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 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	13% 7% 3 5% 5% 3 5% 5% 5% 8% 41%	74895
	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 44%	74934
Transparency	Day	41% 5% 11% 23% 17% 3	74606
	Night	40% 5% 10% 23% 18% 4	74744
Seeing	Day	17% 12% 25% 24% 16% 6%	14168
	Night	18% 11% 25% 27% 17% 3	24054

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	17% 8% 4 7% 5% 3 4 4 4 7% 37%	1517
	11th to 20th	Day	20% 11% 4 7% 6% 2 3 4 5% 5% 32%	1515
	21st to end	Day	14% 9% 3 5% 4 2 3 4 4 6% 44%	1895
February	1st to 10th	Day	17% 11% 5% 6% 5% 2 4 4 5% 6% 36%	1800
	11th to 20th	Day	20% 9% 5% 7% 5% 3 4 3 4 6% 35%	1748
	21st to end	Day	20% 11% 3 7% 4 3 4 4 4 6% 34%	1480
March	1st to 10th	Day	16% 11% 2 6% 5% 3 4 4 4 5% 41%	1800
	11th to 20th	Day	21% 9% 3 6% 5% 3 4 3 4 5% 36%	1844
	21st to end	Day	18% 9% 3 7% 6% 3 5% 4 5% 7% 33%	2376
April	1st to 10th	Day	13% 6% 2 5% 5% 3 4 5% 4 7% 46%	2160
	11th to 20th	Day	16% 7% 2 6% 5% 3 4 3 5% 6% 43%	2134
	21st to end	Day	15% 7% 4 5% 6% 3 4 4 4 7% 41%	2160
May	1st to 10th	Day	17% 6% 2 5% 5% 4 4 4 5% 7% 41%	2321
	11th to 20th	Day	14% 7% 3 6% 5% 3 5% 4 5% 7% 40%	2301
	21st to end	Day	10% 6% 2 6% 7% 4 5% 6% 7% 10% 37%	2531
June	1st to 10th	Day	10% 5% 3 5% 4 3 6% 5% 7% 10% 42%	2482
	11th to 20th	Day	5% 3 2 4 6% 4 6% 6% 7% 10% 47%	2520
	21st to end	Day	5% 3 2 5% 6% 4 7% 7% 8% 11% 43%	2520
July	1st to 10th	Day	9% 5% 3 5% 7% 4 7% 7% 8% 11% 35%	2520
	11th to 20th	Day	9% 5% 2 5% 6% 4 6% 6% 7% 10% 40%	2520
	21st to end	Day	7% 5% 2 5% 6% 4 6% 6% 7% 11% 40%	2609
August	1st to 10th	Day	6% 3 3 5% 7% 4 7% 7% 8% 11% 39%	2148
	11th to 20th	Day	10% 6% 3 5% 6% 4 5% 5% 6% 9% 41%	2125
	21st to end	Day	9% 5% 2 5% 6% 5% 6% 6% 6% 9% 41%	2376
September	1st to 10th	Day	12% 5% 3 5% 5% 3 5% 6% 6% 8% 43%	2160
	11th to 20th	Day	13% 5% 2 5% 5% 4 4 5% 5% 6% 46%	2000
	21st to end	Day	12% 5% 2 4 4 3 3 4 4 7% 51%	1980
October	1st to 10th	Day	10% 5% 1 4 4 3 3 4 4 6% 56%	1980
	11th to 20th	Day	11% 7% 2 5% 3 3 3 3 4 5% 53%	1864
	21st to end	Day	20% 7% 2 4 4 2 3 4 3 5% 45%	1847
November	1st to 10th	Day	13% 5% 2 3 3 2 3 3 4 6% 57%	1672
	11th to 20th	Day	15% 6% 3 5% 4 2 3 3 4 5% 50%	1710
	21st to end	Day	16% 7% 2 5% 5% 2 4 4 3 6% 44%	1710
December	1st to 10th	Day	19% 10% 4 5% 6% 2 4 4 4 5% 38%	1710
	11th to 20th	Day	16% 10% 4 6% 5% 3 4 4 5% 6% 37%	2077
	21st to end	Day	14% 12% 3 7% 6% 3 5% 5% 7% 7% 30%	2783
January	1st to 10th	Night	17% 10% 4 7% 5% 3 4 4 3 5% 39%	2648
	11th to 20th	Night	21% 11% 4 8% 6% 3 4 4 4 4 30%	2581
	21st to end	Night	17% 9% 3 7% 5% 3 3 4 4 5% 41%	2457
February	1st to 10th	Night	22% 11% 3 8% 5% 3 4 4 4 5% 32%	2160
	11th to 20th	Night	20% 9% 4 6% 5% 3 4 4 4 4 37%	2098
	21st to end	Night	22% 10% 5% 6% 5% 2 4 5% 5% 5% 34%	1776
March	1st to 10th	Night	16% 8% 3 6% 5% 3 4 5% 5% 6% 42%	2160
	11th to 20th	Night	19% 9% 3 6% 5% 3 4 3 4 6% 38%	2116
	21st to end	Night	15% 10% 3 7% 5% 4 5% 5% 5% 6% 37%	1994
April	1st to 10th	Night	11% 5% 2 4 5% 3 4 3 5% 7% 50%	1800
	11th to 20th	Night	16% 6% 2 4 4 2 3 4 5% 7% 47%	1775
	21st to end	Night	14% 6% 2 5% 5% 3 4 4 6% 8% 42%	1800
May	1st to 10th	Night	14% 5% 3 6% 5% 2 5% 3 5% 8% 45%	1638
	11th to 20th	Night	11% 5% 3 5% 5% 4 4 5% 7% 8% 43%	1592
	21st to end	Night	6% 4 2 5% 5% 3 5% 6% 7% 10% 46%	1744
June	1st to 10th	Night	10% 6% 3 5% 4 2 4 4 6% 8% 47%	1473
	11th to 20th	Night	5% 3 1 3 4 2 5% 6% 6% 9% 57%	1440
	21st to end	Night	4 2 1 3 4 2 6% 6% 8% 13% 51%	1440
July	1st to 10th	Night	7% 4 2 6% 5% 3 4 6% 8% 10% 44%	1440
	11th to 20th	Night	7% 5% 2 3 4 3 5% 5% 7% 11% 48%	1440
	21st to end	Night	7% 2 1 4 5% 3 5% 7% 6% 11% 49%	1703
August	1st to 10th	Night	5% 4 2 5% 6% 4 4 6% 5% 9% 51%	1790
	11th to 20th	Night	7% 4 2 3 4 3 5% 5% 7% 11% 48%	1776
	21st to end	Night	7% 4 2 5% 5% 4 5% 4 7% 8% 50%	1980
September	1st to 10th	Night	11% 4 2 3 4 2 3 4 5% 7% 53%	1800
	11th to 20th	Night	10% 4 1 5% 4 3 4 5% 5% 6% 53%	1940
	21st to end	Night	9% 4 2 4 4 3 4 3 4 7% 57%	1980
October	1st to 10th	Night	10% 4 1 4 3 2 3 3 5% 6% 61%	1980
	11th to 20th	Night	16% 5% 1 3 4 2 3 3 4 6% 52%	2092
	21st to end	Night	19% 6% 2 5% 3 2 4 4 4 4 47%	2506
November	1st to 10th	Night	13% 5% 2 4 4 1 3 3 4 6% 55%	2406
	11th to 20th	Night	13% 5% 3 4 4 2 4 4 5% 6% 50%	2470
	21st to end	Night	16% 7% 3 5% 3 2 4 4 4 5% 47%	2470
December	1st to 10th	Night	19% 12% 3 6% 5% 2 4 3 4 5% 36%	2470
	11th to 20th	Night	16% 9% 3 5% 4 2 3 4 4 5% 44%	3115
	21st to end	Night	17% 12% 5% 8% 6% 4 5% 4 5% 6% 30%	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	48% 4 8% 23% 14% 3	1518
	11th to 20th	Day	54% 4 8% 19% 14% 1	1515
	21st to end	Day	42% 3 11% 26% 16% 2	1895
February	1st to 10th	Day	50% 4 9% 23% 12% 2	1800
	11th to 20th	Day	52% 3 8% 21% 12% 4	1758
	21st to end	Day	53% 3 8% 16% 14% 5%	1480
March	1st to 10th	Day	47% 3 8% 17% 19% 6%	1800
	11th to 20th	Day	52% 4 9% 18% 14% 2	1844
	21st to end	Day	51% 5% 11% 19% 13% 1	2376
April	1st to 10th	Day	38% 4 10% 23% 21% 3	2160
	11th to 20th	Day	43% 4 7% 21% 21% 4	2134
	21st to end	Day	43% 4 9% 26% 16% 2	2160
May	1st to 10th	Day	43% 5% 12% 25% 14% 2	2321
	11th to 20th	Day	44% 6% 11% 23% 15% 1	2305
	21st to end	Day	40% 8% 14% 24% 12% 2	2531
June	1st to 10th	Day	36% 7% 12% 25% 18% 3	2482
	11th to 20th	Day	31% 7% 15% 27% 18% 3	2520
	21st to end	Day	32% 9% 14% 24% 15% 5%	2520
July	1st to 10th	Day	39% 9% 16% 21% 11% 4	2451
	11th to 20th	Day	38% 7% 18% 21% 13% 4	2445
	21st to end	Day	35% 7% 18% 26% 11% 2	2464
August	1st to 10th	Day	36% 8% 16% 25% 13% 1	2136
	11th to 20th	Day	39% 6% 12% 24% 17% 2	2125
	21st to end	Day	36% 7% 14% 24% 15% 3	2376
September	1st to 10th	Day	38% 6% 11% 25% 17% 3	2160
	11th to 20th	Day	37% 6% 8% 24% 23% 3	2000
	21st to end	Day	33% 4 9% 25% 23% 6%	1980
October	1st to 10th	Day	30% 5% 10% 25% 25% 5%	1980
	11th to 20th	Day	34% 3 7% 23% 25% 7%	1864
	21st to end	Day	43% 3 6% 21% 21% 5%	1847
November	1st to 10th	Day	30% 4 9% 24% 28% 5%	1672
	11th to 20th	Day	37% 3 7% 22% 24% 7%	1710
	21st to end	Day	43% 3 6% 25% 19% 5%	1710
December	1st to 10th	Day	50% 5% 7% 21% 15% 2	1710
	11th to 20th	Day	48% 4 9% 21% 16% 2	2074
	21st to end	Day	51% 5% 11% 21% 11% 1	2783
January	1st to 10th	Night	50% 3 8% 21% 16% 2	2648
	11th to 20th	Night	58% 3 7% 17% 13% 2	2581
	21st to end	Night	47% 3 9% 23% 16% 2	2457
February	1st to 10th	Night	55% 3 9% 17% 14% 1	2160
	11th to 20th	Night	51% 4 6% 21% 15% 4	2099
	21st to end	Night	53% 4 8% 16% 15% 5%	1776
March	1st to 10th	Night	44% 5% 8% 15% 20% 8%	2160
	11th to 20th	Night	49% 3 10% 21% 15% 3	2116
	21st to end	Night	49% 4 10% 21% 14% 2	1994
April	1st to 10th	Night	36% 5% 10% 25% 21% 3	1800
	11th to 20th	Night	37% 5% 8% 22% 21% 7%	1775
	21st to end	Night	40% 5% 10% 24% 18% 2	1800
May	1st to 10th	Night	38% 5% 11% 24% 19% 3	1638
	11th to 20th	Night	37% 8% 15% 23% 15% 2	1593
	21st to end	Night	31% 9% 17% 25% 16% 2	1745
June	1st to 10th	Night	35% 5% 13% 25% 20% 3	1473
	11th to 20th	Night	23% 6% 16% 31% 19% 5%	1440
	21st to end	Night	23% 10% 18% 28% 16% 6%	1440
July	1st to 10th	Night	32% 8% 17% 26% 14% 4	1400
	11th to 20th	Night	28% 8% 19% 27% 13% 5%	1392
	21st to end	Night	27% 8% 18% 31% 15% 1	1608
August	1st to 10th	Night	29% 7% 17% 29% 17% 1	1778
	11th to 20th	Night	29% 7% 18% 27% 17% 2	1776
	21st to end	Night	31% 6% 14% 28% 15% 5%	1980
September	1st to 10th	Night	31% 5% 12% 28% 19% 5%	1800
	11th to 20th	Night	32% 5% 10% 23% 27% 4	1940
	21st to end	Night	28% 4 8% 26% 27% 6%	1980
October	1st to 10th	Night	27% 4 7% 25% 28% 8%	1980
	11th to 20th	Night	35% 3 7% 23% 24% 8%	2092
	21st to end	Night	41% 3 6% 22% 23% 5%	2506
November	1st to 10th	Night	32% 4 6% 24% 27% 7%	2406
	11th to 20th	Night	34% 3 7% 23% 25% 8%	2470
	21st to end	Night	41% 3 7% 23% 22% 4	2470
December	1st to 10th	Night	52% 3 8% 19% 16% 2	2470
	11th to 20th	Night	43% 3 7% 24% 20% 3	3117
	21st to end	Night	55% 4 8% 18% 11% 2	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	27% 27% 32% 11% 3	270
	11th to 20th	Day	30% 26% 33% 9% 2	258
	21st to end	Day	25% 30% 35% 9% 2	264
February	1st to 10th	Day	30% 28% 31% 10% 2	240
	11th to 20th	Day	39% 28% 29% 3 1	240
	21st to end	Day	33% 30% 28% 8% 1	198
March	1st to 10th	Day	23% 29% 35% 12% 1	240
	11th to 20th	Day	29% 28% 27% 13% 2 1	284
	21st to end	Day	26% 22% 30% 16% 5%	462
April	1st to 10th	Day	21% 19% 32% 22% 6% 1	420
	11th to 20th	Day	22% 17% 30% 23% 6% 2	412
	21st to end	Day	22% 16% 34% 18% 9% 1	420
May	1st to 10th	Day	22% 17% 29% 22% 7% 3	423
	11th to 20th	Day	15% 10% 32% 26% 13% 4	447
	21st to end	Day	13% 5% 24% 29% 18% 11%	485
June	1st to 10th	Day	12% 6% 23% 27% 21% 12%	592
	11th to 20th	Day	5% 5% 21% 36% 24% 9%	630
	21st to end	Day	6% 3 26% 33% 22% 10%	630
July	1st to 10th	Day	10% 3 15% 29% 30% 13%	621
	11th to 20th	Day	8% 5% 12% 32% 31% 11%	618
	21st to end	Day	7% 3 17% 31% 29% 13%	556
August	1st to 10th	Day	9% 4 13% 35% 30% 10%	446
	11th to 20th	Day	13% 2 16% 33% 30% 6%	439
	21st to end	Day	12% 3 12% 26% 31% 16%	495
September	1st to 10th	Day	10% 2 16% 31% 29% 12%	450
	11th to 20th	Day	11% 3 14% 32% 28% 13%	450
	21st to end	Day	16% 6% 17% 29% 24% 7%	450
October	1st to 10th	Day	12% 5% 23% 33% 22% 6%	450
	11th to 20th	Day	13% 18% 34% 24% 9% 1	334
	21st to end	Day	29% 11% 34% 20% 6%	297
November	1st to 10th	Day	17% 13% 34% 24% 9% 2	270
	11th to 20th	Day	17% 20% 37% 23% 3 1	270
	21st to end	Day	24% 17% 32% 17% 9% 1	270
December	1st to 10th	Day	26% 22% 30% 18% 4	270
	11th to 20th	Day	22% 23% 33% 16% 6%	270
	21st to end	Day	28% 24% 35% 11% 2	297
January	1st to 10th	Night	24% 21% 31% 21% 3	950
	11th to 20th	Night	28% 21% 29% 19% 4	933
	21st to end	Night	22% 18% 38% 20% 1	873
February	1st to 10th	Night	29% 17% 32% 19% 3	720
	11th to 20th	Night	27% 21% 31% 18% 1	703
	21st to end	Night	30% 21% 32% 15% 2	592
March	1st to 10th	Night	23% 16% 33% 25% 4	720
	11th to 20th	Night	24% 15% 33% 21% 5% 1	720
	21st to end	Night	21% 15% 30% 27% 6%	608
April	1st to 10th	Night	14% 13% 39% 25% 8%	540
	11th to 20th	Night	20% 11% 30% 27% 11% 1	533
	21st to end	Night	18% 10% 35% 27% 10% 1	540
May	1st to 10th	Night	17% 9% 34% 25% 13% 2	540
	11th to 20th	Night	14% 6% 29% 29% 19% 3	531
	21st to end	Night	8% 3 22% 35% 27% 6%	584
June	1st to 10th	Night	12% 3 16% 34% 30% 4	540
	11th to 20th	Night	5% 2 12% 42% 34% 4	540
	21st to end	Night	4 2 17% 38% 34% 6%	540
July	1st to 10th	Night	8% 2 10% 30% 40% 10%	525
	11th to 20th	Night	7% 5% 11% 27% 39% 11%	522
	21st to end	Night	6% 2 9% 36% 37% 9%	591
August	1st to 10th	Night	6% 2 8% 30% 47% 7%	537
	11th to 20th	Night	8% 3 12% 38% 35% 4	534
	21st to end	Night	8% 2 10% 32% 39% 9%	594
September	1st to 10th	Night	12% 2 13% 31% 37% 5%	540
	11th to 20th	Night	12% 3 15% 29% 35% 7%	680
	21st to end	Night	10% 3 20% 32% 31% 4	720
October	1st to 10th	Night	11% 6% 20% 31% 28% 5%	720
	11th to 20th	Night	18% 8% 26% 32% 15% 1	720
	21st to end	Night	22% 10% 26% 29% 11% 2	792
November	1st to 10th	Night	15% 9% 26% 33% 15% 1	740
	11th to 20th	Night	15% 15% 34% 27% 9% 1	760
	21st to end	Night	21% 13% 29% 25% 11%	760
December	1st to 10th	Night	27% 14% 30% 24% 5%	760
	11th to 20th	Night	24% 14% 29% 27% 6%	807
	21st to end	Night	27% 16% 30% 22% 4	1045

Page generated at 2024-01-13 22:56:38. Version ac07

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