Indian Prairie Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Indian Prairie Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	9% 8% 3 7% 5% 4 4 4 5% 7% 45%	130558
	Night	10% 9% 3 8% 5% 5% 5% 4 5% 7% 40%	128851
Transparency	Day	39% 6% 11% 23% 16% 6%	130284
	Night	44% 6% 11% 20% 14% 5%	128702
Seeing	Day	13% 8% 19% 27% 22% 10%	16084
	Night	16% 6% 15% 36% 23% 3	25239

By Month

Percentage Cloud Cover NW (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	18% 22% 8% 15% 7% 6% 4 3 4 4 10%	3639
	11th to 20th	Day	14% 10% 4 11% 6% 6% 6% 4 5% 7% 28%	3616
	21st to end	Day	18% 15% 5% 10% 5% 4 4 4 5% 6% 23%	4004
February	1st to 10th	Day	17% 16% 4 10% 6% 5% 4 3 5% 8% 22%	1614
	11th to 20th	Day	13% 14% 4 10% 6% 6% 5% 4 5% 6% 27%	1862
	21st to end	Day	12% 15% 5% 9% 5% 5% 5% 5% 5% 5% 29%	1650
March	1st to 10th	Day	7% 9% 3 7% 6% 6% 7% 6% 7% 9% 32%	4518
	11th to 20th	Day	8% 8% 3 8% 5% 5% 5% 4 5% 8% 42%	4663
	21st to end	Day	11% 11% 4 10% 7% 6% 7% 5% 6% 8% 24%	5959
April	1st to 10th	Day	11% 9% 4 9% 6% 6% 6% 5% 8% 7% 30%	2400
	11th to 20th	Day	9% 7% 2 8% 6% 5% 6% 4 6% 8% 39%	2427
	21st to end	Day	8% 7% 3 8% 5% 6% 5% 5% 6% 7% 40%	2600
May	1st to 10th	Day	6% 5% 2 5% 5% 5% 4 4 7% 9% 48%	6246
	11th to 20th	Day	8% 6% 3 8% 5% 6% 5% 4 7% 9% 39%	6277
	21st to end	Day	7% 5% 2 7% 5% 5% 5% 4 6% 9% 45%	6903
June	1st to 10th	Day	13% 8% 3 6% 4 4 4 4 6% 7% 42%	2970
	11th to 20th	Day	11% 6% 2 6% 4 5% 6% 5% 6% 8% 42%	3000
	21st to end	Day	6% 4 2 5% 4 4 4 3 5% 6% 58%	2981
July	1st to 10th	Day	3 3 1 3 2 2 3 2 4 4 73%	6320
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 80%	6320
	21st to end	Day	3 2 1 2 2 2 3 2 4 5% 74%	6874
August	1st to 10th	Day	3 2 1 3 3 3 3 2 4 6% 70%	4541
	11th to 20th	Day	3 3 1 3 2 3 3 3 5% 7% 67%	4311
	21st to end	Day	2 3 1 5% 4 3 4 3 5% 6% 64%	4362
September	1st to 10th	Day	4 3 1 3 2 3 3 3 4 6% 68%	2400
	11th to 20th	Day	5% 5% 2 5% 4 4 4 4 6% 7% 55%	2300
	21st to end	Day	6% 4 1 6% 4 4 4 4 7% 8% 51%	2200
October	1st to 10th	Day	6% 5% 2 5% 3 3 3 3 4 7% 58%	3443
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 4 7% 8% 43%	3078
	21st to end	Day	13% 9% 3 7% 4 6% 6% 4 5% 7% 37%	3200
November	1st to 10th	Day	20% 10% 3 9% 6% 4 4 3 5% 6% 29%	1854
	11th to 20th	Day	17% 13% 5% 10% 6% 5% 5% 3 4 5% 26%	1768
	21st to end	Day	15% 19% 4 12% 6% 6% 4 4 4 6% 20%	1566
December	1st to 10th	Day	13% 11% 5% 13% 9% 7% 6% 4 6% 8% 18%	2304
	11th to 20th	Day	19% 21% 7% 13% 8% 5% 5% 3 3 4 12%	2642
	21st to end	Day	17% 20% 7% 12% 7% 6% 5% 4 5% 4 12%	3746
January	1st to 10th	Night	21% 20% 6% 13% 7% 5% 4 3 4 4 13%	6306
	11th to 20th	Night	12% 11% 4 10% 6% 5% 6% 5% 6% 7% 28%	6282
	21st to end	Night	16% 15% 5% 8% 5% 5% 4 5% 9% 6% 22%	6952
February	1st to 10th	Night	14% 16% 4 9% 5% 5% 4 4 5% 6% 27%	2785
	11th to 20th	Night	12% 13% 4 9% 6% 4 4 4 6% 7% 30%	2425
	21st to end	Night	10% 12% 3 8% 6% 5% 4 4 6% 7% 33%	1980
March	1st to 10th	Night	6% 9% 3 7% 5% 4 5% 4 6% 9% 42%	5376
	11th to 20th	Night	9% 11% 3 7% 4 4 4 3 4 6% 45%	5335
	21st to end	Night	10% 8% 4 10% 7% 7% 6% 5% 8% 8% 26%	5012
April	1st to 10th	Night	10% 9% 3 7% 6% 5% 5% 6% 6% 8% 35%	2000
	11th to 20th	Night	8% 9% 3 7% 5% 5% 4 4 5% 6% 44%	1922
	21st to end	Night	7% 8% 3 7% 5% 4 5% 5% 7% 7% 41%	1800
May	1st to 10th	Night	6% 5% 2 6% 5% 5% 5% 4 7% 8% 48%	3714
	11th to 20th	Night	9% 7% 3 7% 5% 4 5% 4 6% 8% 42%	3616
	21st to end	Night	6% 5% 2 5% 4 4 5% 4 6% 9% 49%	3972
June	1st to 10th	Night	10% 6% 2 5% 5% 6% 4 4 5% 8% 46%	1425
	11th to 20th	Night	8% 6% 2 6% 5% 5% 5% 5% 5% 9% 43%	1400
	21st to end	Night	4 4 2 5% 4 4 4 5% 5% 7% 56%	1413
July	1st to 10th	Night	3 2 1 2 2 2 2 2 4 6% 74%	3640
	11th to 20th	Night	2 1 1 2 2 2 2 1 2 4 81%	3640
	21st to end	Night	2 2 1 2 2 1 2 1 3 6% 77%	3969
August	1st to 10th	Night	3 3 1 3 2 2 2 2 4 7% 71%	2609
	11th to 20th	Night	3 3 1 4 3 4 5% 4 5% 6% 63%	2742
	21st to end	Night	3 3 1 4 2 3 4 4 5% 7% 64%	3541
September	1st to 10th	Night	4 2 1 3 2 2 4 3 5% 7% 67%	2000
	11th to 20th	Night	6% 3 1 6% 4 4 4 4 5% 7% 55%	2100
	21st to end	Night	8% 5% 1 5% 4 4 3 3 5% 8% 54%	2200
October	1st to 10th	Night	6% 3 1 3 3 3 3 4 5% 8% 60%	3755
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 7% 51%	4073
	21st to end	Night	12% 6% 2 6% 4 5% 6% 4 6% 7% 43%	4600
November	1st to 10th	Night	18% 11% 3 7% 5% 4 4 4 6% 9% 29%	2665
	11th to 20th	Night	16% 12% 4 9% 6% 5% 7% 4 6% 6% 27%	2851
	21st to end	Night	17% 15% 3 10% 6% 5% 5% 4 5% 7% 24%	3029
December	1st to 10th	Night	9% 9% 5% 11% 7% 7% 6% 7% 6% 7% 26%	4921
	11th to 20th	Night	19% 19% 7% 14% 6% 6% 5% 3 5% 5% 12%	5550
	21st to end	Night	20% 16% 6% 14% 7% 6% 5% 4 5% 4 13%	7251

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	78% 7% 7% 6% 1	3639
	11th to 20th	Day	56% 6% 15% 17% 5% 1	3640
	21st to end	Day	61% 6% 11% 17% 5%	4004
February	1st to 10th	Day	62% 9% 13% 9% 6% 1	1614
	11th to 20th	Day	57% 8% 13% 14% 7%	1871
	21st to end	Day	56% 9% 11% 17% 7% 1	1650
March	1st to 10th	Day	44% 8% 13% 19% 16% 1	4518
	11th to 20th	Day	41% 6% 15% 29% 10%	4663
	21st to end	Day	55% 9% 16% 15% 5%	5959
April	1st to 10th	Day	50% 7% 16% 20% 7% 1	2400
	11th to 20th	Day	42% 6% 17% 21% 13% 2	2427
	21st to end	Day	42% 7% 16% 23% 11% 2	2600
May	1st to 10th	Day	30% 5% 10% 28% 25% 1	6246
	11th to 20th	Day	39% 6% 13% 28% 12% 1	6280
	21st to end	Day	34% 7% 17% 30% 11% 1	6904
June	1st to 10th	Day	40% 6% 10% 23% 18% 3	2970
	11th to 20th	Day	37% 7% 11% 25% 15% 4	3000
	21st to end	Day	26% 5% 11% 31% 21% 6%	2981
July	1st to 10th	Day	17% 3 5% 21% 29% 25%	6251
	11th to 20th	Day	11% 2 4 20% 32% 31%	6245
	21st to end	Day	15% 2 4 27% 33% 19%	6681
August	1st to 10th	Day	16% 3 5% 22% 38% 15%	4524
	11th to 20th	Day	17% 4 8% 39% 27% 5%	4353
	21st to end	Day	21% 3 9% 33% 29% 5%	4361
September	1st to 10th	Day	18% 4 11% 29% 27% 11%	2400
	11th to 20th	Day	28% 6% 13% 26% 23% 5%	2300
	21st to end	Day	30% 7% 16% 30% 12% 4	2200
October	1st to 10th	Day	28% 4 13% 37% 18% 1	3443
	11th to 20th	Day	38% 5% 19% 24% 14%	3078
	21st to end	Day	48% 6% 15% 20% 10% 1	3200
November	1st to 10th	Day	56% 7% 14% 18% 4 1	1854
	11th to 20th	Day	60% 8% 14% 13% 4 1	1768
	21st to end	Day	66% 9% 11% 10% 3 1	1566
December	1st to 10th	Day	66% 7% 14% 10% 2	2304
	11th to 20th	Day	76% 6% 9% 7% 2	2644
	21st to end	Day	73% 7% 10% 8% 2	3746
January	1st to 10th	Night	76% 6% 8% 9% 1	6306
	11th to 20th	Night	56% 7% 14% 15% 6% 1	6320
	21st to end	Night	58% 10% 11% 16% 4	6952
February	1st to 10th	Night	57% 8% 13% 12% 7% 1	2785
	11th to 20th	Night	53% 7% 18% 14% 8% 1	2427
	21st to end	Night	49% 7% 14% 19% 9% 1	1980
March	1st to 10th	Night	39% 6% 14% 24% 16% 1	5376
	11th to 20th	Night	42% 4 12% 27% 14% 1	5336
	21st to end	Night	51% 9% 14% 20% 5%	5012
April	1st to 10th	Night	45% 6% 13% 25% 10% 1	2000
	11th to 20th	Night	40% 6% 10% 24% 17% 2	1922
	21st to end	Night	39% 7% 11% 23% 18% 2	1800
May	1st to 10th	Night	30% 6% 8% 26% 28% 3	3711
	11th to 20th	Night	38% 5% 12% 26% 17% 1	3616
	21st to end	Night	29% 8% 13% 34% 14% 1	3972
June	1st to 10th	Night	35% 7% 8% 24% 21% 5%	1425
	11th to 20th	Night	36% 8% 11% 20% 20% 6%	1400
	21st to end	Night	26% 5% 8% 27% 25% 9%	1413
July	1st to 10th	Night	13% 2 3 19% 32% 31%	3600
	11th to 20th	Night	10% 2 3 16% 29% 40%	3592
	21st to end	Night	13% 1 2 21% 35% 27%	3857
August	1st to 10th	Night	15% 2 3 18% 39% 23%	2601
	11th to 20th	Night	22% 3 5% 28% 32% 10%	2766
	21st to end	Night	20% 4 9% 27% 31% 10%	3541
September	1st to 10th	Night	18% 4 7% 27% 29% 16%	2000
	11th to 20th	Night	27% 5% 10% 24% 24% 9%	2100
	21st to end	Night	28% 4 14% 31% 17% 6%	2200
October	1st to 10th	Night	23% 3 9% 42% 22% 1	3755
	11th to 20th	Night	33% 4 16% 25% 21% 1	4073
	21st to end	Night	41% 6% 13% 29% 9% 2	4600
November	1st to 10th	Night	53% 7% 17% 16% 6% 1	2665
	11th to 20th	Night	57% 8% 14% 13% 6% 2	2851
	21st to end	Night	59% 9% 14% 12% 5%	3029
December	1st to 10th	Night	60% 9% 13% 16% 2	4921
	11th to 20th	Night	76% 6% 9% 7% 2	5547
	21st to end	Night	72% 8% 9% 8% 2	7251

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	29% 13% 11% 24% 18% 7%	200
	11th to 20th	Day	28% 8% 12% 25% 18% 11%	200
	21st to end	Day	26% 5% 14% 33% 19% 4	220
February	1st to 10th	Day	26% 11% 19% 26% 15% 2	214
	11th to 20th	Day	24% 13% 16% 27% 17% 4	450
	21st to end	Day	18% 15% 24% 24% 17% 3	414
March	1st to 10th	Day	17% 15% 23% 25% 16% 5%	500
	11th to 20th	Day	17% 10% 21% 23% 23% 7%	500
	21st to end	Day	18% 15% 22% 27% 14% 4	550
April	1st to 10th	Day	15% 16% 22% 24% 16% 7%	500
	11th to 20th	Day	12% 17% 25% 21% 16% 8%	492
	21st to end	Day	9% 12% 21% 29% 20% 9%	500
May	1st to 10th	Day	10% 8% 24% 30% 23% 6%	503
	11th to 20th	Day	15% 6% 19% 26% 22% 11%	528
	21st to end	Day	11% 8% 17% 26% 24% 14%	573
June	1st to 10th	Day	15% 6% 19% 29% 23% 9%	700
	11th to 20th	Day	12% 7% 29% 31% 16% 6%	730
	21st to end	Day	6% 5% 18% 29% 30% 12%	711
July	1st to 10th	Day	4 4 19% 34% 31% 9%	526
	11th to 20th	Day	3 5% 21% 32% 25% 14%	524
	21st to end	Day	3 3 14% 31% 33% 15%	575
August	1st to 10th	Day	6% 3 13% 31% 28% 18%	527
	11th to 20th	Day	4 2 15% 28% 31% 21%	517
	21st to end	Day	7% 5% 18% 25% 26% 20%	583
September	1st to 10th	Day	8% 4 16% 24% 26% 23%	530
	11th to 20th	Day	8% 6% 18% 27% 26% 15%	530
	21st to end	Day	9% 7% 22% 27% 24% 10%	530
October	1st to 10th	Day	12% 6% 21% 27% 24% 9%	530
	11th to 20th	Day	15% 9% 18% 22% 26% 10%	424
	21st to end	Day	22% 8% 18% 31% 15% 6%	363
November	1st to 10th	Day	23% 10% 15% 29% 20% 2	330
	11th to 20th	Day	28% 12% 15% 20% 18% 7%	266
	21st to end	Day	25% 9% 13% 26% 20% 7%	220
December	1st to 10th	Day	26% 7% 14% 24% 21% 9%	204
	11th to 20th	Day	30% 10% 15% 26% 18% 3	200
	21st to end	Day	31% 7% 19% 23% 15% 5%	220
January	1st to 10th	Night	32% 10% 14% 29% 14% 2	800
	11th to 20th	Night	23% 7% 15% 34% 18% 4	800
	21st to end	Night	21% 6% 12% 36% 21% 2	880
February	1st to 10th	Night	23% 9% 16% 36% 14% 2	800
	11th to 20th	Night	20% 7% 18% 34% 19% 2	784
	21st to end	Night	18% 9% 19% 32% 19% 3	660
March	1st to 10th	Night	19% 7% 17% 37% 19% 2	800
	11th to 20th	Night	17% 10% 18% 31% 21% 3	800
	21st to end	Night	16% 9% 20% 38% 15% 2	880
April	1st to 10th	Night	14% 5% 21% 37% 21% 3	800
	11th to 20th	Night	12% 7% 23% 40% 15% 2	734
	21st to end	Night	9% 8% 22% 37% 21% 3	600
May	1st to 10th	Night	8% 7% 19% 38% 25% 4	435
	11th to 20th	Night	11% 6% 14% 31% 31% 7%	394
	21st to end	Night	10% 5% 17% 33% 30% 4	433
June	1st to 10th	Night	12% 4 16% 40% 24% 4	400
	11th to 20th	Night	11% 6% 21% 37% 23% 3	400
	21st to end	Night	6% 4 13% 43% 31% 4	400
July	1st to 10th	Night	4 2 11% 40% 40% 4	390
	11th to 20th	Night	3 2 15% 44% 35% 2	388
	21st to end	Night	1 1 9% 42% 42% 5%	438
August	1st to 10th	Night	5% 1 9% 38% 42% 5%	398
	11th to 20th	Night	3 1 11% 37% 39% 9%	496
	21st to end	Night	5% 3 11% 39% 38% 5%	839
September	1st to 10th	Night	5% 3 10% 41% 35% 8%	800
	11th to 20th	Night	7% 3 12% 33% 37% 8%	800
	21st to end	Night	10% 3 16% 36% 30% 5%	800
October	1st to 10th	Night	12% 4 12% 41% 28% 4	800
	11th to 20th	Night	13% 7% 16% 33% 26% 4	800
	21st to end	Night	18% 4 20% 36% 18% 3	880
November	1st to 10th	Night	24% 7% 14% 38% 16% 1	820
	11th to 20th	Night	23% 10% 18% 31% 15% 3	840
	21st to end	Night	26% 6% 13% 31% 20% 3	929
December	1st to 10th	Night	21% 6% 14% 36% 21% 2	1011
	11th to 20th	Night	29% 8% 12% 36% 14% 1	1000
	21st to end	Night	31% 9% 14% 32% 11% 2	1010

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