Port Dover
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Dover clear sky chart from 2005-11-06 to 2026-05-17.

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% 9% 4 9% 5% 5% 5% 4 6% 7% 33%	132078
	Night	15% 10% 4 9% 5% 4 5% 4 5% 7% 33%	130723
Transparency	Day	33% 37% 8% 14% 7% 1	135674
	Night	34% 36% 8% 13% 8% 1	135213
Seeing	Day	17% 9% 26% 27% 16% 5%	16373
	Night	19% 9% 28% 28% 13% 2	26002

By Month

Percentage Cloud Cover NE (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% 14% 6% 13% 6% 5% 5% 3 4 5% 24%	4839
	11th to 20th	Day	10% 21% 9% 22% 9% 7% 4 2 4 4 9%	5246
	21st to end	Day	15% 13% 6% 16% 8% 6% 5% 4 4 6% 17%	5780
February	1st to 10th	Day	15% 16% 5% 12% 8% 6% 6% 4 4 4 20%	1890
	11th to 20th	Day	14% 13% 6% 13% 7% 6% 5% 4 5% 5% 22%	1843
	21st to end	Day	18% 13% 5% 12% 6% 6% 4 4 5% 5% 22%	1664
March	1st to 10th	Day	18% 10% 4 12% 6% 5% 5% 3 5% 5% 28%	5037
	11th to 20th	Day	10% 11% 4 9% 5% 5% 5% 4 6% 8% 32%	5090
	21st to end	Day	18% 10% 3 9% 6% 5% 4 3 4 5% 32%	5586
April	1st to 10th	Day	17% 10% 3 8% 6% 5% 5% 3 6% 6% 31%	2436
	11th to 20th	Day	16% 10% 3 8% 4 4 4 3 6% 6% 34%	2602
	21st to end	Day	18% 9% 3 6% 5% 5% 4 3 5% 7% 35%	2730
May	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 4 5% 6% 31%	6018
	11th to 20th	Day	16% 9% 3 8% 6% 5% 5% 4 6% 8% 32%	6117
	21st to end	Day	15% 5% 3 7% 5% 5% 5% 4 6% 7% 39%	6949
June	1st to 10th	Day	11% 6% 3 6% 5% 5% 5% 4 7% 8% 41%	2800
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 8% 8% 40%	2800
	21st to end	Day	11% 5% 2 6% 4 5% 4 4 6% 10% 43%	2800
July	1st to 10th	Day	7% 4 2 6% 5% 5% 5% 5% 6% 10% 44%	6360
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 5% 7% 10% 43%	6113
	21st to end	Day	7% 4 2 6% 4 4 5% 4 6% 9% 48%	6424
August	1st to 10th	Day	15% 5% 2 6% 3 4 5% 4 7% 9% 41%	4234
	11th to 20th	Day	10% 6% 3 7% 4 5% 6% 5% 7% 9% 37%	4174
	21st to end	Day	9% 5% 2 7% 4 5% 5% 5% 6% 10% 43%	4522
September	1st to 10th	Day	12% 5% 2 5% 4 4 5% 4 6% 8% 44%	2400
	11th to 20th	Day	9% 5% 2 4 4 4 4 4 5% 8% 50%	2220
	21st to end	Day	15% 6% 2 7% 4 5% 5% 4 6% 7% 38%	2195
October	1st to 10th	Day	15% 7% 2 6% 5% 4 5% 4 6% 8% 39%	2000
	11th to 20th	Day	18% 7% 3 8% 6% 5% 5% 4 5% 7% 31%	2000
	21st to end	Day	22% 9% 4 9% 5% 4 4 4 7% 7% 25%	2200
November	1st to 10th	Day	15% 7% 3 7% 6% 4 5% 4 6% 8% 36%	1855
	11th to 20th	Day	15% 12% 3 8% 5% 5% 5% 4 6% 7% 30%	1680
	21st to end	Day	20% 14% 3 7% 6% 5% 5% 5% 6% 7% 21%	1680
December	1st to 10th	Day	19% 15% 5% 9% 7% 6% 6% 5% 7% 7% 15%	2680
	11th to 20th	Day	21% 17% 5% 9% 5% 5% 5% 3 5% 6% 20%	3066
	21st to end	Day	25% 18% 6% 11% 5% 4 4 3 4 6% 13%	4048
January	1st to 10th	Night	15% 15% 5% 11% 6% 4 4 4 5% 6% 24%	8054
	11th to 20th	Night	11% 18% 7% 15% 7% 5% 6% 4 5% 6% 17%	7639
	21st to end	Night	16% 14% 6% 14% 8% 5% 5% 3 4 5% 20%	8401
February	1st to 10th	Night	15% 16% 5% 10% 7% 5% 5% 4 5% 6% 24%	2730
	11th to 20th	Night	15% 13% 4 10% 5% 5% 5% 4 6% 6% 25%	2663
	21st to end	Night	17% 13% 4 11% 5% 5% 5% 4 5% 7% 24%	2139
March	1st to 10th	Night	16% 9% 4 9% 6% 5% 5% 4 4 6% 33%	5023
	11th to 20th	Night	11% 8% 3 8% 4 4 4 3 6% 8% 40%	5089
	21st to end	Night	17% 10% 3 8% 4 4 4 3 5% 5% 38%	5588
April	1st to 10th	Night	19% 10% 3 7% 5% 5% 4 4 4 6% 31%	2184
	11th to 20th	Night	18% 8% 3 7% 5% 5% 5% 3 6% 6% 35%	1984
	21st to end	Night	15% 7% 3 7% 5% 5% 5% 4 6% 7% 37%	1890
May	1st to 10th	Night	21% 9% 3 8% 4 4 4 3 4 5% 33%	4143
	11th to 20th	Night	12% 8% 3 8% 6% 5% 5% 4 5% 6% 37%	3904
	21st to end	Night	17% 5% 2 5% 4 3 4 4 6% 8% 41%	3926
June	1st to 10th	Night	11% 6% 3 6% 4 4 4 5% 7% 8% 41%	1600
	11th to 20th	Night	10% 4 2 7% 5% 5% 5% 5% 7% 9% 42%	1600
	21st to end	Night	10% 5% 2 6% 4 4 4 4 5% 9% 46%	1600
July	1st to 10th	Night	6% 5% 2 7% 5% 4 4 4 6% 10% 47%	3600
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 4 6% 8% 47%	3830
	21st to end	Night	7% 5% 2 6% 4 4 4 4 6% 8% 50%	4428
August	1st to 10th	Night	10% 6% 2 6% 5% 4 4 4 5% 9% 44%	2915
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 4 6% 9% 45%	2875
	21st to end	Night	8% 4 2 5% 4 4 5% 4 6% 10% 49%	3341
September	1st to 10th	Night	11% 6% 3 6% 4 4 3 4 4 7% 48%	2000
	11th to 20th	Night	9% 5% 2 5% 3 2 3 3 5% 7% 57%	2177
	21st to end	Night	17% 5% 2 7% 4 4 4 3 7% 6% 40%	2205
October	1st to 10th	Night	15% 6% 2 6% 5% 3 4 4 6% 7% 44%	2400
	11th to 20th	Night	17% 10% 2 7% 5% 5% 4 3 5% 6% 36%	2400
	21st to end	Night	21% 8% 2 7% 4 4 5% 4 5% 5% 34%	2640
November	1st to 10th	Night	14% 8% 3 6% 4 4 5% 4 6% 6% 41%	2657
	11th to 20th	Night	15% 11% 3 8% 5% 5% 4 4 7% 8% 29%	2940
	21st to end	Night	19% 13% 3 9% 5% 5% 5% 4 6% 6% 26%	2940
December	1st to 10th	Night	16% 17% 4 10% 5% 6% 6% 4 6% 6% 21%	4720
	11th to 20th	Night	19% 17% 4 10% 5% 5% 4 3 4 5% 23%	5348
	21st to end	Night	24% 17% 5% 10% 5% 4 5% 3 5% 5% 17%	7150

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	51% 22% 9% 10% 8%	4840
	11th to 20th	Day	66% 22% 4 4 3	5280
	21st to end	Day	54% 24% 8% 9% 5%	5780
February	1st to 10th	Day	28% 60% 3 6% 3	1890
	11th to 20th	Day	27% 59% 4 8% 3	1852
	21st to end	Day	23% 60% 6% 7% 3 1	1664
March	1st to 10th	Day	43% 30% 9% 13% 5%	5040
	11th to 20th	Day	34% 31% 13% 16% 5% 1	5090
	21st to end	Day	39% 30% 5% 17% 8% 1	5586
April	1st to 10th	Day	27% 59% 4 7% 3	2436
	11th to 20th	Day	22% 60% 4 9% 4 1	2602
	21st to end	Day	21% 60% 5% 8% 5% 1	2730
May	1st to 10th	Day	38% 31% 9% 12% 6% 4	6027
	11th to 20th	Day	36% 33% 9% 15% 6% 1	6121
	21st to end	Day	34% 30% 8% 15% 11% 2	6949
June	1st to 10th	Day	17% 62% 4 9% 7% 1	2800
	11th to 20th	Day	17% 64% 6% 8% 5% 1	2800
	21st to end	Day	14% 61% 6% 11% 8%	2800
July	1st to 10th	Day	27% 26% 10% 23% 13% 2	6290
	11th to 20th	Day	26% 26% 10% 23% 13% 1	6039
	21st to end	Day	23% 27% 12% 20% 17% 1	6240
August	1st to 10th	Day	28% 37% 10% 15% 9% 1	4221
	11th to 20th	Day	29% 38% 11% 17% 4	4213
	21st to end	Day	24% 37% 8% 23% 8%	4522
September	1st to 10th	Day	15% 59% 6% 14% 6%	2400
	11th to 20th	Day	11% 58% 6% 14% 10%	2220
	21st to end	Day	19% 58% 6% 11% 5%	2195
October	1st to 10th	Day	20% 37% 10% 21% 12% 1	3260
	11th to 20th	Day	21% 36% 10% 21% 11%	3240
	21st to end	Day	27% 35% 12% 19% 6%	3538
November	1st to 10th	Day	25% 54% 7% 10% 3 1	1855
	11th to 20th	Day	24% 56% 6% 10% 4	1680
	21st to end	Day	29% 56% 7% 5% 2	1680
December	1st to 10th	Day	48% 35% 10% 6% 1	2680
	11th to 20th	Day	46% 33% 9% 5% 6%	3066
	21st to end	Day	56% 28% 6% 7% 3	4048
January	1st to 10th	Night	47% 23% 9% 16% 4	8054
	11th to 20th	Night	53% 24% 8% 8% 7%	7680
	21st to end	Night	54% 23% 5% 9% 9%	8401
February	1st to 10th	Night	25% 59% 5% 7% 3	2730
	11th to 20th	Night	25% 58% 5% 8% 4	2665
	21st to end	Night	25% 60% 4 7% 4	2139
March	1st to 10th	Night	39% 29% 10% 14% 7% 2	5033
	11th to 20th	Night	30% 29% 14% 17% 10% 1	5090
	21st to end	Night	34% 29% 8% 14% 13% 1	5588
April	1st to 10th	Night	26% 60% 3 7% 4 1	2184
	11th to 20th	Night	21% 60% 4 9% 5% 1	1984
	21st to end	Night	18% 59% 6% 11% 5% 1	1890
May	1st to 10th	Night	41% 29% 8% 9% 8% 6%	4150
	11th to 20th	Night	37% 30% 10% 17% 6%	3905
	21st to end	Night	32% 30% 8% 17% 12% 1	3927
June	1st to 10th	Night	16% 63% 4 10% 6% 2	1600
	11th to 20th	Night	15% 63% 6% 11% 5% 1	1600
	21st to end	Night	15% 60% 6% 8% 10% 1	1600
July	1st to 10th	Night	29% 25% 11% 18% 14% 3	3560
	11th to 20th	Night	27% 26% 9% 23% 13% 1	3781
	21st to end	Night	26% 26% 10% 16% 21% 2	4297
August	1st to 10th	Night	25% 37% 8% 17% 11% 2	2904
	11th to 20th	Night	27% 36% 10% 21% 5%	2902
	21st to end	Night	20% 37% 9% 24% 10%	3341
September	1st to 10th	Night	14% 57% 6% 14% 9%	2000
	11th to 20th	Night	11% 57% 6% 16% 10%	2177
	21st to end	Night	18% 58% 5% 11% 7% 1	2205
October	1st to 10th	Night	19% 36% 4 20% 20% 1	3920
	11th to 20th	Night	27% 35% 8% 18% 12%	3899
	21st to end	Night	21% 36% 11% 20% 12%	4252
November	1st to 10th	Night	24% 54% 6% 9% 6%	2657
	11th to 20th	Night	23% 56% 6% 11% 4	2940
	21st to end	Night	28% 56% 7% 7% 2	2940
December	1st to 10th	Night	42% 35% 12% 8% 2	4720
	11th to 20th	Night	45% 33% 5% 10% 7% 1	5348
	21st to end	Night	52% 27% 8% 8% 4	7150

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	21% 17% 35% 19% 6% 1	253
	11th to 20th	Day	25% 15% 40% 14% 5% 1	330
	21st to end	Day	18% 19% 37% 18% 6% 1	363
February	1st to 10th	Day	21% 15% 38% 20% 6% 1	330
	11th to 20th	Day	21% 17% 37% 20% 4	330
	21st to end	Day	21% 25% 33% 20% 2	273
March	1st to 10th	Day	21% 9% 32% 28% 8% 1	535
	11th to 20th	Day	20% 13% 34% 26% 6% 1	540
	21st to end	Day	25% 12% 31% 24% 7%	594
April	1st to 10th	Day	18% 13% 33% 27% 8% 1	540
	11th to 20th	Day	21% 13% 29% 24% 10% 3	532
	21st to end	Day	22% 7% 27% 32% 10% 2	540
May	1st to 10th	Day	22% 6% 26% 31% 10% 4	543
	11th to 20th	Day	15% 6% 21% 32% 22% 3	551
	21st to end	Day	13% 4 22% 34% 21% 6%	573
June	1st to 10th	Day	11% 5% 28% 37% 17% 2	530
	11th to 20th	Day	12% 5% 22% 34% 23% 5%	530
	21st to end	Day	13% 3 20% 41% 19% 4	530
July	1st to 10th	Day	9% 2 16% 30% 33% 10%	525
	11th to 20th	Day	13% 4 16% 31% 24% 12%	524
	21st to end	Day	9% 3 17% 37% 27% 8%	576
August	1st to 10th	Day	9% 3 16% 28% 29% 13%	526
	11th to 20th	Day	10% 4 15% 33% 27% 12%	519
	21st to end	Day	12% 3 17% 31% 27% 10%	583
September	1st to 10th	Day	13% 4 18% 28% 23% 14%	530
	11th to 20th	Day	8% 3 15% 22% 32% 20%	530
	21st to end	Day	17% 8% 22% 27% 19% 8%	530
October	1st to 10th	Day	17% 8% 22% 25% 21% 8%	530
	11th to 20th	Day	18% 13% 30% 25% 10% 4	530
	21st to end	Day	25% 11% 31% 23% 9% 1	583
November	1st to 10th	Day	19% 17% 26% 26% 9% 2	348
	11th to 20th	Day	23% 17% 30% 19% 9% 1	220
	21st to end	Day	25% 15% 37% 20% 3	220
December	1st to 10th	Day	27% 21% 27% 17% 7% 1	220
	11th to 20th	Day	25% 25% 31% 15% 4	220
	21st to end	Day	26% 21% 33% 17% 4	242
January	1st to 10th	Night	24% 17% 37% 17% 5%	840
	11th to 20th	Night	27% 17% 34% 20% 4	840
	21st to end	Night	19% 13% 38% 24% 5%	924
February	1st to 10th	Night	25% 13% 33% 25% 3	840
	11th to 20th	Night	22% 12% 35% 26% 5%	823
	21st to end	Night	23% 14% 36% 24% 3	692
March	1st to 10th	Night	19% 8% 37% 27% 8%	840
	11th to 20th	Night	21% 10% 34% 26% 7%	840
	21st to end	Night	21% 8% 31% 29% 9% 1	924
April	1st to 10th	Night	22% 10% 32% 30% 6%	840
	11th to 20th	Night	21% 9% 30% 29% 10% 1	740
	21st to end	Night	18% 5% 30% 36% 10%	630
May	1st to 10th	Night	19% 5% 26% 33% 16% 1	630
	11th to 20th	Night	15% 4 24% 38% 17% 2	531
	21st to end	Night	14% 4 21% 35% 23% 3	433
June	1st to 10th	Night	12% 4 25% 40% 19% 1	400
	11th to 20th	Night	13% 2 24% 38% 24%	400
	21st to end	Night	13% 3 20% 41% 23% 1	400
July	1st to 10th	Night	7% 2 18% 40% 28% 6%	390
	11th to 20th	Night	10% 3 19% 38% 26% 4	487
	21st to end	Night	8% 2 18% 42% 26% 4	657
August	1st to 10th	Night	9% 3 11% 41% 31% 4	597
	11th to 20th	Night	9% 3 15% 33% 35% 5%	594
	21st to end	Night	10% 2 18% 37% 28% 4	726
September	1st to 10th	Night	13% 5% 17% 32% 26% 7%	800
	11th to 20th	Night	11% 4 14% 28% 35% 8%	800
	21st to end	Night	17% 6% 18% 30% 22% 6%	800
October	1st to 10th	Night	19% 5% 22% 28% 22% 4	800
	11th to 20th	Night	22% 10% 29% 27% 10% 2	800
	21st to end	Night	25% 11% 31% 27% 6%	880
November	1st to 10th	Night	20% 11% 29% 28% 10% 1	820
	11th to 20th	Night	21% 18% 30% 23% 6% 1	840
	21st to end	Night	28% 12% 35% 20% 5%	840
December	1st to 10th	Night	31% 15% 30% 17% 5% 1	840
	11th to 20th	Night	26% 19% 34% 17% 3	840
	21st to end	Night	29% 14% 33% 21% 3	924

Page generated at 2026-05-14 01:58:54. Version ac07

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