 10th	Night	6% 12% 3 8% 6% 5% 5% 4 5% 8% 38%	3736
	11th to 20th	Night	7% 8% 3 6% 4 3 3 3 4 5% 55%	3714
	21st to end	Night	7% 9% 3 9% 5% 6% 6% 5% 7% 10% 34%	3506
April	1st to 10th	Night	8% 8% 3 7% 5% 4 5% 3 5% 7% 43%	1900
	11th to 20th	Night	7% 8% 2 7% 5% 5% 4 3 5% 6% 48%	1879
	21st to end	Night	8% 5% 2 4 4 4 4 4 7% 8% 50%	1715
May	1st to 10th	Night	9% 6% 2 5% 4 4 4 3 5% 6% 51%	2838
	11th to 20th	Night	4 2 1 4 3 2 3 3 4 7% 66%	2516
	21st to end	Night	7% 6% 2 6% 5% 4 4 4 6% 7% 50%	2762
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	6% 5% 2 4 4 3 3 3 4 7% 60%	2131
	11th to 20th	Night	6% 4 2 4 3 4 4 3 4 7% 59%	2339
	21st to end	Night	7% 5% 3 6% 4 4 5% 4 6% 6% 50%	2717
November	1st to 10th	Night	12% 7% 2 6% 5% 5% 5% 4 5% 6% 42%	2418
	11th to 20th	Night	12% 8% 2 6% 5% 4 5% 4 5% 7% 40%	2470
	21st to end	Night	9% 9% 3 7% 4 4 4 4 4 6% 45%	2608
December	1st to 10th	Night	12% 12% 5% 10% 5% 4 4 3 5% 6% 34%	2660
	11th to 20th	Night	16% 13% 5% 11% 6% 5% 5% 3 5% 5% 26%	3276
	21st to end	Night	11% 14% 5% 9% 6% 5% 5% 4 6% 6% 30%	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	65% 6% 12% 13% 4	2539
	11th to 20th	Day	68% 5% 10% 14% 3	2540
	21st to end	Day	72% 6% 11% 8% 2 1	2794
February	1st to 10th	Day	54% 6% 18% 16% 5%	1635
	11th to 20th	Day	50% 8% 14% 20% 7%	1860
	21st to end	Day	44% 8% 16% 20% 11% 1	1570
March	1st to 10th	Day	47% 7% 12% 19% 14% 1	3138
	11th to 20th	Day	28% 4 12% 42% 14%	3265
	21st to end	Day	44% 9% 15% 21% 10% 1	4133
April	1st to 10th	Day	41% 6% 15% 23% 13% 2	2280
	11th to 20th	Day	35% 5% 13% 25% 19% 3	2252
	21st to end	Day	35% 5% 11% 27% 19% 2	2465
May	1st to 10th	Day	32% 5% 8% 27% 24% 4	4128
	11th to 20th	Day	21% 4 6% 31% 27% 11%	4380
	21st to end	Day	28% 7% 13% 33% 16% 3	4814
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	28% 4 10% 27% 26% 5%	2047
	11th to 20th	Day	28% 4 15% 32% 19% 3	1839
	21st to end	Day	35% 5% 16% 28% 14% 1	1881
November	1st to 10th	Day	42% 7% 16% 24% 10% 1	1683
	11th to 20th	Day	46% 8% 19% 20% 6% 1	1710
	21st to end	Day	48% 5% 20% 18% 9% 1	1568
December	1st to 10th	Day	58% 7% 16% 13% 7%	1520
	11th to 20th	Day	71% 5% 10% 9% 4	1928
	21st to end	Day	62% 5% 13% 14% 6%	2794
January	1st to 10th	Night	63% 6% 16% 11% 4	4406
	11th to 20th	Night	64% 5% 12% 16% 3	4420
	21st to end	Night	67% 4 14% 12% 3	4862
February	1st to 10th	Night	46% 7% 21% 19% 6% 1	2542
	11th to 20th	Night	45% 6% 19% 19% 11%	2219
	21st to end	Night	42% 9% 16% 21% 10% 2	1884
March	1st to 10th	Night	43% 6% 12% 25% 13% 1	3736
	11th to 20th	Night	32% 4 9% 37% 18% 1	3714
	21st to end	Night	42% 8% 17% 25% 8%	3506
April	1st to 10th	Night	39% 5% 14% 26% 15% 2	1900
	11th to 20th	Night	36% 5% 10% 29% 16% 4	1879
	21st to end	Night	31% 4 13% 29% 21% 1	1715
May	1st to 10th	Night	33% 4 8% 30% 21% 3	2838
	11th to 20th	Night	19% 3 7% 30% 35% 5%	2516
	21st to end	Night	31% 5% 15% 34% 14% 1	2762
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	26% 3 9% 25% 28% 9%	2131
	11th to 20th	Night	26% 4 11% 29% 26% 4	2339
	21st to end	Night	33% 4 14% 27% 20% 2	2717
November	1st to 10th	Night	40% 6% 13% 24% 16% 1	2418
	11th to 20th	Night	42% 6% 18% 25% 8%	2470
	21st to end	Night	40% 5% 18% 25% 12% 1	2608
December	1st to 10th	Night	50% 7% 16% 18% 8%	2660
	11th to 20th	Night	59% 5% 17% 14% 4	3276
	21st to end	Night	54% 5% 18% 13% 9%	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	23% 1 15% 31% 23% 7%	180
	11th to 20th	Day	27% 3 12% 32% 19% 8%	180
	21st to end	Day	21% 1 15% 37% 18% 8%	198
February	1st to 10th	Day	15% 3 21% 39% 19% 4	293
	11th to 20th	Day	12% 6% 25% 37% 18% 2	455
	21st to end	Day	9% 5% 30% 34% 18% 3	382
March	1st to 10th	Day	15% 5% 22% 37% 19% 2	460
	11th to 20th	Day	13% 5% 23% 31% 22% 5%	460
	21st to end	Day	13% 8% 27% 33% 16% 2	506
April	1st to 10th	Day	12% 13% 24% 28% 18% 3	460
	11th to 20th	Day	9% 10% 32% 31% 12% 6%	452
	21st to end	Day	11% 12% 30% 28% 16% 3	460
May	1st to 10th	Day	10% 12% 27% 30% 16% 5%	463
	11th to 20th	Day	18% 9% 25% 24% 18% 7%	488
	21st to end	Day	10% 6% 21% 31% 20% 12%	529
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% 25% 35% 13% 6%	330
November	1st to 10th	Day	17% 5% 23% 34% 19% 2	273
	11th to 20th	Day	13% 2 21% 37% 23% 4	270
	21st to end	Day	14% 3 19% 36% 25% 3	203
December	1st to 10th	Day	24% 3 13% 34% 16% 10%	180
	11th to 20th	Day	26% 4 19% 36% 14% 2	180
	21st to end	Day	16% 4 14% 40% 22% 4	198
January	1st to 10th	Night	19% 3 16% 35% 22% 5%	760
	11th to 20th	Night	19% 4 18% 32% 23% 5%	760
	21st to end	Night	17% 3 17% 34% 24% 5%	836
February	1st to 10th	Night	14% 5% 26% 37% 16% 3	760
	11th to 20th	Night	14% 4 24% 37% 17% 3	744
	21st to end	Night	11% 5% 27% 39% 15% 3	628
March	1st to 10th	Night	10% 5% 24% 40% 18% 3	760
	11th to 20th	Night	11% 3 24% 39% 20% 3	760
	21st to end	Night	13% 9% 24% 39% 13% 2	836
April	1st to 10th	Night	11% 8% 24% 37% 16% 3	760
	11th to 20th	Night	11% 11% 26% 32% 17% 3	751
	21st to end	Night	11% 9% 29% 35% 13% 2	575
May	1st to 10th	Night	9% 10% 30% 33% 16% 2	561
	11th to 20th	Night	10% 8% 26% 33% 20% 4	374
	21st to end	Night	10% 7% 25% 36% 18% 4	411
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% 4 19% 38% 23% 2	744
	11th to 20th	Night	15% 3 21% 41% 17% 3	760
	21st to end	Night	13% 4 22% 39% 17% 6%	760
December	1st to 10th	Night	17% 2 17% 40% 19% 4	760
	11th to 20th	Night	23% 3 15% 37% 18% 4	760
	21st to end	Night	22% 5% 20% 36% 14% 2	836

Page generated at 2024-10-29 22:22:54. Version ac07

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