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 2024-01-12.

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% 8% 4 7% 6% 3 5% 5% 5% 7% 37%	74779
	Night	14% 9% 4 6% 5% 3 4 4 5% 7% 40%	74949
Transparency	Day	46% 4 12% 23% 13% 2	74480
	Night	45% 3 11% 24% 15% 2	74769
Seeing	Day	17% 9% 28% 26% 15% 5%	14555
	Night	18% 9% 31% 29% 12% 1	24518

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% 9% 8% 5% 4 4 5% 6% 24%	1517
	11th to 20th	Day	15% 18% 6% 10% 8% 3 5% 4 4 6% 20%	1448
	21st to end	Day	14% 14% 7% 12% 8% 4 5% 3 4 5% 25%	1729
February	1st to 10th	Day	16% 13% 6% 10% 6% 4 6% 4 4 5% 28%	1642
	11th to 20th	Day	14% 10% 4 9% 8% 5% 5% 4 5% 7% 31%	1748
	21st to end	Day	16% 13% 5% 10% 5% 3 3 4 5% 6% 27%	1480
March	1st to 10th	Day	15% 13% 5% 6% 6% 4 5% 4 6% 6% 32%	1800
	11th to 20th	Day	13% 10% 4 8% 6% 3 5% 5% 6% 8% 31%	1893
	21st to end	Day	20% 13% 5% 7% 6% 3 5% 5% 5% 6% 25%	2359
April	1st to 10th	Day	15% 9% 4 7% 5% 3 5% 5% 5% 6% 36%	2160
	11th to 20th	Day	15% 10% 4 6% 6% 3 4 3 5% 7% 37%	2134
	21st to end	Day	21% 7% 3 6% 6% 3 4 5% 6% 6% 34%	2177
May	1st to 10th	Day	17% 9% 3 7% 5% 3 5% 5% 6% 7% 32%	2340
	11th to 20th	Day	20% 7% 3 6% 5% 3 5% 5% 5% 7% 35%	2421
	21st to end	Day	13% 6% 3 6% 6% 4 6% 6% 6% 9% 36%	2725
June	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 5% 7% 9% 45%	2520
	11th to 20th	Day	8% 5% 2 5% 5% 3 5% 5% 6% 9% 47%	2520
	21st to end	Day	9% 6% 3 6% 6% 3 6% 6% 7% 10% 39%	2520
July	1st to 10th	Day	8% 4 2 5% 5% 4 5% 6% 7% 11% 43%	2520
	11th to 20th	Day	8% 4 3 4 5% 3 6% 6% 7% 10% 44%	2520
	21st to end	Day	8% 5% 3 5% 5% 3 5% 6% 7% 9% 45%	2743
August	1st to 10th	Day	7% 4 2 5% 5% 3 5% 5% 7% 9% 47%	2360
	11th to 20th	Day	7% 4 2 6% 5% 4 6% 6% 5% 9% 46%	2125
	21st to end	Day	7% 3 2 5% 5% 3 5% 6% 7% 9% 48%	2376
September	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 5% 6% 8% 44%	2160
	11th to 20th	Day	11% 4 2 4 4 3 4 4 5% 6% 52%	2079
	21st to end	Day	9% 6% 3 5% 5% 2 3 4 6% 7% 48%	1980
October	1st to 10th	Day	11% 5% 3 5% 5% 4 5% 4 5% 6% 47%	1887
	11th to 20th	Day	18% 8% 2 5% 5% 3 4 3 5% 6% 41%	1800
	21st to end	Day	19% 8% 3 7% 5% 3 4 4 5% 7% 36%	1840
November	1st to 10th	Day	11% 6% 3 4 4 2 4 4 5% 7% 49%	1672
	11th to 20th	Day	17% 10% 4 6% 4 3 4 4 5% 6% 38%	1710
	21st to end	Day	17% 10% 6% 8% 6% 2 3 3 4 4 37%	1653
December	1st to 10th	Day	14% 12% 6% 8% 6% 3 5% 5% 5% 6% 30%	1520
	11th to 20th	Day	14% 14% 4 10% 6% 3 4 5% 5% 6% 29%	1918
	21st to end	Day	21% 18% 9% 13% 6% 4 3 3 3 4 16%	2783
January	1st to 10th	Night	12% 12% 5% 9% 7% 4 4 5% 5% 5% 33%	2648
	11th to 20th	Night	15% 12% 6% 9% 7% 3 4 4 4 6% 29%	2541
	21st to end	Night	15% 11% 6% 9% 8% 4 5% 5% 4 5% 29%	2624
February	1st to 10th	Night	17% 12% 7% 9% 6% 3 4 4 4 5% 29%	2316
	11th to 20th	Night	13% 10% 4 8% 6% 3 5% 4 5% 6% 35%	2098
	21st to end	Night	17% 13% 4 7% 5% 3 4 3 4 6% 34%	1776
March	1st to 10th	Night	14% 12% 4 6% 6% 4 4 4 5% 6% 35%	2160
	11th to 20th	Night	15% 9% 4 7% 6% 3 4 4 4 7% 38%	2083
	21st to end	Night	20% 10% 4 6% 6% 3 5% 4 4 7% 30%	1995
April	1st to 10th	Night	13% 8% 4 6% 5% 3 4 4 6% 7% 40%	1800
	11th to 20th	Night	16% 7% 2 5% 5% 4 5% 4 4 6% 41%	1775
	21st to end	Night	20% 8% 3 5% 5% 3 4 4 4 7% 37%	1782
May	1st to 10th	Night	16% 7% 3 6% 5% 3 4 5% 6% 9% 36%	1620
	11th to 20th	Night	17% 6% 3 6% 4 3 3 4 4 9% 42%	1470
	21st to end	Night	9% 7% 2 6% 6% 4 5% 5% 7% 10% 38%	1551
June	1st to 10th	Night	9% 4 3 4 5% 3 3 5% 5% 9% 50%	1440
	11th to 20th	Night	7% 5% 2 5% 5% 4 5% 5% 6% 8% 49%	1440
	21st to end	Night	8% 4 2 4 7% 5% 6% 6% 6% 8% 44%	1440
July	1st to 10th	Night	8% 5% 2 4 5% 3 4 5% 6% 9% 50%	1440
	11th to 20th	Night	6% 3 1 4 4 2 4 5% 5% 9% 57%	1440
	21st to end	Night	4 4 3 4 5% 4 4 4 6% 10% 54%	1575
August	1st to 10th	Night	7% 3 2 4 5% 3 5% 4 7% 9% 53%	1571
	11th to 20th	Night	6% 4 2 5% 5% 3 5% 4 4 9% 53%	1776
	21st to end	Night	6% 3 2 3 5% 3 5% 4 5% 7% 57%	1980
September	1st to 10th	Night	12% 4 2 3 4 2 3 5% 5% 7% 53%	1800
	11th to 20th	Night	9% 4 1 4 4 3 4 3 5% 7% 56%	1862
	21st to end	Night	8% 5% 2 5% 5% 3 5% 4 5% 9% 50%	1980
October	1st to 10th	Night	11% 4 2 4 4 3 4 4 5% 6% 53%	2070
	11th to 20th	Night	16% 6% 2 4 4 2 4 4 4 7% 49%	2160
	21st to end	Night	19% 9% 3 5% 5% 2 3 4 4 6% 39%	2515
November	1st to 10th	Night	10% 6% 3 5% 5% 3 4 4 5% 7% 50%	2406
	11th to 20th	Night	13% 8% 3 8% 5% 2 4 4 5% 7% 42%	2470
	21st to end	Night	17% 9% 4 6% 5% 3 3 3 4 6% 38%	2522
December	1st to 10th	Night	15% 12% 4 8% 5% 3 5% 4 5% 6% 34%	2660
	11th to 20th	Night	16% 11% 5% 8% 5% 3 3 4 4 5% 35%	3279
	21st to end	Night	23% 19% 8% 10% 6% 3 4 3 3 4 16%	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	61% 3 8% 17% 11% 1	1518
	11th to 20th	Day	67% 2 11% 13% 6%	1448
	21st to end	Day	64% 2 11% 15% 7%	1729
February	1st to 10th	Day	60% 3 10% 17% 7% 2	1642
	11th to 20th	Day	55% 2 12% 21% 9% 1	1758
	21st to end	Day	59% 3 8% 21% 8% 2	1480
March	1st to 10th	Day	53% 3 11% 22% 9% 1	1800
	11th to 20th	Day	51% 4 13% 23% 9% 1	1893
	21st to end	Day	60% 4 11% 19% 5% 1	2359
April	1st to 10th	Day	49% 3 12% 23% 13%	2160
	11th to 20th	Day	48% 3 10% 24% 13% 3	2134
	21st to end	Day	50% 4 12% 20% 12% 2	2177
May	1st to 10th	Day	50% 5% 13% 23% 8%	2340
	11th to 20th	Day	48% 4 13% 21% 12% 1	2425
	21st to end	Day	44% 5% 13% 20% 16% 2	2725
June	1st to 10th	Day	35% 4 13% 29% 15% 3	2520
	11th to 20th	Day	34% 4 11% 26% 21% 4	2520
	21st to end	Day	39% 6% 14% 22% 16% 3	2520
July	1st to 10th	Day	34% 4 16% 28% 14% 4	2451
	11th to 20th	Day	34% 5% 14% 31% 14% 2	2445
	21st to end	Day	34% 5% 14% 30% 16% 1	2591
August	1st to 10th	Day	33% 4 18% 32% 12% 2	2345
	11th to 20th	Day	34% 5% 13% 30% 16% 2	2125
	21st to end	Day	31% 5% 13% 26% 20% 5%	2376
September	1st to 10th	Day	37% 4 11% 24% 19% 5%	2160
	11th to 20th	Day	33% 3 10% 27% 22% 5%	2079
	21st to end	Day	34% 4 8% 29% 23% 2	1980
October	1st to 10th	Day	38% 3 9% 26% 19% 4	1887
	11th to 20th	Day	45% 2 8% 23% 19% 2	1800
	21st to end	Day	49% 3 10% 22% 13% 2	1840
November	1st to 10th	Day	35% 3 14% 26% 18% 4	1672
	11th to 20th	Day	47% 2 10% 25% 13% 2	1710
	21st to end	Day	52% 2 10% 24% 11% 1	1653
December	1st to 10th	Day	55% 4 11% 21% 8% 1	1520
	11th to 20th	Day	56% 3 9% 24% 7% 1	1915
	21st to end	Day	74% 2 8% 10% 5%	2783
January	1st to 10th	Night	54% 3 9% 19% 15%	2648
	11th to 20th	Night	58% 2 13% 18% 9%	2541
	21st to end	Night	58% 3 11% 19% 9% 1	2624
February	1st to 10th	Night	58% 2 11% 17% 10% 2	2316
	11th to 20th	Night	51% 3 11% 22% 11% 2	2099
	21st to end	Night	53% 2 9% 22% 12% 2	1776
March	1st to 10th	Night	51% 2 10% 23% 13% 1	2160
	11th to 20th	Night	47% 3 11% 22% 14% 2	2083
	21st to end	Night	54% 3 11% 20% 9% 1	1995
April	1st to 10th	Night	43% 3 12% 25% 14% 3	1800
	11th to 20th	Night	45% 3 7% 25% 15% 5%	1775
	21st to end	Night	49% 3 11% 21% 12% 4	1782
May	1st to 10th	Night	45% 4 13% 25% 12%	1620
	11th to 20th	Night	42% 3 14% 23% 17% 1	1471
	21st to end	Night	41% 6% 13% 20% 17% 3	1552
June	1st to 10th	Night	31% 4 13% 30% 18% 5%	1440
	11th to 20th	Night	33% 3 12% 25% 23% 4	1440
	21st to end	Night	35% 6% 15% 23% 17% 3	1440
July	1st to 10th	Night	31% 4 14% 31% 16% 4	1400
	11th to 20th	Night	24% 4 14% 33% 22% 3	1392
	21st to end	Night	26% 4 14% 33% 20% 3	1487
August	1st to 10th	Night	27% 5% 16% 34% 16% 2	1562
	11th to 20th	Night	30% 3 12% 31% 22% 2	1776
	21st to end	Night	27% 4 13% 28% 22% 6%	1980
September	1st to 10th	Night	32% 3 10% 27% 22% 6%	1800
	11th to 20th	Night	28% 2 9% 28% 26% 5%	1862
	21st to end	Night	32% 4 10% 29% 22% 4	1980
October	1st to 10th	Night	34% 3 10% 26% 24% 4	2070
	11th to 20th	Night	37% 3 8% 28% 21% 3	2160
	21st to end	Night	47% 2 9% 23% 17% 2	2515
November	1st to 10th	Night	35% 3 10% 28% 19% 4	2406
	11th to 20th	Night	43% 3 11% 24% 17% 2	2470
	21st to end	Night	49% 2 10% 25% 13% 1	2522
December	1st to 10th	Night	52% 3 12% 22% 10% 1	2660
	11th to 20th	Night	53% 2 10% 22% 11% 1	3281
	21st to end	Night	75% 2 6% 11% 6%	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	24% 16% 34% 19% 6% 1	270
	11th to 20th	Day	26% 18% 39% 12% 4 1	243
	21st to end	Day	20% 19% 36% 20% 4	264
February	1st to 10th	Day	27% 19% 35% 16% 3	240
	11th to 20th	Day	26% 22% 31% 15% 6%	240
	21st to end	Day	29% 18% 39% 14% 1	198
March	1st to 10th	Day	25% 21% 33% 18% 3	240
	11th to 20th	Day	26% 14% 41% 13% 6%	240
	21st to end	Day	33% 11% 32% 18% 5% 1	445
April	1st to 10th	Day	20% 12% 38% 22% 6% 1	420
	11th to 20th	Day	20% 15% 32% 20% 11% 2	412
	21st to end	Day	27% 13% 31% 21% 7% 1	420
May	1st to 10th	Day	22% 11% 35% 21% 9% 2	423
	11th to 20th	Day	23% 8% 28% 27% 11% 3	567
	21st to end	Day	14% 6% 31% 27% 14% 7%	679
June	1st to 10th	Day	9% 4 28% 33% 20% 7%	630
	11th to 20th	Day	11% 5% 26% 33% 20% 5%	630
	21st to end	Day	11% 7% 30% 31% 15% 4	630
July	1st to 10th	Day	11% 1 18% 29% 24% 16%	621
	11th to 20th	Day	11% 2 21% 32% 24% 10%	618
	21st to end	Day	10% 3 19% 36% 26% 7%	684
August	1st to 10th	Day	8% 1 18% 35% 27% 11%	586
	11th to 20th	Day	11% 4 18% 28% 27% 11%	439
	21st to end	Day	8% 2 18% 32% 33% 8%	495
September	1st to 10th	Day	14% 5% 20% 28% 29% 5%	450
	11th to 20th	Day	11% 3 16% 32% 27% 12%	450
	21st to end	Day	12% 8% 25% 24% 23% 7%	450
October	1st to 10th	Day	15% 6% 29% 30% 15% 5%	357
	11th to 20th	Day	20% 18% 33% 21% 6% 1	270
	21st to end	Day	33% 14% 33% 16% 3 1	297
November	1st to 10th	Day	14% 13% 35% 25% 10% 3	270
	11th to 20th	Day	22% 17% 36% 16% 7% 1	270
	21st to end	Day	26% 10% 36% 24% 5%	270
December	1st to 10th	Day	18% 19% 38% 15% 6% 3	270
	11th to 20th	Day	18% 17% 39% 21% 4	270
	21st to end	Day	26% 21% 35% 15% 3	297
January	1st to 10th	Night	19% 15% 39% 22% 5%	950
	11th to 20th	Night	21% 14% 39% 21% 4	908
	21st to end	Night	21% 14% 38% 24% 4	990
February	1st to 10th	Night	24% 11% 39% 21% 4	876
	11th to 20th	Night	21% 14% 36% 24% 4	703
	21st to end	Night	26% 13% 38% 20% 3 1	592
March	1st to 10th	Night	22% 13% 38% 23% 5%	720
	11th to 20th	Night	21% 11% 35% 26% 5% 1	643
	21st to end	Night	25% 8% 36% 24% 6%	594
April	1st to 10th	Night	16% 14% 39% 26% 5%	540
	11th to 20th	Night	19% 12% 33% 26% 9%	533
	21st to end	Night	25% 9% 33% 26% 6% 1	540
May	1st to 10th	Night	19% 7% 35% 30% 8% 1	540
	11th to 20th	Night	20% 5% 26% 35% 13% 1	531
	21st to end	Night	11% 3 32% 31% 22% 2	584
June	1st to 10th	Night	11% 2 24% 45% 16%	540
	11th to 20th	Night	10% 3 26% 44% 16% 1	540
	21st to end	Night	9% 4 27% 39% 20% 1	540
July	1st to 10th	Night	11% 2 14% 36% 32% 5%	525
	11th to 20th	Night	8% 1 19% 42% 28% 3	522
	21st to end	Night	6% 1 15% 46% 30% 2	591
August	1st to 10th	Night	8% 13% 43% 31% 5%	537
	11th to 20th	Night	7% 2 13% 41% 31% 5%	534
	21st to end	Night	8% 2 16% 39% 33% 3	594
September	1st to 10th	Night	15% 3 15% 38% 27% 2	540
	11th to 20th	Night	10% 2 19% 36% 28% 4	602
	21st to end	Night	11% 7% 22% 34% 24% 3	720
October	1st to 10th	Night	13% 4 28% 35% 18% 3	720
	11th to 20th	Night	19% 13% 30% 30% 9% 1	720
	21st to end	Night	26% 11% 35% 21% 7%	792
November	1st to 10th	Night	13% 11% 40% 26% 9% 1	740
	11th to 20th	Night	18% 15% 35% 25% 7% 1	760
	21st to end	Night	21% 13% 36% 24% 5%	812
December	1st to 10th	Night	22% 12% 38% 23% 5%	950
	11th to 20th	Night	23% 12% 34% 26% 5%	950
	21st to end	Night	30% 14% 33% 21% 2	1045

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