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 2024-11-04.

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 6% 7% 32%	101915
	Night	14% 11% 4 9% 6% 5% 5% 4 5% 6% 31%	97085
Transparency	Day	49% 7% 13% 20% 10% 1	105518
	Night	50% 7% 13% 20% 10% 1	101573
Seeing	Day	17% 8% 27% 26% 16% 6%	14754
	Night	19% 8% 29% 30% 12% 1	23753

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	16% 14% 5% 11% 6% 5% 6% 3 4 5% 25%	2517
	11th to 20th	Day	10% 16% 7% 17% 9% 7% 4 4 5% 5% 15%	2715
	21st to end	Day	18% 21% 7% 13% 7% 4 5% 3 4 4 15%	3124
February	1st to 10th	Day	12% 16% 6% 14% 7% 6% 6% 5% 5% 5% 18%	1710
	11th to 20th	Day	11% 13% 6% 13% 7% 5% 5% 4 5% 6% 24%	1663
	21st to end	Day	15% 14% 5% 11% 6% 6% 5% 3 6% 6% 23%	1454
March	1st to 10th	Day	17% 12% 5% 9% 6% 4 4 3 5% 5% 31%	3432
	11th to 20th	Day	9% 16% 6% 11% 6% 6% 5% 4 5% 5% 28%	3480
	21st to end	Day	15% 10% 4 10% 5% 4 4 3 4 5% 36%	3815
April	1st to 10th	Day	17% 13% 4 8% 6% 4 5% 4 5% 5% 29%	2247
	11th to 20th	Day	18% 11% 2 6% 4 4 4 3 5% 6% 36%	2337
	21st to end	Day	19% 10% 3 8% 5% 4 3 3 5% 6% 34%	2470
May	1st to 10th	Day	15% 8% 3 8% 5% 5% 4 4 5% 7% 35%	4126
	11th to 20th	Day	15% 9% 3 8% 5% 4 4 4 6% 7% 34%	4398
	21st to end	Day	12% 6% 2 6% 4 4 4 4 6% 7% 45%	4837
June	1st to 10th	Day	16% 8% 3 7% 4 4 4 4 7% 7% 37%	2660
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 7% 9% 40%	2708
	21st to end	Day	14% 7% 2 6% 5% 5% 5% 4 6% 7% 39%	2850
July	1st to 10th	Day	10% 6% 2 7% 6% 6% 5% 5% 7% 9% 37%	4632
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 9% 42%	4440
	21st to end	Day	8% 6% 2 6% 4 5% 5% 5% 7% 8% 43%	4546
August	1st to 10th	Day	13% 8% 3 6% 4 4 5% 4 6% 8% 37%	4104
	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%	1606
	11th to 20th	Day	13% 17% 5% 10% 6% 5% 6% 4 6% 6% 22%	1520
	21st to end	Day	18% 17% 5% 11% 7% 6% 5% 3 4 6% 17%	1520
December	1st to 10th	Day	15% 20% 7% 16% 7% 6% 5% 3 4 5% 12%	1520
	11th to 20th	Day	17% 14% 4 11% 8% 7% 6% 5% 5% 5% 18%	1906
	21st to end	Day	25% 18% 5% 15% 6% 5% 4 2 4 4 12%	2772
January	1st to 10th	Night	15% 17% 6% 14% 7% 6% 5% 3 5% 5% 16%	4429
	11th to 20th	Night	12% 12% 6% 16% 10% 8% 6% 4 5% 5% 16%	4244
	21st to end	Night	18% 15% 7% 11% 8% 5% 5% 5% 5% 5% 16%	4532
February	1st to 10th	Night	12% 16% 5% 12% 9% 6% 5% 4 5% 6% 19%	2470
	11th to 20th	Night	12% 15% 5% 11% 7% 6% 6% 4 6% 6% 23%	2403
	21st to end	Night	13% 12% 4 12% 7% 6% 4 4 6% 5% 26%	1996
March	1st to 10th	Night	16% 11% 4 9% 6% 5% 4 3 5% 6% 31%	3408
	11th to 20th	Night	12% 12% 5% 9% 5% 5% 5% 4 5% 6% 33%	3479
	21st to end	Night	14% 7% 4 9% 6% 6% 5% 4 5% 6% 33%	3817
April	1st to 10th	Night	19% 12% 4 8% 5% 5% 5% 4 4 6% 30%	1932
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 4 5% 8% 37%	1810
	21st to end	Night	15% 10% 4 8% 5% 4 4 4 4 6% 36%	1710
May	1st to 10th	Night	17% 7% 3 6% 5% 4 4 4 5% 7% 39%	2815
	11th to 20th	Night	14% 7% 3 7% 4 4 4 4 7% 8% 39%	2495
	21st to end	Night	9% 6% 2 5% 4 4 4 4 5% 8% 50%	2738
June	1st to 10th	Night	12% 6% 4 6% 4 5% 5% 4 6% 8% 39%	1520
	11th to 20th	Night	9% 5% 2 6% 5% 6% 5% 4 6% 9% 42%	1462
	21st to end	Night	11% 7% 3 6% 5% 5% 3 4 6% 7% 43%	1330
July	1st to 10th	Night	5% 5% 2 6% 4 5% 5% 5% 6% 7% 50%	2328
	11th to 20th	Night	12% 5% 2 4 4 4 3 4 5% 7% 50%	2520
	21st to end	Night	8% 5% 2 5% 4 4 5% 4 6% 9% 49%	2990
August	1st to 10th	Night	10% 5% 2 6% 4 5% 4 4 6% 8% 47%	2825
	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 8% 5% 4 4 4 6% 6% 32%	2541
	11th to 20th	Night	15% 16% 4 9% 6% 5% 4 3 6% 7% 24%	2660
	21st to end	Night	18% 15% 4 10% 6% 6% 5% 4 6% 6% 20%	2660
December	1st to 10th	Night	17% 18% 6% 14% 7% 5% 5% 3 5% 5% 15%	2660
	11th to 20th	Night	15% 16% 6% 13% 7% 6% 6% 5% 5% 5% 17%	3288
	21st to end	Night	25% 18% 6% 15% 6% 4 4 3 4 4 10%	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	59% 7% 16% 12% 6%	2518
	11th to 20th	Day	70% 6% 11% 9% 4	2715
	21st to end	Day	71% 6% 10% 8% 4 1	3124
February	1st to 10th	Day	67% 7% 8% 13% 4 1	1710
	11th to 20th	Day	60% 6% 11% 15% 7% 1	1672
	21st to end	Day	62% 5% 11% 15% 7%	1454
March	1st to 10th	Day	55% 6% 13% 16% 10%	3435
	11th to 20th	Day	57% 5% 12% 17% 8% 1	3480
	21st to end	Day	50% 5% 11% 18% 15% 2	3815
April	1st to 10th	Day	56% 5% 10% 17% 10% 1	2247
	11th to 20th	Day	48% 6% 11% 20% 15% 1	2337
	21st to end	Day	51% 6% 10% 17% 13% 3	2470
May	1st to 10th	Day	46% 6% 14% 20% 13% 2	4138
	11th to 20th	Day	46% 10% 12% 21% 10% 1	4402
	21st to end	Day	36% 8% 14% 23% 17% 3	4837
June	1st to 10th	Day	44% 8% 12% 21% 15% 1	2660
	11th to 20th	Day	39% 8% 13% 23% 15% 1	2708
	21st to end	Day	43% 7% 14% 22% 13% 1	2850
July	1st to 10th	Day	39% 9% 15% 21% 14% 1	4562
	11th to 20th	Day	34% 9% 16% 31% 10% 1	4365
	21st to end	Day	37% 9% 12% 26% 16% 1	4401
August	1st to 10th	Day	42% 8% 14% 23% 11% 1	4091
	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% 7% 1	1606
	11th to 20th	Day	60% 9% 11% 12% 8% 1	1520
	21st to end	Day	67% 7% 11% 12% 2	1520
December	1st to 10th	Day	72% 7% 10% 9% 2	1520
	11th to 20th	Day	66% 9% 11% 10% 5%	1906
	21st to end	Day	77% 5% 8% 8% 1	2772
January	1st to 10th	Night	65% 8% 13% 10% 3	4429
	11th to 20th	Night	68% 8% 12% 9% 3	4244
	21st to end	Night	67% 9% 10% 10% 4 1	4532
February	1st to 10th	Night	63% 7% 10% 15% 4 1	2470
	11th to 20th	Night	59% 6% 12% 16% 7%	2405
	21st to end	Night	57% 6% 12% 17% 8% 1	1996
March	1st to 10th	Night	54% 5% 11% 21% 8% 1	3418
	11th to 20th	Night	48% 6% 16% 19% 9% 1	3480
	21st to end	Night	47% 7% 11% 20% 14% 1	3817
April	1st to 10th	Night	55% 5% 9% 18% 11% 1	1932
	11th to 20th	Night	47% 6% 9% 21% 16% 2	1810
	21st to end	Night	49% 6% 10% 17% 16% 2	1710
May	1st to 10th	Night	43% 6% 14% 18% 16% 3	2822
	11th to 20th	Night	42% 8% 13% 23% 11% 2	2496
	21st to end	Night	31% 8% 14% 26% 19% 2	2739
June	1st to 10th	Night	41% 7% 10% 24% 15% 2	1520
	11th to 20th	Night	37% 7% 12% 22% 21% 2	1462
	21st to end	Night	39% 7% 14% 24% 15% 1	1330
July	1st to 10th	Night	31% 7% 16% 26% 18% 2	2290
	11th to 20th	Night	32% 7% 13% 33% 14% 1	2472
	21st to end	Night	30% 9% 14% 31% 15% 1	2895
August	1st to 10th	Night	35% 6% 14% 28% 16% 1	2814
	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% 21% 9% 1	2541
	11th to 20th	Night	58% 8% 12% 14% 7% 1	2660
	21st to end	Night	62% 8% 15% 12% 2	2660
December	1st to 10th	Night	70% 6% 11% 10% 2	2660
	11th to 20th	Night	67% 8% 11% 11% 3	3288
	21st to end	Night	77% 6% 8% 7% 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	16% 12% 39% 23% 11% 1	180
	11th to 20th	Day	19% 10% 42% 21% 6% 2	236
	21st to end	Day	21% 9% 35% 27% 6% 1	297
February	1st to 10th	Day	18% 13% 45% 17% 6% 1	270
	11th to 20th	Day	16% 18% 40% 24% 2	270
	21st to end	Day	24% 19% 32% 23% 2	238
March	1st to 10th	Day	19% 10% 38% 25% 8%	460
	11th to 20th	Day	17% 14% 36% 25% 8% 1	460
	21st to end	Day	25% 8% 33% 23% 10% 1	506
April	1st to 10th	Day	20% 13% 32% 23% 10% 2	460
	11th to 20th	Day	27% 10% 28% 19% 12% 5%	452
	21st to end	Day	22% 10% 27% 26% 11% 4	460
May	1st to 10th	Day	17% 6% 26% 30% 15% 5%	463
	11th to 20th	Day	17% 5% 22% 30% 19% 7%	487
	21st to end	Day	14% 5% 22% 30% 21% 7%	529
June	1st to 10th	Day	16% 6% 25% 29% 20% 3	490
	11th to 20th	Day	13% 7% 23% 31% 21% 5%	538
	21st to end	Day	16% 7% 25% 31% 16% 4	680
July	1st to 10th	Day	10% 5% 20% 30% 27% 7%	599
	11th to 20th	Day	12% 3 21% 28% 26% 10%	484
	21st to end	Day	10% 4 17% 37% 23% 10%	532
August	1st to 10th	Day	12% 4 17% 28% 28% 12%	486
	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	19% 17% 29% 21% 12% 2	252
	11th to 20th	Day	21% 19% 31% 22% 6% 2	180
	21st to end	Day	26% 14% 38% 19% 4	180
December	1st to 10th	Day	19% 16% 41% 20% 4 1	180
	11th to 20th	Day	19% 21% 36% 17% 6% 1	180
	21st to end	Day	27% 9% 41% 19% 3	198
January	1st to 10th	Night	21% 12% 39% 20% 7% 1	760
	11th to 20th	Night	23% 10% 34% 26% 7% 1	760
	21st to end	Night	19% 7% 33% 32% 8% 1	836
February	1st to 10th	Night	19% 9% 41% 26% 4	760
	11th to 20th	Night	20% 10% 37% 28% 4	743
	21st to end	Night	19% 10% 37% 28% 5%	628
March	1st to 10th	Night	18% 9% 36% 30% 6%	760
	11th to 20th	Night	18% 11% 37% 27% 7% 1	760
	21st to end	Night	20% 8% 32% 29% 11%	836
April	1st to 10th	Night	23% 9% 33% 29% 6%	760
	11th to 20th	Night	20% 7% 32% 29% 10% 2	686
	21st to end	Night	21% 7% 29% 30% 13%	570
May	1st to 10th	Night	18% 3 24% 37% 16% 2	565
	11th to 20th	Night	16% 4 25% 40% 14% 1	374
	21st to end	Night	11% 4 22% 40% 21% 1	411
June	1st to 10th	Night	14% 6% 27% 38% 14% 1	380
	11th to 20th	Night	8% 7% 23% 45% 16% 1	380
	21st to end	Night	14% 3 22% 41% 19% 1	380
July	1st to 10th	Night	8% 3 16% 48% 24% 1	370
	11th to 20th	Night	12% 3 20% 41% 22% 2	368
	21st to end	Night	9% 3 17% 44% 25% 1	566
August	1st to 10th	Night	11% 2 14% 42% 29% 2	567
	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	24% 10% 28% 26% 12% 1	755
	11th to 20th	Night	23% 14% 33% 21% 9% 1	760
	21st to end	Night	27% 12% 33% 24% 4	760
December	1st to 10th	Night	27% 13% 37% 18% 4 1	760
	11th to 20th	Night	22% 13% 38% 21% 5% 1	760
	21st to end	Night	28% 11% 35% 22% 3	836

Page generated at 2024-11-01 22:47:37. Version ac07

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