Brentwood
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brentwood clear sky chart from 2005-11-06 to 2024-12-02.

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	5% 3 1 3 2 2 3 2 4 5% 68%	105283
	Night	7% 3 1 3 3 3 3 3 4 6% 65%	103103
Transparency	Day	20% 4 8% 18% 25% 25%	105037
	Night	22% 5% 9% 18% 22% 24%	102938
Seeing	Day	7% 9% 23% 31% 22% 8%	15414
	Night	7% 5% 20% 37% 27% 4	24775

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	16% 8% 4 8% 5% 4 4 4 5% 6% 36%	2612
	11th to 20th	Day	15% 12% 4 9% 6% 5% 4 3 4 6% 34%	2860
	21st to end	Day	19% 9% 3 8% 5% 4 5% 3 5% 6% 32%	3146
February	1st to 10th	Day	13% 6% 3 6% 5% 4 4 3 5% 5% 46%	1710
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 7% 48%	1772
	21st to end	Day	11% 5% 1 5% 4 3 4 4 5% 6% 50%	1570
March	1st to 10th	Day	9% 7% 3 9% 6% 6% 6% 5% 7% 7% 34%	3322
	11th to 20th	Day	8% 4 2 4 3 3 4 3 4 6% 59%	3490
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 8% 42%	3917
April	1st to 10th	Day	10% 6% 2 4 4 4 4 3 6% 7% 51%	2280
	11th to 20th	Day	7% 4 1 4 4 3 3 3 5% 5% 62%	2249
	21st to end	Day	4 3 1 3 2 2 3 3 4 8% 67%	2280
May	1st to 10th	Day	4 2 1 2 2 2 1 2 3 5% 77%	3880
	11th to 20th	Day	2 1 1 1 1 1 1 2 3 84%	4251
	21st to end	Day	1 1 1 1 1 2 2 1 3 5% 82%	4813
June	1st to 10th	Day	2 3 1 3 2 2 2 3 5% 7% 71%	2660
	11th to 20th	Day	1 1 1 1 2 2 4 4 85%	2660
	21st to end	Day	1 1 1 1 1 1 1 3 3 86%	2660
July	1st to 10th	Day	1 1 2 94%	4420
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 87%	4245
	21st to end	Day	1 1 1 2 3 91%	4455
August	1st to 10th	Day	1 1 1 1 1 2 2 4 5% 82%	4091
	11th to 20th	Day	1 1 1 1 1 2 3 91%	3791
	21st to end	Day	1 1 1 1 1 1 1 2 3 89%	4180
September	1st to 10th	Day	1 1 1 1 1 1 1 3 89%	2280
	11th to 20th	Day	1 1 1 1 2 2 3 4 6% 78%	2280
	21st to end	Day	2 1 2 1 2 2 1 3 4 82%	2033
October	1st to 10th	Day	1 1 1 2 2 2 3 2 3 5% 78%	3180
	11th to 20th	Day	3 2 1 2 1 2 2 2 4 4 79%	3161
	21st to end	Day	3 2 1 3 2 3 3 3 4 7% 70%	3448
November	1st to 10th	Day	6% 4 1 4 3 3 3 3 5% 8% 59%	1874
	11th to 20th	Day	8% 5% 2 6% 5% 5% 4 4 6% 7% 50%	1800
	21st to end	Day	10% 6% 2 5% 3 3 3 2 5% 7% 54%	1663
December	1st to 10th	Day	11% 8% 2 6% 6% 4 3 4 5% 7% 45%	1528
	11th to 20th	Day	23% 12% 3 7% 5% 4 4 3 5% 4 30%	1928
	21st to end	Day	14% 8% 4 9% 6% 4 5% 4 6% 7% 33%	2794
January	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 47%	4331
	11th to 20th	Night	18% 9% 3 7% 4 4 4 3 6% 6% 35%	4110
	21st to end	Night	17% 7% 3 7% 5% 5% 4 4 6% 7% 36%	4521
February	1st to 10th	Night	14% 5% 2 5% 3 3 4 3 4 5% 52%	2470
	11th to 20th	Night	9% 4 2 5% 4 4 4 4 6% 8% 52%	2295
	21st to end	Night	11% 5% 1 4 3 3 2 3 5% 6% 57%	1884
March	1st to 10th	Night	11% 6% 2 6% 5% 5% 5% 4 6% 7% 42%	3550
	11th to 20th	Night	10% 4 2 3 2 3 3 2 4 5% 63%	3490
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 7% 7% 48%	3723
April	1st to 10th	Night	7% 5% 2 4 4 5% 4 3 5% 7% 55%	1900
	11th to 20th	Night	6% 4 1 3 3 3 4 3 4 6% 63%	1879
	21st to end	Night	3 2 1 2 2 2 3 3 5% 7% 70%	1900
May	1st to 10th	Night	2 1 1 2 1 2 2 3 5% 81%	3094
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 4 85%	2644
	21st to end	Night	1 1 1 1 1 1 1 2 4 86%	2762
June	1st to 10th	Night	2 2 1 2 2 2 2 2 5% 6% 74%	1520
	11th to 20th	Night	1 1 1 1 1 1 2 4 87%	1520
	21st to end	Night	1 1 1 1 1 2 1 3 3 86%	1520
July	1st to 10th	Night	1 1 2 95%	2540
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 88%	2717
	21st to end	Night	1 1 1 1 1 1 3 5% 86%	3109
August	1st to 10th	Night	2 1 1 1 1 1 2 2 3 6% 80%	2848
	11th to 20th	Night	1 1 1 1 1 2 4 91%	3053
	21st to end	Night	1 1 1 1 1 1 1 1 3 90%	3498
September	1st to 10th	Night	1 1 1 1 1 2 4 88%	1900
	11th to 20th	Night	1 2 1 1 1 2 2 3 6% 80%	1900
	21st to end	Night	1 1 2 1 2 1 1 3 4 84%	2143
October	1st to 10th	Night	2 1 1 2 2 2 2 3 5% 80%	3800
	11th to 20th	Night	2 1 2 1 2 2 2 3 6% 79%	3777
	21st to end	Night	4 1 2 2 3 3 3 5% 6% 70%	4121
November	1st to 10th	Night	6% 3 1 3 2 2 3 3 5% 8% 63%	2425
	11th to 20th	Night	7% 4 2 4 3 3 3 3 6% 8% 57%	2600
	21st to end	Night	8% 4 2 3 3 3 3 3 5% 7% 58%	2735
December	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 5% 51%	2686
	11th to 20th	Night	19% 8% 3 7% 5% 4 4 4 7% 7% 34%	3276
	21st to end	Night	15% 6% 2 8% 5% 5% 5% 4 5% 6% 37%	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	47% 8% 12% 24% 8% 1	2612
	11th to 20th	Day	52% 8% 10% 14% 12% 4	2860
	21st to end	Day	51% 8% 10% 16% 12% 3	3146
February	1st to 10th	Day	40% 5% 10% 21% 18% 7%	1710
	11th to 20th	Day	35% 7% 11% 19% 22% 6%	1781
	21st to end	Day	33% 6% 10% 24% 21% 6%	1570
March	1st to 10th	Day	46% 12% 10% 15% 15% 2	3322
	11th to 20th	Day	26% 6% 11% 29% 17% 12%	3490
	21st to end	Day	36% 10% 15% 23% 13% 3	3917
April	1st to 10th	Day	32% 8% 12% 22% 20% 6%	2280
	11th to 20th	Day	24% 5% 14% 27% 24% 7%	2252
	21st to end	Day	16% 5% 13% 29% 31% 6%	2280
May	1st to 10th	Day	12% 3 8% 30% 28% 19%	3880
	11th to 20th	Day	8% 2 4 23% 43% 19%	4254
	21st to end	Day	8% 3 8% 35% 36% 9%	4814
June	1st to 10th	Day	13% 6% 10% 25% 31% 15%	2660
	11th to 20th	Day	5% 2 7% 20% 37% 28%	2660
	21st to end	Day	7% 1 3 10% 32% 47%	2660
July	1st to 10th	Day	1 1 2 5% 13% 78%	4351
	11th to 20th	Day	6% 2 3 6% 13% 70%	4171
	21st to end	Day	2 1 3 4 27% 64%	4315
August	1st to 10th	Day	7% 2 2 7% 28% 55%	4076
	11th to 20th	Day	2 1 2 6% 39% 50%	3828
	21st to end	Day	5% 1 3 10% 35% 47%	4179
September	1st to 10th	Day	5% 1 3 7% 31% 53%	2280
	11th to 20th	Day	8% 3 7% 20% 40% 23%	2280
	21st to end	Day	8% 2 5% 18% 39% 28%	2033
October	1st to 10th	Day	11% 2 4 11% 27% 45%	3180
	11th to 20th	Day	11% 3 6% 26% 33% 21%	3161
	21st to end	Day	16% 3 6% 23% 37% 15%	3448
November	1st to 10th	Day	24% 6% 9% 26% 24% 10%	1874
	11th to 20th	Day	32% 6% 12% 25% 20% 5%	1800
	21st to end	Day	31% 4 12% 20% 26% 7%	1663
December	1st to 10th	Day	37% 8% 12% 20% 16% 7%	1528
	11th to 20th	Day	57% 5% 10% 13% 13% 2	1928
	21st to end	Day	45% 10% 15% 17% 11% 2	2794
January	1st to 10th	Night	36% 9% 18% 24% 10% 3	4331
	11th to 20th	Night	49% 9% 13% 13% 10% 6%	4110
	21st to end	Night	47% 8% 13% 15% 12% 5%	4521
February	1st to 10th	Night	35% 5% 8% 21% 22% 9%	2470
	11th to 20th	Night	29% 7% 12% 21% 23% 9%	2297
	21st to end	Night	29% 5% 11% 28% 21% 6%	1884
March	1st to 10th	Night	39% 12% 16% 15% 15% 4	3550
	11th to 20th	Night	26% 6% 9% 26% 20% 13%	3490
	21st to end	Night	34% 10% 15% 25% 12% 4	3723
April	1st to 10th	Night	29% 6% 14% 22% 21% 8%	1900
	11th to 20th	Night	23% 5% 12% 29% 20% 11%	1879
	21st to end	Night	14% 6% 13% 29% 31% 8%	1900
May	1st to 10th	Night	9% 4 12% 27% 31% 17%	3094
	11th to 20th	Night	6% 3 5% 16% 42% 28%	2644
	21st to end	Night	6% 3 8% 33% 39% 12%	2762
June	1st to 10th	Night	12% 6% 10% 22% 31% 18%	1520
	11th to 20th	Night	5% 2 8% 16% 34% 35%	1520
	21st to end	Night	5% 2 3 11% 29% 50%	1520
July	1st to 10th	Night	2 2 5% 12% 80%	2500
	11th to 20th	Night	5% 2 2 5% 13% 72%	2668
	21st to end	Night	4 1 2 5% 21% 66%	3011
August	1st to 10th	Night	8% 1 2 7% 23% 58%	2839
	11th to 20th	Night	2 1 3 6% 31% 58%	3082
	21st to end	Night	5% 4 8% 31% 52%	3498
September	1st to 10th	Night	3 2 3 8% 26% 59%	1900
	11th to 20th	Night	7% 3 6% 20% 35% 29%	1900
	21st to end	Night	7% 2 5% 17% 34% 34%	2143
October	1st to 10th	Night	9% 2 5% 11% 18% 54%	3800
	11th to 20th	Night	9% 3 6% 28% 27% 26%	3777
	21st to end	Night	16% 3 6% 21% 31% 23%	4121
November	1st to 10th	Night	20% 6% 9% 27% 23% 16%	2425
	11th to 20th	Night	24% 7% 12% 26% 24% 7%	2600
	21st to end	Night	27% 5% 10% 22% 27% 11%	2735
December	1st to 10th	Night	34% 7% 11% 20% 17% 11%	2686
	11th to 20th	Night	49% 6% 11% 14% 16% 3	3276
	21st to end	Night	44% 10% 13% 18% 12% 3	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	27% 10% 23% 22% 16% 3	227
	11th to 20th	Day	16% 8% 23% 26% 21% 6%	370
	21st to end	Day	15% 7% 21% 26% 22% 9%	407
February	1st to 10th	Day	15% 14% 19% 22% 22% 9%	370
	11th to 20th	Day	14% 15% 25% 22% 17% 6%	416
	21st to end	Day	12% 15% 22% 29% 19% 3	382
March	1st to 10th	Day	15% 15% 28% 27% 13% 3	460
	11th to 20th	Day	16% 13% 26% 21% 17% 7%	460
	21st to end	Day	13% 15% 31% 24% 14% 2	506
April	1st to 10th	Day	12% 17% 28% 21% 15% 7%	460
	11th to 20th	Day	9% 16% 30% 25% 14% 6%	452
	21st to end	Day	3 17% 28% 30% 15% 7%	460
May	1st to 10th	Day	5% 12% 34% 30% 14% 5%	463
	11th to 20th	Day	5% 10% 33% 30% 19% 4	488
	21st to end	Day	2 11% 26% 33% 22% 6%	529
June	1st to 10th	Day	3 11% 28% 35% 18% 5%	490
	11th to 20th	Day	11% 27% 39% 19% 3	490
	21st to end	Day	1 4 26% 37% 26% 6%	490
July	1st to 10th	Day	2 24% 43% 26% 6%	486
	11th to 20th	Day	1 15% 43% 32% 7%	484
	21st to end	Day	1 19% 44% 25% 10%	531
August	1st to 10th	Day	1 4 20% 43% 23% 9%	487
	11th to 20th	Day	3 24% 38% 24% 11%	477
	21st to end	Day	1 1 17% 39% 31% 11%	539
September	1st to 10th	Day	4 17% 40% 28% 10%	490
	11th to 20th	Day	2 8% 22% 37% 22% 9%	490
	21st to end	Day	1 5% 19% 34% 28% 12%	490
October	1st to 10th	Day	1 7% 23% 24% 32% 13%	490
	11th to 20th	Day	5% 6% 17% 26% 30% 16%	490
	21st to end	Day	6% 7% 17% 28% 27% 14%	539
November	1st to 10th	Day	7% 7% 22% 21% 30% 13%	410
	11th to 20th	Day	9% 6% 17% 29% 29% 11%	300
	21st to end	Day	11% 6% 18% 32% 28% 5%	231
December	1st to 10th	Day	18% 9% 19% 23% 19% 12%	182
	11th to 20th	Day	17% 12% 17% 22% 22% 11%	180
	21st to end	Day	19% 11% 23% 23% 20% 5%	198
January	1st to 10th	Night	16% 4 17% 34% 22% 7%	760
	11th to 20th	Night	14% 5% 16% 35% 26% 5%	760
	21st to end	Night	15% 4 18% 33% 25% 5%	836
February	1st to 10th	Night	15% 7% 15% 35% 24% 4	760
	11th to 20th	Night	10% 9% 19% 32% 25% 5%	744
	21st to end	Night	11% 8% 22% 36% 19% 3	628
March	1st to 10th	Night	15% 8% 22% 31% 21% 3	760
	11th to 20th	Night	12% 10% 22% 32% 20% 3	760
	21st to end	Night	10% 8% 29% 34% 18% 1	836
April	1st to 10th	Night	9% 8% 26% 36% 19% 2	760
	11th to 20th	Night	7% 11% 31% 30% 18% 4	751
	21st to end	Night	4 8% 32% 32% 22% 3	760
May	1st to 10th	Night	3 6% 32% 37% 19% 4	713
	11th to 20th	Night	3 4 23% 45% 24% 2	450
	21st to end	Night	3 6% 31% 41% 19%	411
June	1st to 10th	Night	2 4 30% 47% 17% 1	380
	11th to 20th	Night	1 5% 26% 49% 18% 1	380
	21st to end	Night	1 2 21% 50% 26% 1	380
July	1st to 10th	Night	1 19% 50% 29% 1	370
	11th to 20th	Night	13% 49% 35% 2	466
	21st to end	Night	1 1 15% 39% 41% 3	624
August	1st to 10th	Night	2 1 19% 39% 35% 5%	567
	11th to 20th	Night	1 20% 42% 33% 4	703
	21st to end	Night	1 1 13% 43% 37% 5%	836
September	1st to 10th	Night	1 2 15% 39% 37% 5%	760
	11th to 20th	Night	1 3 20% 41% 29% 5%	760
	21st to end	Night	2 3 15% 38% 36% 8%	760
October	1st to 10th	Night	3 4 16% 38% 33% 7%	760
	11th to 20th	Night	3 4 15% 36% 35% 6%	760
	21st to end	Night	5% 3 16% 33% 32% 11%	836
November	1st to 10th	Night	6% 4 15% 34% 33% 8%	780
	11th to 20th	Night	8% 2 16% 40% 29% 5%	800
	21st to end	Night	9% 4 16% 36% 29% 6%	800
December	1st to 10th	Night	13% 5% 16% 37% 24% 6%	768
	11th to 20th	Night	19% 5% 15% 35% 23% 4	760
	21st to end	Night	16% 8% 20% 34% 20% 1	836

Page generated at 2024-11-29 07:43:11. Version ac07

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