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 2024-03-08.

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% 8% 3 6% 5% 3 4 4 5% 7% 47%	78480
	Night	10% 8% 3 6% 5% 3 4 4 5% 7% 45%	81102
Transparency	Day	37% 5% 10% 21% 18% 9%	78184
	Night	40% 5% 13% 21% 15% 6%	80919
Seeing	Day	10% 6% 23% 33% 21% 6%	13524
	Night	11% 5% 23% 37% 19% 4	22922

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% 17% 8% 13% 8% 4 5% 4 4 5% 21%	2539
	11th to 20th	Day	17% 19% 5% 11% 8% 3 5% 4 4 4 20%	2540
	21st to end	Day	22% 20% 7% 11% 6% 3 4 4 4 5% 14%	2794
February	1st to 10th	Day	10% 14% 5% 11% 7% 3 5% 4 6% 6% 30%	1635
	11th to 20th	Day	9% 10% 5% 9% 8% 4 7% 6% 6% 7% 30%	1850
	21st to end	Day	6% 10% 4 9% 6% 4 6% 6% 7% 8% 35%	1570
March	1st to 10th	Day	8% 12% 6% 7% 6% 4 6% 5% 6% 8% 32%	1871
	11th to 20th	Day	11% 9% 4 7% 5% 3 6% 6% 5% 9% 37%	1846
	21st to end	Day	10% 8% 4 8% 7% 4 7% 6% 6% 8% 33%	2357
April	1st to 10th	Day	7% 8% 3 6% 6% 4 6% 7% 6% 9% 37%	2160
	11th to 20th	Day	6% 7% 3 6% 6% 4 5% 6% 6% 8% 44%	2129
	21st to end	Day	8% 7% 3 5% 5% 3 5% 4 5% 7% 48%	2335
May	1st to 10th	Day	5% 5% 2 6% 4 2 5% 4 6% 8% 53%	2345
	11th to 20th	Day	10% 5% 2 5% 5% 3 4 4 6% 8% 47%	2477
	21st to end	Day	8% 6% 3 6% 6% 4 5% 6% 6% 9% 40%	2723
June	1st to 10th	Day	9% 4 3 5% 7% 4 5% 5% 7% 10% 42%	2520
	11th to 20th	Day	7% 4 2 4 5% 3 4 5% 5% 7% 53%	2520
	21st to end	Day	3 2 1 3 3 2 4 4 4 6% 67%	2520
July	1st to 10th	Day	3 2 1 2 2 2 3 4 4 7% 70%	2520
	11th to 20th	Day	1 1 1 1 2 1 2 2 3 5% 80%	2520
	21st to end	Day	1 2 1 2 2 1 2 3 3 5% 77%	2741
August	1st to 10th	Day	4 2 1 3 5% 2 3 4 4 6% 64%	2499
	11th to 20th	Day	2 2 1 1 2 2 3 3 4 5% 76%	2246
	21st to end	Day	2 3 1 3 3 1 3 3 3 5% 73%	2376
September	1st to 10th	Day	2 2 1 2 3 2 3 3 4 6% 74%	2160
	11th to 20th	Day	4 3 2 3 3 3 3 4 6% 7% 63%	2048
	21st to end	Day	4 4 2 4 4 2 3 4 4 6% 63%	1980
October	1st to 10th	Day	7% 6% 2 3 5% 2 4 4 5% 6% 55%	1940
	11th to 20th	Day	6% 5% 2 4 4 3 4 5% 5% 8% 56%	1743
	21st to end	Day	9% 6% 3 5% 6% 3 4 5% 4 7% 48%	1782
November	1st to 10th	Day	13% 9% 3 4 5% 3 5% 5% 5% 9% 40%	1674
	11th to 20th	Day	11% 9% 4 9% 5% 3 5% 5% 6% 6% 37%	1710
	21st to end	Day	10% 10% 3 8% 7% 4 6% 5% 8% 7% 33%	1568
December	1st to 10th	Day	15% 13% 7% 10% 7% 3 4 5% 5% 5% 27%	1520
	11th to 20th	Day	17% 15% 7% 14% 9% 5% 4 4 4 4 17%	1928
	21st to end	Day	11% 14% 9% 11% 9% 4 4 5% 4 6% 24%	2794
January	1st to 10th	Night	14% 14% 8% 10% 8% 4 5% 5% 5% 6% 21%	4406
	11th to 20th	Night	18% 15% 6% 9% 6% 5% 5% 4 4 5% 23%	4420
	21st to end	Night	23% 17% 6% 9% 6% 3 4 3 3 4 23%	4862
February	1st to 10th	Night	10% 10% 5% 7% 6% 3 5% 5% 6% 7% 37%	2542
	11th to 20th	Night	9% 9% 4 7% 6% 3 5% 6% 6% 7% 38%	2218
	21st to end	Night	8% 9% 4 7% 6% 4 5% 6% 6% 8% 37%	1884
March	1st to 10th	Night	8% 9% 3 6% 7% 5% 6% 5% 5% 8% 36%	2249
	11th to 20th	Night	8% 8% 3 7% 6% 4 5% 5% 6% 9% 38%	2113
	21st to end	Night	9% 9% 4 7% 5% 4 5% 5% 6% 9% 37%	1998
April	1st to 10th	Night	9% 7% 3 6% 6% 3 5% 5% 5% 8% 43%	1800
	11th to 20th	Night	7% 7% 2 6% 6% 4 5% 4 5% 6% 47%	1779
	21st to end	Night	9% 5% 2 4 4 3 4 5% 6% 8% 50%	1625
May	1st to 10th	Night	5% 5% 2 4 4 2 5% 4 5% 7% 56%	1612
	11th to 20th	Night	8% 3 2 4 6% 3 5% 6% 6% 10% 48%	1416
	21st to end	Night	8% 5% 2 7% 6% 4 5% 5% 7% 9% 43%	1552
June	1st to 10th	Night	7% 4 3 5% 6% 3 6% 5% 6% 8% 46%	1440
	11th to 20th	Night	7% 5% 2 3 4 3 4 4 4 8% 57%	1440
	21st to end	Night	4 2 1 3 3 2 4 4 4 6% 67%	1440
July	1st to 10th	Night	2 2 1 2 3 2 3 4 4 6% 70%	1440
	11th to 20th	Night	1 1 1 1 1 3 3 5% 82%	1440
	21st to end	Night	1 2 1 2 3 1 3 3 5% 5% 76%	1576
August	1st to 10th	Night	3 2 1 4 3 2 4 4 5% 7% 64%	1440
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 6% 75%	1651
	21st to end	Night	2 2 1 2 2 2 2 4 5% 5% 72%	1980
September	1st to 10th	Night	3 2 1 3 2 1 2 3 4 4 75%	1800
	11th to 20th	Night	3 2 1 3 3 2 4 3 4 6% 69%	1911
	21st to end	Night	7% 3 1 3 3 3 3 3 4 6% 64%	1980
October	1st to 10th	Night	6% 5% 2 4 4 3 3 4 4 7% 58%	2018
	11th to 20th	Night	7% 4 2 4 4 3 4 4 3 7% 58%	2215
	21st to end	Night	7% 5% 3 4 5% 3 5% 5% 6% 6% 50%	2574
November	1st to 10th	Night	12% 7% 2 5% 6% 4 5% 6% 5% 6% 42%	2405
	11th to 20th	Night	12% 8% 3 5% 6% 3 5% 6% 5% 7% 41%	2470
	21st to end	Night	9% 9% 3 6% 5% 3 4 5% 4 6% 46%	2608
December	1st to 10th	Night	12% 11% 5% 9% 6% 3 4 4 5% 7% 35%	2660
	11th to 20th	Night	16% 13% 5% 10% 7% 4 5% 4 5% 5% 26%	3276
	21st to end	Night	11% 13% 6% 8% 7% 4 5% 5% 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	49% 5% 15% 21% 8% 1	1871
	11th to 20th	Day	44% 5% 16% 24% 11%	1846
	21st to end	Day	48% 6% 16% 22% 8% 1	2357
April	1st to 10th	Day	41% 6% 14% 22% 13% 2	2160
	11th to 20th	Day	36% 5% 12% 25% 18% 3	2132
	21st to end	Day	36% 5% 11% 27% 19% 2	2335
May	1st to 10th	Day	29% 3 9% 34% 21% 3	2345
	11th to 20th	Day	35% 4 10% 30% 17% 4	2480
	21st to end	Day	39% 5% 14% 27% 14% 1	2724
June	1st to 10th	Day	36% 5% 11% 28% 18% 1	2520
	11th to 20th	Day	28% 5% 8% 26% 24% 9%	2520
	21st to end	Day	19% 3 5% 22% 34% 18%	2520
July	1st to 10th	Day	14% 3 4 17% 29% 33%	2451
	11th to 20th	Day	10% 2 3 12% 31% 43%	2445
	21st to end	Day	11% 3 1 10% 29% 47%	2589
August	1st to 10th	Day	20% 4 4 15% 32% 26%	2482
	11th to 20th	Day	12% 3 2 19% 35% 29%	2246
	21st to end	Day	15% 3 3 19% 38% 22%	2376
September	1st to 10th	Day	14% 3 3 22% 33% 25%	2160
	11th to 20th	Day	20% 4 8% 24% 29% 15%	2048
	21st to end	Day	23% 3 7% 30% 28% 9%	1980
October	1st to 10th	Day	29% 4 11% 27% 24% 4	1940
	11th to 20th	Day	27% 4 15% 32% 19% 3	1743
	21st to end	Day	35% 5% 17% 28% 14% 1	1782
November	1st to 10th	Day	42% 6% 16% 24% 10% 1	1674
	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	44% 7% 13% 24% 11% 2	2249
	11th to 20th	Night	41% 5% 15% 28% 9% 1	2113
	21st to end	Night	43% 6% 16% 26% 8%	1998
April	1st to 10th	Night	39% 5% 14% 25% 15% 2	1800
	11th to 20th	Night	38% 5% 9% 29% 15% 4	1779
	21st to end	Night	32% 4 13% 28% 21% 1	1625
May	1st to 10th	Night	27% 4 10% 33% 24% 2	1612
	11th to 20th	Night	31% 5% 13% 31% 18% 3	1416
	21st to end	Night	37% 4 15% 30% 13% 1	1552
June	1st to 10th	Night	35% 6% 13% 28% 18% 1	1440
	11th to 20th	Night	26% 4 10% 30% 24% 6%	1440
	21st to end	Night	19% 3 5% 26% 35% 12%	1440
July	1st to 10th	Night	15% 3 2 19% 32% 28%	1400
	11th to 20th	Night	7% 2 3 19% 31% 39%	1392
	21st to end	Night	13% 1 2 10% 29% 44%	1488
August	1st to 10th	Night	20% 3 4 18% 31% 23%	1432
	11th to 20th	Night	11% 3 4 17% 32% 33%	1651
	21st to end	Night	14% 3 3 18% 36% 25%	1980
September	1st to 10th	Night	14% 3 3 21% 28% 31%	1800
	11th to 20th	Night	18% 2 6% 24% 28% 22%	1911
	21st to end	Night	22% 3 4 26% 31% 14%	1980
October	1st to 10th	Night	27% 3 9% 26% 28% 7%	2018
	11th to 20th	Night	27% 4 10% 29% 26% 4	2215
	21st to end	Night	33% 5% 14% 26% 20% 2	2574
November	1st to 10th	Night	40% 6% 13% 24% 16% 1	2405
	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	449
	11th to 20th	Day	14% 5% 23% 32% 22% 3	420
	21st to end	Day	13% 8% 27% 33% 16% 2	462
April	1st to 10th	Day	12% 14% 24% 27% 19% 3	420
	11th to 20th	Day	10% 10% 33% 30% 12% 6%	412
	21st to end	Day	11% 12% 30% 27% 15% 4	420
May	1st to 10th	Day	9% 12% 27% 30% 16% 6%	423
	11th to 20th	Day	19% 9% 24% 25% 17% 6%	448
	21st to end	Day	10% 6% 21% 31% 19% 12%	485
June	1st to 10th	Day	8% 6% 25% 28% 24% 9%	450
	11th to 20th	Day	11% 14% 27% 27% 18% 4	450
	21st to end	Day	3 8% 26% 32% 25% 7%	450
July	1st to 10th	Day	4 4 26% 36% 25% 5%	446
	11th to 20th	Day	2 5% 24% 40% 22% 6%	444
	21st to end	Day	3 2 22% 39% 27% 7%	487
August	1st to 10th	Day	6% 2 18% 38% 25% 11%	447
	11th to 20th	Day	3 3 20% 32% 32% 9%	437
	21st to end	Day	4 4 21% 36% 24% 11%	495
September	1st to 10th	Day	4 5% 18% 34% 28% 11%	450
	11th to 20th	Day	7% 4 20% 32% 28% 8%	450
	21st to end	Day	7% 6% 24% 32% 24% 7%	450
October	1st to 10th	Day	9% 6% 23% 25% 29% 7%	450
	11th to 20th	Day	9% 5% 22% 31% 25% 9%	393
	21st to end	Day	12% 8% 23% 36% 14% 7%	297
November	1st to 10th	Day	17% 5% 23% 34% 20% 2	270
	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	750
	11th to 20th	Night	12% 3 23% 39% 20% 4	720
	21st to end	Night	13% 9% 24% 38% 13% 2	792
April	1st to 10th	Night	11% 8% 24% 37% 17% 3	720
	11th to 20th	Night	11% 11% 25% 32% 17% 4	711
	21st to end	Night	11% 9% 28% 35% 14% 2	545
May	1st to 10th	Night	8% 10% 30% 33% 17% 2	532
	11th to 20th	Night	10% 7% 25% 33% 21% 4	354
	21st to end	Night	10% 7% 24% 37% 18% 4	389
June	1st to 10th	Night	8% 9% 30% 30% 22% 1	360
	11th to 20th	Night	10% 13% 31% 29% 14% 3	360
	21st to end	Night	6% 6% 30% 42% 14% 2	360
July	1st to 10th	Night	3 8% 34% 39% 14% 2	350
	11th to 20th	Night	1 6% 45% 36% 12% 1	348
	21st to end	Night	2 4 34% 42% 16% 2	394
August	1st to 10th	Night	4 4 32% 37% 20% 3	366
	11th to 20th	Night	2 4 26% 42% 24% 3	583
	21st to end	Night	3 4 23% 42% 23% 4	792
September	1st to 10th	Night	4 4 21% 41% 25% 5%	720
	11th to 20th	Night	3 3 21% 39% 26% 8%	720
	21st to end	Night	9% 4 20% 34% 24% 9%	720
October	1st to 10th	Night	9% 6% 20% 41% 20% 4	720
	11th to 20th	Night	9% 5% 19% 39% 22% 7%	720
	21st to end	Night	10% 3 20% 39% 21% 6%	792
November	1st to 10th	Night	14% 4 19% 38% 23% 2	740
	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 2024-03-05 04:24:01. Version ac07

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