Ewell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ewell Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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% 4 2 4 3 3 3 3 5% 8% 59%	146796
	Night	6% 5% 2 5% 4 4 4 3 5% 8% 53%	143742
Transparency	Day	23% 6% 11% 19% 24% 17%	141719
	Night	28% 7% 12% 18% 21% 14%	139809
Seeing	Day	8% 11% 25% 31% 20% 6%	17705
	Night	9% 6% 16% 32% 30% 7%	27020

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	12% 6% 3 6% 4 4 4 4 6% 10% 42%	5119
	11th to 20th	Day	7% 5% 3 6% 4 4 3 3 4 7% 55%	5340
	21st to end	Day	12% 8% 3 8% 5% 4 5% 4 6% 8% 38%	5843
February	1st to 10th	Day	11% 8% 2 6% 4 4 4 4 5% 5% 45%	1890
	11th to 20th	Day	11% 8% 3 7% 4 4 5% 3 7% 8% 39%	1934
	21st to end	Day	10% 6% 2 7% 5% 4 4 4 6% 7% 46%	1730
March	1st to 10th	Day	9% 7% 3 8% 5% 5% 7% 5% 7% 9% 35%	5010
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 6% 9% 49%	5110
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	5668
April	1st to 10th	Day	9% 6% 2 5% 4 4 5% 4 6% 8% 47%	2520
	11th to 20th	Day	8% 5% 2 5% 3 4 4 3 6% 7% 54%	2489
	21st to end	Day	4 5% 2 5% 3 3 3 3 6% 10% 56%	2520
May	1st to 10th	Day	3 3 1 4 3 3 3 3 4 6% 68%	7298
	11th to 20th	Day	2 2 1 2 1 1 2 1 3 7% 78%	7697
	21st to end	Day	1 4 1 3 2 2 2 2 5% 9% 67%	8992
June	1st to 10th	Day	4 5% 2 3 2 3 3 3 6% 8% 61%	2940
	11th to 20th	Day	1 2 1 2 1 2 2 2 4 8% 73%	2940
	21st to end	Day	2 3 1 3 2 2 3 2 4 8% 70%	2940
July	1st to 10th	Day	1 1 2 1 2 2 1 3 6% 81%	6419
	11th to 20th	Day	1 1 1 2 2 2 2 2 4 8% 75%	6010
	21st to end	Day	2 1 3 3 3 4 3 6% 11% 63%	6533
August	1st to 10th	Day	1 2 1 2 2 2 3 3 5% 8% 70%	5777
	11th to 20th	Day	2 1 3 2 3 3 2 4 8% 73%	5533
	21st to end	Day	2 1 3 2 2 3 2 4 8% 72%	5984
September	1st to 10th	Day	1 3 1 4 2 2 2 2 3 7% 71%	2400
	11th to 20th	Day	2 5% 2 5% 3 4 3 3 5% 9% 59%	2400
	21st to end	Day	4 2 1 2 2 2 2 2 3 8% 72%	2114
October	1st to 10th	Day	2 2 1 3 3 3 2 3 4 6% 71%	3280
	11th to 20th	Day	3 3 1 4 3 3 3 3 4 8% 64%	3261
	21st to end	Day	4 3 1 5% 4 3 3 3 6% 8% 60%	3558
November	1st to 10th	Day	5% 5% 2 4 4 4 4 4 5% 8% 54%	2040
	11th to 20th	Day	8% 7% 2 6% 4 6% 5% 4 5% 8% 46%	1890
	21st to end	Day	9% 5% 2 6% 4 3 3 3 5% 8% 51%	1717
December	1st to 10th	Day	5% 3 2 6% 4 4 3 3 6% 10% 53%	2700
	11th to 20th	Day	14% 9% 3 8% 7% 6% 5% 4 7% 8% 29%	3108
	21st to end	Day	16% 9% 4 8% 5% 6% 6% 4 7% 7% 29%	4092
January	1st to 10th	Night	12% 7% 3 6% 5% 4 4 4 6% 8% 41%	7791
	11th to 20th	Night	8% 6% 3 6% 4 4 3 3 5% 6% 52%	7650
	21st to end	Night	11% 10% 3 9% 5% 5% 4 3 5% 7% 39%	8371
February	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 5% 6% 47%	2730
	11th to 20th	Night	9% 8% 2 7% 5% 5% 4 4 6% 8% 42%	2572
	21st to end	Night	9% 7% 2 6% 3 3 4 3 5% 8% 48%	2076
March	1st to 10th	Night	10% 9% 3 7% 5% 5% 5% 5% 7% 8% 37%	5100
	11th to 20th	Night	10% 7% 3 5% 4 4 4 2 4 6% 50%	5110
	21st to end	Night	10% 9% 2 7% 5% 4 4 3 5% 7% 43%	5535
April	1st to 10th	Night	7% 6% 2 7% 5% 4 3 4 7% 8% 47%	2100
	11th to 20th	Night	7% 5% 1 5% 4 4 4 3 6% 7% 53%	2079
	21st to end	Night	3 4 1 4 4 3 4 3 5% 10% 59%	2100
May	1st to 10th	Night	3 4 2 6% 4 3 3 2 4 7% 62%	5663
	11th to 20th	Night	1 2 1 2 2 2 2 3 5% 8% 71%	5181
	21st to end	Night	2 3 1 4 3 3 3 3 4 6% 68%	5181
June	1st to 10th	Night	3 4 2 5% 3 3 3 4 6% 10% 57%	1680
	11th to 20th	Night	1 1 1 2 3 2 2 2 5% 9% 72%	1680
	21st to end	Night	1 3 1 3 3 2 3 2 4 7% 69%	1680
July	1st to 10th	Night	1 2 1 1 2 2 4 8% 78%	3756
	11th to 20th	Night	1 1 1 3 2 3 3 3 5% 11% 67%	4190
	21st to end	Night	2 2 7% 6% 5% 6% 6% 9% 13% 44%	4562
August	1st to 10th	Night	1 3 1 4 3 3 3 3 6% 12% 60%	4035
	11th to 20th	Night	2 1 3 3 4 3 3 6% 10% 65%	4328
	21st to end	Night	1 2 1 4 3 3 4 3 6% 10% 63%	5015
September	1st to 10th	Night	1 2 1 3 3 2 3 4 5% 9% 66%	2000
	11th to 20th	Night	3 4 2 5% 3 3 3 3 6% 10% 57%	2000
	21st to end	Night	2 3 1 3 3 3 2 1 4 7% 70%	2281
October	1st to 10th	Night	3 2 1 3 3 3 3 3 5% 6% 69%	3920
	11th to 20th	Night	4 4 1 4 3 3 3 3 6% 9% 59%	3897
	21st to end	Night	4 4 1 5% 3 3 4 3 5% 7% 61%	4253
November	1st to 10th	Night	6% 5% 2 4 4 4 4 3 5% 8% 55%	2476
	11th to 20th	Night	8% 7% 2 6% 5% 4 4 3 6% 8% 48%	2730
	21st to end	Night	7% 5% 2 5% 4 4 4 4 5% 8% 52%	2898
December	1st to 10th	Night	6% 4 2 5% 3 3 4 3 5% 7% 57%	4700
	11th to 20th	Night	16% 8% 3 8% 5% 5% 5% 5% 7% 10% 29%	5316
	21st to end	Night	16% 9% 3 8% 6% 5% 6% 5% 6% 6% 30%	7106

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	36% 6% 18% 22% 14% 3	5120
	11th to 20th	Day	29% 5% 6% 12% 27% 20%	5340
	21st to end	Day	42% 6% 7% 16% 16% 13%	5843
February	1st to 10th	Day	39% 6% 8% 19% 18% 10%	1890
	11th to 20th	Day	40% 9% 11% 16% 18% 6%	1943
	21st to end	Day	39% 6% 9% 23% 16% 6%	1730
March	1st to 10th	Day	42% 11% 9% 19% 17% 2	5010
	11th to 20th	Day	30% 7% 16% 28% 11% 8%	5110
	21st to end	Day	36% 12% 17% 23% 10% 2	5668
April	1st to 10th	Day	35% 8% 13% 21% 18% 4	2520
	11th to 20th	Day	30% 8% 15% 23% 19% 5%	2492
	21st to end	Day	25% 9% 14% 27% 22% 4	2520
May	1st to 10th	Day	18% 5% 9% 25% 31% 11%	7298
	11th to 20th	Day	10% 4 8% 29% 38% 12%	7700
	21st to end	Day	15% 7% 12% 27% 35% 4	8994
June	1st to 10th	Day	22% 10% 13% 22% 24% 8%	2940
	11th to 20th	Day	11% 7% 10% 21% 36% 15%	2940
	21st to end	Day	13% 6% 11% 12% 29% 28%	2940
July	1st to 10th	Day	6% 4 7% 8% 20% 55%	6351
	11th to 20th	Day	8% 4 7% 5% 18% 57%	5941
	21st to end	Day	11% 5% 13% 18% 27% 27%	6348
August	1st to 10th	Day	14% 5% 10% 11% 28% 31%	4206
	11th to 20th	Day	9% 5% 9% 9% 42% 27%	4004
	21st to end	Day	12% 5% 11% 12% 33% 28%	4311
September	1st to 10th	Day	13% 5% 9% 11% 27% 37%	2400
	11th to 20th	Day	21% 8% 11% 20% 25% 15%	2400
	21st to end	Day	14% 5% 7% 21% 32% 22%	2114
October	1st to 10th	Day	15% 3 6% 12% 28% 36%	3280
	11th to 20th	Day	18% 7% 10% 22% 28% 15%	3261
	21st to end	Day	21% 5% 11% 24% 26% 14%	3558
November	1st to 10th	Day	28% 6% 11% 23% 20% 11%	2040
	11th to 20th	Day	35% 8% 10% 22% 19% 6%	1890
	21st to end	Day	31% 5% 12% 17% 27% 9%	1717
December	1st to 10th	Day	26% 6% 7% 25% 29% 7%	2700
	11th to 20th	Day	47% 8% 13% 14% 14% 5%	3108
	21st to end	Day	51% 9% 12% 17% 10% 2	4092
January	1st to 10th	Night	38% 8% 19% 20% 13% 3	7791
	11th to 20th	Night	31% 6% 8% 13% 26% 16%	7650
	21st to end	Night	43% 6% 7% 15% 17% 12%	8371
February	1st to 10th	Night	37% 6% 10% 17% 18% 11%	2730
	11th to 20th	Night	40% 8% 11% 17% 19% 6%	2574
	21st to end	Night	34% 7% 13% 27% 15% 4	2076
March	1st to 10th	Night	43% 10% 11% 20% 14% 2	5100
	11th to 20th	Night	35% 6% 12% 24% 15% 8%	5110
	21st to end	Night	41% 10% 15% 20% 11% 2	5535
April	1st to 10th	Night	34% 8% 15% 22% 17% 4	2100
	11th to 20th	Night	31% 8% 13% 24% 17% 7%	2079
	21st to end	Night	23% 9% 14% 29% 19% 6%	2100
May	1st to 10th	Night	23% 5% 10% 25% 26% 11%	5661
	11th to 20th	Night	15% 5% 13% 25% 30% 13%	5182
	21st to end	Night	17% 7% 15% 25% 34% 3	5182
June	1st to 10th	Night	22% 11% 14% 21% 24% 8%	1680
	11th to 20th	Night	11% 9% 12% 19% 33% 17%	1680
	21st to end	Night	14% 7% 11% 14% 27% 27%	1680
July	1st to 10th	Night	5% 4 7% 11% 20% 53%	3716
	11th to 20th	Night	8% 5% 8% 11% 26% 43%	4136
	21st to end	Night	16% 8% 22% 14% 22% 19%	4435
August	1st to 10th	Night	17% 7% 13% 15% 27% 22%	2929
	11th to 20th	Night	12% 6% 13% 9% 36% 24%	3127
	21st to end	Night	13% 8% 15% 11% 29% 25%	3608
September	1st to 10th	Night	13% 8% 10% 11% 23% 35%	2000
	11th to 20th	Night	22% 7% 16% 18% 23% 13%	2000
	21st to end	Night	15% 6% 9% 20% 27% 23%	2281
October	1st to 10th	Night	16% 5% 6% 11% 23% 39%	3920
	11th to 20th	Night	20% 6% 15% 20% 22% 18%	3897
	21st to end	Night	22% 5% 8% 22% 24% 18%	4253
November	1st to 10th	Night	27% 8% 12% 23% 19% 11%	2476
	11th to 20th	Night	32% 8% 12% 24% 17% 8%	2730
	21st to end	Night	29% 6% 11% 21% 21% 12%	2898
December	1st to 10th	Night	27% 6% 7% 24% 27% 9%	4700
	11th to 20th	Night	47% 6% 12% 15% 16% 4	5316
	21st to end	Night	51% 9% 14% 15% 8% 3	7106

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	22% 13% 28% 20% 13% 4	357
	11th to 20th	Day	13% 10% 21% 32% 20% 3	430
	21st to end	Day	12% 10% 26% 29% 19% 4	473
February	1st to 10th	Day	14% 17% 23% 26% 16% 4	430
	11th to 20th	Day	18% 18% 28% 25% 10% 2	472
	21st to end	Day	11% 16% 27% 32% 13% 1	446
March	1st to 10th	Day	15% 21% 28% 25% 10% 1	540
	11th to 20th	Day	15% 17% 25% 23% 17% 4	540
	21st to end	Day	13% 17% 32% 25% 11% 2	594
April	1st to 10th	Day	12% 16% 32% 24% 12% 4	540
	11th to 20th	Day	12% 17% 31% 26% 10% 5%	532
	21st to end	Day	6% 18% 32% 25% 13% 6%	540
May	1st to 10th	Day	6% 14% 32% 30% 13% 5%	543
	11th to 20th	Day	7% 15% 33% 27% 14% 3	568
	21st to end	Day	8% 14% 28% 29% 15% 6%	617
June	1st to 10th	Day	6% 10% 29% 32% 16% 6%	570
	11th to 20th	Day	2 11% 29% 36% 18% 4	570
	21st to end	Day	2 7% 24% 38% 24% 6%	570
July	1st to 10th	Day	3 5% 19% 46% 20% 7%	566
	11th to 20th	Day	1 2 19% 44% 28% 6%	564
	21st to end	Day	1 2 22% 41% 26% 7%	616
August	1st to 10th	Day	4 5% 23% 37% 23% 9%	527
	11th to 20th	Day	1 3 23% 41% 25% 8%	517
	21st to end	Day	1 3 19% 41% 26% 10%	583
September	1st to 10th	Day	2 4 18% 39% 28% 9%	530
	11th to 20th	Day	3 8% 23% 35% 22% 8%	530
	21st to end	Day	5% 7% 19% 32% 28% 9%	530
October	1st to 10th	Day	3 7% 22% 32% 24% 12%	530
	11th to 20th	Day	6% 8% 19% 30% 27% 10%	530
	21st to end	Day	7% 7% 25% 28% 24% 10%	583
November	1st to 10th	Day	7% 11% 20% 26% 25% 11%	516
	11th to 20th	Day	8% 9% 19% 32% 25% 8%	330
	21st to end	Day	9% 6% 23% 27% 33% 3	239
December	1st to 10th	Day	10% 11% 16% 26% 27% 10%	220
	11th to 20th	Day	10% 13% 20% 22% 25% 9%	220
	21st to end	Day	19% 13% 26% 23% 15% 3	242
January	1st to 10th	Night	16% 8% 17% 31% 23% 5%	840
	11th to 20th	Night	13% 7% 14% 34% 26% 6%	840
	21st to end	Night	16% 5% 14% 36% 24% 4	924
February	1st to 10th	Night	14% 8% 18% 35% 22% 4	840
	11th to 20th	Night	13% 9% 21% 32% 21% 4	824
	21st to end	Night	12% 9% 21% 36% 19% 3	692
March	1st to 10th	Night	15% 11% 21% 30% 21% 3	840
	11th to 20th	Night	15% 8% 18% 32% 24% 4	840
	21st to end	Night	14% 10% 21% 35% 18% 2	924
April	1st to 10th	Night	11% 8% 23% 33% 22% 3	840
	11th to 20th	Night	10% 11% 21% 31% 21% 5%	831
	21st to end	Night	7% 8% 24% 32% 24% 6%	840
May	1st to 10th	Night	6% 4 21% 37% 25% 6%	751
	11th to 20th	Night	6% 5% 20% 34% 28% 7%	585
	21st to end	Night	6% 8% 21% 34% 25% 6%	455
June	1st to 10th	Night	5% 4 21% 36% 29% 5%	420
	11th to 20th	Night	1 2 20% 40% 34% 2	420
	21st to end	Night	4 3 13% 35% 41% 5%	420
July	1st to 10th	Night	3 2 11% 30% 46% 7%	420
	11th to 20th	Night	2 1 6% 28% 48% 16%	612
	21st to end	Night	2 2 10% 24% 51% 11%	687
August	1st to 10th	Night	7% 2 9% 27% 43% 12%	597
	11th to 20th	Night	2 3 11% 33% 44% 8%	714
	21st to end	Night	2 2 11% 30% 43% 12%	880
September	1st to 10th	Night	3 2 11% 30% 43% 12%	800
	11th to 20th	Night	6% 4 14% 30% 37% 9%	800
	21st to end	Night	2 2 13% 31% 41% 12%	800
October	1st to 10th	Night	6% 3 12% 32% 38% 10%	800
	11th to 20th	Night	7% 4 14% 27% 37% 12%	800
	21st to end	Night	7% 4 10% 33% 32% 13%	880
November	1st to 10th	Night	8% 4 15% 27% 34% 11%	820
	11th to 20th	Night	10% 4 13% 35% 30% 8%	840
	21st to end	Night	9% 4 13% 41% 27% 6%	840
December	1st to 10th	Night	12% 6% 15% 34% 27% 6%	840
	11th to 20th	Night	18% 7% 16% 32% 21% 5%	840
	21st to end	Night	18% 10% 22% 29% 19% 2	924

Page generated at 2026-07-28 00:25:24. Version ac07

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