Chelsea
Clear Sky Chart
History

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

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	17% 10% 3 8% 5% 5% 4 4 5% 7% 32%	101832
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 32%	97881
Transparency	Day	29% 45% 6% 13% 6%	105315
	Night	30% 45% 6% 12% 6%	102487
Seeing	Day	24% 15% 31% 19% 8% 2	14531
	Night	24% 13% 28% 25% 9% 1	23652

By Month

Percentage Cloud Cover NE (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% 17% 7% 13% 6% 4 4 3 4 4 21%	2517
	11th to 20th	Day	15% 14% 5% 15% 7% 6% 5% 3 5% 5% 21%	2520
	21st to end	Day	14% 19% 7% 12% 7% 6% 5% 3 4 5% 18%	2880
February	1st to 10th	Day	13% 15% 6% 13% 7% 5% 4 4 5% 5% 24%	1710
	11th to 20th	Day	13% 16% 6% 10% 6% 4 4 4 5% 5% 28%	1663
	21st to end	Day	19% 13% 4 10% 7% 4 4 4 5% 5% 25%	1570
March	1st to 10th	Day	21% 16% 3 8% 5% 5% 3 3 4 5% 25%	3116
	11th to 20th	Day	13% 16% 6% 10% 6% 5% 5% 3 4 5% 28%	3480
	21st to end	Day	14% 8% 2 7% 7% 4 4 3 5% 6% 40%	4026
April	1st to 10th	Day	23% 12% 3 7% 4 4 4 2 4 6% 31%	2280
	11th to 20th	Day	23% 10% 3 7% 4 3 4 3 4 5% 36%	2254
	21st to end	Day	23% 10% 3 8% 5% 5% 4 3 4 5% 31%	2422
May	1st to 10th	Day	22% 8% 2 7% 5% 5% 5% 4 6% 6% 30%	4426
	11th to 20th	Day	18% 8% 3 8% 5% 5% 4 3 5% 7% 34%	4398
	21st to end	Day	17% 7% 3 7% 5% 5% 4 4 6% 7% 36%	4837
June	1st to 10th	Day	22% 8% 2 7% 6% 5% 5% 4 5% 6% 30%	2660
	11th to 20th	Day	15% 6% 2 7% 5% 5% 5% 4 6% 8% 38%	2660
	21st to end	Day	20% 7% 2 6% 5% 5% 6% 5% 7% 6% 31%	2660
July	1st to 10th	Day	12% 6% 2 8% 5% 5% 6% 5% 7% 10% 34%	4440
	11th to 20th	Day	14% 5% 2 7% 5% 5% 5% 4 6% 7% 40%	4440
	21st to end	Day	11% 5% 2 5% 4 3 5% 4 6% 9% 45%	4814
August	1st to 10th	Day	14% 6% 3 8% 4 5% 5% 4 6% 8% 36%	4362
	11th to 20th	Day	17% 5% 2 6% 4 4 4 4 7% 9% 37%	3997
	21st to end	Day	13% 5% 2 6% 4 5% 5% 4 7% 9% 40%	4180
September	1st to 10th	Day	14% 5% 2 6% 5% 4 4 4 5% 8% 43%	2280
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 7% 47%	2280
	21st to end	Day	19% 5% 2 6% 4 3 4 4 5% 8% 40%	2029
October	1st to 10th	Day	21% 8% 3 6% 4 4 4 4 6% 7% 34%	1900
	11th to 20th	Day	23% 9% 2 8% 5% 4 5% 4 5% 7% 28%	1900
	21st to end	Day	26% 9% 3 7% 4 5% 4 3 4 6% 28%	1936
November	1st to 10th	Day	16% 9% 3 7% 5% 4 5% 3 6% 7% 33%	1757
	11th to 20th	Day	20% 13% 3 7% 5% 5% 5% 4 6% 6% 27%	1600
	21st to end	Day	22% 15% 5% 8% 7% 5% 4 4 4 6% 20%	1600
December	1st to 10th	Day	21% 20% 5% 11% 5% 4 3 3 6% 6% 16%	1560
	11th to 20th	Day	21% 13% 4 11% 6% 5% 4 3 4 4 25%	1906
	21st to end	Day	28% 19% 5% 10% 4 3 3 2 3 4 19%	2772
January	1st to 10th	Night	17% 19% 6% 11% 5% 5% 4 3 4 5% 21%	4429
	11th to 20th	Night	13% 16% 5% 12% 7% 5% 5% 4 4 5% 24%	4440
	21st to end	Night	16% 20% 6% 11% 5% 6% 4 3 3 4 21%	4774
February	1st to 10th	Night	14% 14% 5% 12% 7% 5% 4 4 5% 5% 24%	2470
	11th to 20th	Night	13% 14% 6% 10% 7% 4 4 3 4 6% 29%	2403
	21st to end	Night	17% 14% 4 8% 6% 5% 5% 3 5% 5% 29%	1884
March	1st to 10th	Night	22% 12% 4 7% 5% 4 4 3 3 4 31%	3707
	11th to 20th	Night	16% 17% 5% 10% 4 5% 4 3 5% 5% 26%	3479
	21st to end	Night	15% 9% 3 8% 4 4 3 3 4 6% 40%	3605
April	1st to 10th	Night	21% 10% 4 7% 4 4 4 3 4 5% 33%	1900
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 3 4 6% 36%	1875
	21st to end	Night	22% 9% 3 6% 4 4 4 3 5% 6% 34%	1773
May	1st to 10th	Night	21% 9% 3 8% 4 5% 5% 4 6% 6% 30%	2514
	11th to 20th	Night	21% 8% 2 7% 4 3 4 4 5% 6% 36%	2495
	21st to end	Night	16% 6% 3 7% 5% 5% 5% 4 6% 6% 39%	2738
June	1st to 10th	Night	17% 6% 2 7% 5% 5% 4 5% 6% 7% 35%	1520
	11th to 20th	Night	14% 5% 2 6% 4 4 3 4 6% 7% 43%	1520
	21st to end	Night	19% 8% 2 5% 5% 5% 5% 3 5% 8% 35%	1520
July	1st to 10th	Night	13% 6% 3 8% 5% 6% 6% 5% 6% 7% 35%	2520
	11th to 20th	Night	14% 7% 2 6% 4 4 5% 4 5% 7% 42%	2520
	21st to end	Night	11% 5% 2 6% 4 5% 5% 5% 7% 7% 44%	2739
August	1st to 10th	Night	13% 7% 3 8% 4 4 4 4 6% 8% 39%	2564
	11th to 20th	Night	16% 6% 2 6% 4 4 4 4 6% 8% 42%	2799
	21st to end	Night	14% 5% 2 6% 5% 5% 4 4 6% 7% 41%	3476
September	1st to 10th	Night	12% 5% 2 6% 4 3 4 4 5% 7% 48%	1900
	11th to 20th	Night	15% 4 2 5% 3 3 4 4 4 6% 49%	1900
	21st to end	Night	18% 6% 2 7% 4 3 4 2 4 6% 44%	2150
October	1st to 10th	Night	20% 7% 2 6% 4 5% 4 4 4 6% 39%	2280
	11th to 20th	Night	22% 7% 2 7% 6% 5% 5% 4 5% 5% 32%	2280
	21st to end	Night	23% 10% 2 8% 5% 5% 4 3 5% 7% 28%	2661
November	1st to 10th	Night	19% 11% 3 7% 4 4 4 5% 4 7% 33%	2537
	11th to 20th	Night	19% 11% 4 9% 5% 5% 5% 4 4 5% 30%	2800
	21st to end	Night	22% 13% 5% 9% 6% 5% 4 3 5% 5% 24%	2800
December	1st to 10th	Night	21% 17% 4 10% 6% 5% 5% 4 5% 6% 18%	2737
	11th to 20th	Night	23% 14% 5% 9% 6% 3 3 3 3 5% 27%	3288
	21st to end	Night	29% 19% 4 9% 5% 3 3 2 3 3 19%	4884

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	44% 36% 8% 6% 5% 1	2518
	11th to 20th	Day	45% 36% 7% 9% 3	2520
	21st to end	Day	47% 39% 6% 6% 1	2880
February	1st to 10th	Day	24% 64% 2 7% 2 1	1710
	11th to 20th	Day	22% 63% 3 7% 5%	1672
	21st to end	Day	21% 64% 4 4 5% 2	1570
March	1st to 10th	Day	40% 39% 4 9% 7%	3119
	11th to 20th	Day	39% 39% 5% 9% 7% 2	3480
	21st to end	Day	28% 38% 3 16% 13% 1	4026
April	1st to 10th	Day	21% 65% 3 5% 6% 1	2280
	11th to 20th	Day	20% 65% 1 8% 5% 1	2254
	21st to end	Day	21% 64% 2 7% 6%	2422
May	1st to 10th	Day	36% 38% 4 17% 5%	4440
	11th to 20th	Day	34% 38% 10% 14% 4	4402
	21st to end	Day	32% 38% 7% 14% 8%	4837
June	1st to 10th	Day	19% 64% 2 10% 4	2660
	11th to 20th	Day	17% 65% 4 8% 5% 1	2660
	21st to end	Day	18% 63% 3 11% 4	2660
July	1st to 10th	Day	31% 35% 10% 17% 7%	4370
	11th to 20th	Day	29% 35% 9% 23% 4	4365
	21st to end	Day	23% 35% 9% 25% 8%	4661
August	1st to 10th	Day	31% 36% 11% 16% 6%	4348
	11th to 20th	Day	30% 36% 9% 20% 5%	4036
	21st to end	Day	26% 36% 11% 19% 8%	4180
September	1st to 10th	Day	16% 59% 8% 11% 5%	2280
	11th to 20th	Day	14% 60% 5% 15% 6%	2280
	21st to end	Day	15% 59% 6% 13% 7%	2029
October	1st to 10th	Day	33% 36% 6% 23% 3	3160
	11th to 20th	Day	28% 37% 8% 20% 7%	3140
	21st to end	Day	32% 36% 14% 14% 4	3161
November	1st to 10th	Day	27% 56% 5% 9% 2 1	1757
	11th to 20th	Day	25% 57% 4 10% 4 1	1600
	21st to end	Day	31% 57% 5% 4 4 1	1600
December	1st to 10th	Day	29% 58% 4 6% 2	1560
	11th to 20th	Day	35% 48% 7% 6% 4	1906
	21st to end	Day	47% 36% 2 12% 3	2772
January	1st to 10th	Night	43% 35% 12% 8% 3	4429
	11th to 20th	Night	40% 35% 11% 11% 2	4440
	21st to end	Night	48% 39% 6% 4 3	4774
February	1st to 10th	Night	22% 64% 3 8% 3	2470
	11th to 20th	Night	20% 63% 3 9% 5%	2405
	21st to end	Night	20% 64% 2 7% 6%	1884
March	1st to 10th	Night	36% 39% 5% 10% 7% 2	3717
	11th to 20th	Night	40% 39% 6% 9% 6% 1	3480
	21st to end	Night	29% 39% 3 14% 13% 3	3605
April	1st to 10th	Night	20% 64% 3 7% 5% 1	1900
	11th to 20th	Night	18% 65% 2 8% 6%	1875
	21st to end	Night	18% 64% 4 6% 7% 1	1773
May	1st to 10th	Night	37% 39% 6% 15% 4	2520
	11th to 20th	Night	34% 38% 8% 14% 5%	2496
	21st to end	Night	32% 39% 6% 13% 10% 1	2739
June	1st to 10th	Night	16% 65% 2 11% 4 1	1520
	11th to 20th	Night	13% 65% 3 10% 8%	1520
	21st to end	Night	18% 63% 3 10% 6%	1520
July	1st to 10th	Night	35% 35% 9% 11% 9%	2480
	11th to 20th	Night	29% 35% 9% 23% 4	2472
	21st to end	Night	24% 35% 10% 21% 9%	2651
August	1st to 10th	Night	30% 36% 9% 19% 6%	2554
	11th to 20th	Night	31% 36% 10% 20% 3	2826
	21st to end	Night	29% 36% 8% 17% 9% 2	3476
September	1st to 10th	Night	13% 60% 6% 16% 6%	1900
	11th to 20th	Night	14% 60% 5% 15% 5% 1	1900
	21st to end	Night	15% 59% 4 15% 7%	2150
October	1st to 10th	Night	25% 35% 8% 23% 8%	3800
	11th to 20th	Night	26% 36% 10% 21% 7%	3779
	21st to end	Night	32% 36% 12% 13% 7%	4386
November	1st to 10th	Night	26% 55% 6% 9% 4	2537
	11th to 20th	Night	26% 56% 4 10% 5%	2800
	21st to end	Night	27% 57% 5% 8% 4	2800
December	1st to 10th	Night	28% 58% 4 8% 2	2737
	11th to 20th	Night	34% 48% 4 9% 5% 1	3288
	21st to end	Night	49% 35% 2 10% 4	4884

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	29% 16% 33% 18% 4	180
	11th to 20th	Day	29% 17% 38% 12% 3 1	180
	21st to end	Day	23% 19% 36% 16% 5%	231
February	1st to 10th	Day	34% 23% 31% 9% 2	270
	11th to 20th	Day	29% 28% 36% 6% 1	270
	21st to end	Day	33% 22% 29% 14% 2	382
March	1st to 10th	Day	28% 19% 33% 14% 6%	460
	11th to 20th	Day	28% 20% 33% 14% 3 1	460
	21st to end	Day	32% 19% 30% 13% 5% 1	506
April	1st to 10th	Day	31% 22% 27% 14% 5% 1	460
	11th to 20th	Day	36% 19% 24% 14% 7% 1	452
	21st to end	Day	32% 21% 29% 12% 5% 1	460
May	1st to 10th	Day	26% 16% 32% 16% 6% 3	463
	11th to 20th	Day	19% 18% 34% 19% 8% 2	487
	21st to end	Day	23% 15% 33% 20% 8% 2	529
June	1st to 10th	Day	26% 11% 35% 20% 7% 1	490
	11th to 20th	Day	19% 15% 31% 22% 11% 3	490
	21st to end	Day	26% 11% 31% 20% 9% 3	490
July	1st to 10th	Day	14% 8% 32% 32% 11% 3	485
	11th to 20th	Day	14% 10% 35% 27% 11% 3	484
	21st to end	Day	14% 7% 35% 29% 12% 3	532
August	1st to 10th	Day	16% 9% 28% 30% 14% 3	486
	11th to 20th	Day	19% 4 27% 30% 13% 8%	479
	21st to end	Day	17% 11% 29% 25% 12% 6%	539
September	1st to 10th	Day	18% 11% 31% 22% 12% 7%	490
	11th to 20th	Day	14% 10% 26% 21% 19% 10%	490
	21st to end	Day	22% 12% 27% 21% 14% 4	490
October	1st to 10th	Day	25% 16% 23% 22% 10% 3	490
	11th to 20th	Day	27% 18% 32% 16% 7% 1	490
	21st to end	Day	33% 16% 29% 19% 4	457
November	1st to 10th	Day	28% 18% 30% 18% 6% 1	391
	11th to 20th	Day	27% 27% 30% 14% 3	200
	21st to end	Day	31% 23% 30% 12% 3 2	200
December	1st to 10th	Day	34% 12% 38% 12% 4	190
	11th to 20th	Day	28% 22% 37% 11% 2	180
	21st to end	Day	35% 20% 32% 11% 2 1	198
January	1st to 10th	Night	28% 17% 32% 19% 4	760
	11th to 20th	Night	30% 17% 26% 21% 5%	760
	21st to end	Night	20% 15% 35% 25% 5%	836
February	1st to 10th	Night	22% 20% 31% 24% 3	760
	11th to 20th	Night	22% 21% 36% 18% 4	743
	21st to end	Night	25% 19% 29% 23% 4	628
March	1st to 10th	Night	27% 16% 31% 22% 4	760
	11th to 20th	Night	25% 19% 31% 20% 5%	760
	21st to end	Night	25% 17% 29% 22% 7%	836
April	1st to 10th	Night	29% 15% 27% 24% 4	760
	11th to 20th	Night	25% 20% 28% 20% 7% 1	751
	21st to end	Night	27% 15% 27% 23% 8%	633
May	1st to 10th	Night	25% 10% 27% 29% 8% 2	380
	11th to 20th	Night	24% 9% 28% 30% 10%	374
	21st to end	Night	18% 8% 36% 27% 10% 1	411
June	1st to 10th	Night	19% 9% 27% 34% 10%	380
	11th to 20th	Night	16% 9% 26% 34% 14% 1	380
	21st to end	Night	21% 7% 23% 35% 14%	380
July	1st to 10th	Night	13% 3 28% 38% 19%	370
	11th to 20th	Night	13% 4 22% 43% 17% 1	368
	21st to end	Night	10% 5% 24% 42% 18% 2	416
August	1st to 10th	Night	13% 4 19% 41% 22% 1	410
	11th to 20th	Night	14% 5% 18% 36% 24% 3	565
	21st to end	Night	15% 6% 25% 34% 18% 2	836
September	1st to 10th	Night	15% 7% 29% 29% 19% 1	760
	11th to 20th	Night	17% 7% 20% 30% 22% 4	760
	21st to end	Night	22% 8% 25% 26% 18% 2	760
October	1st to 10th	Night	23% 11% 26% 23% 15% 1	760
	11th to 20th	Night	27% 12% 27% 25% 8% 1	760
	21st to end	Night	29% 14% 28% 24% 5%	836
November	1st to 10th	Night	28% 15% 23% 25% 8% 1	780
	11th to 20th	Night	26% 17% 31% 16% 10%	800
	21st to end	Night	30% 19% 23% 22% 5%	800
December	1st to 10th	Night	30% 16% 29% 21% 4	783
	11th to 20th	Night	28% 19% 27% 20% 5% 1	760
	21st to end	Night	34% 16% 31% 16% 4	836

Page generated at 2024-12-03 06:45:12. Version ac07

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