Shady Grove Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shady Grove Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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	11% 6% 2 5% 4 4 4 3 5% 7% 49%	101495
	Night	11% 5% 2 5% 3 3 3 3 5% 6% 54%	98204
Transparency	Day	33% 5% 11% 25% 21% 5%	105186
	Night	30% 5% 10% 24% 24% 8%	102640
Seeing	Day	13% 7% 22% 29% 22% 7%	15442
	Night	13% 8% 27% 34% 17% 2	25305

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	12% 10% 4 7% 3 3 3 3 4 5% 45%	2847
	11th to 20th	Day	9% 6% 2 5% 3 4 3 2 4 6% 57%	2850
	21st to end	Day	21% 10% 2 5% 3 2 2 2 3 5% 45%	3135
February	1st to 10th	Day	17% 9% 3 7% 4 3 3 2 4 5% 42%	1710
	11th to 20th	Day	15% 8% 3 5% 4 4 3 3 4 5% 45%	1811
	21st to end	Day	18% 9% 3 5% 3 3 3 3 4 6% 43%	1633
March	1st to 10th	Day	16% 7% 2 5% 3 4 3 2 3 4 51%	3436
	11th to 20th	Day	13% 7% 2 6% 4 4 4 3 5% 6% 48%	3480
	21st to end	Day	17% 10% 2 7% 4 4 4 3 5% 7% 38%	3814
April	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 5% 7% 48%	2095
	11th to 20th	Day	16% 6% 2 5% 4 4 3 3 5% 6% 46%	2254
	21st to end	Day	17% 7% 2 7% 4 4 3 3 4 6% 42%	2382
May	1st to 10th	Day	19% 8% 3 6% 4 4 3 3 5% 5% 41%	4082
	11th to 20th	Day	12% 6% 2 7% 5% 4 4 4 5% 6% 44%	4082
	21st to end	Day	16% 6% 3 6% 5% 5% 5% 4 5% 7% 37%	4477
June	1st to 10th	Day	8% 5% 2 5% 4 3 4 4 7% 9% 48%	2470
	11th to 20th	Day	6% 4 2 5% 5% 4 4 4 7% 9% 50%	2470
	21st to end	Day	4 3 1 3 4 3 4 3 5% 8% 59%	2470
July	1st to 10th	Day	7% 4 2 5% 4 4 5% 5% 7% 9% 49%	4120
	11th to 20th	Day	4 4 2 5% 3 3 4 4 6% 9% 57%	4120
	21st to end	Day	5% 4 1 5% 4 4 5% 5% 7% 10% 50%	4465
August	1st to 10th	Day	3 3 1 5% 3 4 4 3 6% 8% 60%	4104
	11th to 20th	Day	8% 3 2 5% 3 4 4 4 6% 8% 53%	4041
	21st to end	Day	4 3 1 4 4 4 5% 4 6% 8% 55%	4250
September	1st to 10th	Day	11% 5% 1 4 3 4 4 4 5% 6% 53%	2134
	11th to 20th	Day	11% 4 2 6% 4 4 5% 3 6% 7% 50%	2090
	21st to end	Day	8% 3 1 5% 3 4 5% 3 5% 6% 57%	2090
October	1st to 10th	Day	6% 4 1 5% 4 4 3 3 5% 6% 59%	1962
	11th to 20th	Day	11% 4 1 4 2 2 3 1 3 4 63%	1900
	21st to end	Day	16% 6% 2 5% 3 3 3 2 4 4 54%	2090
November	1st to 10th	Day	11% 6% 2 4 3 2 3 4 5% 5% 56%	1958
	11th to 20th	Day	10% 5% 2 4 2 3 3 2 4 5% 61%	1871
	21st to end	Day	15% 6% 2 3 2 2 3 3 4 6% 54%	1796
December	1st to 10th	Day	14% 6% 1 6% 4 2 3 3 4 6% 50%	1710
	11th to 20th	Day	16% 8% 3 6% 4 4 4 2 4 5% 45%	2161
	21st to end	Day	17% 9% 2 6% 5% 4 5% 3 5% 6% 38%	3135
January	1st to 10th	Night	13% 7% 3 7% 4 4 3 3 4 5% 47%	4108
	11th to 20th	Night	10% 5% 2 4 3 4 3 2 4 6% 57%	4120
	21st to end	Night	23% 7% 2 4 2 2 2 2 4 4 47%	4532
February	1st to 10th	Night	15% 8% 2 6% 4 3 3 2 4 6% 47%	2470
	11th to 20th	Night	15% 8% 3 5% 3 3 2 2 4 6% 49%	2271
	21st to end	Night	14% 9% 2 6% 3 3 3 2 4 5% 49%	1819
March	1st to 10th	Night	18% 7% 2 6% 4 3 3 3 4 5% 46%	3419
	11th to 20th	Night	12% 6% 2 4 3 3 3 2 4 5% 56%	3480
	21st to end	Night	14% 6% 2 6% 3 4 4 3 5% 6% 46%	3817
April	1st to 10th	Night	11% 5% 3 4 4 3 3 4 5% 6% 51%	2085
	11th to 20th	Night	12% 6% 1 5% 4 3 3 2 4 7% 53%	1875
	21st to end	Night	14% 6% 2 6% 4 3 3 2 5% 7% 49%	1794
May	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 5% 8% 43%	2823
	11th to 20th	Night	13% 5% 2 7% 4 4 4 3 5% 7% 47%	2823
	21st to end	Night	12% 5% 2 5% 4 4 4 4 6% 9% 46%	3090
June	1st to 10th	Night	4 3 1 5% 3 3 3 3 6% 9% 59%	1710
	11th to 20th	Night	5% 3 1 3 3 3 3 3 5% 9% 61%	1710
	21st to end	Night	2 2 1 3 3 3 3 3 5% 8% 68%	1710
July	1st to 10th	Night	8% 4 2 5% 4 4 3 4 5% 8% 54%	2850
	11th to 20th	Night	3 2 1 3 2 2 2 3 5% 9% 68%	2850
	21st to end	Night	4 3 1 4 3 3 4 4 7% 10% 57%	3098
August	1st to 10th	Night	2 1 1 3 3 4 4 4 5% 8% 67%	2836
	11th to 20th	Night	4 3 1 3 3 4 3 4 6% 10% 60%	2798
	21st to end	Night	2 2 1 4 3 3 3 4 5% 8% 63%	3405
September	1st to 10th	Night	8% 5% 1 4 3 3 4 3 4 6% 59%	2042
	11th to 20th	Night	10% 4 2 5% 4 3 4 4 6% 6% 53%	2090
	21st to end	Night	7% 3 2 4 2 3 3 3 4 5% 63%	2090
October	1st to 10th	Night	7% 3 2 4 3 3 3 3 4 6% 62%	2198
	11th to 20th	Night	12% 4 1 4 3 2 2 2 3 3 63%	2280
	21st to end	Night	16% 5% 2 4 2 3 2 2 3 4 57%	2508
November	1st to 10th	Night	11% 5% 2 3 3 2 2 3 3 5% 61%	2340
	11th to 20th	Night	8% 5% 2 4 3 3 3 3 4 5% 61%	2525
	21st to end	Night	14% 6% 1 4 3 2 3 3 4 5% 56%	2600
December	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 5% 5% 52%	2470
	11th to 20th	Night	14% 6% 3 6% 3 3 3 3 3 5% 51%	3036
	21st to end	Night	16% 8% 2 5% 3 3 3 3 4 5% 46%	4532

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	42% 3 7% 23% 23% 2	2848
	11th to 20th	Day	30% 6% 13% 25% 23% 3	2850
	21st to end	Day	45% 4 6% 18% 24% 3	3135
February	1st to 10th	Day	46% 4 10% 19% 17% 4	1710
	11th to 20th	Day	42% 4 8% 23% 18% 5%	1821
	21st to end	Day	44% 3 6% 17% 22% 8%	1633
March	1st to 10th	Day	38% 4 7% 17% 22% 12%	3436
	11th to 20th	Day	37% 6% 11% 24% 17% 5%	3480
	21st to end	Day	45% 6% 9% 26% 12% 2	3814
April	1st to 10th	Day	36% 5% 11% 22% 19% 7%	2095
	11th to 20th	Day	39% 4 11% 22% 18% 6%	2254
	21st to end	Day	43% 6% 10% 20% 19% 2	2382
May	1st to 10th	Day	44% 6% 9% 21% 17% 3	4120
	11th to 20th	Day	38% 7% 12% 24% 18% 1	4086
	21st to end	Day	44% 8% 12% 19% 14% 4	4489
June	1st to 10th	Day	30% 8% 16% 29% 14% 3	2470
	11th to 20th	Day	29% 7% 13% 29% 20% 2	2470
	21st to end	Day	23% 6% 15% 30% 22% 4	2470
July	1st to 10th	Day	30% 6% 16% 31% 15% 2	4056
	11th to 20th	Day	23% 4 11% 36% 23% 2	4051
	21st to end	Day	27% 7% 16% 34% 16%	4325
August	1st to 10th	Day	21% 6% 12% 27% 29% 5%	4090
	11th to 20th	Day	28% 5% 16% 33% 15% 2	4080
	21st to end	Day	24% 7% 15% 38% 14% 2	4249
September	1st to 10th	Day	31% 5% 8% 23% 23% 10%	2134
	11th to 20th	Day	32% 7% 10% 23% 21% 7%	2090
	21st to end	Day	26% 6% 9% 23% 29% 8%	2090
October	1st to 10th	Day	17% 3 6% 12% 39% 22%	3261
	11th to 20th	Day	17% 2 4 19% 39% 19%	3140
	21st to end	Day	28% 3 7% 29% 27% 7%	3428
November	1st to 10th	Day	28% 5% 9% 24% 27% 7%	1958
	11th to 20th	Day	28% 3 8% 26% 25% 10%	1871
	21st to end	Day	31% 3 10% 22% 27% 7%	1796
December	1st to 10th	Day	37% 4 10% 25% 20% 3	1710
	11th to 20th	Day	42% 4 6% 19% 24% 6%	2159
	21st to end	Day	46% 7% 8% 19% 18% 3	3135
January	1st to 10th	Night	40% 5% 6% 21% 25% 4	4108
	11th to 20th	Night	34% 5% 11% 18% 26% 4	4120
	21st to end	Night	42% 4 7% 19% 24% 5%	4532
February	1st to 10th	Night	40% 3 10% 22% 18% 6%	2470
	11th to 20th	Night	40% 4 7% 23% 21% 6%	2271
	21st to end	Night	37% 4 7% 18% 23% 10%	1819
March	1st to 10th	Night	40% 5% 7% 18% 18% 12%	3419
	11th to 20th	Night	32% 4 10% 30% 17% 7%	3480
	21st to end	Night	38% 6% 10% 25% 19% 2	3817
April	1st to 10th	Night	32% 6% 11% 20% 19% 12%	2085
	11th to 20th	Night	33% 5% 9% 25% 17% 11%	1875
	21st to end	Night	36% 6% 9% 25% 22% 3	1794
May	1st to 10th	Night	37% 8% 12% 21% 16% 6%	2850
	11th to 20th	Night	36% 7% 13% 26% 16% 2	2824
	21st to end	Night	34% 8% 15% 23% 16% 4	3099
June	1st to 10th	Night	21% 7% 16% 32% 20% 4	1710
	11th to 20th	Night	20% 5% 13% 32% 25% 4	1710
	21st to end	Night	15% 5% 15% 31% 27% 7%	1710
July	1st to 10th	Night	28% 6% 16% 29% 17% 4	2805
	11th to 20th	Night	15% 4 10% 42% 26% 4	2796
	21st to end	Night	21% 5% 19% 33% 20% 1	2999
August	1st to 10th	Night	15% 5% 12% 26% 33% 9%	2826
	11th to 20th	Night	20% 4 17% 34% 22% 3	2825
	21st to end	Night	18% 5% 12% 39% 22% 3	3405
September	1st to 10th	Night	26% 5% 9% 25% 23% 12%	2042
	11th to 20th	Night	29% 6% 10% 21% 24% 9%	2090
	21st to end	Night	23% 4 7% 24% 32% 9%	2090
October	1st to 10th	Night	14% 2 6% 14% 36% 27%	3666
	11th to 20th	Night	19% 2 3 13% 43% 20%	3779
	21st to end	Night	28% 3 4 30% 25% 10%	4120
November	1st to 10th	Night	28% 4 7% 24% 27% 10%	2340
	11th to 20th	Night	28% 3 8% 22% 28% 11%	2525
	21st to end	Night	32% 4 8% 21% 28% 7%	2600
December	1st to 10th	Night	35% 4 10% 23% 22% 8%	2470
	11th to 20th	Night	38% 4 7% 20% 24% 7%	3037
	21st to end	Night	39% 6% 9% 21% 19% 6%	4532

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	20% 14% 34% 23% 8% 1	270
	11th to 20th	Day	21% 19% 35% 20% 5%	270
	21st to end	Day	19% 13% 37% 22% 8% 1	297
February	1st to 10th	Day	27% 12% 34% 20% 7%	270
	11th to 20th	Day	23% 19% 35% 18% 4	270
	21st to end	Day	27% 17% 40% 13% 3	225
March	1st to 10th	Day	19% 15% 36% 23% 7% 1	270
	11th to 20th	Day	24% 16% 36% 17% 5% 1	270
	21st to end	Day	20% 15% 41% 18% 5% 1	297
April	1st to 10th	Day	17% 16% 39% 19% 7% 2	275
	11th to 20th	Day	22% 12% 34% 26% 6%	452
	21st to end	Day	19% 14% 34% 23% 8% 2	562
May	1st to 10th	Day	15% 8% 35% 30% 11% 1	653
	11th to 20th	Day	16% 7% 25% 33% 16% 4	675
	21st to end	Day	13% 6% 28% 26% 20% 7%	734
June	1st to 10th	Day	8% 2 14% 35% 32% 9%	680
	11th to 20th	Day	7% 2 15% 42% 26% 7%	680
	21st to end	Day	3 2 17% 39% 32% 9%	680
July	1st to 10th	Day	8% 1 9% 31% 35% 16%	671
	11th to 20th	Day	5% 1 6% 34% 42% 13%	668
	21st to end	Day	5% 1 5% 28% 42% 18%	739
August	1st to 10th	Day	5% 1 5% 37% 42% 9%	676
	11th to 20th	Day	9% 1 6% 34% 39% 10%	665
	21st to end	Day	5% 2 4 27% 40% 22%	583
September	1st to 10th	Day	10% 1 8% 20% 39% 23%	344
	11th to 20th	Day	3 3 8% 33% 36% 17%	300
	21st to end	Day	11% 3 15% 25% 33% 13%	300
October	1st to 10th	Day	5% 3 24% 33% 23% 12%	300
	11th to 20th	Day	11% 5% 32% 32% 18% 2	300
	21st to end	Day	25% 9% 26% 24% 13% 3	330
November	1st to 10th	Day	18% 5% 37% 26% 12% 3	300
	11th to 20th	Day	15% 10% 32% 30% 11% 2	300
	21st to end	Day	18% 12% 38% 23% 8%	299
December	1st to 10th	Day	14% 16% 31% 28% 10% 1	270
	11th to 20th	Day	26% 9% 38% 21% 6%	270
	21st to end	Day	21% 14% 36% 23% 6%	297
January	1st to 10th	Night	15% 12% 35% 30% 7%	950
	11th to 20th	Night	17% 16% 36% 27% 4	950
	21st to end	Night	16% 9% 36% 33% 5%	1045
February	1st to 10th	Night	19% 12% 34% 29% 5%	950
	11th to 20th	Night	21% 15% 36% 25% 3	796
	21st to end	Night	20% 14% 39% 25% 3	563
March	1st to 10th	Night	18% 10% 34% 34% 4	570
	11th to 20th	Night	19% 10% 36% 30% 5%	570
	21st to end	Night	15% 12% 38% 28% 7%	627
April	1st to 10th	Night	13% 12% 40% 29% 6%	570
	11th to 20th	Night	14% 12% 34% 30% 9%	563
	21st to end	Night	16% 8% 36% 29% 9% 1	570
May	1st to 10th	Night	13% 6% 36% 35% 9% 1	570
	11th to 20th	Night	13% 4 26% 40% 16% 2	561
	21st to end	Night	10% 3 26% 36% 22% 3	617
June	1st to 10th	Night	5% 3 15% 38% 36% 3	570
	11th to 20th	Night	6% 1 18% 50% 25% 1	570
	21st to end	Night	3 1 14% 46% 31% 5%	570
July	1st to 10th	Night	4 1 7% 42% 40% 6%	555
	11th to 20th	Night	2 1 7% 39% 44% 6%	552
	21st to end	Night	3 2 4 40% 45% 6%	624
August	1st to 10th	Night	2 1 6% 40% 46% 5%	567
	11th to 20th	Night	6% 1 6% 39% 44% 4	564
	21st to end	Night	4 1 5% 36% 48% 6%	627
September	1st to 10th	Night	9% 1 7% 36% 43% 4	570
	11th to 20th	Night	12% 1 10% 38% 36% 5%	570
	21st to end	Night	8% 1 16% 37% 32% 5%	570
October	1st to 10th	Night	8% 3 20% 37% 28% 4	678
	11th to 20th	Night	14% 6% 28% 37% 15%	760
	21st to end	Night	19% 7% 27% 32% 14% 1	836
November	1st to 10th	Night	14% 5% 32% 36% 13% 1	780
	11th to 20th	Night	10% 12% 31% 36% 10%	925
	21st to end	Night	17% 10% 35% 29% 8%	1000
December	1st to 10th	Night	16% 12% 32% 33% 7%	950
	11th to 20th	Night	21% 10% 34% 30% 5%	950
	21st to end	Night	18% 12% 38% 27% 5%	1045

Page generated at 2024-11-27 16:30:55. Version ac07

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