Bloomington
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bloomington clear sky chart from 2005-11-06 to 2026-06-16.

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% 9% 3 7% 5% 4 5% 4 6% 8% 36%	137022
	Night	14% 10% 3 8% 5% 4 4 3 5% 7% 37%	135172
Transparency	Day	45% 4 10% 24% 15% 2	140630
	Night	46% 3 10% 23% 15% 2	139638
Seeing	Day	21% 13% 28% 21% 12% 4	17685
	Night	20% 13% 27% 25% 13% 2	27522

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	14% 14% 6% 10% 4 4 4 3 6% 7% 29%	4691
	11th to 20th	Day	10% 12% 6% 13% 7% 4 4 3 5% 5% 31%	4710
	21st to end	Day	17% 14% 5% 11% 7% 5% 4 3 4 4 25%	5540
February	1st to 10th	Day	17% 12% 5% 9% 5% 4 3 3 5% 6% 31%	2100
	11th to 20th	Day	15% 12% 5% 9% 4 4 4 4 4 7% 31%	2048
	21st to end	Day	16% 12% 5% 10% 4 3 3 3 5% 6% 33%	1730
March	1st to 10th	Day	19% 14% 5% 7% 4 3 3 2 3 4 38%	4580
	11th to 20th	Day	12% 10% 3 6% 4 4 4 3 6% 8% 41%	4887
	21st to end	Day	16% 11% 3 8% 5% 4 4 4 6% 7% 32%	5900
April	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 3 5% 8% 36%	2520
	11th to 20th	Day	16% 9% 3 6% 4 4 4 3 5% 6% 38%	2494
	21st to end	Day	19% 7% 3 7% 5% 4 4 3 5% 7% 36%	2662
May	1st to 10th	Day	17% 9% 3 7% 5% 5% 5% 4 7% 9% 29%	7616
	11th to 20th	Day	17% 8% 3 6% 4 4 4 3 5% 7% 38%	7626
	21st to end	Day	15% 8% 3 9% 6% 5% 6% 5% 7% 9% 27%	8547
June	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 4 5% 9% 43%	2940
	11th to 20th	Day	8% 5% 3 6% 5% 5% 6% 5% 7% 9% 42%	2871
	21st to end	Day	9% 6% 3 6% 5% 4 6% 5% 7% 8% 42%	2800
July	1st to 10th	Day	11% 6% 3 6% 4 4 5% 4 7% 11% 41%	6360
	11th to 20th	Day	8% 6% 2 7% 5% 6% 6% 5% 7% 11% 37%	6360
	21st to end	Day	10% 7% 3 8% 5% 6% 7% 6% 8% 9% 32%	6708
August	1st to 10th	Day	8% 5% 2 5% 3 4 4 4 7% 10% 48%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 38%	3894
	21st to end	Day	5% 4 2 5% 4 4 4 3 6% 9% 54%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 4 5% 3 6% 7% 47%	2380
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 7% 8% 49%	2200
	21st to end	Day	13% 4 3 5% 4 3 4 3 5% 7% 49%	2200
October	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 4 7% 51%	2200
	11th to 20th	Day	13% 6% 2 6% 3 4 4 3 5% 7% 45%	2056
	21st to end	Day	19% 9% 3 7% 3 4 4 3 4 6% 39%	1980
November	1st to 10th	Day	9% 6% 2 5% 4 3 4 4 5% 6% 52%	1852
	11th to 20th	Day	16% 10% 3 7% 4 3 3 3 5% 7% 39%	1890
	21st to end	Day	19% 12% 3 7% 4 3 3 2 4 5% 37%	1890
December	1st to 10th	Day	13% 12% 4 8% 3 3 3 2 4 5% 43%	3020
	11th to 20th	Day	16% 14% 5% 9% 4 5% 5% 4 4 5% 29%	3154
	21st to end	Day	24% 19% 7% 8% 4 4 4 3 4 5% 18%	4070
January	1st to 10th	Night	15% 14% 6% 11% 5% 5% 4 3 4 6% 26%	8232
	11th to 20th	Night	11% 15% 6% 14% 7% 6% 5% 3 4 4 26%	8280
	21st to end	Night	17% 11% 5% 8% 6% 5% 4 3 5% 6% 31%	8643
February	1st to 10th	Night	18% 13% 5% 9% 5% 4 4 4 6% 5% 27%	2520
	11th to 20th	Night	15% 12% 4 9% 6% 4 4 3 5% 6% 32%	2458
	21st to end	Night	18% 12% 3 9% 4 4 4 2 5% 5% 33%	2076
March	1st to 10th	Night	20% 15% 3 6% 3 2 3 3 3 5% 36%	5493
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 5% 7% 46%	5292
	21st to end	Night	16% 9% 3 8% 5% 6% 5% 4 5% 7% 32%	5314
April	1st to 10th	Night	14% 9% 3 7% 4 4 4 4 5% 7% 40%	2100
	11th to 20th	Night	15% 9% 3 6% 4 4 3 4 5% 7% 40%	2075
	21st to end	Night	15% 7% 3 6% 4 4 4 4 7% 8% 37%	1958
May	1st to 10th	Night	19% 9% 3 8% 4 4 5% 3 5% 8% 31%	5237
	11th to 20th	Night	15% 7% 3 6% 5% 4 5% 4 6% 8% 38%	5234
	21st to end	Night	15% 8% 3 8% 5% 6% 6% 4 6% 9% 31%	5596
June	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 5% 10% 45%	1680
	11th to 20th	Night	10% 4 2 7% 4 4 4 3 5% 10% 47%	1640
	21st to end	Night	6% 3 1 5% 3 4 4 5% 8% 11% 49%	1600
July	1st to 10th	Night	8% 5% 2 6% 5% 4 4 4 6% 8% 47%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 5% 8% 10% 44%	3617
	21st to end	Night	7% 6% 2 7% 5% 6% 6% 5% 8% 10% 38%	4151
August	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 5% 8% 51%	2915
	11th to 20th	Night	6% 5% 2 6% 4 5% 5% 4 7% 8% 47%	3138
	21st to end	Night	4 3 1 5% 4 4 5% 4 5% 9% 58%	3586
September	1st to 10th	Night	10% 6% 2 6% 3 4 4 4 4 7% 51%	2000
	11th to 20th	Night	7% 4 2 5% 3 3 3 4 6% 7% 56%	2199
	21st to end	Night	13% 5% 2 6% 4 3 3 3 4 7% 49%	2200
October	1st to 10th	Night	9% 4 1 4 4 3 3 3 5% 6% 58%	2200
	11th to 20th	Night	15% 7% 2 4 3 3 3 3 4 6% 49%	2337
	21st to end	Night	18% 8% 2 6% 4 4 4 3 4 6% 41%	2860
November	1st to 10th	Night	11% 7% 2 5% 4 3 4 3 5% 7% 48%	2666
	11th to 20th	Night	14% 9% 3 7% 4 4 4 3 5% 6% 41%	2730
	21st to end	Night	20% 10% 4 8% 4 4 3 2 4 6% 35%	2730
December	1st to 10th	Night	18% 11% 3 7% 5% 4 3 2 4 4 38%	4380
	11th to 20th	Night	18% 13% 5% 10% 4 4 4 3 5% 5% 28%	5267
	21st to end	Night	27% 18% 6% 9% 4 4 3 3 3 4 18%	7150

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	55% 2 11% 23% 8% 1	4692
	11th to 20th	Day	56% 2 9% 17% 14% 2	4710
	21st to end	Day	63% 2 7% 19% 5% 5%	5540
February	1st to 10th	Day	55% 3 8% 22% 9% 2	2100
	11th to 20th	Day	53% 3 9% 23% 11% 2	2058
	21st to end	Day	54% 3 8% 23% 10% 3	1730
March	1st to 10th	Day	53% 1 5% 19% 17% 5%	4580
	11th to 20th	Day	43% 2 8% 31% 14% 2	4887
	21st to end	Day	53% 2 12% 21% 10% 1	5902
April	1st to 10th	Day	48% 3 12% 21% 15% 1	2520
	11th to 20th	Day	46% 3 9% 25% 13% 3	2494
	21st to end	Day	48% 3 9% 25% 12% 3	2662
May	1st to 10th	Day	51% 4 12% 21% 11%	7651
	11th to 20th	Day	47% 3 11% 24% 15%	7630
	21st to end	Day	52% 5% 11% 18% 13% 2	8561
June	1st to 10th	Day	38% 4 10% 27% 17% 3	2940
	11th to 20th	Day	37% 4 13% 24% 19% 3	2871
	21st to end	Day	37% 6% 14% 21% 18% 4	2800
July	1st to 10th	Day	37% 5% 10% 27% 20% 2	6291
	11th to 20th	Day	39% 7% 13% 26% 14%	6285
	21st to end	Day	45% 7% 14% 22% 11% 1	6516
August	1st to 10th	Day	30% 5% 14% 34% 16% 1	4221
	11th to 20th	Day	36% 5% 13% 29% 16% 1	3931
	21st to end	Day	27% 4 14% 34% 17% 4	4311
September	1st to 10th	Day	36% 3 11% 26% 20% 4	2380
	11th to 20th	Day	33% 4 9% 30% 21% 3	2200
	21st to end	Day	35% 3 8% 25% 25% 2	2200
October	1st to 10th	Day	22% 3 6% 31% 37% 2	3590
	11th to 20th	Day	30% 3 8% 28% 29% 2	3324
	21st to end	Day	39% 2 8% 24% 25% 2	3180
November	1st to 10th	Day	33% 4 10% 31% 19% 4	1852
	11th to 20th	Day	45% 3 9% 23% 16% 4	1890
	21st to end	Day	51% 2 9% 23% 15% 1	1890
December	1st to 10th	Day	46% 2 10% 27% 14%	3020
	11th to 20th	Day	58% 3 7% 22% 8% 1	3151
	21st to end	Day	70% 2 10% 11% 6%	4070
January	1st to 10th	Night	59% 2 11% 20% 7%	8233
	11th to 20th	Night	63% 2 8% 13% 12% 3	8280
	21st to end	Night	55% 2 10% 22% 10% 1	8643
February	1st to 10th	Night	58% 4 11% 19% 7% 1	2520
	11th to 20th	Night	54% 3 9% 21% 12% 1	2459
	21st to end	Night	55% 3 9% 20% 11% 2	2076
March	1st to 10th	Night	53% 2 5% 24% 12% 4	5493
	11th to 20th	Night	39% 2 9% 26% 21% 1	5292
	21st to end	Night	52% 3 9% 27% 8% 1	5314
April	1st to 10th	Night	44% 3 10% 27% 14% 2	2100
	11th to 20th	Night	44% 3 10% 21% 17% 5%	2075
	21st to end	Night	45% 3 9% 27% 13% 3	1958
May	1st to 10th	Night	52% 4 11% 23% 8% 2	5264
	11th to 20th	Night	44% 4 12% 23% 16%	5235
	21st to end	Night	50% 4 13% 20% 10% 2	5606
June	1st to 10th	Night	36% 3 10% 25% 22% 3	1680
	11th to 20th	Night	33% 3 12% 25% 21% 6%	1640
	21st to end	Night	27% 8% 14% 25% 21% 5%	1600
July	1st to 10th	Night	35% 6% 10% 27% 20% 3	3580
	11th to 20th	Night	33% 7% 16% 28% 15% 1	3572
	21st to end	Night	39% 7% 16% 25% 12% 2	4030
August	1st to 10th	Night	32% 4 14% 36% 12% 1	2904
	11th to 20th	Night	31% 5% 14% 30% 18% 1	3166
	21st to end	Night	24% 5% 12% 32% 23% 4	3586
September	1st to 10th	Night	33% 4 9% 28% 20% 7%	2000
	11th to 20th	Night	27% 4 7% 29% 29% 4	2199
	21st to end	Night	36% 3 10% 22% 24% 5%	2200
October	1st to 10th	Night	19% 3 7% 22% 47% 3	3590
	11th to 20th	Night	26% 2 7% 26% 36% 3	3808
	21st to end	Night	35% 3 7% 24% 26% 5%	4610
November	1st to 10th	Night	37% 3 8% 26% 22% 4	2666
	11th to 20th	Night	44% 2 8% 23% 19% 3	2730
	21st to end	Night	52% 2 9% 22% 13% 1	2730
December	1st to 10th	Night	50% 2 11% 24% 13%	4380
	11th to 20th	Night	58% 3 10% 18% 10%	5269
	21st to end	Night	72% 2 8% 12% 5%	7150

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	32% 21% 30% 14% 4	330
	11th to 20th	Day	34% 26% 31% 8% 1	330
	21st to end	Day	32% 26% 30% 10% 1 1	363
February	1st to 10th	Day	30% 25% 32% 11% 2	330
	11th to 20th	Day	35% 25% 31% 7% 2 1	330
	21st to end	Day	32% 22% 39% 7%	273
March	1st to 10th	Day	32% 25% 34% 8% 1	330
	11th to 20th	Day	31% 22% 34% 9% 4	441
	21st to end	Day	32% 21% 30% 14% 3	594
April	1st to 10th	Day	29% 19% 30% 15% 6% 1	540
	11th to 20th	Day	29% 21% 23% 19% 8% 1	532
	21st to end	Day	27% 19% 34% 13% 7% 1	540
May	1st to 10th	Day	26% 15% 34% 17% 6% 2	543
	11th to 20th	Day	26% 13% 29% 20% 7% 4	567
	21st to end	Day	18% 12% 29% 21% 14% 6%	669
June	1st to 10th	Day	14% 9% 32% 26% 16% 4	780
	11th to 20th	Day	12% 8% 29% 30% 16% 5%	756
	21st to end	Day	13% 8% 32% 24% 17% 6%	730
July	1st to 10th	Day	12% 5% 21% 27% 24% 11%	721
	11th to 20th	Day	8% 6% 27% 32% 19% 8%	718
	21st to end	Day	10% 5% 22% 34% 21% 7%	704
August	1st to 10th	Day	11% 5% 17% 33% 25% 10%	526
	11th to 20th	Day	14% 5% 24% 29% 22% 6%	519
	21st to end	Day	8% 4 20% 34% 23% 11%	583
September	1st to 10th	Day	11% 6% 20% 32% 24% 6%	530
	11th to 20th	Day	10% 3 17% 31% 27% 12%	530
	21st to end	Day	18% 10% 22% 22% 21% 7%	530
October	1st to 10th	Day	10% 9% 28% 32% 16% 6%	530
	11th to 20th	Day	18% 18% 33% 18% 10% 3	440
	21st to end	Day	33% 13% 39% 14% 1	363
November	1st to 10th	Day	14% 23% 35% 22% 5% 1	330
	11th to 20th	Day	29% 21% 29% 16% 5% 1	330
	21st to end	Day	31% 13% 38% 13% 4	330
December	1st to 10th	Day	24% 24% 34% 14% 3 1	330
	11th to 20th	Day	31% 18% 36% 11% 3	330
	21st to end	Day	37% 19% 32% 9% 2 1	363
January	1st to 10th	Night	27% 22% 29% 18% 3	1050
	11th to 20th	Night	29% 23% 32% 14% 2	1050
	21st to end	Night	26% 20% 35% 17% 2	1067
February	1st to 10th	Night	28% 18% 34% 18% 2	840
	11th to 20th	Night	25% 24% 32% 16% 2	823
	21st to end	Night	28% 20% 34% 17% 1	692
March	1st to 10th	Night	28% 19% 30% 20% 2	840
	11th to 20th	Night	25% 19% 32% 18% 5%	840
	21st to end	Night	26% 17% 30% 22% 5%	805
April	1st to 10th	Night	20% 16% 33% 25% 5% 1	630
	11th to 20th	Night	21% 17% 29% 24% 9%	623
	21st to end	Night	21% 11% 36% 24% 7% 1	630
May	1st to 10th	Night	21% 12% 33% 25% 9% 1	630
	11th to 20th	Night	16% 10% 28% 28% 15% 3	621
	21st to end	Night	12% 7% 25% 33% 21% 3	683
June	1st to 10th	Night	13% 5% 23% 35% 22% 1	630
	11th to 20th	Night	11% 6% 23% 41% 20% 1	615
	21st to end	Night	6% 4 26% 37% 25% 2	600
July	1st to 10th	Night	10% 2 14% 33% 30% 10%	585
	11th to 20th	Night	7% 5% 17% 34% 31% 6%	582
	21st to end	Night	9% 4 14% 39% 31% 5%	657
August	1st to 10th	Night	9% 2 13% 38% 32% 6%	597
	11th to 20th	Night	8% 2 14% 37% 35% 5%	594
	21st to end	Night	5% 1 17% 35% 35% 7%	660
September	1st to 10th	Night	13% 5% 15% 35% 29% 4	600
	11th to 20th	Night	8% 5% 17% 31% 33% 6%	799
	21st to end	Night	16% 8% 20% 28% 24% 4	800
October	1st to 10th	Night	12% 7% 23% 31% 24% 4	800
	11th to 20th	Night	18% 13% 28% 25% 14% 2	800
	21st to end	Night	24% 15% 31% 21% 7%	880
November	1st to 10th	Night	16% 12% 35% 25% 11%	820
	11th to 20th	Night	24% 17% 29% 22% 7%	840
	21st to end	Night	27% 18% 25% 23% 6%	840
December	1st to 10th	Night	28% 16% 33% 19% 4	840
	11th to 20th	Night	28% 16% 30% 21% 5%	1004
	21st to end	Night	36% 16% 30% 15% 3	1155

Page generated at 2026-06-13 02:31:44. Version ac07

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