Island Grove Observatory.
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Island Grove Observatory. clear sky chart from 2005-11-06 to 2025-08-05.

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	14% 10% 4 9% 5% 5% 5% 4 5% 7% 32%	126544
	Night	14% 11% 4 10% 6% 5% 5% 4 5% 6% 30%	121497
Transparency	Day	50% 7% 12% 19% 11% 1	130137
	Night	51% 7% 12% 18% 10% 1	125994
Seeing	Day	17% 9% 28% 25% 15% 5%	15682
	Night	19% 9% 30% 30% 12% 1	24662

By Month

Percentage Cloud Cover NE (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% 12% 5% 15% 6% 5% 6% 4 5% 5% 24%	3597
	11th to 20th	Day	8% 15% 7% 18% 10% 9% 5% 4 5% 5% 12%	3859
	21st to end	Day	13% 18% 7% 15% 7% 6% 6% 4 5% 5% 15%	4466
February	1st to 10th	Day	12% 16% 6% 14% 7% 7% 6% 5% 5% 5% 18%	1800
	11th to 20th	Day	11% 13% 6% 14% 8% 5% 5% 3 5% 6% 23%	1753
	21st to end	Day	15% 14% 5% 10% 6% 6% 5% 4 6% 5% 22%	1529
March	1st to 10th	Day	17% 11% 5% 10% 6% 4 4 3 5% 6% 30%	4932
	11th to 20th	Day	9% 13% 5% 10% 6% 6% 5% 4 6% 7% 28%	4980
	21st to end	Day	17% 12% 4 10% 5% 4 4 3 4 5% 32%	5465
April	1st to 10th	Day	17% 13% 4 8% 6% 4 5% 4 5% 5% 29%	2366
	11th to 20th	Day	18% 12% 2 6% 4 4 4 3 5% 6% 36%	2462
	21st to end	Day	18% 10% 4 8% 5% 4 4 3 5% 6% 34%	2600
May	1st to 10th	Day	16% 9% 4 8% 4 4 3 4 5% 7% 35%	5906
	11th to 20th	Day	13% 10% 3 8% 5% 5% 4 3 6% 6% 36%	6318
	21st to end	Day	17% 8% 3 7% 4 4 4 4 5% 7% 38%	6949
June	1st to 10th	Day	15% 8% 3 7% 4 4 4 4 6% 7% 37%	2800
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	2851
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	3000
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 5% 6% 9% 38%	6635
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 4 6% 9% 45%	6360
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 6% 8% 48%	6546
August	1st to 10th	Day	13% 8% 3 6% 4 4 5% 4 6% 8% 38%	4156
	11th to 20th	Day	12% 7% 3 7% 5% 4 4 4 7% 10% 36%	4044
	21st to end	Day	9% 6% 2 7% 4 4 5% 5% 7% 8% 44%	4372
September	1st to 10th	Day	13% 7% 2 6% 4 5% 5% 3 6% 7% 41%	2280
	11th to 20th	Day	12% 6% 2 7% 4 4 4 4 5% 8% 43%	2173
	21st to end	Day	13% 7% 3 7% 6% 5% 5% 4 6% 7% 37%	2047
October	1st to 10th	Day	18% 10% 3 7% 5% 5% 4 4 5% 6% 34%	1900
	11th to 20th	Day	19% 9% 3 10% 7% 5% 5% 3 6% 6% 27%	1900
	21st to end	Day	24% 14% 4 10% 5% 5% 4 3 5% 6% 21%	2090
November	1st to 10th	Day	14% 14% 4 8% 5% 5% 5% 4 6% 7% 28%	1662
	11th to 20th	Day	13% 16% 5% 10% 7% 5% 6% 4 6% 6% 23%	1600
	21st to end	Day	18% 17% 5% 12% 7% 6% 5% 4 4 6% 16%	1600
December	1st to 10th	Day	15% 23% 8% 16% 8% 6% 5% 3 4 4 9%	2600
	11th to 20th	Day	16% 18% 5% 12% 7% 6% 5% 4 4 5% 19%	2986
	21st to end	Day	22% 16% 6% 16% 6% 5% 5% 3 4 5% 12%	3960
January	1st to 10th	Night	13% 14% 6% 17% 7% 5% 6% 4 5% 6% 17%	6349
	11th to 20th	Night	11% 12% 7% 17% 10% 7% 6% 4 5% 5% 14%	6025
	21st to end	Night	13% 14% 6% 13% 8% 6% 6% 5% 6% 6% 18%	6490
February	1st to 10th	Night	12% 16% 5% 12% 9% 7% 5% 4 5% 7% 19%	2600
	11th to 20th	Night	13% 14% 5% 11% 7% 6% 5% 4 6% 6% 22%	2533
	21st to end	Night	13% 12% 4 12% 7% 6% 4 4 6% 6% 25%	2097
March	1st to 10th	Night	14% 11% 5% 10% 6% 5% 5% 4 5% 6% 28%	4908
	11th to 20th	Night	9% 10% 4 8% 5% 5% 6% 5% 7% 8% 33%	4979
	21st to end	Night	18% 8% 4 10% 6% 5% 5% 4 4 5% 31%	5467
April	1st to 10th	Night	18% 12% 4 8% 5% 5% 5% 4 4 6% 30%	2033
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 4 5% 7% 36%	1906
	21st to end	Night	15% 10% 4 8% 5% 5% 4 4 5% 6% 36%	1800
May	1st to 10th	Night	19% 8% 3 7% 5% 4 4 3 5% 7% 35%	4035
	11th to 20th	Night	11% 6% 3 7% 4 4 5% 4 6% 8% 41%	3575
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 7% 42%	3926
June	1st to 10th	Night	12% 6% 4 6% 4 5% 5% 4 6% 8% 40%	1600
	11th to 20th	Night	9% 5% 2 7% 5% 6% 5% 4 6% 9% 42%	1539
	21st to end	Night	11% 7% 3 6% 5% 5% 3 4 6% 7% 43%	1400
July	1st to 10th	Night	8% 6% 3 6% 4 5% 5% 5% 6% 7% 45%	3330
	11th to 20th	Night	11% 6% 2 4 4 4 4 4 5% 7% 50%	3600
	21st to end	Night	8% 5% 2 5% 3 4 4 3 6% 8% 50%	4289
August	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 47%	2861
	11th to 20th	Night	13% 5% 2 6% 3 3 4 3 5% 8% 47%	2785
	21st to end	Night	9% 6% 3 6% 4 4 4 4 6% 8% 46%	3251
September	1st to 10th	Night	11% 7% 2 5% 5% 4 5% 4 5% 8% 45%	1900
	11th to 20th	Night	12% 6% 2 5% 3 3 3 4 5% 7% 51%	2004
	21st to end	Night	16% 8% 2 6% 5% 4 3 3 5% 5% 42%	2131
October	1st to 10th	Night	16% 7% 3 8% 5% 5% 5% 3 4 7% 37%	2280
	11th to 20th	Night	17% 10% 4 9% 5% 5% 4 3 6% 6% 31%	2280
	21st to end	Night	21% 10% 4 8% 6% 5% 5% 4 5% 6% 25%	2508
November	1st to 10th	Night	17% 11% 3 9% 5% 4 4 4 6% 6% 32%	2632
	11th to 20th	Night	15% 16% 4 9% 6% 5% 4 3 6% 7% 24%	2800
	21st to end	Night	18% 15% 4 10% 6% 6% 5% 4 6% 6% 20%	2800
December	1st to 10th	Night	15% 19% 7% 15% 8% 6% 5% 4 5% 4 12%	4580
	11th to 20th	Night	14% 16% 6% 13% 7% 6% 6% 5% 5% 5% 17%	5208
	21st to end	Night	21% 18% 6% 16% 6% 5% 6% 3 4 4 11%	6996

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	58% 7% 14% 13% 8%	3598
	11th to 20th	Day	72% 6% 11% 7% 4	3893
	21st to end	Day	68% 6% 11% 8% 6%	4466
February	1st to 10th	Day	67% 7% 8% 14% 4 1	1800
	11th to 20th	Day	61% 6% 11% 15% 7% 1	1762
	21st to end	Day	62% 5% 10% 16% 6%	1529
March	1st to 10th	Day	55% 6% 14% 13% 13%	4935
	11th to 20th	Day	53% 6% 14% 18% 7% 2	4980
	21st to end	Day	55% 5% 8% 17% 14% 1	5465
April	1st to 10th	Day	56% 5% 10% 17% 10% 1	2366
	11th to 20th	Day	49% 6% 10% 20% 15% 1	2462
	21st to end	Day	51% 6% 11% 17% 13% 3	2600
May	1st to 10th	Day	48% 6% 13% 18% 10% 5%	5915
	11th to 20th	Day	47% 8% 10% 16% 18% 1	6322
	21st to end	Day	45% 7% 12% 20% 14% 2	6949
June	1st to 10th	Day	44% 8% 12% 21% 15% 1	2800
	11th to 20th	Day	39% 8% 14% 24% 15% 1	2851
	21st to end	Day	42% 7% 14% 23% 13% 1	3000
July	1st to 10th	Day	40% 9% 13% 23% 14% 1	6566
	11th to 20th	Day	32% 8% 14% 28% 16% 1	6285
	21st to end	Day	32% 8% 13% 32% 15%	6359
August	1st to 10th	Day	42% 8% 14% 23% 12% 1	4143
	11th to 20th	Day	41% 8% 17% 25% 9%	4083
	21st to end	Day	34% 7% 14% 32% 11% 1	4372
September	1st to 10th	Day	40% 8% 13% 27% 11% 1	2280
	11th to 20th	Day	38% 7% 13% 28% 13% 1	2173
	21st to end	Day	45% 8% 13% 25% 9%	2047
October	1st to 10th	Day	43% 7% 14% 25% 11% 1	3160
	11th to 20th	Day	47% 6% 13% 21% 12%	3140
	21st to end	Day	55% 8% 15% 17% 4	3428
November	1st to 10th	Day	53% 8% 14% 17% 8% 1	1662
	11th to 20th	Day	60% 8% 11% 12% 8% 1	1600
	21st to end	Day	68% 7% 11% 12% 2	1600
December	1st to 10th	Day	77% 8% 8% 6% 1	2600
	11th to 20th	Day	67% 7% 10% 10% 7%	2986
	21st to end	Day	74% 7% 7% 8% 4	3960
January	1st to 10th	Night	63% 9% 11% 12% 4	6349
	11th to 20th	Night	67% 9% 11% 9% 4	6066
	21st to end	Night	62% 8% 11% 10% 8% 1	6490
February	1st to 10th	Night	63% 7% 10% 15% 4 1	2600
	11th to 20th	Night	60% 6% 12% 16% 7%	2535
	21st to end	Night	57% 6% 11% 17% 8% 1	2097
March	1st to 10th	Night	55% 5% 10% 17% 11% 2	4918
	11th to 20th	Night	44% 7% 20% 20% 9% 1	4980
	21st to end	Night	52% 6% 10% 17% 14% 1	5467
April	1st to 10th	Night	56% 5% 9% 18% 11% 1	2033
	11th to 20th	Night	47% 6% 9% 21% 16% 2	1906
	21st to end	Night	49% 6% 10% 17% 16% 2	1800
May	1st to 10th	Night	48% 6% 12% 14% 14% 6%	4042
	11th to 20th	Night	39% 9% 12% 22% 15% 2	3576
	21st to end	Night	41% 7% 12% 24% 14% 2	3927
June	1st to 10th	Night	41% 7% 10% 24% 16% 2	1600
	11th to 20th	Night	37% 8% 12% 22% 20% 2	1539
	21st to end	Night	39% 7% 14% 24% 15% 1	1400
July	1st to 10th	Night	35% 7% 15% 25% 16% 1	3292
	11th to 20th	Night	33% 7% 13% 31% 17% 1	3552
	21st to end	Night	30% 7% 11% 33% 17% 1	4162
August	1st to 10th	Night	34% 6% 14% 28% 16% 1	2850
	11th to 20th	Night	34% 6% 19% 28% 12% 1	2812
	21st to end	Night	34% 8% 13% 32% 12% 1	3251
September	1st to 10th	Night	36% 7% 15% 27% 13% 1	1900
	11th to 20th	Night	32% 6% 13% 31% 17% 1	2004
	21st to end	Night	43% 7% 13% 26% 11%	2131
October	1st to 10th	Night	34% 8% 11% 26% 20% 1	3800
	11th to 20th	Night	48% 6% 15% 21% 11%	3779
	21st to end	Night	46% 9% 15% 23% 7%	4120
November	1st to 10th	Night	50% 6% 13% 20% 9% 1	2632
	11th to 20th	Night	58% 8% 11% 14% 7% 1	2800
	21st to end	Night	63% 8% 15% 12% 2	2800
December	1st to 10th	Night	72% 9% 11% 7% 2	4580
	11th to 20th	Night	66% 8% 9% 11% 5%	5208
	21st to end	Night	74% 7% 9% 7% 2	6996

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	15% 14% 40% 21% 10% 1	200
	11th to 20th	Day	18% 12% 42% 20% 6% 2	263
	21st to end	Day	20% 13% 35% 25% 6% 1	330
February	1st to 10th	Day	19% 15% 43% 17% 5% 1	300
	11th to 20th	Day	19% 20% 38% 22% 2	300
	21st to end	Day	24% 19% 33% 23% 2	263
March	1st to 10th	Day	20% 11% 37% 24% 8% 1	500
	11th to 20th	Day	18% 15% 35% 24% 7% 1	500
	21st to end	Day	26% 9% 32% 23% 9% 1	550
April	1st to 10th	Day	21% 14% 32% 22% 9% 2	500
	11th to 20th	Day	27% 11% 28% 19% 11% 4	492
	21st to end	Day	22% 11% 27% 26% 11% 4	500
May	1st to 10th	Day	16% 8% 28% 30% 14% 5%	503
	11th to 20th	Day	17% 7% 22% 29% 19% 6%	527
	21st to end	Day	16% 5% 22% 29% 20% 7%	573
June	1st to 10th	Day	16% 7% 26% 28% 19% 3	530
	11th to 20th	Day	13% 7% 24% 31% 20% 5%	581
	21st to end	Day	16% 8% 25% 31% 16% 4	730
July	1st to 10th	Day	10% 5% 21% 30% 27% 7%	645
	11th to 20th	Day	12% 3 21% 28% 27% 10%	524
	21st to end	Day	10% 4 20% 36% 22% 9%	576
August	1st to 10th	Day	11% 4 17% 28% 28% 12%	502
	11th to 20th	Day	10% 4 16% 28% 28% 14%	479
	21st to end	Day	12% 4 22% 28% 23% 10%	539
September	1st to 10th	Day	13% 7% 20% 24% 24% 11%	490
	11th to 20th	Day	10% 4 17% 26% 25% 19%	490
	21st to end	Day	14% 7% 26% 24% 20% 9%	490
October	1st to 10th	Day	20% 7% 28% 22% 15% 8%	490
	11th to 20th	Day	22% 10% 35% 23% 8% 2	490
	21st to end	Day	29% 10% 32% 22% 7% 1	539
November	1st to 10th	Day	21% 18% 29% 20% 11% 2	266
	11th to 20th	Day	21% 20% 33% 20% 5% 2	200
	21st to end	Day	25% 16% 37% 19% 4	200
December	1st to 10th	Day	20% 17% 39% 20% 5% 1	200
	11th to 20th	Day	21% 21% 36% 16% 6% 1	200
	21st to end	Day	26% 9% 40% 19% 5%	220
January	1st to 10th	Night	20% 13% 40% 19% 7% 1	800
	11th to 20th	Night	23% 10% 34% 25% 7% 1	800
	21st to end	Night	19% 8% 33% 31% 8% 1	880
February	1st to 10th	Night	19% 10% 41% 25% 4	800
	11th to 20th	Night	20% 11% 36% 28% 4	783
	21st to end	Night	18% 11% 38% 28% 5%	660
March	1st to 10th	Night	19% 10% 36% 29% 6%	800
	11th to 20th	Night	17% 12% 37% 27% 7% 1	800
	21st to end	Night	21% 9% 32% 28% 11%	880
April	1st to 10th	Night	23% 10% 32% 29% 6%	800
	11th to 20th	Night	20% 8% 32% 29% 10% 2	722
	21st to end	Night	20% 8% 30% 30% 12%	600
May	1st to 10th	Night	18% 4 24% 37% 16% 2	595
	11th to 20th	Night	16% 5% 25% 40% 14% 1	394
	21st to end	Night	12% 5% 22% 40% 20% 1	433
June	1st to 10th	Night	14% 6% 28% 38% 14% 1	400
	11th to 20th	Night	9% 8% 23% 45% 16% 1	400
	21st to end	Night	14% 4 22% 41% 19% 1	400
July	1st to 10th	Night	8% 3 18% 47% 23% 1	390
	11th to 20th	Night	11% 4 19% 42% 22% 2	388
	21st to end	Night	10% 4 18% 44% 24% 1	596
August	1st to 10th	Night	11% 2 13% 42% 30% 2	579
	11th to 20th	Night	7% 3 15% 37% 34% 4	564
	21st to end	Night	12% 4 20% 35% 26% 3	702
September	1st to 10th	Night	12% 5% 22% 34% 21% 6%	760
	11th to 20th	Night	14% 3 19% 31% 25% 8%	760
	21st to end	Night	19% 6% 20% 31% 19% 5%	760
October	1st to 10th	Night	18% 6% 28% 28% 18% 2	760
	11th to 20th	Night	23% 11% 29% 28% 9% 1	760
	21st to end	Night	26% 11% 31% 25% 6%	836
November	1st to 10th	Night	23% 11% 28% 26% 11% 1	780
	11th to 20th	Night	23% 14% 33% 21% 8% 1	800
	21st to end	Night	27% 13% 33% 23% 4	800
December	1st to 10th	Night	26% 13% 38% 18% 4 1	800
	11th to 20th	Night	22% 14% 38% 21% 5% 1	800
	21st to end	Night	28% 11% 35% 22% 4	880

Page generated at 2025-08-02 01:32:23. Version ac07

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