Dundalk
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dundalk clear sky chart from 2005-11-06 to 2024-01-17.

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	15% 12% 5% 9% 7% 4 5% 5% 5% 6% 28%	75002
	Night	14% 12% 5% 10% 7% 4 5% 4 5% 6% 28%	74771
Transparency	Day	55% 8% 11% 16% 9% 1	74711
	Night	55% 9% 11% 16% 9% 1	74597
Seeing	Day	21% 20% 31% 18% 8% 3	13839
	Night	19% 20% 32% 20% 7% 1	23033

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	13% 16% 9% 18% 12% 4 5% 5% 3 4 11%	1371
	11th to 20th	Day	14% 18% 10% 15% 10% 5% 4 5% 4 4 11%	1664
	21st to end	Day	10% 17% 9% 17% 11% 5% 7% 5% 4 5% 11%	1782
February	1st to 10th	Day	12% 17% 9% 16% 14% 7% 6% 3 3 3 11%	1620
	11th to 20th	Day	12% 13% 8% 15% 13% 4 6% 4 5% 5% 14%	1573
	21st to end	Day	14% 17% 9% 12% 9% 5% 6% 3 4 3 18%	1409
March	1st to 10th	Day	13% 13% 6% 11% 10% 5% 5% 4 4 4 25%	1949
	11th to 20th	Day	11% 13% 8% 11% 7% 4 6% 4 4 5% 28%	1980
	21st to end	Day	16% 12% 5% 7% 7% 3 5% 5% 4 5% 30%	2178
April	1st to 10th	Day	17% 15% 5% 9% 7% 4 4 4 5% 5% 25%	2092
	11th to 20th	Day	17% 13% 4 6% 5% 3 4 4 4 6% 34%	2266
	21st to end	Day	21% 11% 5% 7% 4 3 4 4 5% 6% 31%	2340
May	1st to 10th	Day	17% 10% 4 7% 6% 3 5% 4 5% 7% 33%	2340
	11th to 20th	Day	17% 9% 3 6% 6% 3 4 4 5% 7% 36%	2420
	21st to end	Day	14% 6% 3 5% 5% 3 5% 4 6% 9% 40%	2725
June	1st to 10th	Day	16% 7% 3 6% 5% 4 5% 5% 6% 8% 36%	2522
	11th to 20th	Day	13% 6% 3 5% 5% 3 5% 5% 6% 9% 40%	2700
	21st to end	Day	16% 7% 2 5% 5% 3 5% 5% 6% 8% 38%	2700
July	1st to 10th	Day	11% 6% 2 5% 5% 3 6% 5% 6% 10% 42%	2700
	11th to 20th	Day	13% 6% 3 6% 6% 4 6% 6% 6% 9% 36%	2560
	21st to end	Day	11% 7% 3 7% 7% 4 6% 5% 7% 9% 34%	2547
August	1st to 10th	Day	11% 6% 3 7% 6% 4 6% 6% 6% 9% 36%	2327
	11th to 20th	Day	12% 6% 3 7% 7% 5% 6% 6% 6% 9% 33%	2303
	21st to end	Day	13% 7% 3 7% 6% 3 6% 5% 6% 8% 35%	2522
September	1st to 10th	Day	16% 8% 3 6% 6% 3 5% 5% 5% 7% 36%	2160
	11th to 20th	Day	13% 6% 3 6% 5% 3 5% 5% 6% 8% 39%	1998
	21st to end	Day	17% 8% 4 7% 7% 3 4 6% 5% 8% 32%	1978
October	1st to 10th	Day	18% 11% 3 7% 6% 4 4 4 5% 6% 31%	1800
	11th to 20th	Day	22% 15% 3 8% 7% 4 5% 6% 5% 4 21%	1800
	21st to end	Day	25% 19% 5% 7% 6% 3 5% 4 4 5% 17%	1963
November	1st to 10th	Day	14% 18% 6% 8% 6% 3 5% 4 5% 6% 25%	1603
	11th to 20th	Day	14% 18% 7% 9% 7% 4 5% 5% 6% 6% 19%	1520
	21st to end	Day	16% 21% 8% 11% 6% 3 6% 5% 4 5% 14%	1520
December	1st to 10th	Day	13% 23% 12% 18% 7% 3 6% 4 4 4 8%	1520
	11th to 20th	Day	16% 20% 9% 13% 8% 3 5% 4 4 5% 12%	1906
	21st to end	Day	22% 21% 10% 17% 8% 3 4 3 2 2 7%	2644
January	1st to 10th	Night	14% 15% 9% 16% 11% 5% 4 4 4 6% 11%	2784
	11th to 20th	Night	17% 15% 8% 16% 10% 6% 6% 4 3 4 11%	2438
	21st to end	Night	11% 13% 8% 15% 11% 7% 6% 6% 6% 6% 12%	2574
February	1st to 10th	Night	12% 17% 8% 12% 12% 6% 6% 5% 5% 5% 12%	2340
	11th to 20th	Night	11% 13% 7% 15% 9% 5% 6% 5% 5% 6% 18%	2273
	21st to end	Night	13% 15% 6% 13% 8% 4 6% 5% 5% 6% 19%	1844
March	1st to 10th	Night	12% 9% 6% 12% 7% 4 6% 5% 4 6% 28%	1997
	11th to 20th	Night	11% 13% 6% 9% 8% 3 5% 4 4 5% 32%	1979
	21st to end	Night	13% 10% 4 8% 7% 4 4 5% 4 5% 34%	2178
April	1st to 10th	Night	18% 12% 5% 8% 6% 3 5% 4 5% 6% 29%	1868
	11th to 20th	Night	15% 10% 4 6% 5% 4 5% 5% 4 6% 37%	1660
	21st to end	Night	14% 13% 4 9% 5% 4 5% 5% 4 6% 31%	1620
May	1st to 10th	Night	15% 9% 3 6% 5% 3 4 6% 6% 6% 37%	1620
	11th to 20th	Night	14% 6% 3 5% 5% 2 5% 4 5% 7% 43%	1470
	21st to end	Night	8% 6% 2 4 4 3 4 5% 5% 9% 48%	1550
June	1st to 10th	Night	13% 6% 3 6% 6% 3 5% 4 5% 8% 42%	1433
	11th to 20th	Night	10% 7% 2 5% 5% 3 5% 5% 7% 9% 44%	1260
	21st to end	Night	13% 6% 3 4 4 2 5% 4 5% 9% 44%	1260
July	1st to 10th	Night	7% 6% 2 4 4 2 3 4 4 8% 56%	1260
	11th to 20th	Night	9% 6% 2 5% 6% 3 5% 4 6% 8% 46%	1392
	21st to end	Night	8% 6% 3 5% 5% 3 5% 4 6% 8% 48%	1771
August	1st to 10th	Night	10% 5% 2 5% 6% 2 4 4 6% 8% 49%	1611
	11th to 20th	Night	8% 5% 2 5% 5% 3 5% 5% 5% 9% 49%	1598
	21st to end	Night	11% 6% 3 6% 7% 3 5% 5% 4 7% 44%	1814
September	1st to 10th	Night	11% 6% 2 7% 5% 3 4 5% 6% 8% 45%	1800
	11th to 20th	Night	12% 6% 2 4 6% 2 3 4 5% 7% 49%	1957
	21st to end	Night	16% 9% 3 6% 6% 4 4 3 4 7% 39%	1981
October	1st to 10th	Night	18% 9% 3 6% 7% 3 4 4 5% 7% 35%	2160
	11th to 20th	Night	20% 12% 4 8% 6% 4 5% 5% 5% 6% 26%	2160
	21st to end	Night	20% 16% 4 8% 7% 4 5% 5% 5% 5% 21%	2389
November	1st to 10th	Night	16% 15% 4 8% 7% 4 5% 5% 5% 6% 27%	2473
	11th to 20th	Night	15% 15% 6% 8% 6% 4 5% 5% 6% 7% 22%	2660
	21st to end	Night	17% 16% 8% 11% 7% 4 5% 4 5% 6% 17%	2660
December	1st to 10th	Night	17% 18% 11% 15% 7% 4 4 4 4 5% 10%	2660
	11th to 20th	Night	13% 20% 10% 16% 8% 4 4 5% 4 5% 13%	3288
	21st to end	Night	21% 21% 9% 17% 8% 3 4 3 4 3 6%	4989

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	77% 7% 8% 6% 2	1372
	11th to 20th	Day	76% 8% 5% 5% 4 1	1664
	21st to end	Day	73% 7% 8% 6% 5% 1	1782
February	1st to 10th	Day	78% 6% 5% 8% 3 1	1620
	11th to 20th	Day	68% 8% 8% 10% 5% 1	1582
	21st to end	Day	69% 7% 7% 10% 6%	1409
March	1st to 10th	Day	61% 7% 11% 12% 8% 1	1952
	11th to 20th	Day	58% 6% 9% 16% 9% 2	1980
	21st to end	Day	55% 7% 10% 14% 12% 2	2178
April	1st to 10th	Day	59% 7% 9% 16% 8% 1	2092
	11th to 20th	Day	50% 6% 11% 19% 14% 1	2266
	21st to end	Day	53% 7% 10% 17% 11% 2	2340
May	1st to 10th	Day	50% 6% 9% 21% 13% 1	2340
	11th to 20th	Day	47% 7% 10% 21% 12% 1	2424
	21st to end	Day	40% 8% 15% 22% 14% 2	2725
June	1st to 10th	Day	44% 8% 13% 20% 14% 1	2522
	11th to 20th	Day	40% 8% 12% 24% 15% 1	2700
	21st to end	Day	42% 9% 13% 23% 12% 1	2700
July	1st to 10th	Day	36% 8% 14% 23% 17% 2	2626
	11th to 20th	Day	41% 9% 13% 26% 9% 1	2481
	21st to end	Day	44% 9% 14% 20% 12% 1	2405
August	1st to 10th	Day	42% 10% 16% 21% 10% 1	2314
	11th to 20th	Day	43% 10% 16% 20% 9%	2303
	21st to end	Day	44% 9% 14% 21% 10% 1	2522
September	1st to 10th	Day	46% 8% 13% 22% 10% 2	2160
	11th to 20th	Day	41% 8% 15% 26% 11%	1998
	21st to end	Day	48% 10% 13% 21% 7% 1	1978
October	1st to 10th	Day	53% 8% 11% 16% 11% 1	1800
	11th to 20th	Day	61% 9% 10% 13% 5% 1	1800
	21st to end	Day	68% 7% 10% 9% 5%	1963
November	1st to 10th	Day	59% 7% 12% 15% 7%	1603
	11th to 20th	Day	63% 9% 10% 11% 6% 1	1520
	21st to end	Day	70% 10% 10% 8% 2	1520
December	1st to 10th	Day	78% 9% 6% 5% 1	1520
	11th to 20th	Day	73% 8% 9% 8% 1	1906
	21st to end	Day	82% 7% 5% 5% 1	2644
January	1st to 10th	Night	72% 11% 8% 6% 3	2784
	11th to 20th	Night	76% 8% 6% 5% 4 1	2438
	21st to end	Night	68% 11% 10% 6% 4 1	2574
February	1st to 10th	Night	70% 9% 8% 10% 2	2340
	11th to 20th	Night	64% 9% 10% 12% 5%	2275
	21st to end	Night	62% 10% 10% 11% 7% 1	1844
March	1st to 10th	Night	56% 8% 10% 16% 9% 1	2007
	11th to 20th	Night	54% 7% 9% 17% 11% 2	1980
	21st to end	Night	50% 7% 9% 18% 13% 2	2178
April	1st to 10th	Night	55% 6% 9% 17% 12% 1	1868
	11th to 20th	Night	47% 7% 8% 21% 16% 2	1660
	21st to end	Night	52% 7% 10% 14% 15% 2	1620
May	1st to 10th	Night	44% 7% 11% 20% 15% 2	1620
	11th to 20th	Night	40% 6% 10% 25% 16% 3	1471
	21st to end	Night	31% 8% 16% 25% 15% 4	1551
June	1st to 10th	Night	40% 7% 11% 24% 17% 2	1433
	11th to 20th	Night	35% 8% 14% 24% 18% 1	1260
	21st to end	Night	37% 7% 14% 26% 14% 1	1260
July	1st to 10th	Night	26% 8% 12% 28% 22% 3	1225
	11th to 20th	Night	33% 8% 14% 30% 16% 1	1348
	21st to end	Night	33% 9% 14% 27% 15% 2	1672
August	1st to 10th	Night	31% 10% 16% 29% 14% 1	1600
	11th to 20th	Night	30% 9% 16% 29% 15% 1	1598
	21st to end	Night	38% 9% 12% 26% 13% 1	1814
September	1st to 10th	Night	35% 8% 16% 27% 13% 2	1800
	11th to 20th	Night	34% 7% 14% 29% 16% 1	1957
	21st to end	Night	46% 8% 13% 24% 10%	1981
October	1st to 10th	Night	47% 9% 12% 17% 14% 1	2160
	11th to 20th	Night	55% 9% 13% 16% 6%	2160
	21st to end	Night	62% 9% 10% 11% 7%	2389
November	1st to 10th	Night	56% 9% 11% 16% 7% 1	2473
	11th to 20th	Night	58% 10% 13% 12% 5% 2	2660
	21st to end	Night	65% 10% 13% 9% 2 1	2660
December	1st to 10th	Night	73% 10% 8% 6% 2	2660
	11th to 20th	Night	72% 9% 10% 8% 2	3288
	21st to end	Night	82% 8% 5% 5% 1	4989

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	22% 31% 29% 14% 2 1	180
	11th to 20th	Day	21% 36% 32% 6% 4	253
	21st to end	Day	23% 31% 34% 10% 2	264
February	1st to 10th	Day	30% 35% 28% 6%	240
	11th to 20th	Day	25% 36% 34% 4	240
	21st to end	Day	33% 38% 25% 4	198
March	1st to 10th	Day	25% 27% 33% 11% 4	392
	11th to 20th	Day	23% 26% 33% 14% 4	420
	21st to end	Day	28% 22% 32% 13% 5%	462
April	1st to 10th	Day	25% 23% 34% 13% 4	420
	11th to 20th	Day	27% 25% 24% 13% 8% 3	412
	21st to end	Day	27% 19% 34% 14% 4 2	420
May	1st to 10th	Day	23% 18% 33% 20% 5% 1	423
	11th to 20th	Day	20% 16% 33% 20% 7% 3	447
	21st to end	Day	18% 15% 29% 25% 10% 4	485
June	1st to 10th	Day	19% 17% 31% 24% 8% 1	452
	11th to 20th	Day	16% 17% 32% 27% 7% 2	630
	21st to end	Day	16% 14% 38% 22% 7% 2	630
July	1st to 10th	Day	13% 12% 30% 28% 14% 4	621
	11th to 20th	Day	16% 11% 31% 25% 11% 6%	499
	21st to end	Day	11% 10% 39% 27% 10% 4	488
August	1st to 10th	Day	10% 11% 30% 28% 15% 7%	446
	11th to 20th	Day	13% 9% 25% 27% 18% 8%	439
	21st to end	Day	14% 12% 31% 25% 15% 4	495
September	1st to 10th	Day	16% 15% 28% 24% 11% 6%	450
	11th to 20th	Day	12% 12% 26% 26% 16% 7%	450
	21st to end	Day	17% 18% 29% 19% 13% 4	450
October	1st to 10th	Day	23% 20% 26% 16% 12% 2	450
	11th to 20th	Day	25% 26% 33% 12% 3	450
	21st to end	Day	33% 24% 27% 12% 3	478
November	1st to 10th	Day	26% 28% 31% 11% 4	237
	11th to 20th	Day	30% 23% 36% 6% 4 2	180
	21st to end	Day	23% 29% 41% 7% 1	180
December	1st to 10th	Day	27% 35% 28% 9%	180
	11th to 20th	Day	28% 34% 29% 7% 1 1	180
	21st to end	Day	29% 36% 28% 6% 1	198
January	1st to 10th	Night	21% 33% 31% 13% 2	896
	11th to 20th	Night	24% 26% 33% 14% 2 1	748
	21st to end	Night	16% 21% 39% 19% 4	792
February	1st to 10th	Night	22% 32% 34% 12% 1	720
	11th to 20th	Night	21% 27% 34% 14% 3	703
	21st to end	Night	22% 31% 31% 13% 3	592
March	1st to 10th	Night	19% 26% 35% 17% 3	720
	11th to 20th	Night	20% 27% 34% 15% 3	720
	21st to end	Night	20% 20% 33% 20% 6% 1	792
April	1st to 10th	Night	23% 22% 36% 17% 2	720
	11th to 20th	Night	21% 22% 28% 20% 8% 1	596
	21st to end	Night	21% 18% 31% 21% 8%	540
May	1st to 10th	Night	17% 15% 31% 28% 9% 1	540
	11th to 20th	Night	18% 12% 32% 28% 10%	409
	21st to end	Night	11% 13% 32% 32% 12% 1	389
June	1st to 10th	Night	14% 9% 39% 29% 8%	360
	11th to 20th	Night	12% 12% 29% 36% 11%	360
	21st to end	Night	13% 12% 31% 30% 13% 1	360
July	1st to 10th	Night	7% 7% 29% 39% 17% 1	350
	11th to 20th	Night	9% 8% 35% 30% 17% 1	364
	21st to end	Night	10% 6% 32% 36% 15% 1	591
August	1st to 10th	Night	10% 5% 29% 34% 21% 1	537
	11th to 20th	Night	8% 7% 22% 37% 22% 3	534
	21st to end	Night	13% 10% 29% 27% 20% 2	626
September	1st to 10th	Night	13% 11% 30% 26% 16% 4	720
	11th to 20th	Night	14% 10% 29% 29% 14% 4	720
	21st to end	Night	19% 13% 24% 28% 12% 4	720
October	1st to 10th	Night	21% 16% 32% 18% 12% 2	720
	11th to 20th	Night	25% 21% 31% 16% 6% 1	720
	21st to end	Night	26% 25% 32% 14% 3	792
November	1st to 10th	Night	24% 21% 30% 19% 5%	740
	11th to 20th	Night	23% 28% 29% 15% 4 1	760
	21st to end	Night	25% 25% 33% 14% 2	760
December	1st to 10th	Night	27% 28% 32% 10% 2 1	760
	11th to 20th	Night	21% 30% 36% 10% 3	760
	21st to end	Night	28% 31% 29% 11% 1	902

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