Dover
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dover clear sky chart from 2005-11-06 to 2025-10-04.

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	14% 6% 2 6% 4 4 4 4 6% 8% 43%	126416
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	122972
Transparency	Day	25% 38% 8% 16% 11% 2	129975
	Night	25% 38% 7% 15% 12% 3	127482
Seeing	Day	16% 8% 24% 27% 18% 6%	16259
	Night	17% 8% 28% 31% 14% 1	25565

By Month

Percentage Cloud Cover SE (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	13% 5% 2 4 3 3 3 3 4 6% 53%	3597
	11th to 20th	Day	14% 8% 3 7% 4 4 3 3 3 5% 46%	3851
	21st to end	Day	16% 7% 3 5% 4 4 4 3 5% 5% 45%	4466
February	1st to 10th	Day	14% 8% 3 6% 4 4 5% 4 6% 8% 41%	1800
	11th to 20th	Day	18% 7% 2 5% 3 3 4 4 6% 6% 42%	1908
	21st to end	Day	17% 9% 3 6% 4 3 4 3 4 6% 41%	1650
March	1st to 10th	Day	20% 5% 1 4 3 3 3 3 4 6% 47%	4560
	11th to 20th	Day	10% 8% 2 5% 3 3 4 4 5% 7% 50%	4980
	21st to end	Day	17% 6% 2 6% 4 4 5% 3 4 6% 43%	5756
April	1st to 10th	Day	14% 7% 2 5% 4 4 4 4 6% 6% 43%	2400
	11th to 20th	Day	13% 6% 2 5% 5% 4 4 4 5% 7% 46%	2374
	21st to end	Day	15% 5% 2 6% 4 4 4 4 6% 8% 43%	2400
May	1st to 10th	Day	21% 8% 3 6% 4 4 4 4 6% 8% 32%	5625
	11th to 20th	Day	24% 7% 2 6% 4 4 4 4 6% 8% 33%	6318
	21st to end	Day	17% 6% 2 6% 4 4 5% 4 6% 9% 38%	6934
June	1st to 10th	Day	10% 6% 2 5% 5% 4 5% 5% 7% 9% 41%	2800
	11th to 20th	Day	12% 4 2 5% 6% 4 5% 4 6% 9% 43%	2800
	21st to end	Day	9% 4 2 4 4 4 5% 4 8% 11% 45%	2800
July	1st to 10th	Day	7% 4 2 6% 5% 5% 5% 5% 8% 10% 41%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 6% 6% 5% 10% 11% 32%	6360
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 9% 41%	6635
August	1st to 10th	Day	16% 6% 2 6% 4 4 4 5% 8% 10% 35%	4216
	11th to 20th	Day	9% 6% 2 7% 4 4 5% 4 7% 9% 42%	3850
	21st to end	Day	8% 5% 2 5% 4 4 5% 5% 6% 8% 48%	4312
September	1st to 10th	Day	14% 5% 2 5% 5% 3 4 4 6% 8% 45%	2400
	11th to 20th	Day	12% 5% 2 6% 3 3 4 3 5% 7% 50%	2380
	21st to end	Day	19% 7% 3 7% 4 4 4 4 5% 6% 37%	2185
October	1st to 10th	Day	17% 4 2 5% 3 3 4 3 6% 7% 46%	1930
	11th to 20th	Day	17% 4 1 3 3 3 3 3 4 7% 51%	1900
	21st to end	Day	15% 6% 2 5% 4 3 3 3 4 6% 49%	2063
November	1st to 10th	Day	12% 6% 2 4 3 3 5% 2 4 6% 52%	1761
	11th to 20th	Day	13% 6% 2 4 4 3 4 4 6% 7% 48%	1800
	21st to end	Day	13% 5% 2 6% 4 5% 4 3 4 6% 47%	1695
December	1st to 10th	Day	11% 5% 2 6% 4 3 4 3 5% 8% 50%	2600
	11th to 20th	Day	19% 9% 2 5% 3 3 3 3 4 6% 43%	2990
	21st to end	Day	13% 6% 2 6% 4 4 4 3 6% 8% 44%	3960
January	1st to 10th	Night	13% 7% 2 5% 4 3 4 3 4 5% 50%	6349
	11th to 20th	Night	14% 7% 2 4 3 3 3 3 4 5% 51%	6107
	21st to end	Night	15% 8% 2 6% 3 3 4 3 5% 5% 46%	6490
February	1st to 10th	Night	16% 8% 2 5% 4 5% 4 3 4 6% 42%	2600
	11th to 20th	Night	19% 8% 2 5% 4 4 3 3 5% 5% 43%	2377
	21st to end	Night	16% 7% 2 5% 4 3 5% 3 5% 6% 44%	1980
March	1st to 10th	Night	16% 8% 2 5% 3 3 3 3 4 6% 47%	5290
	11th to 20th	Night	11% 9% 2 5% 3 3 3 3 5% 6% 49%	4980
	21st to end	Night	15% 8% 3 6% 4 4 3 3 4 5% 45%	5174
April	1st to 10th	Night	16% 8% 3 7% 4 4 3 3 4 7% 41%	2000
	11th to 20th	Night	13% 7% 2 6% 5% 5% 4 4 5% 6% 44%	1975
	21st to end	Night	18% 4 2 5% 4 4 4 3 6% 8% 44%	2000
May	1st to 10th	Night	17% 10% 3 8% 4 4 5% 4 6% 8% 31%	4299
	11th to 20th	Night	21% 9% 3 6% 4 4 5% 4 5% 7% 32%	3575
	21st to end	Night	18% 7% 3 6% 4 4 4 4 6% 8% 36%	3920
June	1st to 10th	Night	14% 5% 2 5% 5% 3 3 4 6% 9% 43%	1600
	11th to 20th	Night	11% 6% 2 6% 6% 5% 5% 4 6% 11% 40%	1600
	21st to end	Night	9% 5% 1 5% 5% 5% 4 5% 7% 11% 42%	1600
July	1st to 10th	Night	8% 6% 2 7% 5% 6% 5% 5% 8% 10% 38%	3598
	11th to 20th	Night	10% 7% 3 8% 5% 5% 5% 5% 7% 12% 33%	3597
	21st to end	Night	9% 7% 2 6% 5% 5% 5% 4 7% 8% 41%	4198
August	1st to 10th	Night	16% 7% 3 7% 4 5% 5% 4 6% 10% 33%	2915
	11th to 20th	Night	11% 6% 3 7% 4 5% 5% 4 7% 8% 41%	3183
	21st to end	Night	10% 5% 2 5% 3 4 4 3 5% 7% 51%	3586
September	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 4 7% 45%	2000
	11th to 20th	Night	12% 4 2 4 4 3 3 3 5% 8% 53%	2020
	21st to end	Night	21% 5% 3 6% 4 4 4 4 5% 6% 39%	2210
October	1st to 10th	Night	18% 6% 2 6% 4 3 4 3 4 6% 45%	2317
	11th to 20th	Night	16% 5% 2 4 3 3 3 3 4 5% 53%	2280
	21st to end	Night	14% 6% 2 5% 3 3 4 3 4 6% 49%	2533
November	1st to 10th	Night	11% 5% 2 4 3 3 4 4 5% 6% 53%	2537
	11th to 20th	Night	15% 6% 3 5% 4 2 4 3 5% 5% 49%	2600
	21st to end	Night	12% 6% 1 4 4 4 4 4 5% 6% 48%	2701
December	1st to 10th	Night	12% 5% 2 4 3 3 3 3 4 7% 52%	4580
	11th to 20th	Night	21% 6% 2 5% 3 3 4 3 6% 7% 40%	5205
	21st to end	Night	15% 8% 3 6% 4 4 5% 4 5% 7% 41%	6996

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	24% 26% 4 20% 23% 4	3598
	11th to 20th	Day	33% 26% 4 18% 14% 6%	3851
	21st to end	Day	30% 27% 9% 21% 10% 2	4466
February	1st to 10th	Day	18% 62% 8% 7% 4 1	1800
	11th to 20th	Day	18% 60% 3 9% 8% 2	1917
	21st to end	Day	17% 62% 4 9% 6% 2	1650
March	1st to 10th	Day	29% 28% 6% 15% 15% 7%	4560
	11th to 20th	Day	24% 29% 6% 22% 15% 5%	4980
	21st to end	Day	30% 28% 8% 11% 17% 5%	5756
April	1st to 10th	Day	18% 62% 5% 8% 6% 2	2400
	11th to 20th	Day	16% 62% 5% 10% 6% 1	2374
	21st to end	Day	15% 62% 5% 9% 8% 1	2400
May	1st to 10th	Day	38% 30% 12% 13% 6% 1	5657
	11th to 20th	Day	37% 29% 9% 18% 6%	6322
	21st to end	Day	32% 29% 9% 16% 12% 1	6949
June	1st to 10th	Day	15% 63% 4 9% 7% 1	2800
	11th to 20th	Day	14% 63% 4 10% 8% 1	2800
	21st to end	Day	11% 61% 7% 11% 9% 1	2800
July	1st to 10th	Day	27% 32% 16% 14% 10% 2	6290
	11th to 20th	Day	33% 31% 17% 14% 4 1	6285
	21st to end	Day	27% 28% 8% 19% 18%	6445
August	1st to 10th	Day	29% 38% 11% 17% 4	4203
	11th to 20th	Day	23% 38% 10% 22% 7%	3886
	21st to end	Day	17% 36% 9% 23% 14% 1	4312
September	1st to 10th	Day	17% 57% 7% 11% 7% 1	2400
	11th to 20th	Day	16% 57% 4 12% 9% 2	2380
	21st to end	Day	24% 57% 4 9% 5%	2185
October	1st to 10th	Day	18% 37% 6% 13% 22% 4	3190
	11th to 20th	Day	11% 36% 5% 17% 22% 9%	3140
	21st to end	Day	9% 35% 8% 22% 20% 6%	3377
November	1st to 10th	Day	16% 56% 6% 11% 7% 3	1761
	11th to 20th	Day	14% 56% 3 11% 13% 2	1800
	21st to end	Day	15% 56% 4 16% 7% 1	1695
December	1st to 10th	Day	22% 36% 13% 17% 8% 3	2600
	11th to 20th	Day	31% 31% 8% 20% 8% 2	2986
	21st to end	Day	29% 27% 11% 16% 16% 1	3960
January	1st to 10th	Night	27% 26% 5% 18% 19% 5%	6349
	11th to 20th	Night	26% 26% 5% 20% 17% 7%	6107
	21st to end	Night	31% 28% 5% 20% 14% 2	6490
February	1st to 10th	Night	17% 61% 7% 9% 4 1	2600
	11th to 20th	Night	18% 60% 2 8% 10% 2	2379
	21st to end	Night	17% 61% 4 8% 6% 3	1980
March	1st to 10th	Night	32% 29% 4 17% 9% 9%	5290
	11th to 20th	Night	24% 29% 9% 20% 14% 5%	4980
	21st to end	Night	31% 29% 6% 13% 20% 3	5174
April	1st to 10th	Night	19% 61% 3 8% 8% 1	2000
	11th to 20th	Night	17% 62% 4 8% 7% 2	1975
	21st to end	Night	16% 62% 4 8% 9% 1	2000
May	1st to 10th	Night	39% 30% 11% 14% 5% 2	4329
	11th to 20th	Night	40% 30% 7% 17% 6%	3576
	21st to end	Night	35% 28% 10% 15% 10% 2	3927
June	1st to 10th	Night	16% 62% 4 8% 9% 1	1600
	11th to 20th	Night	15% 64% 4 11% 6%	1600
	21st to end	Night	13% 61% 7% 10% 8% 1	1600
July	1st to 10th	Night	32% 30% 13% 14% 10% 1	3560
	11th to 20th	Night	35% 30% 17% 15% 3	3552
	21st to end	Night	30% 27% 11% 17% 15%	4074
August	1st to 10th	Night	34% 38% 11% 13% 4	2904
	11th to 20th	Night	25% 37% 9% 21% 8%	3213
	21st to end	Night	19% 36% 6% 22% 16% 1	3586
September	1st to 10th	Night	16% 58% 6% 12% 7% 1	2000
	11th to 20th	Night	15% 57% 5% 12% 10% 2	2020
	21st to end	Night	24% 57% 5% 8% 6%	2210
October	1st to 10th	Night	25% 37% 7% 15% 13% 3	3837
	11th to 20th	Night	10% 36% 8% 14% 25% 7%	3779
	21st to end	Night	10% 36% 7% 23% 18% 6%	4169
November	1st to 10th	Night	16% 56% 5% 11% 9% 4	2537
	11th to 20th	Night	16% 56% 3 9% 13% 3	2600
	21st to end	Night	14% 56% 5% 13% 10% 1	2701
December	1st to 10th	Night	21% 35% 9% 22% 10% 2	4580
	11th to 20th	Night	31% 31% 9% 13% 12% 4	5208
	21st to end	Night	33% 27% 10% 15% 15% 1	6996

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	18% 19% 41% 16% 6% 1	200
	11th to 20th	Day	21% 25% 28% 20% 5%	255
	21st to end	Day	21% 16% 36% 19% 8% 1	330
February	1st to 10th	Day	21% 14% 37% 22% 6% 1	300
	11th to 20th	Day	24% 18% 41% 15% 2	456
	21st to end	Day	22% 15% 34% 23% 5%	414
March	1st to 10th	Day	16% 19% 34% 24% 6%	500
	11th to 20th	Day	19% 15% 35% 22% 7% 2	500
	21st to end	Day	21% 11% 35% 25% 7% 1	550
April	1st to 10th	Day	19% 14% 37% 23% 5% 2	500
	11th to 20th	Day	15% 14% 31% 26% 11% 2	492
	21st to end	Day	19% 11% 28% 27% 15% 1	500
May	1st to 10th	Day	25% 9% 28% 24% 12% 3	503
	11th to 20th	Day	20% 6% 26% 32% 14% 3	527
	21st to end	Day	19% 3 17% 29% 23% 8%	573
June	1st to 10th	Day	14% 4 22% 29% 22% 9%	530
	11th to 20th	Day	12% 5% 20% 29% 25% 8%	530
	21st to end	Day	10% 2 16% 31% 31% 10%	530
July	1st to 10th	Day	10% 3 10% 33% 32% 12%	525
	11th to 20th	Day	5% 2 10% 32% 36% 15%	524
	21st to end	Day	8% 3 12% 35% 32% 9%	576
August	1st to 10th	Day	13% 3 11% 31% 29% 12%	526
	11th to 20th	Day	13% 1 12% 26% 33% 13%	519
	21st to end	Day	10% 2 13% 32% 29% 14%	583
September	1st to 10th	Day	17% 2 16% 29% 23% 13%	530
	11th to 20th	Day	12% 4 16% 29% 28% 11%	530
	21st to end	Day	23% 5% 20% 30% 17% 4	530
October	1st to 10th	Day	21% 4 18% 26% 20% 11%	502
	11th to 20th	Day	15% 7% 26% 26% 21% 5%	490
	21st to end	Day	17% 7% 29% 29% 15% 4	524
November	1st to 10th	Day	14% 7% 31% 26% 18% 5%	395
	11th to 20th	Day	12% 13% 34% 28% 12% 1	400
	21st to end	Day	15% 11% 35% 26% 12% 2	295
December	1st to 10th	Day	16% 12% 38% 25% 8% 2	200
	11th to 20th	Day	24% 17% 27% 23% 10% 1	200
	21st to end	Day	21% 10% 32% 23% 13%	220
January	1st to 10th	Night	19% 15% 40% 22% 4	800
	11th to 20th	Night	22% 14% 34% 26% 5%	800
	21st to end	Night	18% 12% 40% 28% 3	880
February	1st to 10th	Night	20% 10% 38% 29% 4	800
	11th to 20th	Night	22% 17% 41% 18% 2	783
	21st to end	Night	20% 14% 38% 24% 3	660
March	1st to 10th	Night	17% 13% 40% 26% 5%	800
	11th to 20th	Night	18% 12% 38% 24% 7% 1	800
	21st to end	Night	19% 9% 38% 26% 7% 1	880
April	1st to 10th	Night	21% 11% 36% 25% 6%	800
	11th to 20th	Night	15% 10% 32% 36% 7%	791
	21st to end	Night	18% 9% 31% 33% 9%	800
May	1st to 10th	Night	20% 8% 25% 36% 10% 1	726
	11th to 20th	Night	20% 4 27% 36% 13%	394
	21st to end	Night	17% 2 17% 38% 23% 2	433
June	1st to 10th	Night	17% 2 20% 42% 18% 2	400
	11th to 20th	Night	11% 2 21% 44% 23% 1	400
	21st to end	Night	10% 4 15% 47% 24% 1	400
July	1st to 10th	Night	9% 2 14% 43% 30% 2	390
	11th to 20th	Night	7% 3 9% 42% 36% 3	388
	21st to end	Night	9% 2 13% 43% 31% 2	558
August	1st to 10th	Night	12% 2 11% 42% 31% 2	597
	11th to 20th	Night	13% 2 10% 40% 30% 4	776
	21st to end	Night	14% 3 10% 38% 29% 6%	880
September	1st to 10th	Night	16% 2 16% 36% 26% 4	800
	11th to 20th	Night	13% 3 14% 37% 28% 5%	800
	21st to end	Night	23% 4 18% 33% 20% 2	800
October	1st to 10th	Night	21% 3 19% 31% 20% 5%	773
	11th to 20th	Night	17% 7% 27% 32% 16% 1	760
	21st to end	Night	18% 6% 32% 28% 14% 1	836
November	1st to 10th	Night	15% 5% 32% 34% 12% 2	780
	11th to 20th	Night	19% 10% 32% 30% 9% 1	800
	21st to end	Night	14% 14% 35% 26% 10% 1	800
December	1st to 10th	Night	22% 10% 37% 21% 9% 1	800
	11th to 20th	Night	20% 10% 36% 28% 6%	800
	21st to end	Night	19% 9% 36% 32% 4	880

Page generated at 2025-10-01 01:39:01. Version ac07

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