Shamrock Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shamrock Observatory clear sky chart from 2005-11-06 to 2024-12-24.

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	12% 5% 2 5% 4 3 4 3 5% 7% 50%	101630
	Night	11% 5% 2 4 3 3 3 3 5% 7% 55%	98583
Transparency	Day	32% 5% 10% 25% 21% 7%	105360
	Night	29% 4 9% 23% 24% 10%	102980
Seeing	Day	12% 8% 22% 28% 23% 7%	15289
	Night	13% 7% 27% 35% 16% 2	25211

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	13% 5% 2 5% 4 2 3 3 4 6% 53%	2847
	11th to 20th	Day	12% 5% 1 3 3 3 2 2 4 6% 57%	2850
	21st to end	Day	23% 10% 3 5% 2 2 1 2 2 4 45%	3135
February	1st to 10th	Day	18% 8% 3 8% 3 2 3 3 3 4 45%	1795
	11th to 20th	Day	19% 7% 3 5% 4 3 3 2 4 4 46%	1849
	21st to end	Day	21% 6% 2 4 3 3 2 3 3 4 50%	1625
March	1st to 10th	Day	15% 8% 2 4 3 2 3 3 5% 7% 46%	3436
	11th to 20th	Day	19% 7% 2 6% 4 4 4 3 5% 6% 39%	3480
	21st to end	Day	16% 8% 2 5% 4 4 4 4 6% 7% 41%	3814
April	1st to 10th	Day	14% 6% 2 5% 4 4 3 5% 5% 6% 46%	2090
	11th to 20th	Day	15% 7% 2 6% 4 4 3 3 6% 7% 44%	2187
	21st to end	Day	13% 5% 2 6% 4 5% 4 4 5% 6% 46%	2365
May	1st to 10th	Day	20% 8% 3 7% 5% 4 4 3 5% 6% 36%	4082
	11th to 20th	Day	15% 6% 2 6% 4 4 3 3 5% 5% 47%	4082
	21st to end	Day	15% 8% 3 8% 4 5% 5% 5% 6% 8% 33%	4477
June	1st to 10th	Day	6% 4 2 5% 4 5% 4 4 6% 9% 50%	2470
	11th to 20th	Day	5% 4 1 4 5% 5% 5% 4 6% 8% 52%	2470
	21st to end	Day	5% 3 2 5% 5% 3 4 4 5% 9% 54%	2470
July	1st to 10th	Day	6% 5% 1 5% 5% 4 5% 4 6% 8% 51%	4120
	11th to 20th	Day	3 3 1 4 3 3 4 3 6% 9% 61%	4120
	21st to end	Day	7% 6% 2 6% 5% 5% 5% 5% 8% 10% 41%	4465
August	1st to 10th	Day	2 1 1 2 2 2 3 3 5% 7% 71%	4104
	11th to 20th	Day	4 2 1 3 3 3 3 4 7% 9% 61%	4041
	21st to end	Day	3 3 1 4 4 4 5% 5% 7% 11% 53%	4095
September	1st to 10th	Day	7% 6% 2 5% 4 4 4 4 4 8% 52%	2090
	11th to 20th	Day	10% 5% 2 7% 4 4 4 4 5% 6% 48%	2090
	21st to end	Day	8% 4 1 4 3 4 3 2 5% 7% 59%	2090
October	1st to 10th	Day	5% 3 2 5% 3 3 4 3 4 6% 63%	2015
	11th to 20th	Day	13% 4 1 4 2 2 3 2 4 5% 60%	1900
	21st to end	Day	15% 4 2 4 2 2 2 2 3 4 59%	2090
November	1st to 10th	Day	13% 5% 2 4 3 3 4 2 4 6% 54%	1958
	11th to 20th	Day	8% 4 2 6% 4 2 3 3 4 6% 58%	1915
	21st to end	Day	16% 6% 2 4 3 3 3 2 3 6% 52%	1800
December	1st to 10th	Day	15% 6% 1 5% 3 3 4 3 5% 6% 49%	1800
	11th to 20th	Day	16% 7% 2 6% 5% 3 4 3 5% 5% 45%	2251
	21st to end	Day	17% 7% 2 5% 3 2 3 3 4 5% 48%	3162
January	1st to 10th	Night	14% 5% 2 5% 2 3 3 2 4 5% 55%	4108
	11th to 20th	Night	13% 4 1 4 2 2 2 2 3 4 62%	4120
	21st to end	Night	21% 7% 3 5% 4 3 3 2 3 4 47%	4532
February	1st to 10th	Night	15% 7% 2 5% 3 3 2 2 4 5% 50%	2401
	11th to 20th	Night	18% 6% 2 5% 4 3 3 3 4 5% 48%	2218
	21st to end	Night	19% 5% 2 4 3 3 3 2 4 5% 51%	1827
March	1st to 10th	Night	16% 8% 3 7% 4 3 4 3 4 5% 42%	3419
	11th to 20th	Night	15% 6% 3 6% 3 4 4 3 5% 6% 44%	3480
	21st to end	Night	15% 6% 2 6% 3 3 4 3 5% 7% 47%	3817
April	1st to 10th	Night	9% 5% 2 4 4 4 3 3 6% 8% 52%	2090
	11th to 20th	Night	14% 5% 2 5% 3 4 4 3 5% 6% 49%	1941
	21st to end	Night	12% 5% 2 4 3 3 3 3 5% 7% 54%	1811
May	1st to 10th	Night	19% 9% 3 7% 4 3 4 2 5% 7% 36%	2823
	11th to 20th	Night	11% 6% 2 7% 4 4 4 3 4 6% 49%	2823
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 7% 47%	3090
June	1st to 10th	Night	5% 3 1 3 3 4 4 3 6% 9% 59%	1710
	11th to 20th	Night	3 4 1 3 2 3 4 3 4 9% 65%	1710
	21st to end	Night	4 3 1 3 4 3 4 4 6% 10% 58%	1710
July	1st to 10th	Night	4 3 2 4 3 3 4 4 6% 10% 58%	2850
	11th to 20th	Night	3 3 1 3 2 1 3 2 5% 10% 67%	2850
	21st to end	Night	4 4 2 5% 4 4 5% 5% 7% 9% 52%	3098
August	1st to 10th	Night	1 1 1 1 1 2 2 4 9% 77%	2836
	11th to 20th	Night	3 2 1 3 2 2 2 3 5% 9% 67%	2798
	21st to end	Night	3 2 1 3 3 3 4 4 7% 10% 62%	3555
September	1st to 10th	Night	6% 5% 2 5% 4 3 3 3 5% 8% 56%	2090
	11th to 20th	Night	9% 5% 1 4 3 4 3 3 4 8% 55%	2090
	21st to end	Night	6% 2 1 3 3 4 3 3 5% 7% 63%	2090
October	1st to 10th	Night	6% 2 1 3 2 2 3 3 5% 7% 65%	2144
	11th to 20th	Night	12% 4 1 3 2 2 3 2 3 5% 63%	2280
	21st to end	Night	15% 5% 2 3 2 2 2 2 3 3 61%	2508
November	1st to 10th	Night	11% 5% 2 4 2 3 3 2 3 5% 60%	2340
	11th to 20th	Night	8% 4 2 4 3 3 3 3 5% 6% 59%	2480
	21st to end	Night	15% 5% 2 4 3 3 3 2 4 6% 53%	2600
December	1st to 10th	Night	14% 6% 2 5% 3 2 3 3 4 7% 52%	2600
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 4 5% 50%	3166
	21st to end	Night	17% 6% 2 5% 2 2 3 2 3 5% 53%	4578

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	33% 3 7% 19% 33% 5%	2848
	11th to 20th	Day	30% 4 8% 23% 30% 5%	2850
	21st to end	Day	46% 2 4 18% 22% 8%	3135
February	1st to 10th	Day	45% 3 6% 17% 20% 9%	1795
	11th to 20th	Day	44% 3 5% 18% 21% 9%	1859
	21st to end	Day	41% 3 5% 15% 23% 13%	1625
March	1st to 10th	Day	37% 6% 6% 17% 27% 7%	3436
	11th to 20th	Day	45% 5% 8% 21% 17% 4	3480
	21st to end	Day	41% 6% 10% 19% 18% 5%	3814
April	1st to 10th	Day	38% 5% 9% 19% 18% 10%	2090
	11th to 20th	Day	40% 6% 8% 17% 21% 8%	2187
	21st to end	Day	38% 5% 8% 20% 23% 5%	2365
May	1st to 10th	Day	49% 6% 7% 22% 13% 4	4120
	11th to 20th	Day	39% 5% 7% 25% 22% 1	4086
	21st to end	Day	45% 9% 12% 19% 11% 4	4489
June	1st to 10th	Day	28% 8% 15% 29% 17% 3	2470
	11th to 20th	Day	27% 7% 13% 29% 19% 5%	2470
	21st to end	Day	26% 7% 12% 30% 22% 3	2470
July	1st to 10th	Day	29% 8% 18% 34% 10%	4056
	11th to 20th	Day	18% 5% 13% 42% 20% 2	4051
	21st to end	Day	34% 9% 15% 31% 11%	4325
August	1st to 10th	Day	11% 4 12% 32% 36% 4	4090
	11th to 20th	Day	18% 5% 15% 41% 17% 2	4080
	21st to end	Day	23% 9% 19% 35% 13% 2	4094
September	1st to 10th	Day	31% 6% 9% 23% 20% 11%	2090
	11th to 20th	Day	35% 7% 11% 18% 22% 8%	2090
	21st to end	Day	25% 4 9% 23% 26% 13%	2090
October	1st to 10th	Day	14% 3 6% 14% 32% 31%	3353
	11th to 20th	Day	17% 2 4 22% 31% 23%	3140
	21st to end	Day	24% 3 6% 29% 29% 9%	3428
November	1st to 10th	Day	33% 4 6% 23% 24% 10%	1958
	11th to 20th	Day	28% 4 9% 19% 28% 12%	1915
	21st to end	Day	37% 3 5% 21% 25% 10%	1800
December	1st to 10th	Day	36% 5% 9% 20% 23% 7%	1800
	11th to 20th	Day	41% 4 6% 15% 25% 9%	2249
	21st to end	Day	39% 3 6% 23% 22% 7%	3162
January	1st to 10th	Night	32% 3 6% 19% 31% 9%	4108
	11th to 20th	Night	28% 4 7% 19% 30% 13%	4120
	21st to end	Night	43% 3 5% 15% 26% 8%	4532
February	1st to 10th	Night	38% 3 7% 17% 21% 13%	2401
	11th to 20th	Night	40% 3 7% 16% 25% 10%	2218
	21st to end	Night	38% 4 6% 15% 21% 16%	1827
March	1st to 10th	Night	44% 4 6% 13% 21% 11%	3419
	11th to 20th	Night	40% 6% 9% 19% 20% 6%	3480
	21st to end	Night	37% 5% 8% 18% 26% 5%	3817
April	1st to 10th	Night	30% 5% 11% 22% 19% 14%	2090
	11th to 20th	Night	35% 5% 9% 20% 18% 14%	1941
	21st to end	Night	30% 5% 9% 23% 28% 5%	1811
May	1st to 10th	Night	46% 7% 10% 19% 13% 6%	2850
	11th to 20th	Night	36% 6% 13% 19% 25% 2	2824
	21st to end	Night	34% 9% 13% 26% 12% 6%	3099
June	1st to 10th	Night	20% 8% 16% 31% 21% 4	1710
	11th to 20th	Night	18% 6% 14% 35% 20% 8%	1710
	21st to end	Night	21% 7% 12% 30% 25% 6%	1710
July	1st to 10th	Night	22% 6% 18% 40% 12% 1	2805
	11th to 20th	Night	14% 4 12% 44% 23% 3	2796
	21st to end	Night	26% 8% 18% 35% 13% 1	2999
August	1st to 10th	Night	7% 3 11% 33% 41% 6%	2826
	11th to 20th	Night	15% 4 13% 42% 22% 4	2825
	21st to end	Night	17% 7% 21% 32% 21% 2	3555
September	1st to 10th	Night	27% 5% 12% 23% 22% 11%	2090
	11th to 20th	Night	30% 5% 12% 21% 23% 9%	2090
	21st to end	Night	21% 3 9% 21% 29% 16%	2090
October	1st to 10th	Night	11% 3 6% 15% 26% 39%	3573
	11th to 20th	Night	16% 2 4 19% 31% 27%	3779
	21st to end	Night	24% 2 4 22% 31% 17%	4120
November	1st to 10th	Night	29% 3 7% 20% 27% 14%	2340
	11th to 20th	Night	27% 4 6% 20% 28% 16%	2480
	21st to end	Night	35% 3 8% 17% 26% 11%	2600
December	1st to 10th	Night	34% 4 8% 20% 23% 11%	2600
	11th to 20th	Night	38% 3 6% 16% 27% 11%	3167
	21st to end	Night	36% 3 6% 17% 27% 11%	4578

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% 13% 35% 24% 6%	270
	11th to 20th	Day	21% 17% 35% 20% 6% 1	270
	21st to end	Day	23% 14% 34% 19% 9% 1	297
February	1st to 10th	Day	23% 16% 37% 19% 6%	270
	11th to 20th	Day	27% 17% 40% 12% 4	270
	21st to end	Day	31% 24% 33% 12% 1	225
March	1st to 10th	Day	23% 17% 35% 23% 3	270
	11th to 20th	Day	24% 19% 37% 14% 4 1	270
	21st to end	Day	16% 20% 39% 19% 5% 1	297
April	1st to 10th	Day	24% 14% 36% 20% 6%	270
	11th to 20th	Day	20% 15% 33% 23% 8%	385
	21st to end	Day	15% 16% 36% 23% 9% 1	545
May	1st to 10th	Day	13% 14% 33% 31% 8% 1	653
	11th to 20th	Day	16% 6% 29% 31% 15% 3	675
	21st to end	Day	13% 7% 26% 35% 16% 3	734
June	1st to 10th	Day	7% 3 16% 36% 29% 10%	680
	11th to 20th	Day	5% 2 17% 41% 29% 6%	680
	21st to end	Day	4 1 12% 37% 34% 11%	680
July	1st to 10th	Day	6% 2 11% 30% 40% 11%	671
	11th to 20th	Day	2 1 6% 35% 44% 13%	668
	21st to end	Day	5% 1 6% 29% 46% 13%	739
August	1st to 10th	Day	3 7% 28% 48% 13%	676
	11th to 20th	Day	8% 1 5% 32% 43% 11%	665
	21st to end	Day	5% 1 5% 25% 44% 20%	493
September	1st to 10th	Day	8% 1 6% 23% 40% 22%	300
	11th to 20th	Day	5% 9% 29% 36% 21%	300
	21st to end	Day	11% 4 10% 25% 32% 18%	300
October	1st to 10th	Day	4 2 17% 35% 32% 11%	300
	11th to 20th	Day	11% 6% 26% 31% 19% 6%	300
	21st to end	Day	22% 12% 20% 25% 18% 3	330
November	1st to 10th	Day	20% 7% 29% 29% 10% 5%	300
	11th to 20th	Day	12% 9% 30% 32% 14% 2	300
	21st to end	Day	19% 14% 33% 24% 9% 2	300
December	1st to 10th	Day	19% 10% 34% 24% 10% 3	300
	11th to 20th	Day	22% 16% 35% 22% 5%	300
	21st to end	Day	22% 13% 36% 22% 7% 1	306
January	1st to 10th	Night	16% 10% 34% 32% 7%	950
	11th to 20th	Night	19% 13% 31% 30% 5% 1	950
	21st to end	Night	18% 10% 35% 31% 6%	1045
February	1st to 10th	Night	20% 12% 36% 29% 4	881
	11th to 20th	Night	21% 13% 38% 25% 3	744
	21st to end	Night	22% 15% 42% 19% 3	571
March	1st to 10th	Night	20% 11% 36% 28% 5%	570
	11th to 20th	Night	16% 12% 38% 28% 6%	570
	21st to end	Night	15% 13% 39% 27% 5%	627
April	1st to 10th	Night	11% 11% 45% 29% 4	570
	11th to 20th	Night	16% 11% 37% 27% 8% 1	563
	21st to end	Night	15% 10% 38% 27% 9% 1	570
May	1st to 10th	Night	10% 11% 38% 32% 8% 1	570
	11th to 20th	Night	10% 4 34% 36% 16% 1	561
	21st to end	Night	9% 5% 29% 39% 17% 2	617
June	1st to 10th	Night	6% 2 18% 45% 25% 4	570
	11th to 20th	Night	4 1 22% 54% 18% 1	570
	21st to end	Night	4 2 15% 41% 33% 5%	570
July	1st to 10th	Night	3 1 11% 43% 36% 5%	555
	11th to 20th	Night	3 1 9% 43% 41% 4	552
	21st to end	Night	2 1 5% 48% 41% 4	624
August	1st to 10th	Night	3 1 5% 44% 44% 4	567
	11th to 20th	Night	7% 4 41% 41% 7%	564
	21st to end	Night	4 4 40% 43% 8%	627
September	1st to 10th	Night	6% 1 10% 40% 38% 4	570
	11th to 20th	Night	12% 2 7% 40% 35% 4	570
	21st to end	Night	8% 1 12% 42% 32% 6%	570
October	1st to 10th	Night	6% 2 16% 43% 29% 4	624
	11th to 20th	Night	13% 4 25% 39% 17% 2	760
	21st to end	Night	18% 7% 28% 30% 16% 1	836
November	1st to 10th	Night	12% 5% 28% 40% 14% 1	780
	11th to 20th	Night	10% 8% 33% 37% 12% 1	880
	21st to end	Night	19% 10% 32% 28% 10% 1	1000
December	1st to 10th	Night	16% 9% 34% 34% 7%	1000
	11th to 20th	Night	21% 8% 32% 33% 6%	1000
	21st to end	Night	20% 9% 33% 32% 6%	1063

Page generated at 2024-12-21 03:13:48. Version ac07

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