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 2024-11-05.

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 5% 4 4 4 4 6% 8% 42%	101217
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	97763
Transparency	Day	22% 46% 7% 14% 9% 2	104821
	Night	22% 46% 6% 14% 9% 2	102298
Seeing	Day	16% 8% 25% 27% 18% 6%	15079
	Night	17% 8% 28% 31% 14% 1	24409

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	15% 7% 2 4 3 3 3 3 4 6% 49%	2517
	11th to 20th	Day	16% 9% 3 6% 5% 4 4 3 4 6% 41%	2687
	21st to end	Day	19% 9% 3 6% 4 3 4 4 5% 6% 39%	3124
February	1st to 10th	Day	14% 7% 3 6% 4 4 5% 4 6% 8% 41%	1710
	11th to 20th	Day	17% 6% 2 5% 3 3 4 4 6% 6% 43%	1810
	21st to end	Day	17% 9% 3 6% 4 3 4 3 4 7% 39%	1570
March	1st to 10th	Day	24% 6% 2 5% 4 3 3 3 4 6% 40%	3172
	11th to 20th	Day	10% 4 1 4 3 3 4 5% 6% 8% 52%	3480
	21st to end	Day	21% 6% 2 5% 3 4 4 3 4 5% 43%	4022
April	1st to 10th	Day	13% 7% 2 5% 4 3 4 4 6% 7% 44%	2280
	11th to 20th	Day	12% 6% 2 4 5% 4 4 4 5% 7% 47%	2254
	21st to end	Day	15% 5% 2 6% 4 4 4 4 6% 8% 42%	2280
May	1st to 10th	Day	23% 8% 3 6% 4 3 4 4 5% 7% 31%	3915
	11th to 20th	Day	24% 7% 2 6% 4 4 5% 3 5% 7% 32%	4398
	21st to end	Day	11% 5% 2 5% 4 4 5% 3 6% 10% 45%	4823
June	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 5% 7% 10% 41%	2660
	11th to 20th	Day	10% 4 2 5% 6% 4 5% 4 6% 10% 43%	2660
	21st to end	Day	9% 4 2 4 4 4 5% 4 8% 11% 45%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 6% 6% 6% 8% 10% 40%	4440
	11th to 20th	Day	10% 5% 2 5% 5% 6% 5% 5% 9% 11% 38%	4440
	21st to end	Day	9% 6% 3 7% 6% 6% 7% 6% 9% 8% 33%	4607
August	1st to 10th	Day	16% 6% 2 6% 4 4 4 5% 8% 10% 34%	4087
	11th to 20th	Day	9% 6% 2 7% 4 5% 5% 4 7% 9% 42%	3730
	21st to end	Day	8% 5% 2 5% 4 4 5% 5% 6% 8% 47%	4180
September	1st to 10th	Day	14% 5% 2 5% 5% 3 5% 4 6% 8% 44%	2280
	11th to 20th	Day	12% 5% 2 6% 3 3 4 3 5% 7% 50%	2261
	21st to end	Day	19% 7% 3 6% 4 4 5% 4 5% 6% 38%	2076
October	1st to 10th	Day	17% 4 2 5% 3 3 4 3 6% 6% 46%	1900
	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%	1708
	11th to 20th	Day	14% 6% 2 4 4 3 4 4 6% 7% 48%	1710
	21st to end	Day	13% 5% 2 5% 4 5% 4 3 5% 7% 47%	1611
December	1st to 10th	Day	16% 6% 2 6% 4 4 5% 3 5% 7% 43%	1520
	11th to 20th	Day	17% 8% 2 5% 3 4 3 3 4 6% 44%	1910
	21st to end	Day	15% 6% 2 7% 4 4 5% 3 6% 7% 40%	2772
January	1st to 10th	Night	16% 8% 2 5% 4 3 4 3 4 5% 44%	4429
	11th to 20th	Night	19% 7% 2 5% 4 3 3 3 4 5% 44%	4271
	21st to end	Night	20% 9% 3 6% 3 3 4 3 5% 5% 40%	4532
February	1st to 10th	Night	16% 8% 2 6% 4 4 4 3 4 6% 43%	2470
	11th to 20th	Night	18% 9% 2 5% 4 4 3 3 5% 5% 43%	2256
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 6% 44%	1884
March	1st to 10th	Night	21% 9% 3 6% 3 3 3 3 4 6% 38%	3678
	11th to 20th	Night	12% 6% 1 5% 4 4 4 3 6% 7% 49%	3480
	21st to end	Night	17% 9% 3 7% 4 3 3 3 3 4 45%	3608
April	1st to 10th	Night	16% 9% 2 6% 4 4 3 3 4 7% 42%	1900
	11th to 20th	Night	12% 7% 2 7% 5% 4 4 4 5% 6% 45%	1875
	21st to end	Night	18% 4 2 5% 4 3 4 3 6% 7% 43%	1900
May	1st to 10th	Night	19% 11% 3 8% 4 4 4 3 5% 7% 31%	2995
	11th to 20th	Night	24% 9% 3 7% 4 4 5% 4 4 6% 30%	2495
	21st to end	Night	11% 6% 2 6% 4 4 4 4 7% 10% 43%	2732
June	1st to 10th	Night	14% 5% 2 5% 5% 3 3 4 6% 9% 44%	1520
	11th to 20th	Night	9% 6% 2 6% 6% 5% 5% 4 6% 11% 41%	1520
	21st to end	Night	9% 5% 1 5% 5% 5% 4 5% 7% 11% 42%	1520
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 5% 8% 10% 40%	2520
	11th to 20th	Night	10% 6% 2 7% 4 4 5% 5% 7% 11% 39%	2520
	21st to end	Night	12% 8% 3 7% 5% 5% 5% 4 7% 8% 36%	2928
August	1st to 10th	Night	16% 7% 3 7% 4 4 5% 4 6% 10% 33%	2825
	11th to 20th	Night	10% 6% 3 7% 4 5% 5% 4 7% 8% 41%	3084
	21st to end	Night	10% 5% 2 6% 3 4 4 3 5% 7% 51%	3476
September	1st to 10th	Night	15% 6% 2 6% 4 4 5% 3 4 7% 44%	1900
	11th to 20th	Night	11% 5% 2 4 4 3 3 3 5% 8% 53%	1919
	21st to end	Night	20% 5% 3 6% 3 4 4 4 6% 6% 40%	2099
October	1st to 10th	Night	18% 6% 2 6% 4 3 4 3 4 6% 45%	2280
	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% 6% 2 5% 3 3 4 4 4 6% 53%	2470
	11th to 20th	Night	15% 6% 3 5% 4 2 4 3 5% 5% 49%	2470
	21st to end	Night	13% 6% 1 4 4 4 4 4 5% 7% 48%	2565
December	1st to 10th	Night	18% 7% 3 6% 4 4 4 4 5% 7% 40%	2660
	11th to 20th	Night	17% 5% 2 6% 4 3 4 3 6% 7% 43%	3285
	21st to end	Night	15% 8% 3 7% 4 4 5% 4 6% 7% 38%	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	23% 37% 5% 21% 11% 4	2518
	11th to 20th	Day	32% 37% 4 15% 8% 4	2687
	21st to end	Day	30% 39% 10% 14% 6% 1	3124
February	1st to 10th	Day	16% 65% 7% 6% 4 1	1710
	11th to 20th	Day	15% 64% 3 9% 7% 2	1819
	21st to end	Day	17% 65% 4 8% 5% 1	1570
March	1st to 10th	Day	32% 40% 4 14% 6% 3	3172
	11th to 20th	Day	14% 40% 5% 24% 14% 2	3480
	21st to end	Day	27% 39% 6% 11% 13% 3	4022
April	1st to 10th	Day	16% 65% 4 8% 6% 1	2280
	11th to 20th	Day	14% 65% 5% 10% 6% 1	2254
	21st to end	Day	15% 65% 5% 8% 7% 1	2280
May	1st to 10th	Day	38% 40% 8% 10% 3 1	3950
	11th to 20th	Day	35% 40% 11% 12% 2	4402
	21st to end	Day	21% 40% 8% 17% 13% 1	4837
June	1st to 10th	Day	14% 66% 4 9% 6% 1	2660
	11th to 20th	Day	12% 66% 4 10% 8% 1	2660
	21st to end	Day	10% 64% 6% 11% 8% 1	2660
July	1st to 10th	Day	24% 42% 14% 11% 7% 2	4370
	11th to 20th	Day	27% 38% 13% 15% 6% 1	4365
	21st to end	Day	30% 38% 9% 13% 10%	4460
August	1st to 10th	Day	29% 40% 11% 16% 4	4074
	11th to 20th	Day	22% 38% 10% 22% 7%	3766
	21st to end	Day	17% 37% 9% 23% 13% 1	4180
September	1st to 10th	Day	16% 60% 6% 10% 7% 1	2280
	11th to 20th	Day	15% 60% 4 11% 9% 2	2261
	21st to end	Day	22% 60% 3 9% 5%	2076
October	1st to 10th	Day	19% 37% 6% 13% 21% 4	3160
	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% 58% 6% 11% 7% 3	1708
	11th to 20th	Day	13% 59% 3 9% 13% 2	1710
	21st to end	Day	14% 59% 4 16% 6% 2	1611
December	1st to 10th	Day	21% 60% 6% 7% 5% 2	1520
	11th to 20th	Day	22% 47% 4 17% 7% 3	1906
	21st to end	Day	28% 37% 11% 14% 9%	2772
January	1st to 10th	Night	28% 36% 5% 18% 10% 3	4429
	11th to 20th	Night	28% 37% 5% 17% 11% 3	4271
	21st to end	Night	33% 40% 3 16% 7% 1	4532
February	1st to 10th	Night	15% 64% 7% 9% 3 1	2470
	11th to 20th	Night	17% 63% 2 7% 10% 2	2258
	21st to end	Night	15% 64% 4 8% 5% 3	1884
March	1st to 10th	Night	35% 41% 4 14% 6% 1	3678
	11th to 20th	Night	17% 40% 7% 21% 12% 3	3480
	21st to end	Night	28% 40% 3 10% 16% 3	3608
April	1st to 10th	Night	17% 64% 2 8% 8% 1	1900
	11th to 20th	Night	15% 65% 5% 8% 6% 2	1875
	21st to end	Night	15% 65% 4 7% 8% 1	1900
May	1st to 10th	Night	36% 41% 7% 14% 3	3025
	11th to 20th	Night	38% 41% 7% 12% 3	2496
	21st to end	Night	22% 39% 12% 13% 11% 3	2739
June	1st to 10th	Night	14% 65% 4 8% 8% 1	1520
	11th to 20th	Night	13% 66% 4 11% 6%	1520
	21st to end	Night	12% 64% 6% 9% 7% 1	1520
July	1st to 10th	Night	26% 40% 15% 11% 7% 1	2480
	11th to 20th	Night	25% 37% 15% 19% 3	2472
	21st to end	Night	32% 36% 10% 12% 9%	2834
August	1st to 10th	Night	34% 39% 11% 12% 4	2814
	11th to 20th	Night	24% 39% 8% 21% 8%	3114
	21st to end	Night	18% 37% 6% 22% 16% 1	3476
September	1st to 10th	Night	15% 61% 5% 11% 6% 1	1900
	11th to 20th	Night	13% 60% 4 11% 10% 2	1919
	21st to end	Night	22% 60% 4 8% 6%	2099
October	1st to 10th	Night	25% 37% 7% 15% 13% 3	3800
	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% 57% 5% 11% 7% 4	2470
	11th to 20th	Night	15% 59% 3 8% 12% 3	2470
	21st to end	Night	13% 59% 5% 13% 8% 1	2565
December	1st to 10th	Night	21% 60% 5% 10% 4 1	2660
	11th to 20th	Night	20% 49% 5% 10% 12% 3	3288
	21st to end	Night	30% 37% 11% 16% 6% 1	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	19% 18% 38% 17% 6% 1	180
	11th to 20th	Day	22% 28% 27% 18% 5%	229
	21st to end	Day	22% 18% 37% 15% 8% 1	297
February	1st to 10th	Day	21% 15% 36% 21% 6% 1	270
	11th to 20th	Day	23% 17% 41% 16% 3	418
	21st to end	Day	23% 16% 35% 20% 5%	382
March	1st to 10th	Day	17% 20% 33% 24% 6%	460
	11th to 20th	Day	20% 15% 36% 22% 5% 2	460
	21st to end	Day	22% 11% 36% 23% 8% 1	506
April	1st to 10th	Day	19% 13% 38% 22% 6% 2	460
	11th to 20th	Day	14% 14% 31% 26% 12% 3	452
	21st to end	Day	20% 12% 29% 27% 12% 1	460
May	1st to 10th	Day	26% 9% 28% 22% 11% 3	463
	11th to 20th	Day	20% 6% 25% 33% 14% 3	487
	21st to end	Day	18% 3 16% 29% 25% 9%	529
June	1st to 10th	Day	13% 4 23% 30% 21% 9%	490
	11th to 20th	Day	11% 5% 20% 31% 24% 9%	490
	21st to end	Day	10% 3 17% 33% 30% 8%	490
July	1st to 10th	Day	10% 3 10% 33% 31% 12%	485
	11th to 20th	Day	5% 2 11% 33% 35% 14%	484
	21st to end	Day	9% 3 13% 37% 31% 8%	532
August	1st to 10th	Day	13% 3 12% 32% 29% 12%	486
	11th to 20th	Day	13% 1 13% 28% 32% 13%	479
	21st to end	Day	10% 2 14% 32% 28% 15%	539
September	1st to 10th	Day	17% 2 17% 28% 23% 14%	490
	11th to 20th	Day	12% 4 17% 29% 26% 12%	490
	21st to end	Day	23% 5% 22% 29% 16% 4	490
October	1st to 10th	Day	22% 4 19% 27% 19% 10%	490
	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% 32% 23% 18% 5%	378
	11th to 20th	Day	13% 14% 35% 26% 12% 1	370
	21st to end	Day	15% 10% 34% 26% 13% 3	271
December	1st to 10th	Day	16% 13% 39% 22% 8% 2	180
	11th to 20th	Day	24% 18% 27% 21% 9% 1	180
	21st to end	Day	23% 10% 33% 23% 11% 1	198
January	1st to 10th	Night	20% 13% 40% 23% 5%	760
	11th to 20th	Night	22% 14% 33% 26% 5%	760
	21st to end	Night	19% 12% 39% 28% 3	836
February	1st to 10th	Night	20% 10% 38% 28% 4	760
	11th to 20th	Night	22% 16% 42% 18% 2	743
	21st to end	Night	21% 14% 38% 23% 3	628
March	1st to 10th	Night	17% 12% 40% 26% 5%	760
	11th to 20th	Night	19% 12% 39% 23% 6% 1	760
	21st to end	Night	19% 9% 38% 25% 7% 1	836
April	1st to 10th	Night	21% 11% 37% 25% 6%	760
	11th to 20th	Night	15% 10% 32% 36% 8%	751
	21st to end	Night	19% 9% 31% 32% 9%	760
May	1st to 10th	Night	21% 8% 25% 35% 10% 1	690
	11th to 20th	Night	20% 4 27% 36% 13%	374
	21st to end	Night	16% 2 16% 39% 24% 2	411
June	1st to 10th	Night	17% 2 20% 41% 18% 2	380
	11th to 20th	Night	9% 2 22% 44% 23% 1	380
	21st to end	Night	9% 4 16% 49% 22% 1	380
July	1st to 10th	Night	8% 2 14% 44% 29% 2	370
	11th to 20th	Night	7% 3 9% 43% 35% 3	368
	21st to end	Night	9% 2 13% 44% 30% 2	530
August	1st to 10th	Night	11% 2 11% 42% 31% 2	567
	11th to 20th	Night	12% 2 10% 42% 29% 4	737
	21st to end	Night	14% 3 10% 38% 29% 6%	836
September	1st to 10th	Night	16% 2 16% 35% 26% 4	760
	11th to 20th	Night	13% 3 14% 36% 29% 5%	760
	21st to end	Night	23% 4 19% 32% 19% 2	760
October	1st to 10th	Night	22% 3 19% 31% 20% 5%	760
	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% 33% 13% 2	760
	11th to 20th	Night	19% 10% 32% 30% 9% 1	760
	21st to end	Night	14% 13% 35% 26% 11% 1	760
December	1st to 10th	Night	23% 10% 36% 21% 9% 1	760
	11th to 20th	Night	19% 10% 37% 28% 6%	760
	21st to end	Night	19% 9% 37% 31% 4	836

Page generated at 2024-11-03 02:11:18. Version ac07

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