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 2025-10-04.

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	13% 9% 3 8% 5% 5% 5% 4 6% 8% 37%	126539
	Night	13% 10% 4 8% 4 4 4 3 5% 6% 39%	123079
Transparency	Day	45% 4 11% 24% 14% 2	130072
	Night	45% 3 10% 24% 15% 2	127607
Seeing	Day	17% 10% 29% 25% 15% 5%	17015
	Night	18% 9% 31% 29% 12% 1	26772

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	13% 12% 7% 14% 7% 6% 5% 3 5% 6% 22%	3617
	11th to 20th	Day	10% 11% 5% 14% 8% 8% 7% 4 6% 5% 21%	3620
	21st to end	Day	16% 17% 7% 11% 7% 4 4 3 3 5% 24%	4365
February	1st to 10th	Day	15% 13% 6% 10% 5% 6% 4 4 4 6% 29%	1825
	11th to 20th	Day	13% 11% 4 10% 6% 6% 4 3 4 7% 32%	1948
	21st to end	Day	15% 13% 4 11% 4 4 3 3 5% 6% 31%	1650
March	1st to 10th	Day	18% 15% 3 5% 3 3 3 3 4 4 39%	4480
	11th to 20th	Day	7% 8% 3 7% 4 4 5% 4 6% 9% 42%	4767
	21st to end	Day	18% 12% 4 10% 5% 5% 4 4 5% 6% 27%	5936
April	1st to 10th	Day	15% 11% 3 7% 5% 4 5% 4 5% 6% 35%	2400
	11th to 20th	Day	15% 11% 4 7% 5% 3 4 3 5% 7% 37%	2374
	21st to end	Day	20% 7% 3 8% 4 4 4 4 6% 6% 34%	2419
May	1st to 10th	Day	18% 8% 2 7% 4 4 5% 4 6% 8% 32%	5862
	11th to 20th	Day	16% 7% 2 7% 4 4 4 4 5% 7% 40%	6177
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	6935
June	1st to 10th	Day	8% 5% 2 7% 4 4 5% 5% 7% 9% 44%	2800
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 4 6% 9% 47%	2800
	21st to end	Day	9% 6% 3 6% 6% 5% 6% 4 7% 9% 40%	2800
July	1st to 10th	Day	10% 5% 2 7% 4 4 5% 4 7% 12% 40%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	6360
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 4 8% 9% 37%	6924
August	1st to 10th	Day	6% 5% 2 6% 4 4 4 4 7% 10% 48%	4282
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 10% 41%	3849
	21st to end	Day	5% 4 2 6% 4 5% 6% 4 7% 10% 48%	4312
September	1st to 10th	Day	11% 5% 1 6% 4 4 4 4 6% 7% 48%	2400
	11th to 20th	Day	10% 4 2 5% 4 5% 3 3 5% 7% 52%	2309
	21st to end	Day	10% 7% 2 6% 4 3 3 4 6% 7% 47%	2200
October	1st to 10th	Day	11% 5% 3 6% 4 4 4 3 5% 6% 48%	2024
	11th to 20th	Day	16% 8% 2 6% 4 4 3 3 5% 5% 43%	1900
	21st to end	Day	18% 8% 3 8% 4 4 4 3 5% 7% 36%	1942
November	1st to 10th	Day	11% 8% 2 5% 4 3 4 3 5% 7% 48%	1762
	11th to 20th	Day	16% 12% 3 7% 3 4 4 3 5% 6% 38%	1800
	21st to end	Day	16% 11% 5% 9% 6% 3 3 3 4 5% 35%	1740
December	1st to 10th	Day	8% 10% 4 8% 4 3 4 3 5% 7% 44%	2610
	11th to 20th	Day	13% 16% 6% 12% 5% 5% 5% 4 5% 6% 24%	3008
	21st to end	Day	25% 19% 7% 13% 5% 4 3 2 3 4 14%	3982
January	1st to 10th	Night	14% 14% 6% 12% 5% 4 3 3 4 4 30%	6348
	11th to 20th	Night	11% 15% 6% 13% 7% 6% 5% 4 5% 4 23%	6360
	21st to end	Night	18% 14% 6% 9% 5% 4 4 3 3 4 31%	6610
February	1st to 10th	Night	15% 12% 6% 9% 5% 4 4 3 4 6% 31%	2573
	11th to 20th	Night	13% 11% 4 9% 5% 4 4 3 4 6% 35%	2338
	21st to end	Night	16% 12% 3 8% 4 4 4 3 4 6% 37%	1980
March	1st to 10th	Night	17% 14% 4 6% 3 3 3 3 4 5% 39%	5373
	11th to 20th	Night	11% 8% 3 6% 4 4 3 3 5% 7% 47%	5223
	21st to end	Night	17% 11% 4 9% 5% 5% 5% 4 5% 6% 29%	4991
April	1st to 10th	Night	13% 10% 3 7% 4 4 5% 3 6% 7% 40%	2000
	11th to 20th	Night	17% 7% 2 6% 5% 4 5% 3 4 6% 41%	1975
	21st to end	Night	19% 8% 3 6% 4 4 4 3 5% 7% 37%	1980
May	1st to 10th	Night	19% 10% 3 7% 5% 4 3 3 5% 7% 34%	4033
	11th to 20th	Night	14% 6% 2 6% 3 4 4 4 7% 10% 40%	3728
	21st to end	Night	13% 9% 3 7% 4 4 5% 3 5% 8% 39%	3942
June	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 6% 9% 49%	1600
	11th to 20th	Night	8% 5% 2 6% 4 5% 4 4 6% 8% 47%	1600
	21st to end	Night	7% 5% 1 5% 6% 6% 6% 4 7% 8% 45%	1600
July	1st to 10th	Night	9% 6% 2 7% 4 4 4 4 6% 9% 44%	3618
	11th to 20th	Night	6% 5% 2 6% 5% 4 5% 5% 7% 9% 47%	3617
	21st to end	Night	9% 7% 3 7% 5% 4 4 3 6% 9% 42%	3947
August	1st to 10th	Night	5% 4 2 5% 3 4 5% 3 7% 9% 52%	2867
	11th to 20th	Night	6% 5% 2 5% 4 4 5% 3 5% 8% 52%	3200
	21st to end	Night	5% 4 2 5% 4 4 4 4 6% 8% 55%	3586
September	1st to 10th	Night	11% 4 2 4 3 3 3 3 5% 7% 55%	2000
	11th to 20th	Night	8% 4 1 4 3 4 3 3 6% 7% 56%	2070
	21st to end	Night	8% 6% 2 6% 4 4 5% 3 5% 8% 49%	2200
October	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 6% 54%	2219
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 4 6% 51%	2280
	21st to end	Night	19% 9% 2 6% 4 3 3 3 5% 7% 39%	2655
November	1st to 10th	Night	11% 7% 2 5% 4 3 4 3 5% 6% 49%	2536
	11th to 20th	Night	13% 9% 3 8% 4 3 4 3 5% 7% 41%	2600
	21st to end	Night	17% 11% 4 7% 4 4 4 3 5% 6% 37%	2655
December	1st to 10th	Night	11% 9% 3 8% 4 4 4 3 4 5% 45%	4580
	11th to 20th	Night	16% 12% 5% 10% 5% 4 4 3 4 5% 32%	5199
	21st to end	Night	25% 19% 7% 10% 5% 4 4 3 4 4 16%	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	64% 2 8% 18% 7% 1	3618
	11th to 20th	Day	65% 2 9% 12% 10% 3	3620
	21st to end	Day	65% 2 9% 16% 7% 1	4365
February	1st to 10th	Day	58% 3 11% 18% 8% 2	1825
	11th to 20th	Day	54% 2 11% 21% 10% 1	1958
	21st to end	Day	55% 3 8% 22% 10% 2	1650
March	1st to 10th	Day	51% 2 7% 21% 16% 3	4480
	11th to 20th	Day	39% 3 10% 33% 15% 1	4767
	21st to end	Day	58% 3 13% 19% 6% 1	5938
April	1st to 10th	Day	50% 3 11% 23% 12%	2400
	11th to 20th	Day	48% 3 10% 24% 13% 3	2374
	21st to end	Day	49% 4 12% 21% 12% 2	2419
May	1st to 10th	Day	49% 4 11% 23% 12% 1	5897
	11th to 20th	Day	45% 4 10% 27% 12% 3	6181
	21st to end	Day	42% 5% 11% 23% 17% 2	6949
June	1st to 10th	Day	35% 5% 13% 29% 15% 3	2800
	11th to 20th	Day	34% 4 10% 27% 21% 4	2800
	21st to end	Day	38% 6% 14% 22% 17% 3	2800
July	1st to 10th	Day	36% 4 14% 29% 15% 2	6291
	11th to 20th	Day	40% 7% 14% 26% 14% 1	6285
	21st to end	Day	41% 6% 17% 23% 11% 2	6728
August	1st to 10th	Day	30% 4 19% 32% 13% 1	4267
	11th to 20th	Day	36% 5% 14% 28% 16% 1	3885
	21st to end	Day	31% 5% 13% 30% 17% 4	4311
September	1st to 10th	Day	35% 4 10% 25% 21% 5%	2400
	11th to 20th	Day	32% 3 10% 26% 24% 5%	2309
	21st to end	Day	36% 4 8% 28% 23% 2	2200
October	1st to 10th	Day	26% 2 6% 25% 37% 3	3336
	11th to 20th	Day	30% 2 9% 22% 34% 3	3140
	21st to end	Day	42% 2 8% 24% 21% 2	3180
November	1st to 10th	Day	36% 3 14% 26% 17% 3	1762
	11th to 20th	Day	48% 2 10% 24% 13% 3	1800
	21st to end	Day	53% 3 10% 23% 10% 1	1740
December	1st to 10th	Day	41% 3 13% 32% 10%	2610
	11th to 20th	Day	62% 3 10% 19% 6% 1	3005
	21st to end	Day	77% 2 9% 8% 5%	3982
January	1st to 10th	Night	58% 2 9% 22% 8%	6348
	11th to 20th	Night	65% 1 8% 12% 11% 2	6360
	21st to end	Night	59% 2 8% 19% 11% 1	6610
February	1st to 10th	Night	56% 2 11% 18% 11% 2	2573
	11th to 20th	Night	51% 3 10% 22% 12% 2	2339
	21st to end	Night	50% 2 9% 23% 14% 2	1980
March	1st to 10th	Night	50% 1 8% 24% 13% 3	5373
	11th to 20th	Night	39% 3 9% 28% 18% 3	5223
	21st to end	Night	56% 3 10% 22% 8% 1	4991
April	1st to 10th	Night	44% 3 11% 25% 15% 3	2000
	11th to 20th	Night	45% 3 7% 26% 15% 5%	1975
	21st to end	Night	49% 3 11% 22% 12% 4	1980
May	1st to 10th	Night	51% 3 11% 21% 11% 3	4060
	11th to 20th	Night	40% 4 13% 27% 14% 3	3729
	21st to end	Night	44% 4 11% 23% 15% 3	3950
June	1st to 10th	Night	33% 4 13% 29% 17% 4	1600
	11th to 20th	Night	35% 3 12% 24% 22% 4	1600
	21st to end	Night	34% 6% 15% 24% 17% 3	1600
July	1st to 10th	Night	37% 3 11% 30% 16% 2	3580
	11th to 20th	Night	30% 7% 15% 27% 19% 1	3572
	21st to end	Night	38% 6% 16% 22% 14% 3	3832
August	1st to 10th	Night	27% 5% 17% 33% 17% 2	2858
	11th to 20th	Night	31% 4 14% 32% 18% 1	3230
	21st to end	Night	27% 4 14% 33% 18% 3	3586
September	1st to 10th	Night	30% 3 9% 28% 24% 6%	2000
	11th to 20th	Night	28% 3 10% 28% 27% 5%	2070
	21st to end	Night	34% 4 9% 28% 22% 3	2200
October	1st to 10th	Night	22% 2 6% 28% 38% 3	3674
	11th to 20th	Night	25% 2 8% 25% 35% 4	3779
	21st to end	Night	38% 2 10% 23% 23% 4	4367
November	1st to 10th	Night	36% 3 10% 28% 19% 4	2536
	11th to 20th	Night	43% 3 11% 23% 17% 2	2600
	21st to end	Night	50% 2 9% 24% 13% 1	2655
December	1st to 10th	Night	42% 2 12% 31% 12% 1	4580
	11th to 20th	Night	55% 2 11% 20% 11% 1	5201
	21st to end	Night	73% 2 8% 11% 5%	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	24% 17% 34% 18% 5% 1	300
	11th to 20th	Day	25% 19% 40% 11% 3 1	300
	21st to end	Day	23% 18% 36% 19% 4	330
February	1st to 10th	Day	24% 19% 36% 18% 3	300
	11th to 20th	Day	23% 24% 34% 14% 5%	300
	21st to end	Day	27% 18% 41% 14% 1	249
March	1st to 10th	Day	25% 21% 35% 16% 3	300
	11th to 20th	Day	25% 19% 38% 13% 5%	300
	21st to end	Day	34% 12% 33% 16% 4 1	532
April	1st to 10th	Day	24% 13% 36% 20% 6% 1	500
	11th to 20th	Day	22% 16% 31% 20% 10% 2	492
	21st to end	Day	26% 13% 33% 21% 6% 1	500
May	1st to 10th	Day	22% 11% 34% 21% 10% 2	503
	11th to 20th	Day	23% 9% 26% 28% 11% 3	661
	21st to end	Day	15% 8% 32% 26% 13% 6%	789
June	1st to 10th	Day	10% 6% 28% 31% 18% 6%	730
	11th to 20th	Day	11% 5% 27% 33% 19% 5%	730
	21st to end	Day	11% 7% 32% 32% 15% 4	730
July	1st to 10th	Day	11% 2 21% 30% 23% 14%	721
	11th to 20th	Day	11% 3 23% 31% 23% 10%	718
	21st to end	Day	11% 3 18% 37% 25% 7%	794
August	1st to 10th	Day	8% 2 19% 36% 25% 10%	681
	11th to 20th	Day	10% 5% 19% 29% 27% 10%	519
	21st to end	Day	7% 2 17% 33% 33% 8%	583
September	1st to 10th	Day	12% 5% 20% 30% 28% 5%	530
	11th to 20th	Day	9% 3 15% 31% 28% 12%	530
	21st to end	Day	15% 7% 25% 23% 23% 8%	530
October	1st to 10th	Day	14% 6% 28% 30% 18% 5%	403
	11th to 20th	Day	18% 18% 32% 22% 8% 1	300
	21st to end	Day	32% 15% 32% 16% 5% 1	330
November	1st to 10th	Day	16% 14% 33% 25% 10% 3	300
	11th to 20th	Day	22% 18% 36% 16% 7% 1	300
	21st to end	Day	25% 11% 37% 22% 5%	300
December	1st to 10th	Day	18% 20% 40% 14% 6% 3	300
	11th to 20th	Day	19% 19% 38% 20% 4	300
	21st to end	Day	28% 19% 35% 15% 3	330
January	1st to 10th	Night	19% 15% 39% 23% 4	1000
	11th to 20th	Night	21% 16% 38% 20% 4	1000
	21st to end	Night	22% 14% 38% 23% 3	1100
February	1st to 10th	Night	23% 13% 39% 21% 4	973
	11th to 20th	Night	21% 15% 36% 24% 4	783
	21st to end	Night	25% 13% 39% 20% 2	660
March	1st to 10th	Night	22% 14% 38% 22% 4	800
	11th to 20th	Night	20% 13% 36% 25% 5% 1	713
	21st to end	Night	25% 10% 36% 23% 5%	660
April	1st to 10th	Night	18% 15% 38% 25% 5%	600
	11th to 20th	Night	20% 13% 33% 25% 8%	593
	21st to end	Night	24% 11% 33% 26% 6% 1	600
May	1st to 10th	Night	19% 8% 35% 29% 8% 1	600
	11th to 20th	Night	19% 6% 27% 35% 13% 1	591
	21st to end	Night	11% 4 33% 30% 20% 1	650
June	1st to 10th	Night	11% 3 26% 44% 15%	600
	11th to 20th	Night	11% 4 26% 44% 14% 1	600
	21st to end	Night	9% 4 28% 39% 20% 1	600
July	1st to 10th	Night	11% 3 14% 36% 31% 5%	585
	11th to 20th	Night	8% 1 19% 41% 28% 3	582
	21st to end	Night	7% 1 16% 43% 30% 2	657
August	1st to 10th	Night	8% 1 15% 43% 30% 4	597
	11th to 20th	Night	7% 3 13% 43% 30% 5%	594
	21st to end	Night	7% 2 16% 42% 32% 2	660
September	1st to 10th	Night	14% 3 15% 38% 28% 2	600
	11th to 20th	Night	9% 2 17% 37% 30% 4	670
	21st to end	Night	11% 7% 22% 33% 24% 2	800
October	1st to 10th	Night	12% 4 27% 36% 18% 3	773
	11th to 20th	Night	18% 12% 30% 30% 10% 1	760
	21st to end	Night	25% 11% 35% 22% 7%	836
November	1st to 10th	Night	15% 11% 39% 26% 8% 1	780
	11th to 20th	Night	18% 15% 36% 25% 6% 1	800
	21st to end	Night	22% 14% 36% 23% 5%	855
December	1st to 10th	Night	22% 12% 39% 22% 5%	1000
	11th to 20th	Night	23% 12% 34% 26% 5%	1000
	21st to end	Night	31% 14% 33% 21% 2	1100

Page generated at 2025-10-01 00:39:08. Version ac07

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