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 2025-03-27.

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	6% 5% 2 5% 3 3 3 3 5% 8% 57%	113008
	Night	7% 6% 2 5% 4 4 4 3 5% 8% 52%	115176
Transparency	Day	25% 7% 11% 17% 23% 18%	112769
	Night	29% 8% 12% 17% 20% 15%	115005
Seeing	Day	8% 11% 25% 32% 19% 6%	15882
	Night	9% 6% 16% 32% 30% 7%	25348

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	11% 6% 3 6% 4 4 4 4 6% 10% 43%	3938
	11th to 20th	Day	9% 6% 3 7% 5% 4 4 3 3 5% 50%	4100
	21st to end	Day	14% 9% 3 7% 5% 4 4 4 5% 7% 38%	4510
February	1st to 10th	Day	12% 9% 2 6% 4 4 4 4 5% 5% 44%	1800
	11th to 20th	Day	11% 8% 3 7% 4 4 4 4 7% 9% 41%	1840
	21st to end	Day	10% 6% 2 7% 5% 4 4 4 6% 6% 46%	1650
March	1st to 10th	Day	10% 9% 4 9% 6% 6% 7% 5% 7% 8% 29%	3513
	11th to 20th	Day	7% 5% 2 5% 4 4 4 4 6% 8% 51%	3600
	21st to end	Day	11% 6% 2 7% 6% 5% 5% 5% 6% 10% 37%	3940
April	1st to 10th	Day	10% 7% 2 5% 4 4 5% 4 6% 8% 46%	2280
	11th to 20th	Day	8% 5% 2 5% 3 4 3 3 6% 7% 55%	2249
	21st to end	Day	4 5% 1 4 3 3 3 3 6% 10% 57%	2280
May	1st to 10th	Day	5% 3 1 3 2 2 2 2 4 6% 71%	3937
	11th to 20th	Day	3 3 1 3 2 2 2 2 4 7% 71%	4121
	21st to end	Day	2 7% 2 5% 2 3 2 2 5% 9% 61%	4813
June	1st to 10th	Day	4 5% 2 4 3 3 3 3 6% 8% 59%	2660
	11th to 20th	Day	1 3 1 2 1 2 2 3 4 8% 73%	2660
	21st to end	Day	2 3 1 3 2 2 3 2 4 9% 69%	2660
July	1st to 10th	Day	2 1 2 2 2 2 1 3 6% 80%	4394
	11th to 20th	Day	1 2 1 2 2 3 3 2 5% 8% 72%	4110
	21st to end	Day	2 1 2 3 3 2 2 4 9% 71%	4455
August	1st to 10th	Day	1 3 1 3 3 3 4 4 6% 9% 63%	4091
	11th to 20th	Day	2 1 3 2 2 3 2 4 8% 74%	3841
	21st to end	Day	1 2 1 4 3 3 3 2 4 7% 71%	4180
September	1st to 10th	Day	1 3 1 4 2 2 2 2 3 7% 73%	2280
	11th to 20th	Day	2 5% 2 5% 3 4 3 3 5% 9% 59%	2280
	21st to end	Day	3 2 1 2 2 2 2 2 3 8% 73%	2009
October	1st to 10th	Day	2 2 1 3 3 3 2 3 4 6% 72%	3180
	11th to 20th	Day	3 3 1 4 3 3 3 3 5% 8% 64%	3161
	21st to end	Day	4 3 1 5% 4 3 3 3 6% 8% 60%	3448
November	1st to 10th	Day	6% 5% 2 4 3 4 4 4 5% 8% 54%	1941
	11th to 20th	Day	8% 6% 3 6% 4 6% 5% 4 5% 7% 48%	1800
	21st to end	Day	10% 5% 2 5% 4 3 3 3 5% 8% 51%	1635
December	1st to 10th	Day	6% 4 2 6% 4 4 3 3 6% 10% 53%	2620
	11th to 20th	Day	14% 9% 3 8% 7% 6% 5% 4 7% 8% 29%	3028
	21st to end	Day	15% 9% 4 8% 5% 6% 6% 5% 7% 7% 29%	4004
January	1st to 10th	Night	9% 7% 3 6% 4 4 4 4 7% 9% 44%	6016
	11th to 20th	Night	11% 8% 3 7% 5% 4 4 3 4 4 46%	5880
	21st to end	Night	13% 10% 3 8% 5% 4 4 2 4 6% 40%	6468
February	1st to 10th	Night	12% 8% 2 6% 4 4 4 3 5% 6% 46%	2600
	11th to 20th	Night	9% 8% 2 7% 5% 5% 4 4 6% 9% 42%	2447
	21st to end	Night	9% 7% 2 6% 3 3 4 3 6% 8% 49%	1980
March	1st to 10th	Night	11% 10% 3 8% 5% 5% 5% 5% 6% 7% 34%	3577
	11th to 20th	Night	9% 8% 3 5% 4 3 3 2 4 6% 52%	3600
	21st to end	Night	10% 11% 3 8% 6% 5% 5% 4 5% 6% 38%	3850
April	1st to 10th	Night	7% 7% 2 7% 5% 3 3 4 7% 8% 47%	1900
	11th to 20th	Night	8% 5% 1 5% 4 4 3 3 6% 7% 54%	1879
	21st to end	Night	3 4 1 4 4 3 3 3 5% 10% 60%	1900
May	1st to 10th	Night	3 4 1 4 3 3 2 2 4 7% 67%	3034
	11th to 20th	Night	2 3 1 3 3 3 3 4 5% 9% 64%	2775
	21st to end	Night	2 4 1 4 3 3 4 3 4 7% 66%	2762
June	1st to 10th	Night	4 4 2 5% 4 3 3 4 6% 10% 56%	1520
	11th to 20th	Night	1 1 1 3 3 2 2 2 5% 9% 71%	1520
	21st to end	Night	1 3 1 3 3 2 3 3 4 7% 69%	1520
July	1st to 10th	Night	2 1 2 2 2 2 2 4 8% 77%	2562
	11th to 20th	Night	1 2 1 3 2 2 3 3 5% 11% 66%	2860
	21st to end	Night	2 2 6% 4 3 5% 5% 8% 13% 50%	3109
August	1st to 10th	Night	2 4 1 5% 4 4 4 5% 8% 15% 49%	2848
	11th to 20th	Night	3 1 3 3 4 3 3 6% 9% 64%	3004
	21st to end	Night	1 2 1 5% 3 3 3 3 5% 10% 63%	3498
September	1st to 10th	Night	1 2 1 3 3 2 3 3 5% 9% 67%	1900
	11th to 20th	Night	3 4 2 5% 3 3 3 3 6% 10% 57%	1900
	21st to end	Night	2 3 1 3 3 3 2 1 4 8% 71%	2166
October	1st to 10th	Night	2 2 1 3 3 3 3 3 5% 6% 70%	3800
	11th to 20th	Night	4 3 1 4 3 3 3 3 6% 9% 59%	3777
	21st to end	Night	4 4 1 5% 3 3 4 3 5% 7% 62%	4121
November	1st to 10th	Night	6% 5% 2 4 4 3 4 3 5% 8% 55%	2355
	11th to 20th	Night	8% 6% 2 5% 5% 4 4 3 6% 8% 49%	2600
	21st to end	Night	7% 6% 2 4 4 4 4 4 6% 8% 52%	2760
December	1st to 10th	Night	6% 4 2 5% 3 3 4 4 5% 6% 57%	4560
	11th to 20th	Night	17% 8% 3 8% 5% 5% 5% 5% 7% 10% 28%	5176
	21st to end	Night	16% 9% 3 8% 6% 5% 6% 5% 6% 6% 30%	6952

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	35% 7% 18% 21% 16% 4	3938
	11th to 20th	Day	36% 7% 8% 13% 22% 16%	4100
	21st to end	Day	44% 7% 6% 12% 13% 17%	4510
February	1st to 10th	Day	40% 6% 8% 18% 18% 9%	1800
	11th to 20th	Day	39% 8% 11% 16% 19% 6%	1849
	21st to end	Day	39% 6% 8% 24% 17% 6%	1650
March	1st to 10th	Day	49% 13% 9% 14% 13% 2	3513
	11th to 20th	Day	29% 8% 13% 27% 13% 10%	3600
	21st to end	Day	41% 14% 15% 19% 10% 2	3940
April	1st to 10th	Day	37% 9% 13% 20% 18% 4	2280
	11th to 20th	Day	30% 8% 15% 23% 20% 5%	2252
	21st to end	Day	23% 9% 14% 26% 22% 4	2280
May	1st to 10th	Day	17% 7% 11% 27% 26% 11%	3937
	11th to 20th	Day	15% 7% 9% 24% 34% 11%	4124
	21st to end	Day	22% 11% 13% 24% 27% 3	4814
June	1st to 10th	Day	24% 11% 14% 22% 22% 8%	2660
	11th to 20th	Day	12% 8% 11% 22% 34% 13%	2660
	21st to end	Day	13% 7% 12% 12% 27% 28%	2660
July	1st to 10th	Day	7% 5% 9% 6% 11% 63%	4325
	11th to 20th	Day	9% 6% 10% 6% 14% 54%	4041
	21st to end	Day	8% 4 9% 10% 32% 36%	4315
August	1st to 10th	Day	14% 6% 11% 11% 27% 31%	4076
	11th to 20th	Day	9% 5% 9% 8% 42% 27%	3880
	21st to end	Day	12% 5% 11% 12% 32% 29%	4179
September	1st to 10th	Day	12% 5% 8% 10% 27% 38%	2280
	11th to 20th	Day	21% 8% 11% 20% 25% 15%	2280
	21st to end	Day	13% 5% 7% 20% 33% 22%	2009
October	1st to 10th	Day	15% 3 5% 12% 28% 37%	3180
	11th to 20th	Day	18% 7% 10% 21% 28% 16%	3161
	21st to end	Day	21% 4 11% 24% 26% 14%	3448
November	1st to 10th	Day	28% 6% 11% 24% 20% 11%	1941
	11th to 20th	Day	34% 7% 10% 23% 20% 7%	1800
	21st to end	Day	31% 4 12% 17% 27% 9%	1635
December	1st to 10th	Day	26% 6% 8% 24% 30% 6%	2620
	11th to 20th	Day	47% 8% 13% 14% 14% 4	3028
	21st to end	Day	51% 9% 12% 17% 10% 2	4004
January	1st to 10th	Night	33% 9% 20% 21% 13% 3	6016
	11th to 20th	Night	38% 8% 10% 13% 20% 11%	5880
	21st to end	Night	45% 6% 7% 11% 17% 15%	6468
February	1st to 10th	Night	38% 6% 10% 18% 18% 11%	2600
	11th to 20th	Night	39% 8% 11% 17% 19% 6%	2449
	21st to end	Night	33% 7% 13% 27% 15% 5%	1980
March	1st to 10th	Night	47% 12% 12% 14% 12% 3	3577
	11th to 20th	Night	35% 6% 10% 22% 18% 9%	3600
	21st to end	Night	47% 11% 13% 17% 9% 3	3850
April	1st to 10th	Night	35% 8% 15% 21% 16% 4	1900
	11th to 20th	Night	31% 9% 12% 24% 17% 7%	1879
	21st to end	Night	22% 10% 13% 29% 20% 6%	1900
May	1st to 10th	Night	21% 7% 11% 24% 24% 13%	3034
	11th to 20th	Night	23% 8% 15% 16% 24% 14%	2776
	21st to end	Night	22% 10% 12% 27% 27% 2	2762
June	1st to 10th	Night	24% 12% 14% 20% 22% 8%	1520
	11th to 20th	Night	11% 9% 13% 19% 32% 15%	1520
	21st to end	Night	16% 7% 11% 13% 26% 27%	1520
July	1st to 10th	Night	6% 5% 9% 6% 14% 60%	2522
	11th to 20th	Night	10% 7% 10% 10% 17% 46%	2806
	21st to end	Night	13% 7% 14% 15% 24% 27%	3011
August	1st to 10th	Night	17% 7% 14% 14% 26% 22%	2839
	11th to 20th	Night	12% 6% 13% 9% 37% 24%	3031
	21st to end	Night	13% 8% 15% 11% 28% 25%	3498
September	1st to 10th	Night	12% 8% 9% 11% 23% 37%	1900
	11th to 20th	Night	23% 7% 15% 18% 24% 14%	1900
	21st to end	Night	14% 6% 9% 20% 28% 24%	2166
October	1st to 10th	Night	15% 5% 6% 11% 23% 40%	3800
	11th to 20th	Night	20% 6% 15% 19% 22% 18%	3777
	21st to end	Night	22% 5% 8% 22% 24% 19%	4121
November	1st to 10th	Night	27% 8% 12% 23% 19% 12%	2355
	11th to 20th	Night	31% 8% 12% 24% 17% 8%	2600
	21st to end	Night	29% 6% 11% 20% 22% 13%	2760
December	1st to 10th	Night	28% 6% 7% 23% 27% 9%	4560
	11th to 20th	Night	48% 6% 12% 15% 16% 3	5176
	21st to end	Night	51% 9% 14% 15% 8% 3	6952

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	23% 12% 27% 21% 14% 4	330
	11th to 20th	Day	15% 11% 23% 31% 17% 4	400
	21st to end	Day	13% 10% 26% 27% 20% 5%	440
February	1st to 10th	Day	15% 18% 22% 25% 17% 4	400
	11th to 20th	Day	17% 18% 28% 25% 11% 2	438
	21st to end	Day	11% 17% 26% 31% 13% 1	414
March	1st to 10th	Day	16% 21% 28% 24% 10% 1	500
	11th to 20th	Day	16% 18% 26% 23% 14% 3	500
	21st to end	Day	13% 18% 33% 24% 10% 2	531
April	1st to 10th	Day	13% 17% 34% 23% 11% 3	460
	11th to 20th	Day	13% 18% 31% 26% 9% 4	452
	21st to end	Day	6% 20% 31% 25% 13% 5%	460
May	1st to 10th	Day	6% 15% 33% 30% 12% 4	463
	11th to 20th	Day	8% 15% 33% 28% 14% 2	488
	21st to end	Day	9% 15% 27% 29% 14% 5%	529
June	1st to 10th	Day	7% 11% 31% 31% 15% 4	490
	11th to 20th	Day	2 12% 29% 37% 17% 4	490
	21st to end	Day	2 6% 24% 40% 24% 5%	490
July	1st to 10th	Day	3 4 18% 47% 21% 6%	486
	11th to 20th	Day	2 19% 46% 28% 5%	484
	21st to end	Day	1 1 21% 42% 27% 8%	531
August	1st to 10th	Day	4 4 23% 38% 24% 7%	487
	11th to 20th	Day	1 3 24% 41% 25% 7%	477
	21st to end	Day	1 3 20% 42% 25% 9%	539
September	1st to 10th	Day	2 4 17% 40% 29% 9%	490
	11th to 20th	Day	4 9% 24% 35% 21% 7%	490
	21st to end	Day	4 7% 18% 33% 29% 9%	490
October	1st to 10th	Day	3 7% 22% 33% 24% 11%	490
	11th to 20th	Day	6% 8% 18% 31% 28% 10%	490
	21st to end	Day	7% 7% 24% 28% 24% 9%	539
November	1st to 10th	Day	7% 12% 21% 27% 24% 10%	477
	11th to 20th	Day	6% 8% 18% 33% 26% 8%	300
	21st to end	Day	10% 7% 24% 26% 32% 1	217
December	1st to 10th	Day	11% 13% 17% 26% 26% 9%	200
	11th to 20th	Day	12% 14% 20% 24% 24% 7%	200
	21st to end	Day	17% 12% 27% 24% 16% 3	220
January	1st to 10th	Night	16% 8% 16% 32% 23% 5%	800
	11th to 20th	Night	14% 7% 15% 33% 26% 7%	800
	21st to end	Night	17% 5% 14% 35% 25% 5%	880
February	1st to 10th	Night	15% 8% 18% 34% 22% 4	800
	11th to 20th	Night	13% 8% 20% 32% 22% 5%	784
	21st to end	Night	12% 9% 20% 36% 20% 3	660
March	1st to 10th	Night	15% 11% 20% 30% 21% 3	800
	11th to 20th	Night	15% 9% 18% 31% 23% 4	800
	21st to end	Night	14% 11% 21% 34% 19% 2	862
April	1st to 10th	Night	11% 9% 23% 32% 22% 3	760
	11th to 20th	Night	11% 12% 20% 30% 21% 6%	751
	21st to end	Night	7% 9% 24% 31% 24% 6%	760
May	1st to 10th	Night	7% 5% 21% 36% 25% 6%	679
	11th to 20th	Night	6% 5% 18% 35% 29% 7%	529
	21st to end	Night	7% 9% 20% 34% 24% 6%	411
June	1st to 10th	Night	5% 4 21% 37% 29% 4	380
	11th to 20th	Night	1 2 19% 41% 34% 3	380
	21st to end	Night	4 2 12% 35% 42% 5%	380
July	1st to 10th	Night	3 2 10% 28% 48% 8%	379
	11th to 20th	Night	2 1 6% 29% 47% 16%	552
	21st to end	Night	2 2 10% 22% 52% 12%	624
August	1st to 10th	Night	8% 2 8% 26% 45% 11%	567
	11th to 20th	Night	2 3 10% 32% 45% 8%	678
	21st to end	Night	2 2 11% 30% 42% 13%	836
September	1st to 10th	Night	3 2 10% 29% 44% 12%	760
	11th to 20th	Night	6% 3 14% 30% 38% 9%	760
	21st to end	Night	2 2 12% 30% 42% 12%	760
October	1st to 10th	Night	6% 2 12% 31% 38% 10%	760
	11th to 20th	Night	7% 3 13% 27% 38% 12%	760
	21st to end	Night	7% 4 10% 33% 33% 13%	836
November	1st to 10th	Night	8% 5% 15% 28% 34% 11%	780
	11th to 20th	Night	10% 3 13% 35% 31% 8%	800
	21st to end	Night	10% 4 13% 40% 27% 6%	800
December	1st to 10th	Night	13% 7% 15% 33% 26% 6%	800
	11th to 20th	Night	19% 8% 17% 32% 20% 5%	800
	21st to end	Night	18% 10% 22% 28% 20% 2	880

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