Idlewild Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Idlewild Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	5% 5% 2 5% 4 4 4 4 6% 8% 55%	124096
	Night	5% 6% 2 5% 3 3 3 3 5% 7% 58%	123930
Transparency	Day	26% 5% 9% 23% 25% 11%	123841
	Night	26% 4 10% 25% 26% 10%	123773
Seeing	Day	8% 7% 25% 33% 21% 7%	16572
	Night	7% 3 26% 47% 16% 1	26569

By Month

Percentage Cloud Cover SW (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	1 6% 3 6% 3 3 3 3 5% 8% 60%	3638
	11th to 20th	Day	1 5% 3 7% 4 4 4 3 5% 7% 57%	3751
	21st to end	Day	1 5% 3 6% 3 3 3 3 5% 7% 61%	4510
February	1st to 10th	Day	3 7% 3 7% 4 3 4 3 5% 6% 55%	1800
	11th to 20th	Day	1 5% 3 6% 4 4 4 4 5% 8% 56%	1933
	21st to end	Day	2 7% 2 7% 3 3 3 3 5% 6% 59%	1650
March	1st to 10th	Day	4 10% 2 4 3 3 3 2 4 6% 59%	4500
	11th to 20th	Day	5% 8% 2 6% 4 4 4 3 6% 8% 50%	4926
	21st to end	Day	4 6% 2 4 4 3 4 3 5% 8% 55%	5830
April	1st to 10th	Day	6% 6% 2 4 3 4 4 3 6% 8% 54%	2400
	11th to 20th	Day	9% 8% 3 5% 3 4 4 4 5% 8% 48%	2371
	21st to end	Day	10% 7% 1 4 4 4 4 4 7% 9% 47%	2400
May	1st to 10th	Day	10% 6% 2 5% 4 4 5% 4 6% 7% 45%	5750
	11th to 20th	Day	8% 6% 2 5% 5% 4 5% 4 7% 8% 46%	6254
	21st to end	Day	7% 6% 2 5% 4 5% 5% 5% 6% 9% 46%	6925
June	1st to 10th	Day	6% 4 2 4 4 4 5% 5% 8% 10% 50%	2800
	11th to 20th	Day	5% 3 1 4 4 5% 5% 6% 6% 9% 52%	2800
	21st to end	Day	5% 3 1 4 5% 4 5% 5% 8% 9% 50%	2800
July	1st to 10th	Day	5% 3 1 4 3 4 4 4 6% 8% 57%	4570
	11th to 20th	Day	3 2 1 4 4 5% 6% 5% 9% 12% 48%	4570
	21st to end	Day	5% 4 2 5% 4 4 5% 4 7% 8% 54%	4721
August	1st to 10th	Day	9% 4 1 6% 5% 6% 7% 6% 8% 10% 38%	4074
	11th to 20th	Day	4 3 1 5% 4 5% 6% 5% 7% 9% 51%	3726
	21st to end	Day	4 3 2 5% 4 4 5% 5% 8% 9% 51%	4180
September	1st to 10th	Day	5% 3 1 4 3 3 4 3 5% 7% 61%	2280
	11th to 20th	Day	6% 3 1 4 3 3 3 3 6% 7% 60%	2255
	21st to end	Day	5% 4 1 3 2 3 3 2 4 6% 66%	2090
October	1st to 10th	Day	4 2 1 2 2 2 2 2 3 4 76%	3206
	11th to 20th	Day	4 5% 2 4 3 3 3 3 5% 7% 61%	3150
	21st to end	Day	6% 6% 2 3 2 2 2 2 4 5% 65%	3315
November	1st to 10th	Day	3 5% 1 3 2 2 3 2 5% 8% 64%	1763
	11th to 20th	Day	2 5% 2 4 3 3 2 2 5% 8% 64%	1800
	21st to end	Day	3 6% 2 5% 2 3 3 3 5% 7% 60%	1708
December	1st to 10th	Day	1 4 2 4 3 3 2 2 3 6% 69%	2620
	11th to 20th	Day	2 4 1 4 2 2 3 3 4 7% 68%	3026
	21st to end	Day	2 6% 3 7% 4 4 4 4 5% 7% 54%	4004
January	1st to 10th	Night	2 9% 4 7% 3 3 3 2 4 8% 55%	6327
	11th to 20th	Night	1 5% 2 7% 4 3 3 2 4 7% 61%	6271
	21st to end	Night	3 6% 3 6% 4 3 3 3 3 5% 60%	6468
February	1st to 10th	Night	3 8% 2 6% 3 3 4 3 4 8% 56%	2600
	11th to 20th	Night	2 8% 4 7% 5% 4 4 3 4 6% 52%	2353
	21st to end	Night	2 8% 3 6% 3 3 3 3 4 5% 60%	1980
March	1st to 10th	Night	5% 9% 2 5% 3 3 4 3 5% 5% 56%	5374
	11th to 20th	Night	7% 9% 2 5% 3 4 4 3 4 6% 54%	5055
	21st to end	Night	8% 9% 3 5% 4 4 3 3 5% 7% 50%	5128
April	1st to 10th	Night	7% 7% 2 5% 3 3 4 3 5% 6% 56%	2000
	11th to 20th	Night	12% 8% 2 5% 3 3 3 3 4 6% 50%	1977
	21st to end	Night	13% 8% 2 5% 3 3 3 3 5% 6% 49%	2000
May	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 5% 8% 46%	4228
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 4 6% 8% 48%	3644
	21st to end	Night	10% 6% 2 4 3 3 3 3 5% 6% 55%	3972
June	1st to 10th	Night	6% 2 1 4 2 3 3 4 6% 10% 59%	1600
	11th to 20th	Night	6% 3 1 2 3 3 3 4 7% 7% 62%	1600
	21st to end	Night	5% 3 1 4 3 3 5% 4 7% 8% 57%	1600
July	1st to 10th	Night	4 3 1 4 3 5% 4 3 6% 8% 59%	2620
	11th to 20th	Night	2 2 1 5% 3 5% 5% 5% 8% 10% 53%	2620
	21st to end	Night	4 3 1 4 4 3 4 5% 8% 9% 56%	2839
August	1st to 10th	Night	8% 5% 2 6% 4 5% 5% 5% 8% 10% 42%	2864
	11th to 20th	Night	3 3 1 4 4 4 5% 5% 8% 12% 51%	3113
	21st to end	Night	3 3 2 5% 4 4 6% 5% 8% 11% 51%	3487
September	1st to 10th	Night	6% 3 1 4 3 4 3 3 5% 7% 58%	1900
	11th to 20th	Night	7% 3 1 2 3 3 3 2 4 8% 65%	1924
	21st to end	Night	7% 3 1 3 3 2 3 2 4 6% 65%	2090
October	1st to 10th	Night	5% 3 1 3 2 2 2 2 4 4 72%	3759
	11th to 20th	Night	6% 3 2 4 2 2 3 3 4 8% 62%	3778
	21st to end	Night	5% 7% 2 3 2 2 2 2 3 5% 66%	4209
November	1st to 10th	Night	3 4 2 3 3 2 3 3 4 6% 67%	2535
	11th to 20th	Night	3 5% 2 4 3 3 3 3 4 5% 66%	2600
	21st to end	Night	4 6% 2 5% 3 3 3 3 4 6% 62%	2690
December	1st to 10th	Night	1 4 2 4 3 1 2 2 3 5% 72%	4570
	11th to 20th	Night	1 5% 2 4 2 3 3 2 5% 8% 65%	5181
	21st to end	Night	5% 8% 3 6% 4 4 4 3 4 6% 53%	6974

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	23% 3 8% 33% 24% 9%	3639
	11th to 20th	Day	25% 6% 9% 22% 28% 10%	3751
	21st to end	Day	23% 4 7% 28% 26% 12%	4510
February	1st to 10th	Day	30% 3 10% 27% 21% 10%	1800
	11th to 20th	Day	25% 3 8% 29% 27% 8%	1943
	21st to end	Day	25% 3 6% 23% 27% 15%	1650
March	1st to 10th	Day	26% 3 5% 17% 31% 17%	4500
	11th to 20th	Day	30% 4 10% 22% 28% 6%	4926
	21st to end	Day	26% 6% 9% 36% 20% 3	5830
April	1st to 10th	Day	28% 5% 10% 25% 22% 11%	2400
	11th to 20th	Day	34% 6% 9% 21% 21% 10%	2374
	21st to end	Day	32% 6% 10% 25% 23% 4	2400
May	1st to 10th	Day	34% 7% 9% 25% 18% 6%	5750
	11th to 20th	Day	32% 7% 10% 20% 21% 9%	6258
	21st to end	Day	30% 8% 13% 24% 19% 7%	6926
June	1st to 10th	Day	27% 7% 15% 24% 19% 9%	2800
	11th to 20th	Day	25% 7% 10% 18% 23% 17%	2800
	21st to end	Day	26% 7% 10% 20% 24% 14%	2800
July	1st to 10th	Day	23% 5% 8% 16% 29% 19%	4501
	11th to 20th	Day	23% 8% 10% 22% 22% 13%	4495
	21st to end	Day	25% 7% 10% 19% 21% 18%	4572
August	1st to 10th	Day	37% 9% 14% 21% 15% 5%	4060
	11th to 20th	Day	28% 7% 13% 16% 21% 15%	3762
	21st to end	Day	26% 6% 10% 19% 25% 13%	4179
September	1st to 10th	Day	22% 5% 8% 20% 25% 20%	2280
	11th to 20th	Day	22% 4 9% 18% 28% 19%	2255
	21st to end	Day	21% 4 5% 18% 37% 15%	2090
October	1st to 10th	Day	14% 2 5% 15% 36% 27%	3206
	11th to 20th	Day	23% 4 8% 23% 32% 9%	3150
	21st to end	Day	21% 2 5% 23% 36% 13%	3315
November	1st to 10th	Day	19% 3 8% 25% 29% 17%	1763
	11th to 20th	Day	19% 4 5% 26% 35% 11%	1800
	21st to end	Day	24% 2 7% 21% 31% 14%	1708
December	1st to 10th	Day	18% 2 7% 32% 30% 10%	2620
	11th to 20th	Day	16% 3 6% 30% 34% 10%	3024
	21st to end	Day	28% 3 8% 25% 27% 9%	4004
January	1st to 10th	Night	29% 4 7% 30% 25% 7%	6327
	11th to 20th	Night	23% 5% 9% 25% 24% 14%	6271
	21st to end	Night	26% 3 9% 25% 26% 11%	6468
February	1st to 10th	Night	28% 4 9% 27% 25% 8%	2600
	11th to 20th	Night	31% 3 9% 28% 23% 7%	2353
	21st to end	Night	26% 3 5% 23% 28% 14%	1980
March	1st to 10th	Night	29% 3 5% 18% 35% 10%	5374
	11th to 20th	Night	31% 4 10% 14% 30% 12%	5055
	21st to end	Night	33% 5% 8% 34% 18% 2	5128
April	1st to 10th	Night	27% 6% 10% 24% 23% 10%	2000
	11th to 20th	Night	35% 4 10% 22% 21% 9%	1977
	21st to end	Night	35% 5% 10% 25% 20% 5%	2000
May	1st to 10th	Night	36% 7% 12% 26% 11% 7%	4225
	11th to 20th	Night	33% 6% 15% 21% 18% 8%	3645
	21st to end	Night	29% 6% 18% 26% 17% 5%	3972
June	1st to 10th	Night	20% 7% 19% 32% 15% 8%	1600
	11th to 20th	Night	20% 5% 13% 23% 23% 17%	1600
	21st to end	Night	21% 6% 11% 24% 27% 11%	1600
July	1st to 10th	Night	22% 6% 13% 18% 31% 10%	2580
	11th to 20th	Night	20% 7% 15% 28% 22% 7%	2572
	21st to end	Night	19% 7% 14% 24% 21% 15%	2750
August	1st to 10th	Night	34% 9% 15% 22% 15% 4	2855
	11th to 20th	Night	23% 9% 19% 20% 19% 10%	3143
	21st to end	Night	24% 7% 14% 24% 25% 6%	3487
September	1st to 10th	Night	24% 5% 10% 21% 25% 15%	1900
	11th to 20th	Night	20% 4 9% 23% 27% 17%	1924
	21st to end	Night	22% 4 6% 21% 35% 13%	2090
October	1st to 10th	Night	17% 3 6% 20% 32% 21%	3759
	11th to 20th	Night	21% 4 10% 27% 28% 10%	3778
	21st to end	Night	22% 2 6% 24% 33% 13%	4209
November	1st to 10th	Night	19% 3 8% 24% 29% 16%	2535
	11th to 20th	Night	21% 2 6% 26% 33% 11%	2600
	21st to end	Night	24% 3 8% 23% 31% 11%	2690
December	1st to 10th	Night	16% 2 7% 35% 32% 7%	4570
	11th to 20th	Night	18% 3 6% 31% 36% 5%	5182
	21st to end	Night	31% 3 8% 24% 26% 8%	6974

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	4 6% 30% 42% 15% 3	300
	11th to 20th	Day	6% 6% 31% 43% 12% 1	300
	21st to end	Day	4 6% 38% 38% 12% 2	330
February	1st to 10th	Day	4 10% 45% 32% 8% 1	300
	11th to 20th	Day	4 3 45% 37% 11% 1	300
	21st to end	Day	4 12% 35% 39% 10%	249
March	1st to 10th	Day	4 9% 34% 36% 14% 4	300
	11th to 20th	Day	14% 10% 31% 30% 13% 2	465
	21st to end	Day	12% 19% 30% 27% 11% 2	550
April	1st to 10th	Day	10% 16% 32% 28% 12% 2	500
	11th to 20th	Day	14% 21% 28% 25% 10% 2	492
	21st to end	Day	16% 12% 31% 26% 14% 2	500
May	1st to 10th	Day	15% 10% 29% 29% 15% 3	631
	11th to 20th	Day	15% 7% 22% 30% 20% 6%	725
	21st to end	Day	10% 8% 23% 31% 20% 7%	789
June	1st to 10th	Day	6% 4 20% 32% 26% 12%	730
	11th to 20th	Day	6% 5% 25% 33% 24% 6%	730
	21st to end	Day	5% 5% 23% 32% 26% 7%	730
July	1st to 10th	Day	7% 3 16% 28% 34% 13%	721
	11th to 20th	Day	3 2 15% 29% 38% 13%	718
	21st to end	Day	6% 2 11% 27% 37% 16%	694
August	1st to 10th	Day	8% 2 18% 32% 26% 14%	487
	11th to 20th	Day	5% 2 18% 35% 26% 14%	477
	21st to end	Day	6% 4 18% 34% 27% 12%	539
September	1st to 10th	Day	8% 3 17% 32% 29% 11%	490
	11th to 20th	Day	6% 5% 21% 35% 25% 9%	465
	21st to end	Day	6% 4 20% 30% 25% 15%	300
October	1st to 10th	Day	9% 4 20% 27% 25% 15%	300
	11th to 20th	Day	3 6% 25% 36% 24% 6%	300
	21st to end	Day	10% 8% 25% 36% 18% 3	330
November	1st to 10th	Day	7% 8% 22% 42% 15% 6%	300
	11th to 20th	Day	6% 7% 23% 44% 17% 3	300
	21st to end	Day	8% 4 30% 40% 14% 4	300
December	1st to 10th	Day	4 6% 32% 39% 16% 3	300
	11th to 20th	Day	5% 5% 35% 42% 13%	300
	21st to end	Day	5% 7% 33% 43% 9% 3	330
January	1st to 10th	Night	3 3 33% 49% 11% 2	1000
	11th to 20th	Night	4 4 36% 46% 9% 2	970
	21st to end	Night	5% 2 32% 51% 9% 1	880
February	1st to 10th	Night	6% 3 41% 44% 6%	800
	11th to 20th	Night	5% 3 39% 46% 6%	784
	21st to end	Night	7% 4 38% 47% 4	660
March	1st to 10th	Night	5% 3 36% 46% 10%	800
	11th to 20th	Night	10% 4 28% 49% 10%	800
	21st to end	Night	10% 5% 35% 42% 7%	726
April	1st to 10th	Night	9% 6% 36% 43% 6%	600
	11th to 20th	Night	16% 7% 29% 39% 9%	593
	21st to end	Night	16% 5% 28% 40% 10% 1	600
May	1st to 10th	Night	16% 5% 24% 42% 12% 1	600
	11th to 20th	Night	17% 3 16% 45% 18% 1	591
	21st to end	Night	11% 4 24% 39% 21% 1	650
June	1st to 10th	Night	6% 3 15% 47% 26% 2	600
	11th to 20th	Night	7% 2 20% 52% 18% 1	600
	21st to end	Night	5% 2 15% 51% 27% 1	600
July	1st to 10th	Night	6% 3 13% 41% 35% 2	585
	11th to 20th	Night	3 1 10% 45% 38% 3	582
	21st to end	Night	6% 2 8% 40% 40% 4	624
August	1st to 10th	Night	6% 1 11% 41% 38% 2	567
	11th to 20th	Night	6% 2 11% 49% 30% 2	564
	21st to end	Night	5% 2 13% 47% 32% 2	627
September	1st to 10th	Night	6% 2 11% 46% 30% 5%	570
	11th to 20th	Night	7% 1 16% 48% 25% 3	570
	21st to end	Night	7% 3 16% 50% 22% 3	570
October	1st to 10th	Night	11% 2 18% 47% 18% 3	732
	11th to 20th	Night	5% 1 26% 50% 17% 1	760
	21st to end	Night	9% 3 23% 48% 16% 1	889
November	1st to 10th	Night	5% 3 26% 50% 13% 2	975
	11th to 20th	Night	5% 4 29% 52% 9% 1	1000
	21st to end	Night	6% 3 27% 49% 14% 1	1000
December	1st to 10th	Night	4 3 31% 52% 10%	1000
	11th to 20th	Night	5% 3 33% 48% 10% 1	1000
	21st to end	Night	7% 3 35% 46% 8% 1	1100

Page generated at 2025-07-17 23:10:03. Version ac07

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