Erin
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Erin clear sky chart from 2005-11-06 to 2026-04-17.

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	13% 7% 3 7% 4 4 4 4 5% 8% 41%	131373
	Night	14% 8% 3 7% 4 4 4 3 5% 7% 43%	130927
Transparency	Day	41% 4 10% 24% 17% 3	135036
	Night	41% 4 10% 23% 18% 4	135338
Seeing	Day	17% 13% 26% 24% 15% 5%	17183
	Night	18% 11% 26% 27% 16% 2	27043

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% 12% 6% 10% 4 4 3 3 4 6% 33%	4691
	11th to 20th	Day	12% 8% 3 9% 3 3 2 2 3 4 50%	4807
	21st to end	Day	20% 9% 3 6% 4 4 3 3 4 6% 39%	6182
February	1st to 10th	Day	17% 11% 4 7% 4 3 4 3 5% 7% 36%	2100
	11th to 20th	Day	20% 10% 4 8% 4 4 3 2 4 5% 35%	2048
	21st to end	Day	18% 11% 3 8% 4 4 4 3 4 5% 36%	1730
March	1st to 10th	Day	16% 14% 3 8% 4 3 4 2 4 4 37%	4580
	11th to 20th	Day	13% 9% 2 6% 4 3 4 3 4 5% 48%	4750
	21st to end	Day	12% 7% 2 7% 5% 5% 4 3 5% 7% 43%	6099
April	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 4 6% 45%	2520
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 3 5% 6% 42%	2446
	21st to end	Day	14% 8% 3 6% 5% 5% 4 3 5% 7% 40%	2400
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 7% 8% 27%	5815
	11th to 20th	Day	15% 9% 3 9% 6% 5% 4 4 6% 7% 32%	5861
	21st to end	Day	16% 7% 2 7% 5% 4 4 4 5% 8% 38%	6433
June	1st to 10th	Day	10% 6% 3 5% 4 5% 6% 4 7% 9% 42%	2758
	11th to 20th	Day	6% 4 2 6% 6% 6% 5% 5% 7% 9% 45%	2800
	21st to end	Day	5% 4 1 6% 5% 6% 7% 5% 8% 11% 42%	2800
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 11% 40%	6360
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 12% 39%	6360
	21st to end	Day	11% 7% 3 8% 6% 5% 6% 5% 7% 11% 32%	6557
August	1st to 10th	Day	5% 3 2 4 4 4 5% 4 7% 9% 54%	3905
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 6% 10% 43%	3849
	21st to end	Day	5% 4 1 5% 4 5% 4 3 6% 8% 56%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 4 5% 8% 44%	2400
	11th to 20th	Day	13% 5% 2 5% 5% 5% 5% 3 5% 6% 47%	2222
	21st to end	Day	12% 6% 2 6% 4 4 4 3 4 7% 49%	2200
October	1st to 10th	Day	10% 5% 1 4 3 3 3 3 4 6% 58%	2200
	11th to 20th	Day	11% 6% 2 5% 2 3 3 3 4 5% 56%	2070
	21st to end	Day	20% 7% 2 4 3 3 3 3 3 5% 46%	2051
November	1st to 10th	Day	13% 6% 2 4 3 3 3 3 4 6% 54%	1852
	11th to 20th	Day	14% 7% 3 5% 3 3 4 3 4 6% 49%	1890
	21st to end	Day	16% 8% 2 6% 4 4 4 3 4 6% 44%	1890
December	1st to 10th	Day	15% 9% 3 6% 4 3 4 3 4 5% 44%	3020
	11th to 20th	Day	18% 10% 4 8% 4 3 4 3 4 6% 35%	3345
	21st to end	Day	16% 11% 3 9% 5% 5% 5% 4 7% 6% 29%	4070
January	1st to 10th	Night	17% 11% 3 8% 3 3 3 3 4 5% 39%	8232
	11th to 20th	Night	14% 9% 4 9% 4 4 3 2 3 4 44%	8180
	21st to end	Night	20% 10% 5% 8% 5% 3 3 2 3 4 37%	8008
February	1st to 10th	Night	20% 11% 3 9% 5% 4 4 3 4 5% 33%	2520
	11th to 20th	Night	21% 10% 4 7% 4 3 3 3 4 5% 36%	2458
	21st to end	Night	20% 10% 4 6% 4 3 4 4 5% 5% 37%	2076
March	1st to 10th	Night	18% 12% 3 7% 3 3 4 3 5% 6% 36%	5493
	11th to 20th	Night	12% 7% 2 5% 4 4 4 3 5% 5% 49%	5430
	21st to end	Night	13% 8% 2 7% 4 4 4 3 5% 8% 40%	5101
April	1st to 10th	Night	12% 6% 2 5% 5% 4 4 3 5% 7% 47%	2100
	11th to 20th	Night	15% 7% 2 5% 3 3 3 3 5% 7% 48%	2035
	21st to end	Night	14% 7% 2 6% 5% 4 3 3 6% 8% 41%	2000
May	1st to 10th	Night	16% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	4079
	11th to 20th	Night	15% 9% 3 9% 5% 5% 5% 4 6% 7% 34%	4032
	21st to end	Night	14% 7% 3 7% 4 4 4 4 7% 8% 38%	4421
June	1st to 10th	Night	11% 6% 3 6% 3 4 4 3 6% 8% 46%	1636
	11th to 20th	Night	5% 4 2 4 4 4 5% 5% 7% 8% 54%	1600
	21st to end	Night	4 3 1 4 3 4 6% 5% 8% 12% 50%	1600
July	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 5% 7% 10% 44%	3618
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 8% 13% 45%	3617
	21st to end	Night	8% 5% 2 7% 5% 5% 6% 5% 8% 14% 34%	4268
August	1st to 10th	Night	4 3 1 4 3 3 4 3 5% 7% 62%	3244
	11th to 20th	Night	5% 4 2 5% 3 4 5% 4 7% 11% 51%	3200
	21st to end	Night	4 3 1 4 3 4 4 3 6% 8% 59%	3586
September	1st to 10th	Night	11% 5% 2 4 3 3 4 3 5% 7% 54%	2000
	11th to 20th	Night	10% 4 1 6% 3 3 4 4 5% 6% 53%	2156
	21st to end	Night	10% 5% 2 5% 4 4 4 2 4 7% 54%	2200
October	1st to 10th	Night	10% 3 1 4 3 3 2 3 4 5% 62%	2200
	11th to 20th	Night	15% 5% 1 4 3 2 3 3 4 5% 56%	2325
	21st to end	Night	18% 6% 2 5% 3 3 3 3 4 4 48%	2786
November	1st to 10th	Night	14% 6% 2 5% 4 2 3 2 4 6% 53%	2666
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 6% 50%	2730
	21st to end	Night	17% 8% 3 5% 3 3 3 3 4 5% 46%	2730
December	1st to 10th	Night	17% 11% 2 8% 3 3 3 2 3 5% 42%	4380
	11th to 20th	Night	19% 10% 2 6% 4 3 3 3 4 4 42%	5070
	21st to end	Night	17% 13% 5% 9% 5% 5% 5% 3 5% 5% 28%	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	54% 3 10% 22% 10% 1	4692
	11th to 20th	Day	40% 3 4 28% 25% 1	4807
	21st to end	Day	47% 2 7% 23% 15% 6%	6182
February	1st to 10th	Day	50% 3 9% 23% 13% 2	2100
	11th to 20th	Day	52% 3 8% 20% 13% 4	2058
	21st to end	Day	51% 3 8% 17% 15% 5%	1730
March	1st to 10th	Day	53% 2 4 16% 16% 8%	4580
	11th to 20th	Day	41% 3 8% 22% 20% 7%	4750
	21st to end	Day	41% 4 10% 26% 18% 1	6101
April	1st to 10th	Day	39% 4 10% 23% 20% 3	2520
	11th to 20th	Day	43% 3 8% 21% 20% 4	2446
	21st to end	Day	43% 4 9% 26% 16% 2	2400
May	1st to 10th	Day	54% 6% 12% 19% 9% 1	5851
	11th to 20th	Day	51% 6% 13% 19% 10% 1	5865
	21st to end	Day	44% 5% 8% 23% 16% 3	6447
June	1st to 10th	Day	37% 7% 11% 24% 18% 3	2758
	11th to 20th	Day	33% 6% 14% 26% 18% 3	2800
	21st to end	Day	32% 9% 15% 24% 16% 4	2800
July	1st to 10th	Day	36% 6% 16% 28% 11% 2	6291
	11th to 20th	Day	35% 6% 20% 30% 7% 1	6285
	21st to end	Day	45% 5% 21% 22% 6% 1	6370
August	1st to 10th	Day	25% 6% 15% 30% 23% 2	3893
	11th to 20th	Day	35% 5% 13% 27% 18% 2	3885
	21st to end	Day	25% 6% 10% 33% 24% 2	4311
September	1st to 10th	Day	37% 6% 10% 24% 18% 4	2400
	11th to 20th	Day	36% 5% 8% 24% 23% 2	2222
	21st to end	Day	36% 4 9% 23% 22% 6%	2200
October	1st to 10th	Day	20% 3 6% 17% 44% 10%	3590
	11th to 20th	Day	23% 2 7% 25% 29% 14%	3349
	21st to end	Day	32% 2 5% 25% 29% 7%	3289
November	1st to 10th	Day	33% 4 8% 25% 26% 5%	1852
	11th to 20th	Day	38% 3 8% 22% 24% 6%	1890
	21st to end	Day	43% 3 6% 25% 19% 4	1890
December	1st to 10th	Day	45% 3 5% 27% 18% 1	3020
	11th to 20th	Day	51% 3 8% 21% 14% 3	3342
	21st to end	Day	53% 4 10% 23% 9% 1	4070
January	1st to 10th	Night	50% 2 12% 24% 12% 1	8233
	11th to 20th	Night	45% 3 5% 21% 23% 3	8180
	21st to end	Night	53% 2 7% 20% 14% 4	8008
February	1st to 10th	Night	55% 3 9% 17% 14% 2	2520
	11th to 20th	Night	52% 3 5% 20% 15% 3	2459
	21st to end	Night	49% 4 8% 16% 18% 4	2076
March	1st to 10th	Night	50% 3 6% 15% 18% 7%	5493
	11th to 20th	Night	38% 3 8% 21% 17% 13%	5430
	21st to end	Night	43% 3 9% 25% 17% 3	5101
April	1st to 10th	Night	38% 4 10% 24% 20% 3	2100
	11th to 20th	Night	37% 4 9% 22% 21% 7%	2035
	21st to end	Night	41% 5% 11% 24% 18% 2	2000
May	1st to 10th	Night	51% 5% 13% 20% 10% 1	4106
	11th to 20th	Night	51% 5% 14% 18% 11% 1	4033
	21st to end	Night	43% 7% 12% 25% 13% 1	4429
June	1st to 10th	Night	36% 5% 12% 24% 20% 3	1636
	11th to 20th	Night	25% 6% 16% 29% 21% 4	1600
	21st to end	Night	23% 10% 18% 28% 16% 5%	1600
July	1st to 10th	Night	33% 7% 16% 28% 15% 2	3580
	11th to 20th	Night	29% 8% 21% 33% 8% 2	3572
	21st to end	Night	40% 6% 22% 23% 7% 1	4142
August	1st to 10th	Night	21% 6% 12% 39% 22% 1	3232
	11th to 20th	Night	26% 6% 18% 32% 17% 2	3230
	21st to end	Night	23% 5% 11% 33% 25% 3	3586
September	1st to 10th	Night	30% 5% 12% 27% 21% 6%	2000
	11th to 20th	Night	32% 5% 9% 23% 27% 4	2156
	21st to end	Night	31% 4 9% 24% 26% 6%	2200
October	1st to 10th	Night	19% 3 5% 18% 43% 12%	3590
	11th to 20th	Night	21% 2 4 22% 35% 16%	3785
	21st to end	Night	29% 2 6% 25% 28% 11%	4498
November	1st to 10th	Night	34% 3 6% 25% 25% 6%	2666
	11th to 20th	Night	34% 3 7% 23% 25% 8%	2730
	21st to end	Night	42% 3 7% 24% 20% 4	2730
December	1st to 10th	Night	47% 3 7% 20% 22% 2	4380
	11th to 20th	Night	46% 2 6% 24% 19% 2	5072
	21st to end	Night	58% 4 8% 20% 9% 2	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	28% 25% 33% 12% 3	330
	11th to 20th	Day	30% 26% 34% 8% 1	330
	21st to end	Day	28% 28% 34% 9% 1	363
February	1st to 10th	Day	27% 28% 33% 11% 2	330
	11th to 20th	Day	37% 30% 28% 4 2	330
	21st to end	Day	33% 29% 29% 8% 1	273
March	1st to 10th	Day	27% 28% 32% 11% 2	330
	11th to 20th	Day	29% 28% 29% 12% 2 1	382
	21st to end	Day	25% 22% 35% 14% 4	594
April	1st to 10th	Day	23% 19% 31% 21% 5% 1	540
	11th to 20th	Day	22% 20% 29% 21% 5% 1	516
	21st to end	Day	22% 17% 34% 18% 9% 1	500
May	1st to 10th	Day	23% 17% 29% 21% 7% 2	503
	11th to 20th	Day	16% 11% 32% 25% 13% 4	527
	21st to end	Day	14% 8% 27% 26% 16% 9%	573
June	1st to 10th	Day	12% 7% 25% 27% 19% 11%	688
	11th to 20th	Day	6% 5% 21% 35% 24% 9%	730
	21st to end	Day	6% 3 24% 34% 22% 10%	730
July	1st to 10th	Day	10% 3 16% 31% 29% 12%	721
	11th to 20th	Day	8% 5% 14% 33% 30% 10%	718
	21st to end	Day	9% 4 16% 30% 29% 13%	651
August	1st to 10th	Day	9% 4 13% 34% 29% 10%	526
	11th to 20th	Day	11% 3 18% 31% 30% 7%	519
	21st to end	Day	10% 3 14% 27% 31% 16%	583
September	1st to 10th	Day	9% 3 19% 32% 27% 11%	530
	11th to 20th	Day	11% 3 13% 33% 28% 12%	530
	21st to end	Day	17% 7% 19% 28% 22% 6%	530
October	1st to 10th	Day	11% 5% 22% 34% 23% 6%	530
	11th to 20th	Day	12% 16% 33% 26% 12% 2	400
	21st to end	Day	28% 11% 37% 18% 6%	363
November	1st to 10th	Day	20% 15% 32% 23% 8% 2	330
	11th to 20th	Day	17% 22% 36% 22% 3 1	330
	21st to end	Day	24% 17% 34% 16% 8% 1	330
December	1st to 10th	Day	26% 22% 32% 16% 3	330
	11th to 20th	Day	24% 22% 35% 15% 5%	330
	21st to end	Day	28% 23% 36% 11% 2	363
January	1st to 10th	Night	24% 21% 31% 22% 2	1050
	11th to 20th	Night	27% 22% 30% 18% 3	1050
	21st to end	Night	23% 18% 38% 20% 1	1018
February	1st to 10th	Night	28% 17% 34% 19% 3	840
	11th to 20th	Night	30% 20% 31% 18% 1	823
	21st to end	Night	29% 21% 32% 17% 2	692
March	1st to 10th	Night	24% 17% 32% 24% 4	840
	11th to 20th	Night	24% 15% 35% 21% 5% 1	840
	21st to end	Night	20% 16% 34% 25% 6%	709
April	1st to 10th	Night	17% 14% 39% 23% 7%	630
	11th to 20th	Night	20% 12% 31% 27% 9% 1	611
	21st to end	Night	18% 11% 35% 26% 10% 1	600
May	1st to 10th	Night	17% 9% 34% 25% 13% 2	600
	11th to 20th	Night	15% 7% 28% 30% 18% 3	591
	21st to end	Night	9% 4 23% 34% 25% 5%	650
June	1st to 10th	Night	12% 4 18% 34% 29% 3	600
	11th to 20th	Night	6% 3 13% 41% 34% 4	600
	21st to end	Night	4 2 16% 38% 35% 6%	600
July	1st to 10th	Night	8% 2 10% 31% 40% 10%	585
	11th to 20th	Night	7% 5% 11% 29% 39% 10%	582
	21st to end	Night	7% 3 9% 36% 36% 9%	657
August	1st to 10th	Night	6% 2 9% 30% 46% 7%	597
	11th to 20th	Night	8% 3 13% 38% 35% 4	594
	21st to end	Night	8% 2 10% 34% 37% 9%	660
September	1st to 10th	Night	12% 2 14% 33% 35% 5%	600
	11th to 20th	Night	12% 2 14% 29% 36% 6%	756
	21st to end	Night	11% 3 20% 32% 29% 4	800
October	1st to 10th	Night	10% 6% 20% 32% 28% 5%	800
	11th to 20th	Night	17% 7% 25% 34% 16% 1	800
	21st to end	Night	22% 9% 27% 29% 12% 1	880
November	1st to 10th	Night	16% 9% 27% 33% 14% 1	820
	11th to 20th	Night	15% 14% 34% 28% 9% 1	840
	21st to end	Night	22% 13% 29% 25% 10%	840
December	1st to 10th	Night	27% 14% 30% 24% 5%	840
	11th to 20th	Night	24% 14% 31% 26% 6%	893
	21st to end	Night	28% 16% 30% 22% 4	1155

Page generated at 2026-04-14 01:44:31. Version ac07

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