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 2025-02-21.

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% 4 4 4 4 6% 7% 38%	108473
	Night	15% 10% 4 7% 4 4 4 3 5% 6% 38%	110228
Transparency	Day	44% 3 10% 24% 15% 2	112125
	Night	46% 3 10% 23% 16% 2	114715
Seeing	Day	20% 13% 28% 22% 12% 4	15861
	Night	20% 13% 27% 25% 13% 2	25808

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% 15% 6% 11% 3 4 4 4 5% 5% 28%	3617
	11th to 20th	Day	12% 13% 7% 13% 7% 4 4 3 4 4 28%	3620
	21st to end	Day	20% 14% 6% 9% 5% 4 4 2 4 4 29%	4272
February	1st to 10th	Day	18% 13% 5% 8% 5% 4 3 3 5% 6% 30%	2000
	11th to 20th	Day	15% 11% 5% 10% 4 4 4 4 4 7% 31%	1948
	21st to end	Day	17% 13% 5% 10% 4 3 3 3 5% 6% 32%	1570
March	1st to 10th	Day	23% 16% 3 7% 4 3 3 2 3 4 33%	3120
	11th to 20th	Day	14% 8% 3 6% 3 4 4 3 5% 6% 44%	3338
	21st to end	Day	15% 10% 4 8% 6% 5% 4 4 5% 7% 33%	4027
April	1st to 10th	Day	15% 10% 3 7% 5% 4 4 3 5% 8% 37%	2280
	11th to 20th	Day	17% 9% 3 6% 4 4 4 3 5% 6% 39%	2254
	21st to end	Day	19% 7% 3 7% 5% 4 4 3 5% 7% 36%	2408
May	1st to 10th	Day	18% 9% 3 7% 5% 5% 5% 4 7% 9% 28%	4082
	11th to 20th	Day	20% 7% 3 6% 4 4 4 3 5% 6% 37%	4081
	21st to end	Day	10% 7% 3 7% 6% 5% 5% 5% 7% 8% 37%	4562
June	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 5% 9% 42%	2660
	11th to 20th	Day	8% 5% 2 5% 5% 5% 6% 4 7% 9% 44%	2660
	21st to end	Day	9% 6% 3 5% 5% 5% 6% 5% 7% 8% 42%	2660
July	1st to 10th	Day	14% 6% 3 6% 4 4 5% 4 7% 11% 38%	4440
	11th to 20th	Day	6% 5% 2 5% 4 4 5% 5% 7% 11% 47%	4440
	21st to end	Day	9% 7% 3 7% 5% 6% 7% 6% 8% 10% 31%	4666
August	1st to 10th	Day	8% 5% 2 5% 3 4 4 4 7% 10% 48%	4104
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	3772
	21st to end	Day	5% 4 2 5% 4 4 4 3 6% 9% 54%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 3 6% 8% 46%	2261
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 7% 8% 48%	2090
	21st to end	Day	13% 4 2 5% 4 3 4 3 5% 7% 49%	2090
October	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 4 7% 51%	2090
	11th to 20th	Day	13% 6% 2 6% 3 4 4 3 5% 7% 45%	1954
	21st to end	Day	19% 9% 3 7% 3 4 4 3 4 6% 38%	1881
November	1st to 10th	Day	9% 5% 2 5% 4 3 4 4 5% 6% 52%	1762
	11th to 20th	Day	16% 10% 3 7% 4 3 3 3 5% 7% 39%	1800
	21st to end	Day	19% 12% 3 6% 5% 3 3 3 4 6% 38%	1800
December	1st to 10th	Day	14% 12% 4 7% 3 3 3 2 4 5% 44%	2930
	11th to 20th	Day	16% 14% 5% 9% 4 6% 5% 4 4 5% 29%	3072
	21st to end	Day	25% 19% 7% 8% 4 4 4 3 4 5% 18%	3982
January	1st to 10th	Night	16% 15% 7% 11% 6% 5% 4 4 4 6% 23%	6348
	11th to 20th	Night	13% 17% 7% 13% 6% 6% 5% 3 3 4 23%	6360
	21st to end	Night	20% 13% 5% 7% 5% 4 4 3 4 5% 31%	6679
February	1st to 10th	Night	18% 13% 5% 9% 5% 4 4 3 6% 6% 27%	2400
	11th to 20th	Night	15% 12% 4 10% 6% 4 4 3 5% 6% 31%	2338
	21st to end	Night	20% 13% 4 9% 3 4 4 2 5% 5% 31%	1885
March	1st to 10th	Night	22% 16% 3 6% 4 2 3 3 4 5% 32%	3733
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 48%	3621
	21st to end	Night	16% 8% 3 7% 5% 4 4 3 5% 6% 38%	3633
April	1st to 10th	Night	12% 9% 3 7% 4 4 4 4 5% 7% 41%	1900
	11th to 20th	Night	16% 9% 2 6% 4 4 3 4 5% 7% 41%	1875
	21st to end	Night	16% 7% 4 6% 4 4 4 4 7% 7% 37%	1772
May	1st to 10th	Night	18% 11% 4 8% 5% 4 5% 3 5% 8% 28%	2813
	11th to 20th	Night	18% 7% 2 5% 4 4 4 3 5% 8% 40%	2812
	21st to end	Night	10% 5% 2 6% 5% 5% 5% 3 6% 10% 43%	2992
June	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 5% 10% 46%	1520
	11th to 20th	Night	9% 4 2 6% 4 3 4 3 5% 11% 48%	1520
	21st to end	Night	6% 3 1 5% 3 4 4 5% 8% 11% 49%	1520
July	1st to 10th	Night	9% 5% 3 6% 5% 4 4 4 5% 8% 48%	2530
	11th to 20th	Night	5% 4 1 5% 4 3 4 5% 7% 9% 54%	2530
	21st to end	Night	8% 7% 3 8% 6% 6% 5% 5% 7% 9% 37%	2889
August	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 5% 8% 51%	2825
	11th to 20th	Night	6% 5% 2 7% 4 5% 5% 4 7% 8% 47%	3040
	21st to end	Night	4 3 1 5% 4 4 5% 5% 5% 8% 57%	3476
September	1st to 10th	Night	10% 6% 2 6% 3 4 4 4 4 7% 51%	1900
	11th to 20th	Night	7% 4 2 5% 3 3 3 4 6% 8% 55%	2089
	21st to end	Night	13% 5% 2 6% 4 3 3 3 4 7% 49%	2090
October	1st to 10th	Night	9% 4 1 4 4 3 3 3 5% 6% 58%	2090
	11th to 20th	Night	15% 7% 2 4 3 3 3 3 4 6% 49%	2220
	21st to end	Night	18% 8% 2 6% 4 4 4 3 4 6% 41%	2717
November	1st to 10th	Night	12% 7% 2 5% 4 4 4 3 4 7% 48%	2536
	11th to 20th	Night	15% 9% 3 7% 4 4 4 3 5% 6% 41%	2600
	21st to end	Night	19% 10% 4 8% 4 4 3 2 4 6% 35%	2600
December	1st to 10th	Night	18% 11% 3 7% 5% 4 3 2 4 4 39%	4250
	11th to 20th	Night	19% 13% 5% 10% 4 4 4 3 5% 5% 28%	5129
	21st to end	Night	27% 19% 6% 9% 4 4 3 3 3 4 18%	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% 1 7% 24% 8% 1	3618
	11th to 20th	Day	61% 1 8% 15% 12% 2	3620
	21st to end	Day	61% 2 6% 21% 6% 4	4272
February	1st to 10th	Day	56% 3 8% 21% 10% 2	2000
	11th to 20th	Day	52% 3 9% 23% 11% 2	1958
	21st to end	Day	55% 3 7% 23% 10% 3	1570
March	1st to 10th	Day	57% 1 6% 20% 12% 4	3120
	11th to 20th	Day	42% 2 9% 30% 16% 2	3338
	21st to end	Day	53% 2 13% 22% 9% 1	4027
April	1st to 10th	Day	47% 3 12% 22% 16% 1	2280
	11th to 20th	Day	46% 3 9% 26% 14% 3	2254
	21st to end	Day	49% 3 9% 23% 12% 4	2408
May	1st to 10th	Day	53% 3 13% 23% 7%	4120
	11th to 20th	Day	49% 2 11% 26% 12% 1	4085
	21st to end	Day	42% 4 11% 20% 20% 3	4575
June	1st to 10th	Day	38% 4 10% 27% 17% 3	2660
	11th to 20th	Day	35% 4 13% 24% 20% 3	2660
	21st to end	Day	38% 6% 14% 21% 18% 4	2660
July	1st to 10th	Day	41% 4 12% 24% 18% 2	4371
	11th to 20th	Day	29% 6% 13% 32% 20% 1	4365
	21st to end	Day	44% 7% 15% 19% 13% 2	4518
August	1st to 10th	Day	30% 5% 14% 34% 16% 1	4091
	11th to 20th	Day	36% 5% 12% 30% 16% 1	3809
	21st to end	Day	27% 4 14% 33% 17% 4	4179
September	1st to 10th	Day	37% 4 10% 25% 19% 5%	2261
	11th to 20th	Day	34% 4 9% 29% 21% 3	2090
	21st to end	Day	35% 3 8% 27% 24% 2	2090
October	1st to 10th	Day	22% 3 6% 31% 37% 2	3480
	11th to 20th	Day	30% 3 8% 28% 29% 2	3222
	21st to end	Day	38% 2 8% 25% 25% 2	3081
November	1st to 10th	Day	33% 4 10% 30% 19% 4	1762
	11th to 20th	Day	45% 3 9% 23% 17% 4	1800
	21st to end	Day	50% 2 9% 23% 15% 1	1800
December	1st to 10th	Day	45% 2 10% 28% 15%	2930
	11th to 20th	Day	59% 3 7% 22% 8% 1	3069
	21st to end	Day	71% 2 10% 12% 6%	3982
January	1st to 10th	Night	62% 2 10% 18% 8%	6348
	11th to 20th	Night	67% 1 8% 12% 9% 3	6360
	21st to end	Night	58% 2 8% 21% 10% 1	6679
February	1st to 10th	Night	58% 4 11% 18% 7% 1	2400
	11th to 20th	Night	55% 3 9% 20% 12% 1	2339
	21st to end	Night	56% 3 9% 19% 10% 3	1885
March	1st to 10th	Night	56% 2 6% 22% 12% 1	3733
	11th to 20th	Night	39% 2 11% 25% 22% 1	3621
	21st to end	Night	49% 2 10% 30% 9% 1	3633
April	1st to 10th	Night	43% 3 10% 27% 14% 2	1900
	11th to 20th	Night	44% 2 9% 21% 18% 6%	1875
	21st to end	Night	46% 3 9% 25% 13% 3	1772
May	1st to 10th	Night	55% 3 12% 21% 7% 1	2840
	11th to 20th	Night	44% 4 13% 28% 11% 1	2813
	21st to end	Night	38% 4 13% 24% 16% 4	3000
June	1st to 10th	Night	35% 3 10% 26% 21% 4	1520
	11th to 20th	Night	31% 4 13% 25% 21% 6%	1520
	21st to end	Night	27% 8% 14% 25% 21% 5%	1520
July	1st to 10th	Night	36% 5% 9% 27% 20% 3	2490
	11th to 20th	Night	25% 6% 13% 33% 21% 2	2482
	21st to end	Night	41% 6% 14% 23% 13% 3	2797
August	1st to 10th	Night	32% 4 14% 36% 12% 1	2814
	11th to 20th	Night	32% 5% 14% 29% 18% 1	3068
	21st to end	Night	24% 5% 13% 31% 23% 4	3476
September	1st to 10th	Night	34% 4 10% 26% 19% 7%	1900
	11th to 20th	Night	28% 4 8% 29% 28% 5%	2089
	21st to end	Night	36% 3 10% 23% 23% 5%	2090
October	1st to 10th	Night	19% 3 7% 22% 47% 3	3480
	11th to 20th	Night	26% 2 7% 26% 36% 3	3691
	21st to end	Night	34% 3 7% 25% 26% 5%	4467
November	1st to 10th	Night	37% 3 8% 25% 22% 5%	2536
	11th to 20th	Night	44% 2 8% 23% 19% 4	2600
	21st to end	Night	52% 2 9% 22% 13% 1	2600
December	1st to 10th	Night	49% 2 11% 24% 14%	4250
	11th to 20th	Night	58% 4 10% 18% 10%	5131
	21st to end	Night	72% 2 8% 12% 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	31% 21% 29% 15% 3	300
	11th to 20th	Day	35% 24% 32% 8% 1	300
	21st to end	Day	31% 26% 31% 10% 1 1	330
February	1st to 10th	Day	31% 25% 32% 10% 2	300
	11th to 20th	Day	35% 25% 31% 7% 2 1	300
	21st to end	Day	34% 22% 37% 7%	225
March	1st to 10th	Day	31% 25% 34% 9% 1	270
	11th to 20th	Day	30% 20% 35% 10% 5% 1	370
	21st to end	Day	31% 22% 29% 14% 4	506
April	1st to 10th	Day	28% 19% 30% 16% 6% 1	460
	11th to 20th	Day	27% 22% 22% 19% 8% 1	452
	21st to end	Day	28% 20% 33% 12% 7% 1	460
May	1st to 10th	Day	25% 15% 34% 17% 7% 2	463
	11th to 20th	Day	25% 12% 29% 22% 8% 5%	487
	21st to end	Day	15% 12% 29% 22% 15% 7%	576
June	1st to 10th	Day	13% 8% 31% 27% 16% 4	680
	11th to 20th	Day	11% 7% 28% 31% 17% 5%	680
	21st to end	Day	13% 8% 33% 24% 16% 6%	680
July	1st to 10th	Day	13% 6% 21% 27% 22% 11%	671
	11th to 20th	Day	8% 6% 27% 33% 19% 8%	668
	21st to end	Day	10% 6% 24% 35% 19% 7%	654
August	1st to 10th	Day	12% 4 17% 33% 24% 10%	486
	11th to 20th	Day	15% 5% 25% 29% 21% 6%	479
	21st to end	Day	9% 4 19% 33% 24% 12%	539
September	1st to 10th	Day	12% 6% 20% 33% 23% 6%	490
	11th to 20th	Day	11% 3 18% 32% 26% 9%	490
	21st to end	Day	18% 10% 22% 23% 20% 6%	490
October	1st to 10th	Day	10% 9% 29% 32% 14% 6%	490
	11th to 20th	Day	19% 20% 34% 16% 9% 2	405
	21st to end	Day	32% 13% 39% 15% 1	330
November	1st to 10th	Day	13% 23% 34% 23% 6% 1	300
	11th to 20th	Day	27% 21% 31% 15% 5% 1	300
	21st to end	Day	29% 13% 40% 13% 5%	300
December	1st to 10th	Day	24% 25% 31% 15% 3 1	300
	11th to 20th	Day	30% 18% 37% 12% 3	300
	21st to end	Day	38% 19% 31% 10% 2 1	330
January	1st to 10th	Night	27% 22% 29% 18% 3	1000
	11th to 20th	Night	30% 23% 32% 14% 2	1000
	21st to end	Night	26% 19% 36% 18% 2	1016
February	1st to 10th	Night	29% 19% 33% 17% 2	800
	11th to 20th	Night	25% 25% 33% 16% 2	783
	21st to end	Night	29% 20% 33% 16% 1	629
March	1st to 10th	Night	27% 19% 31% 20% 2	760
	11th to 20th	Night	24% 19% 33% 18% 6% 1	760
	21st to end	Night	26% 16% 30% 21% 6%	729
April	1st to 10th	Night	19% 17% 31% 26% 5% 1	570
	11th to 20th	Night	21% 17% 29% 24% 10%	563
	21st to end	Night	22% 12% 36% 22% 7% 1	570
May	1st to 10th	Night	21% 12% 33% 25% 9% 1	570
	11th to 20th	Night	17% 9% 28% 27% 16% 3	561
	21st to end	Night	11% 6% 24% 34% 22% 3	617
June	1st to 10th	Night	13% 5% 23% 36% 22% 1	570
	11th to 20th	Night	10% 5% 23% 42% 20% 1	570
	21st to end	Night	6% 4 26% 37% 24% 2	570
July	1st to 10th	Night	11% 2 15% 34% 29% 10%	555
	11th to 20th	Night	7% 5% 17% 34% 31% 6%	552
	21st to end	Night	8% 4 14% 39% 30% 5%	624
August	1st to 10th	Night	8% 2 13% 38% 32% 7%	567
	11th to 20th	Night	8% 2 14% 38% 33% 5%	564
	21st to end	Night	5% 1 18% 33% 36% 7%	627
September	1st to 10th	Night	13% 5% 14% 35% 29% 4	570
	11th to 20th	Night	8% 5% 18% 31% 33% 5%	759
	21st to end	Night	16% 8% 21% 28% 23% 4	760
October	1st to 10th	Night	11% 6% 24% 31% 23% 4	760
	11th to 20th	Night	19% 13% 29% 24% 13% 2	760
	21st to end	Night	24% 15% 31% 22% 8%	836
November	1st to 10th	Night	17% 12% 34% 25% 12% 1	780
	11th to 20th	Night	24% 17% 29% 23% 7% 1	800
	21st to end	Night	27% 18% 26% 23% 6%	800
December	1st to 10th	Night	28% 16% 33% 19% 5%	800
	11th to 20th	Night	28% 15% 30% 21% 6% 1	956
	21st to end	Night	36% 16% 29% 16% 3	1100

Page generated at 2025-02-18 03:26:31. Version ac07

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