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 2024-02-15.

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	7% 5% 2 5% 4 3 4 4 5% 8% 54%	78486
	Night	8% 6% 2 5% 5% 3 4 4 5% 8% 49%	80626
Transparency	Day	29% 8% 12% 18% 21% 13%	78207
	Night	32% 9% 12% 17% 17% 12%	80427
Seeing	Day	8% 11% 25% 32% 19% 5%	14401
	Night	10% 6% 15% 31% 31% 7%	23877

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	14% 7% 3 6% 5% 3 5% 5% 6% 8% 37%	2740
	11th to 20th	Day	14% 8% 5% 8% 8% 4 5% 5% 4 7% 32%	2860
	21st to end	Day	19% 10% 3 7% 7% 4 4 5% 6% 7% 28%	3146
February	1st to 10th	Day	12% 7% 3 5% 5% 3 4 5% 5% 6% 45%	1710
	11th to 20th	Day	11% 6% 4 5% 4 3 4 4 6% 9% 43%	1688
	21st to end	Day	11% 5% 3 6% 5% 3 4 4 5% 7% 47%	1480
March	1st to 10th	Day	11% 7% 4 7% 5% 3 6% 5% 7% 10% 37%	1953
	11th to 20th	Day	11% 6% 3 7% 7% 4 5% 6% 6% 8% 38%	1980
	21st to end	Day	10% 5% 2 5% 6% 3 5% 5% 5% 9% 46%	2200
April	1st to 10th	Day	10% 6% 3 5% 5% 3 5% 5% 6% 8% 45%	2160
	11th to 20th	Day	9% 4 2 4 4 3 3 3 5% 7% 56%	2129
	21st to end	Day	4 4 2 4 3 2 3 4 5% 10% 57%	2160
May	1st to 10th	Day	3 4 2 5% 3 2 3 3 5% 9% 60%	2232
	11th to 20th	Day	5% 4 2 4 4 2 3 3 5% 9% 59%	2325
	21st to end	Day	4 6% 3 5% 4 3 3 4 5% 9% 55%	2723
June	1st to 10th	Day	5% 4 3 3 3 2 4 4 6% 9% 59%	2520
	11th to 20th	Day	1 2 1 2 2 1 2 3 4 8% 72%	2520
	21st to end	Day	2 3 1 3 2 2 3 3 4 9% 68%	2520
July	1st to 10th	Day	1 3 2 3 3 2 3 3 4 9% 67%	2507
	11th to 20th	Day	1 3 2 3 3 3 4 3 6% 8% 65%	2340
	21st to end	Day	1 2 1 3 3 2 3 3 4 9% 69%	2546
August	1st to 10th	Day	2 4 2 4 4 3 3 3 4 9% 61%	2328
	11th to 20th	Day	1 3 1 3 3 2 3 3 4 8% 69%	2191
	21st to end	Day	1 3 2 4 3 2 2 3 4 9% 67%	2376
September	1st to 10th	Day	1 3 1 3 3 2 2 3 3 6% 73%	2160
	11th to 20th	Day	2 5% 2 4 4 3 3 3 5% 9% 60%	2160
	21st to end	Day	3 2 2 2 3 1 2 3 3 8% 72%	1904
October	1st to 10th	Day	3 3 2 4 4 2 3 3 3 6% 67%	1800
	11th to 20th	Day	5% 3 2 4 4 3 4 4 4 8% 60%	1800
	21st to end	Day	6% 4 1 4 4 2 3 4 5% 6% 59%	1980
November	1st to 10th	Day	6% 5% 3 4 4 3 4 5% 5% 9% 53%	1842
	11th to 20th	Day	7% 6% 3 6% 5% 5% 5% 5% 4 7% 48%	1710
	21st to end	Day	9% 4 3 5% 5% 2 3 4 5% 8% 52%	1554
December	1st to 10th	Day	9% 6% 3 7% 5% 3 4 5% 6% 8% 43%	1520
	11th to 20th	Day	17% 10% 4 7% 6% 4 4 4 6% 7% 30%	1928
	21st to end	Day	15% 9% 4 7% 5% 3 6% 5% 6% 8% 33%	2794
January	1st to 10th	Night	13% 8% 3 5% 5% 3 4 5% 6% 8% 39%	4207
	11th to 20th	Night	15% 10% 4 8% 8% 4 5% 5% 5% 5% 31%	4110
	21st to end	Night	18% 11% 4 8% 6% 4 5% 4 4 6% 30%	4521
February	1st to 10th	Night	12% 6% 3 5% 4 3 4 3 5% 7% 47%	2470
	11th to 20th	Night	8% 7% 2 6% 5% 3 5% 5% 6% 9% 44%	2253
	21st to end	Night	10% 6% 3 5% 4 3 4 4 5% 8% 49%	1776
March	1st to 10th	Night	13% 7% 3 6% 5% 4 4 5% 6% 7% 40%	2003
	11th to 20th	Night	12% 7% 4 6% 5% 3 4 4 6% 8% 41%	1980
	21st to end	Night	10% 7% 3 5% 4 3 4 4 4 5% 50%	2155
April	1st to 10th	Night	8% 6% 3 7% 6% 3 4 4 6% 8% 46%	1800
	11th to 20th	Night	8% 5% 2 4 4 3 4 3 5% 8% 54%	1779
	21st to end	Night	3 3 1 3 4 3 4 3 5% 10% 60%	1800
May	1st to 10th	Night	3 4 2 5% 4 3 3 3 4 10% 58%	1724
	11th to 20th	Night	4 5% 2 5% 4 2 4 5% 6% 10% 55%	1563
	21st to end	Night	4 5% 1 4 5% 3 4 5% 5% 9% 53%	1552
June	1st to 10th	Night	4 3 2 4 5% 2 4 4 6% 11% 55%	1440
	11th to 20th	Night	1 1 1 2 3 1 3 3 5% 10% 70%	1440
	21st to end	Night	1 3 2 3 4 2 3 3 4 7% 68%	1440
July	1st to 10th	Night	1 4 1 3 3 2 3 4 5% 12% 63%	1448
	11th to 20th	Night	1 3 2 3 3 2 4 4 6% 13% 59%	1620
	21st to end	Night	3 2 5% 5% 3 5% 7% 7% 14% 48%	1773
August	1st to 10th	Night	2 6% 1 4 5% 4 5% 5% 7% 14% 47%	1611
	11th to 20th	Night	3 1 3 4 4 4 5% 8% 15% 53%	1710
	21st to end	Night	1 2 2 4 4 2 3 4 7% 14% 57%	1980
September	1st to 10th	Night	1 2 1 3 3 2 3 4 5% 9% 67%	1800
	11th to 20th	Night	3 4 2 4 4 2 3 4 6% 10% 57%	1800
	21st to end	Night	2 2 1 2 4 2 2 2 4 8% 71%	2051
October	1st to 10th	Night	4 3 2 4 3 2 3 3 4 6% 64%	2160
	11th to 20th	Night	5% 4 2 4 5% 3 4 4 5% 7% 59%	2160
	21st to end	Night	5% 5% 1 5% 4 2 3 5% 5% 7% 59%	2376
November	1st to 10th	Night	7% 5% 3 3 4 3 4 4 5% 8% 54%	2234
	11th to 20th	Night	8% 6% 3 5% 5% 3 4 4 6% 8% 49%	2470
	21st to end	Night	7% 5% 1 3 4 3 4 5% 5% 8% 54%	2622
December	1st to 10th	Night	11% 7% 3 6% 5% 3 4 4 5% 7% 45%	2660
	11th to 20th	Night	22% 9% 3 6% 6% 3 4 5% 6% 7% 29%	3276
	21st to end	Night	15% 8% 3 6% 7% 4 5% 7% 6% 6% 32%	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	42% 9% 13% 26% 8% 3	2740
	11th to 20th	Day	51% 9% 11% 12% 11% 7%	2860
	21st to end	Day	53% 9% 7% 14% 12% 5%	3146
February	1st to 10th	Day	39% 6% 8% 19% 18% 9%	1710
	11th to 20th	Day	38% 8% 10% 16% 21% 7%	1697
	21st to end	Day	39% 6% 8% 24% 18% 7%	1480
March	1st to 10th	Day	43% 9% 11% 19% 14% 4	1953
	11th to 20th	Day	43% 8% 11% 18% 16% 4	1980
	21st to end	Day	37% 9% 14% 20% 16% 4	2200
April	1st to 10th	Day	38% 9% 13% 19% 18% 4	2160
	11th to 20th	Day	30% 8% 15% 23% 20% 5%	2132
	21st to end	Day	24% 9% 14% 26% 22% 5%	2160
May	1st to 10th	Day	23% 11% 11% 22% 28% 5%	2232
	11th to 20th	Day	23% 11% 10% 25% 23% 7%	2328
	21st to end	Day	28% 14% 14% 21% 20% 3	2724
June	1st to 10th	Day	24% 11% 14% 21% 21% 8%	2520
	11th to 20th	Day	12% 8% 12% 21% 33% 13%	2520
	21st to end	Day	14% 7% 13% 13% 28% 26%	2520
July	1st to 10th	Day	12% 9% 15% 10% 19% 35%	2438
	11th to 20th	Day	14% 9% 17% 10% 18% 32%	2271
	21st to end	Day	10% 7% 12% 13% 25% 33%	2405
August	1st to 10th	Day	18% 8% 16% 12% 24% 22%	2312
	11th to 20th	Day	12% 8% 10% 12% 30% 29%	2191
	21st to end	Day	13% 8% 14% 14% 27% 25%	2376
September	1st to 10th	Day	13% 5% 8% 10% 27% 38%	2160
	11th to 20th	Day	21% 8% 11% 19% 25% 16%	2160
	21st to end	Day	14% 5% 7% 21% 32% 22%	1904
October	1st to 10th	Day	20% 4 9% 17% 27% 23%	1800
	11th to 20th	Day	22% 8% 10% 17% 22% 22%	1800
	21st to end	Day	25% 6% 11% 19% 21% 19%	1980
November	1st to 10th	Day	30% 7% 10% 23% 19% 12%	1842
	11th to 20th	Day	34% 7% 10% 22% 20% 7%	1710
	21st to end	Day	30% 5% 12% 16% 27% 9%	1554
December	1st to 10th	Day	37% 8% 11% 18% 16% 10%	1520
	11th to 20th	Day	53% 7% 10% 13% 14% 4	1928
	21st to end	Day	48% 8% 11% 18% 12% 3	2794
January	1st to 10th	Night	40% 10% 17% 22% 8% 3	4207
	11th to 20th	Night	51% 10% 12% 10% 9% 7%	4110
	21st to end	Night	56% 7% 8% 11% 11% 6%	4521
February	1st to 10th	Night	37% 6% 10% 18% 18% 11%	2470
	11th to 20th	Night	37% 8% 10% 17% 21% 7%	2255
	21st to end	Night	35% 7% 13% 26% 15% 4	1776
March	1st to 10th	Night	43% 8% 13% 17% 14% 5%	2003
	11th to 20th	Night	42% 7% 8% 21% 18% 5%	1980
	21st to end	Night	38% 8% 13% 20% 16% 5%	2155
April	1st to 10th	Night	36% 8% 15% 21% 16% 4	1800
	11th to 20th	Night	31% 8% 12% 24% 17% 8%	1779
	21st to end	Night	22% 9% 14% 29% 20% 6%	1800
May	1st to 10th	Night	25% 10% 11% 22% 23% 10%	1724
	11th to 20th	Night	26% 12% 14% 19% 22% 7%	1564
	21st to end	Night	29% 14% 16% 19% 20% 3	1552
June	1st to 10th	Night	24% 12% 15% 20% 22% 8%	1440
	11th to 20th	Night	12% 10% 14% 19% 31% 15%	1440
	21st to end	Night	16% 8% 11% 13% 26% 25%	1440
July	1st to 10th	Night	11% 9% 16% 10% 21% 33%	1408
	11th to 20th	Night	13% 12% 17% 13% 14% 30%	1566
	21st to end	Night	17% 11% 19% 10% 16% 26%	1674
August	1st to 10th	Night	23% 11% 19% 11% 19% 18%	1602
	11th to 20th	Night	15% 10% 19% 8% 21% 26%	1710
	21st to end	Night	16% 13% 16% 12% 21% 23%	1980
September	1st to 10th	Night	13% 8% 9% 11% 23% 36%	1800
	11th to 20th	Night	23% 8% 15% 17% 23% 14%	1800
	21st to end	Night	14% 6% 9% 20% 27% 23%	2051
October	1st to 10th	Night	21% 7% 9% 15% 23% 24%	2160
	11th to 20th	Night	24% 8% 9% 17% 19% 23%	2160
	21st to end	Night	24% 8% 8% 19% 21% 20%	2376
November	1st to 10th	Night	28% 8% 12% 22% 18% 12%	2234
	11th to 20th	Night	31% 8% 12% 23% 17% 8%	2470
	21st to end	Night	28% 7% 11% 20% 22% 13%	2622
December	1st to 10th	Night	38% 8% 9% 17% 17% 12%	2660
	11th to 20th	Night	53% 6% 10% 13% 15% 3	3276
	21st to end	Night	46% 12% 13% 15% 10% 4	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	24% 12% 26% 20% 14% 3	303
	11th to 20th	Day	16% 12% 21% 31% 17% 3	370
	21st to end	Day	13% 11% 26% 27% 20% 4	407
February	1st to 10th	Day	14% 17% 22% 25% 18% 4	370
	11th to 20th	Day	16% 16% 28% 25% 12% 2	382
	21st to end	Day	12% 18% 26% 28% 14% 1	346
March	1st to 10th	Day	16% 20% 28% 25% 10% 1	420
	11th to 20th	Day	15% 17% 27% 23% 14% 4	420
	21st to end	Day	13% 18% 32% 26% 10% 1	462
April	1st to 10th	Day	14% 16% 34% 23% 11% 2	420
	11th to 20th	Day	13% 18% 32% 25% 8% 4	412
	21st to end	Day	5% 20% 30% 25% 12% 6%	420
May	1st to 10th	Day	6% 15% 32% 30% 13% 4	423
	11th to 20th	Day	8% 16% 35% 26% 12% 2	448
	21st to end	Day	8% 16% 27% 30% 13% 6%	485
June	1st to 10th	Day	8% 11% 30% 31% 16% 5%	450
	11th to 20th	Day	2 12% 28% 37% 16% 4	450
	21st to end	Day	2 6% 25% 39% 23% 5%	450
July	1st to 10th	Day	4 5% 19% 49% 20% 4	446
	11th to 20th	Day	2 19% 47% 27% 5%	444
	21st to end	Day	1 1 21% 42% 27% 8%	487
August	1st to 10th	Day	4 5% 24% 39% 23% 6%	447
	11th to 20th	Day	1 2 22% 42% 25% 8%	437
	21st to end	Day	1 2 20% 43% 25% 8%	495
September	1st to 10th	Day	2 4 16% 42% 27% 9%	450
	11th to 20th	Day	3 8% 23% 36% 22% 7%	450
	21st to end	Day	4 7% 19% 34% 28% 8%	450
October	1st to 10th	Day	3 7% 24% 33% 23% 10%	450
	11th to 20th	Day	6% 8% 18% 31% 27% 10%	450
	21st to end	Day	8% 7% 24% 28% 24% 9%	495
November	1st to 10th	Day	8% 12% 19% 28% 25% 9%	438
	11th to 20th	Day	5% 8% 17% 33% 28% 9%	270
	21st to end	Day	7% 8% 23% 26% 34% 2	196
December	1st to 10th	Day	12% 14% 16% 24% 27% 7%	180
	11th to 20th	Day	12% 13% 21% 24% 23% 6%	180
	21st to end	Day	19% 12% 27% 22% 18% 4	198
January	1st to 10th	Night	17% 8% 16% 31% 24% 5%	760
	11th to 20th	Night	14% 7% 14% 32% 26% 7%	760
	21st to end	Night	18% 6% 14% 34% 25% 5%	836
February	1st to 10th	Night	14% 8% 17% 34% 23% 4	760
	11th to 20th	Night	13% 8% 20% 31% 24% 5%	722
	21st to end	Night	13% 9% 21% 34% 20% 3	592
March	1st to 10th	Night	15% 11% 19% 30% 22% 3	720
	11th to 20th	Night	15% 8% 17% 31% 24% 5%	720
	21st to end	Night	14% 10% 20% 34% 19% 3	792
April	1st to 10th	Night	12% 8% 23% 33% 21% 3	720
	11th to 20th	Night	11% 12% 21% 29% 21% 6%	711
	21st to end	Night	7% 8% 24% 31% 24% 7%	720
May	1st to 10th	Night	7% 5% 20% 36% 26% 6%	644
	11th to 20th	Night	6% 5% 18% 35% 29% 7%	501
	21st to end	Night	7% 9% 19% 34% 24% 7%	389
June	1st to 10th	Night	5% 4 21% 36% 30% 5%	360
	11th to 20th	Night	1 3 18% 41% 35% 3	360
	21st to end	Night	5% 2 11% 35% 41% 5%	360
July	1st to 10th	Night	4 2 10% 28% 49% 7%	358
	11th to 20th	Night	2 1 6% 28% 47% 16%	522
	21st to end	Night	2 2 9% 21% 52% 13%	591
August	1st to 10th	Night	8% 2 9% 26% 44% 11%	537
	11th to 20th	Night	2 3 8% 32% 47% 9%	642
	21st to end	Night	2 2 10% 30% 43% 13%	792
September	1st to 10th	Night	3 2 10% 29% 44% 13%	720
	11th to 20th	Night	6% 3 13% 29% 39% 9%	720
	21st to end	Night	2 3 13% 29% 42% 13%	720
October	1st to 10th	Night	7% 2 12% 32% 37% 11%	720
	11th to 20th	Night	7% 3 13% 26% 39% 12%	720
	21st to end	Night	8% 4 9% 32% 33% 14%	792
November	1st to 10th	Night	9% 4 15% 27% 35% 11%	740
	11th to 20th	Night	10% 3 12% 34% 32% 8%	760
	21st to end	Night	10% 4 13% 40% 28% 6%	760
December	1st to 10th	Night	13% 7% 16% 32% 26% 6%	760
	11th to 20th	Night	19% 8% 17% 32% 19% 5%	760
	21st to end	Night	18% 10% 22% 28% 20% 2	836

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