Brighton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brighton clear sky chart from 2005-11-06 to 2024-11-05.

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% 5% 2 5% 4 4 4 4 6% 8% 54%	105195
	Night	5% 6% 2 5% 4 3 4 3 5% 7% 55%	102557
Transparency	Day	26% 7% 11% 23% 24% 11%	104937
	Night	27% 6% 12% 24% 23% 8%	102405
Seeing	Day	8% 6% 22% 33% 23% 8%	15618
	Night	7% 3 26% 47% 16% 1	25609

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	2 5% 3 6% 3 3 4 3 6% 7% 58%	2538
	11th to 20th	Day	2 6% 3 7% 4 4 4 4 6% 8% 52%	2572
	21st to end	Day	2 6% 2 6% 4 3 4 3 5% 8% 57%	3146
February	1st to 10th	Day	3 7% 4 7% 4 3 4 4 7% 7% 50%	1710
	11th to 20th	Day	1 6% 3 6% 4 3 4 4 5% 7% 56%	1834
	21st to end	Day	3 6% 3 6% 4 3 4 3 5% 6% 56%	1570
March	1st to 10th	Day	3 10% 2 5% 4 3 4 3 5% 8% 54%	3130
	11th to 20th	Day	6% 8% 3 6% 4 4 4 3 5% 6% 51%	3416
	21st to end	Day	4 6% 2 6% 4 4 4 4 6% 9% 49%	4086
April	1st to 10th	Day	7% 6% 2 5% 3 4 4 3 5% 8% 53%	2280
	11th to 20th	Day	10% 10% 3 6% 3 4 4 3 6% 6% 46%	2251
	21st to end	Day	9% 7% 2 5% 4 5% 5% 4 7% 9% 44%	2280
May	1st to 10th	Day	10% 8% 3 7% 4 6% 5% 4 6% 7% 41%	4046
	11th to 20th	Day	9% 6% 2 5% 4 5% 5% 5% 8% 10% 42%	4370
	21st to end	Day	6% 4 1 4 3 4 5% 4 7% 10% 51%	4825
June	1st to 10th	Day	4 4 1 5% 4 5% 5% 5% 7% 9% 51%	2660
	11th to 20th	Day	6% 3 1 4 4 4 5% 5% 8% 9% 51%	2660
	21st to end	Day	4 3 1 4 4 4 6% 4 9% 11% 51%	2660
July	1st to 10th	Day	5% 4 1 5% 4 4 4 3 6% 8% 57%	4430
	11th to 20th	Day	3 2 1 4 4 5% 7% 6% 10% 11% 48%	4430
	21st to end	Day	4 3 1 5% 3 4 4 4 6% 8% 57%	4775
August	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 6% 8% 10% 39%	4073
	11th to 20th	Day	3 3 1 4 4 4 5% 5% 7% 10% 53%	3726
	21st to end	Day	3 3 1 4 4 4 5% 5% 7% 9% 54%	4180
September	1st to 10th	Day	5% 4 1 3 2 4 3 4 5% 7% 61%	2280
	11th to 20th	Day	5% 3 1 3 4 4 3 4 5% 7% 61%	2247
	21st to end	Day	5% 4 1 4 2 3 3 2 3 5% 67%	2090
October	1st to 10th	Day	4 2 1 2 2 2 2 2 2 4 77%	3215
	11th to 20th	Day	5% 4 2 4 3 3 3 3 4 5% 63%	3150
	21st to end	Day	5% 6% 2 3 2 3 3 2 4 6% 64%	3286
November	1st to 10th	Day	3 4 1 3 2 3 4 4 5% 8% 63%	1709
	11th to 20th	Day	2 7% 2 4 3 3 3 2 5% 8% 60%	1710
	21st to end	Day	3 7% 3 4 3 3 3 4 5% 6% 59%	1620
December	1st to 10th	Day	2 6% 3 5% 4 3 3 3 6% 9% 58%	1520
	11th to 20th	Day	3 6% 2 4 4 4 3 4 5% 7% 58%	1926
	21st to end	Day	5% 10% 3 7% 4 4 3 3 4 6% 50%	2794
January	1st to 10th	Night	2 9% 3 7% 4 4 3 2 5% 6% 55%	4417
	11th to 20th	Night	1 5% 3 6% 4 4 4 3 5% 7% 59%	4426
	21st to end	Night	3 7% 3 8% 4 3 4 3 5% 6% 54%	4521
February	1st to 10th	Night	2 10% 3 7% 4 4 4 3 4 7% 53%	2470
	11th to 20th	Night	2 9% 3 8% 4 4 3 3 5% 6% 51%	2231
	21st to end	Night	3 10% 3 5% 3 3 3 3 4 7% 56%	1884
March	1st to 10th	Night	4 9% 2 5% 4 4 4 3 5% 5% 54%	3734
	11th to 20th	Night	11% 10% 2 5% 3 3 3 2 4 5% 51%	3559
	21st to end	Night	7% 8% 3 6% 4 4 4 3 6% 8% 48%	3557
April	1st to 10th	Night	7% 8% 2 5% 4 3 3 3 5% 6% 53%	1900
	11th to 20th	Night	12% 8% 3 5% 4 4 3 4 5% 6% 47%	1877
	21st to end	Night	13% 8% 2 6% 4 4 4 3 5% 7% 46%	1900
May	1st to 10th	Night	11% 7% 3 7% 4 4 5% 4 6% 7% 43%	2923
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 7% 9% 45%	2531
	21st to end	Night	8% 5% 2 5% 4 3 4 4 7% 7% 52%	2762
June	1st to 10th	Night	5% 3 1 4 3 3 4 4 6% 10% 56%	1520
	11th to 20th	Night	6% 3 1 4 4 3 4 3 6% 8% 59%	1520
	21st to end	Night	5% 3 2 4 4 3 4 5% 7% 9% 55%	1520
July	1st to 10th	Night	4 4 2 5% 4 4 4 3 5% 8% 58%	2540
	11th to 20th	Night	2 3 1 4 4 4 5% 5% 7% 11% 54%	2540
	21st to end	Night	3 4 2 5% 3 4 4 4 7% 10% 54%	2784
August	1st to 10th	Night	9% 5% 2 6% 5% 5% 6% 6% 8% 9% 41%	2864
	11th to 20th	Night	3 3 1 5% 4 5% 6% 5% 8% 11% 50%	3113
	21st to end	Night	3 3 2 5% 3 4 5% 5% 8% 11% 50%	3487
September	1st to 10th	Night	6% 3 2 4 5% 3 5% 3 6% 7% 56%	1900
	11th to 20th	Night	6% 3 2 4 2 3 3 3 5% 7% 62%	1932
	21st to end	Night	7% 3 1 4 3 3 3 3 4 6% 63%	2090
October	1st to 10th	Night	5% 3 2 3 2 3 2 2 3 4 71%	3736
	11th to 20th	Night	6% 4 2 4 3 3 4 3 5% 7% 59%	3778
	21st to end	Night	6% 6% 2 4 2 2 3 3 4 6% 63%	4240
November	1st to 10th	Night	4 4 1 4 3 2 3 4 5% 7% 64%	2470
	11th to 20th	Night	3 6% 2 5% 3 3 3 3 4 6% 61%	2470
	21st to end	Night	3 8% 2 5% 3 3 3 2 4 6% 59%	2557
December	1st to 10th	Night	4 6% 3 4 4 3 3 3 5% 7% 59%	2660
	11th to 20th	Night	2 6% 2 5% 4 3 3 4 6% 8% 57%	3271
	21st to end	Night	6% 10% 3 6% 4 3 4 3 5% 6% 48%	4873

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	22% 5% 12% 29% 23% 8%	2539
	11th to 20th	Day	26% 7% 14% 29% 18% 6%	2572
	21st to end	Day	23% 6% 13% 30% 23% 5%	3146
February	1st to 10th	Day	30% 4 14% 26% 20% 6%	1710
	11th to 20th	Day	26% 5% 9% 28% 26% 6%	1844
	21st to end	Day	27% 4 10% 24% 26% 10%	1570
March	1st to 10th	Day	26% 5% 10% 19% 28% 11%	3130
	11th to 20th	Day	31% 8% 14% 29% 15% 4	3416
	21st to end	Day	26% 8% 14% 30% 18% 3	4086
April	1st to 10th	Day	27% 6% 13% 24% 23% 9%	2280
	11th to 20th	Day	35% 6% 11% 20% 19% 9%	2254
	21st to end	Day	33% 7% 12% 24% 21% 3	2280
May	1st to 10th	Day	34% 10% 13% 23% 15% 5%	4046
	11th to 20th	Day	31% 12% 12% 25% 18% 3	4374
	21st to end	Day	24% 10% 15% 24% 20% 7%	4825
June	1st to 10th	Day	26% 10% 15% 23% 18% 9%	2660
	11th to 20th	Day	26% 8% 11% 17% 23% 15%	2660
	21st to end	Day	22% 8% 12% 21% 24% 13%	2660
July	1st to 10th	Day	24% 6% 8% 15% 30% 18%	4361
	11th to 20th	Day	22% 8% 13% 21% 23% 13%	4355
	21st to end	Day	23% 6% 10% 19% 21% 21%	4624
August	1st to 10th	Day	35% 8% 15% 20% 15% 7%	4059
	11th to 20th	Day	24% 7% 13% 19% 21% 16%	3762
	21st to end	Day	24% 6% 10% 18% 27% 15%	4179
September	1st to 10th	Day	21% 4 8% 21% 25% 20%	2280
	11th to 20th	Day	20% 6% 9% 17% 29% 19%	2247
	21st to end	Day	20% 4 7% 18% 36% 14%	2090
October	1st to 10th	Day	14% 3 5% 15% 36% 26%	3215
	11th to 20th	Day	22% 4 7% 24% 31% 10%	3150
	21st to end	Day	22% 2 6% 22% 36% 12%	3286
November	1st to 10th	Day	18% 4 11% 25% 28% 14%	1709
	11th to 20th	Day	21% 4 8% 25% 33% 8%	1710
	21st to end	Day	24% 4 8% 22% 30% 13%	1620
December	1st to 10th	Day	25% 5% 11% 30% 21% 8%	1520
	11th to 20th	Day	23% 5% 10% 24% 32% 5%	1924
	21st to end	Day	34% 4 9% 22% 23% 7%	2794
January	1st to 10th	Night	28% 6% 10% 27% 26% 4	4417
	11th to 20th	Night	21% 7% 15% 26% 22% 9%	4426
	21st to end	Night	29% 6% 12% 30% 17% 5%	4521
February	1st to 10th	Night	31% 5% 11% 26% 22% 5%	2470
	11th to 20th	Night	34% 3 9% 27% 21% 6%	2231
	21st to end	Night	28% 4 7% 23% 26% 12%	1884
March	1st to 10th	Night	27% 5% 9% 19% 31% 8%	3734
	11th to 20th	Night	33% 7% 14% 19% 24% 3	3559
	21st to end	Night	31% 6% 15% 30% 15% 3	3557
April	1st to 10th	Night	30% 6% 12% 22% 21% 9%	1900
	11th to 20th	Night	35% 7% 12% 21% 17% 7%	1877
	21st to end	Night	36% 5% 11% 25% 17% 5%	1900
May	1st to 10th	Night	34% 11% 13% 21% 14% 8%	2923
	11th to 20th	Night	31% 11% 18% 23% 14% 3	2532
	21st to end	Night	26% 10% 20% 21% 17% 6%	2762
June	1st to 10th	Night	21% 9% 19% 30% 13% 8%	1520
	11th to 20th	Night	22% 7% 13% 21% 23% 14%	1520
	21st to end	Night	23% 6% 13% 25% 27% 7%	1520
July	1st to 10th	Night	24% 6% 11% 19% 30% 9%	2500
	11th to 20th	Night	22% 7% 15% 25% 24% 8%	2492
	21st to end	Night	22% 7% 14% 23% 21% 13%	2697
August	1st to 10th	Night	35% 8% 16% 20% 15% 4	2855
	11th to 20th	Night	24% 9% 19% 20% 18% 10%	3143
	21st to end	Night	25% 6% 15% 23% 25% 7%	3487
September	1st to 10th	Night	25% 5% 12% 19% 23% 16%	1900
	11th to 20th	Night	20% 5% 11% 22% 25% 16%	1932
	21st to end	Night	22% 4 9% 20% 33% 12%	2090
October	1st to 10th	Night	18% 3 8% 17% 31% 23%	3736
	11th to 20th	Night	24% 3 10% 26% 28% 9%	3778
	21st to end	Night	23% 3 6% 25% 31% 12%	4240
November	1st to 10th	Night	19% 5% 10% 24% 28% 14%	2470
	11th to 20th	Night	22% 4 8% 27% 30% 9%	2470
	21st to end	Night	26% 3 8% 23% 30% 10%	2557
December	1st to 10th	Night	25% 4 13% 26% 23% 9%	2660
	11th to 20th	Night	24% 5% 10% 27% 31% 4	3272
	21st to end	Night	34% 4 10% 24% 23% 5%	4873

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	5% 6% 19% 46% 19% 6%	270
	11th to 20th	Day	8% 6% 25% 43% 16% 2	270
	21st to end	Day	5% 2 30% 42% 16% 5%	297
February	1st to 10th	Day	5% 10% 30% 43% 9% 2	270
	11th to 20th	Day	3 3 36% 45% 10% 3	270
	21st to end	Day	3 7% 34% 46% 8% 1	225
March	1st to 10th	Day	4 9% 31% 36% 17% 4	270
	11th to 20th	Day	10% 13% 26% 32% 14% 4	412
	21st to end	Day	11% 13% 29% 33% 11% 2	506
April	1st to 10th	Day	10% 12% 28% 34% 13% 3	460
	11th to 20th	Day	16% 18% 29% 24% 10% 3	452
	21st to end	Day	14% 12% 27% 26% 16% 4	460
May	1st to 10th	Day	16% 7% 27% 34% 14% 3	605
	11th to 20th	Day	16% 7% 18% 30% 22% 6%	675
	21st to end	Day	10% 8% 22% 30% 22% 7%	734
June	1st to 10th	Day	6% 3 18% 31% 29% 13%	680
	11th to 20th	Day	7% 4 24% 35% 25% 6%	680
	21st to end	Day	5% 4 20% 33% 30% 8%	680
July	1st to 10th	Day	8% 2 14% 25% 37% 13%	671
	11th to 20th	Day	4 2 12% 28% 39% 14%	668
	21st to end	Day	5% 2 12% 23% 42% 16%	724
August	1st to 10th	Day	6% 3 18% 28% 29% 16%	487
	11th to 20th	Day	5% 3 17% 31% 29% 15%	477
	21st to end	Day	5% 3 16% 28% 33% 15%	539
September	1st to 10th	Day	6% 3 13% 31% 31% 16%	490
	11th to 20th	Day	5% 3 18% 33% 28% 13%	457
	21st to end	Day	6% 3 15% 32% 28% 16%	300
October	1st to 10th	Day	11% 3 18% 23% 28% 17%	300
	11th to 20th	Day	4 5% 20% 37% 27% 7%	300
	21st to end	Day	10% 7% 23% 35% 20% 5%	330
November	1st to 10th	Day	4 5% 24% 39% 20% 8%	282
	11th to 20th	Day	5% 7% 26% 41% 15% 6%	270
	21st to end	Day	6% 6% 26% 43% 16% 3	270
December	1st to 10th	Day	4 8% 27% 40% 18% 2	270
	11th to 20th	Day	4 4 30% 46% 16% 1	270
	21st to end	Day	6% 5% 34% 42% 11% 2	297
January	1st to 10th	Night	5% 3 29% 48% 12% 2	950
	11th to 20th	Night	4 4 33% 48% 10% 1	946
	21st to end	Night	6% 2 30% 50% 11% 1	836
February	1st to 10th	Night	7% 2 36% 47% 7%	760
	11th to 20th	Night	5% 3 36% 47% 9%	744
	21st to end	Night	6% 6% 37% 49% 3	628
March	1st to 10th	Night	5% 4 37% 45% 9%	760
	11th to 20th	Night	10% 4 28% 46% 11%	760
	21st to end	Night	9% 7% 32% 44% 9%	681
April	1st to 10th	Night	10% 8% 36% 42% 5%	570
	11th to 20th	Night	16% 6% 33% 38% 7%	563
	21st to end	Night	16% 6% 27% 39% 12%	570
May	1st to 10th	Night	12% 7% 26% 41% 12% 1	570
	11th to 20th	Night	16% 3 18% 44% 18% 1	561
	21st to end	Night	10% 5% 21% 43% 19% 1	617
June	1st to 10th	Night	6% 3 17% 47% 24% 3	570
	11th to 20th	Night	7% 3 22% 52% 16%	570
	21st to end	Night	5% 2 19% 48% 26% 1	570
July	1st to 10th	Night	8% 2 15% 41% 33% 1	555
	11th to 20th	Night	3 2 13% 47% 34% 2	552
	21st to end	Night	6% 1 10% 43% 38% 2	624
August	1st to 10th	Night	6% 2 14% 41% 36% 1	567
	11th to 20th	Night	4 2 12% 54% 27% 1	564
	21st to end	Night	5% 1 13% 49% 29% 2	627
September	1st to 10th	Night	6% 2 14% 48% 26% 4	570
	11th to 20th	Night	6% 1 15% 49% 26% 2	570
	21st to end	Night	7% 2 19% 47% 23% 2	570
October	1st to 10th	Night	10% 4 20% 43% 20% 2	722
	11th to 20th	Night	6% 2 25% 49% 17% 1	760
	21st to end	Night	10% 4 23% 48% 14% 2	907
November	1st to 10th	Night	4 3 26% 52% 13% 1	950
	11th to 20th	Night	7% 3 28% 51% 9% 1	950
	21st to end	Night	7% 3 27% 48% 14% 1	950
December	1st to 10th	Night	6% 3 33% 49% 10%	950
	11th to 20th	Night	5% 3 32% 49% 10% 1	950
	21st to end	Night	7% 4 34% 46% 8% 1	1045

Page generated at 2024-11-03 00:11:16. Version ac07

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