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 2024-11-10.

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%	105191
	Night	11% 9% 3 7% 5% 4 4 4 5% 7% 41%	102532
Transparency	Day	38% 6% 12% 22% 16% 6%	104959
	Night	42% 7% 11% 20% 14% 6%	102410
Seeing	Day	13% 8% 19% 27% 22% 10%	14755
	Night	16% 6% 15% 36% 23% 3	23883

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% 5% 11% 7% 6% 5% 4 5% 5% 13%	2539
	11th to 20th	Day	20% 13% 5% 12% 7% 5% 5% 3 5% 5% 20%	2516
	21st to end	Day	23% 20% 7% 13% 6% 4 4 3 3 4 12%	2794
February	1st to 10th	Day	18% 16% 4 9% 6% 5% 4 3 5% 8% 21%	1533
	11th to 20th	Day	13% 14% 4 9% 6% 6% 5% 4 6% 6% 28%	1766
	21st to end	Day	12% 15% 5% 8% 5% 5% 5% 5% 5% 5% 29%	1570
March	1st to 10th	Day	10% 12% 3 7% 5% 6% 6% 6% 8% 9% 29%	3138
	11th to 20th	Day	8% 7% 2 6% 5% 4 4 3 5% 8% 47%	3257
	21st to end	Day	11% 12% 4 9% 8% 6% 7% 5% 5% 7% 25%	4155
April	1st to 10th	Day	11% 9% 4 9% 6% 6% 6% 5% 8% 7% 29%	2280
	11th to 20th	Day	10% 8% 3 8% 6% 5% 6% 4 6% 7% 38%	2304
	21st to end	Day	8% 7% 3 9% 5% 6% 5% 5% 6% 7% 39%	2470
May	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 8% 43%	4372
	11th to 20th	Day	6% 5% 2 6% 4 5% 5% 4 6% 11% 47%	4377
	21st to end	Day	7% 7% 3 8% 6% 6% 7% 5% 7% 9% 36%	4813
June	1st to 10th	Day	14% 8% 3 6% 4 4 4 4 6% 7% 41%	2821
	11th to 20th	Day	11% 6% 2 6% 4 5% 6% 5% 7% 7% 41%	2850
	21st to end	Day	5% 4 2 5% 4 4 4 3 5% 6% 59%	2832
July	1st to 10th	Day	1 2 1 3 2 2 3 3 3 4 77%	4420
	11th to 20th	Day	2 1 2 2 2 2 2 3 4 80%	4420
	21st to end	Day	2 1 1 2 2 2 2 2 4 4 77%	4790
August	1st to 10th	Day	3 2 1 3 2 3 3 2 4 6% 71%	4401
	11th to 20th	Day	3 3 1 3 2 3 3 3 5% 7% 67%	4176
	21st to end	Day	2 3 1 5% 3 3 4 3 5% 6% 65%	4228
September	1st to 10th	Day	4 3 1 3 2 3 3 3 4 6% 67%	2280
	11th to 20th	Day	5% 5% 2 5% 4 4 4 4 6% 8% 54%	2185
	21st to end	Day	6% 4 1 6% 4 4 4 4 7% 8% 50%	2090
October	1st to 10th	Day	6% 5% 2 5% 3 3 3 3 4 7% 57%	3338
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 4 7% 8% 43%	2984
	21st to end	Day	13% 9% 3 7% 4 6% 6% 4 5% 7% 37%	3101
November	1st to 10th	Day	20% 10% 3 9% 6% 4 4 3 5% 6% 29%	1755
	11th to 20th	Day	17% 13% 5% 10% 6% 5% 6% 3 4 5% 27%	1600
	21st to end	Day	15% 18% 4 12% 6% 6% 5% 4 4 6% 20%	1418
December	1st to 10th	Day	16% 14% 5% 12% 7% 6% 5% 4 5% 7% 18%	1330
	11th to 20th	Day	22% 19% 7% 14% 8% 5% 5% 3 3 3 10%	1682
	21st to end	Day	15% 20% 6% 11% 6% 6% 5% 4 5% 5% 15%	2606
January	1st to 10th	Night	20% 20% 5% 11% 7% 6% 4 3 4 5% 15%	4406
	11th to 20th	Night	17% 14% 5% 11% 6% 4 4 4 5% 7% 23%	4382
	21st to end	Night	22% 19% 6% 10% 5% 6% 4 4 4 5% 14%	4862
February	1st to 10th	Night	15% 16% 4 9% 5% 5% 4 4 5% 6% 27%	2646
	11th to 20th	Night	12% 13% 4 9% 6% 5% 4 4 6% 7% 31%	2301
	21st to end	Night	10% 12% 3 8% 6% 5% 5% 4 6% 7% 33%	1884
March	1st to 10th	Night	8% 9% 3 7% 5% 5% 6% 4 7% 9% 39%	3736
	11th to 20th	Night	8% 9% 2 5% 4 4 4 3 4 6% 53%	3721
	21st to end	Night	10% 8% 4 9% 6% 7% 6% 6% 7% 9% 28%	3494
April	1st to 10th	Night	10% 9% 3 7% 6% 5% 5% 6% 6% 8% 35%	1900
	11th to 20th	Night	8% 9% 3 7% 5% 5% 4 4 5% 6% 42%	1825
	21st to end	Night	7% 8% 3 7% 5% 4 5% 5% 7% 8% 41%	1710
May	1st to 10th	Night	8% 6% 3 7% 6% 6% 5% 5% 8% 9% 38%	2586
	11th to 20th	Night	5% 5% 1 5% 4 3 5% 4 7% 10% 50%	2516
	21st to end	Night	8% 7% 2 7% 5% 5% 6% 5% 6% 9% 40%	2762
June	1st to 10th	Night	10% 6% 2 5% 5% 6% 4 4 5% 8% 45%	1354
	11th to 20th	Night	8% 6% 2 6% 5% 5% 6% 6% 5% 8% 42%	1330
	21st to end	Night	5% 4 2 5% 4 4 4 5% 5% 8% 56%	1342
July	1st to 10th	Night	2 1 1 2 2 3 3 2 4 6% 74%	2540
	11th to 20th	Night	1 1 2 2 1 2 1 2 4 83%	2540
	21st to end	Night	1 1 1 2 2 1 2 1 3 4 81%	2762
August	1st to 10th	Night	3 3 1 3 2 2 2 2 4 7% 72%	2529
	11th to 20th	Night	3 3 1 4 3 4 5% 4 5% 6% 63%	2657
	21st to end	Night	2 3 1 4 2 3 4 3 5% 7% 65%	3433
September	1st to 10th	Night	4 2 1 2 2 2 4 3 5% 7% 67%	1900
	11th to 20th	Night	6% 3 1 6% 4 4 4 4 5% 7% 54%	1995
	21st to end	Night	8% 5% 1 5% 4 4 3 3 5% 8% 53%	2090
October	1st to 10th	Night	6% 3 1 3 3 4 3 4 5% 8% 60%	3640
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 7% 51%	3948
	21st to end	Night	12% 6% 2 6% 4 5% 6% 4 6% 7% 43%	4457
November	1st to 10th	Night	18% 11% 3 7% 5% 4 4 4 6% 9% 29%	2528
	11th to 20th	Night	15% 12% 4 9% 6% 5% 7% 4 6% 5% 28%	2579
	21st to end	Night	16% 14% 3 9% 6% 5% 5% 4 5% 7% 25%	2740
December	1st to 10th	Night	15% 13% 5% 12% 6% 5% 4 4 6% 6% 23%	2850
	11th to 20th	Night	20% 17% 6% 13% 6% 6% 5% 3 5% 5% 14%	3510
	21st to end	Night	18% 16% 5% 13% 7% 7% 5% 4 5% 5% 15%	5077

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	71% 10% 9% 8% 2	2539
	11th to 20th	Day	64% 7% 11% 13% 3 2	2540
	21st to end	Day	75% 8% 9% 7% 2	2794
February	1st to 10th	Day	62% 9% 14% 9% 5% 1	1533
	11th to 20th	Day	56% 9% 13% 14% 7%	1775
	21st to end	Day	55% 9% 11% 17% 8% 1	1570
March	1st to 10th	Day	46% 10% 12% 19% 12% 1	3138
	11th to 20th	Day	34% 7% 18% 35% 7%	3257
	21st to end	Day	54% 11% 16% 14% 5%	4155
April	1st to 10th	Day	51% 7% 16% 20% 6% 1	2280
	11th to 20th	Day	44% 6% 17% 19% 13% 2	2304
	21st to end	Day	43% 7% 16% 22% 10% 2	2470
May	1st to 10th	Day	35% 7% 12% 30% 15% 1	4372
	11th to 20th	Day	28% 6% 15% 34% 14% 2	4380
	21st to end	Day	40% 8% 18% 24% 8% 1	4814
June	1st to 10th	Day	42% 6% 11% 23% 16% 2	2821
	11th to 20th	Day	38% 7% 12% 26% 14% 4	2850
	21st to end	Day	26% 5% 11% 31% 21% 6%	2832
July	1st to 10th	Day	13% 3 5% 17% 31% 30%	4351
	11th to 20th	Day	11% 2 5% 12% 28% 43%	4345
	21st to end	Day	12% 2 5% 19% 36% 26%	4639
August	1st to 10th	Day	16% 3 5% 22% 39% 16%	4384
	11th to 20th	Day	17% 4 8% 40% 27% 5%	4218
	21st to end	Day	20% 3 10% 34% 29% 5%	4227
September	1st to 10th	Day	19% 4 11% 29% 27% 11%	2280
	11th to 20th	Day	28% 6% 13% 26% 23% 5%	2185
	21st to end	Day	31% 7% 17% 29% 12% 4	2090
October	1st to 10th	Day	28% 4 13% 36% 18% 1	3338
	11th to 20th	Day	38% 5% 19% 24% 14%	2984
	21st to end	Day	48% 6% 15% 20% 10% 1	3101
November	1st to 10th	Day	56% 8% 13% 18% 4 1	1755
	11th to 20th	Day	59% 8% 15% 13% 4 2	1600
	21st to end	Day	65% 10% 11% 11% 3 1	1418
December	1st to 10th	Day	66% 8% 12% 10% 4	1330
	11th to 20th	Day	77% 8% 8% 4 2	1684
	21st to end	Day	67% 9% 12% 8% 3	2606
January	1st to 10th	Night	72% 7% 9% 10% 2 1	4406
	11th to 20th	Night	60% 6% 14% 15% 3 2	4420
	21st to end	Night	69% 12% 10% 7% 2 1	4862
February	1st to 10th	Night	58% 8% 14% 12% 7% 1	2646
	11th to 20th	Night	52% 7% 18% 14% 8% 1	2303
	21st to end	Night	49% 7% 14% 19% 9% 1	1884
March	1st to 10th	Night	40% 7% 15% 25% 12% 1	3736
	11th to 20th	Night	34% 5% 16% 33% 12% 1	3722
	21st to end	Night	48% 10% 15% 20% 5% 1	3494
April	1st to 10th	Night	44% 6% 13% 25% 10% 1	1900
	11th to 20th	Night	42% 6% 11% 23% 17% 2	1825
	21st to end	Night	40% 7% 11% 23% 16% 2	1710
May	1st to 10th	Night	36% 8% 11% 26% 17% 2	2586
	11th to 20th	Night	26% 6% 12% 33% 22% 2	2516
	21st to end	Night	36% 11% 14% 26% 14%	2762
June	1st to 10th	Night	36% 7% 8% 24% 21% 4	1354
	11th to 20th	Night	37% 8% 11% 21% 18% 6%	1330
	21st to end	Night	25% 5% 8% 27% 26% 9%	1342
July	1st to 10th	Night	13% 3 4 15% 30% 35%	2500
	11th to 20th	Night	8% 2 4 12% 20% 55%	2492
	21st to end	Night	10% 1 2 16% 33% 38%	2674
August	1st to 10th	Night	14% 2 3 17% 40% 23%	2521
	11th to 20th	Night	22% 3 5% 28% 33% 10%	2681
	21st to end	Night	19% 4 9% 27% 31% 10%	3433
September	1st to 10th	Night	17% 4 7% 27% 30% 15%	1900
	11th to 20th	Night	28% 5% 10% 23% 24% 10%	1995
	21st to end	Night	29% 4 14% 31% 16% 6%	2090
October	1st to 10th	Night	23% 3 9% 41% 22% 1	3640
	11th to 20th	Night	33% 4 17% 25% 21% 1	3948
	21st to end	Night	41% 6% 13% 29% 9% 2	4457
November	1st to 10th	Night	53% 8% 16% 17% 6% 1	2528
	11th to 20th	Night	56% 8% 14% 13% 6% 2	2579
	21st to end	Night	57% 9% 14% 13% 6%	2740
December	1st to 10th	Night	59% 9% 12% 16% 4	2850
	11th to 20th	Night	72% 9% 10% 7% 2	3507
	21st to end	Night	67% 10% 12% 8% 3	5077

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% 14% 11% 22% 17% 8%	180
	11th to 20th	Day	31% 8% 11% 24% 16% 10%	180
	21st to end	Day	28% 6% 15% 31% 19% 2	198
February	1st to 10th	Day	26% 11% 19% 26% 15% 3	193
	11th to 20th	Day	23% 14% 16% 26% 16% 4	414
	21st to end	Day	18% 15% 24% 23% 17% 4	382
March	1st to 10th	Day	18% 16% 23% 24% 15% 5%	460
	11th to 20th	Day	16% 10% 20% 23% 24% 8%	460
	21st to end	Day	17% 15% 22% 27% 14% 5%	506
April	1st to 10th	Day	16% 17% 22% 24% 15% 7%	460
	11th to 20th	Day	13% 18% 26% 19% 16% 9%	452
	21st to end	Day	10% 12% 22% 30% 19% 7%	460
May	1st to 10th	Day	10% 8% 24% 29% 22% 6%	463
	11th to 20th	Day	15% 6% 18% 26% 23% 12%	488
	21st to end	Day	11% 8% 16% 28% 24% 12%	529
June	1st to 10th	Day	16% 6% 20% 29% 23% 7%	651
	11th to 20th	Day	13% 7% 28% 30% 16% 6%	680
	21st to end	Day	6% 6% 18% 29% 31% 11%	662
July	1st to 10th	Day	3 4 19% 34% 32% 8%	486
	11th to 20th	Day	4 6% 20% 32% 25% 14%	484
	21st to end	Day	2 4 15% 32% 32% 15%	531
August	1st to 10th	Day	6% 4 13% 30% 28% 19%	487
	11th to 20th	Day	3 2 15% 27% 32% 21%	477
	21st to end	Day	7% 5% 19% 25% 24% 19%	539
September	1st to 10th	Day	8% 4 17% 26% 26% 19%	490
	11th to 20th	Day	7% 6% 19% 27% 25% 14%	490
	21st to end	Day	10% 7% 23% 28% 24% 9%	490
October	1st to 10th	Day	13% 7% 21% 26% 24% 9%	490
	11th to 20th	Day	15% 8% 17% 22% 27% 11%	390
	21st to end	Day	24% 8% 16% 32% 14% 6%	330
November	1st to 10th	Day	23% 8% 16% 30% 22% 2	297
	11th to 20th	Day	27% 12% 16% 20% 19% 7%	218
	21st to end	Day	23% 10% 14% 22% 24% 8%	180
December	1st to 10th	Day	27% 8% 16% 23% 18% 8%	180
	11th to 20th	Day	28% 11% 14% 27% 17% 3	180
	21st to end	Day	29% 8% 18% 25% 16% 5%	198
January	1st to 10th	Night	32% 11% 14% 28% 14% 2	760
	11th to 20th	Night	25% 7% 15% 32% 18% 4	760
	21st to end	Night	22% 7% 13% 36% 21% 2	836
February	1st to 10th	Night	23% 10% 15% 36% 14% 2	760
	11th to 20th	Night	19% 7% 19% 34% 19% 2	744
	21st to end	Night	17% 9% 20% 33% 19% 3	628
March	1st to 10th	Night	19% 7% 17% 36% 19% 2	760
	11th to 20th	Night	17% 10% 17% 31% 22% 3	760
	21st to end	Night	16% 9% 20% 38% 15% 2	836
April	1st to 10th	Night	14% 5% 20% 37% 20% 3	760
	11th to 20th	Night	12% 7% 23% 40% 15% 2	697
	21st to end	Night	10% 8% 22% 37% 20% 3	570
May	1st to 10th	Night	8% 7% 19% 37% 25% 4	413
	11th to 20th	Night	10% 6% 13% 31% 32% 7%	374
	21st to end	Night	10% 6% 17% 34% 30% 4	411
June	1st to 10th	Night	12% 3 16% 40% 24% 4	380
	11th to 20th	Night	12% 6% 20% 37% 23% 3	380
	21st to end	Night	6% 4 13% 42% 31% 4	380
July	1st to 10th	Night	3 2 11% 40% 41% 4	370
	11th to 20th	Night	3 2 15% 44% 34% 2	368
	21st to end	Night	1 1 9% 42% 41% 5%	416
August	1st to 10th	Night	4 1 9% 38% 42% 5%	378
	11th to 20th	Night	3 1 11% 36% 40% 9%	471
	21st to end	Night	4 3 12% 39% 38% 5%	797
September	1st to 10th	Night	5% 3 10% 41% 35% 7%	760
	11th to 20th	Night	7% 3 13% 33% 36% 8%	760
	21st to end	Night	10% 3 16% 36% 29% 6%	760
October	1st to 10th	Night	12% 4 12% 40% 28% 4	760
	11th to 20th	Night	13% 7% 16% 33% 28% 4	760
	21st to end	Night	18% 4 20% 36% 19% 3	836
November	1st to 10th	Night	24% 6% 14% 38% 17% 1	778
	11th to 20th	Night	22% 10% 18% 32% 15% 3	760
	21st to end	Night	25% 7% 14% 31% 21% 3	840
December	1st to 10th	Night	22% 7% 14% 36% 20% 2	950
	11th to 20th	Night	29% 7% 12% 37% 14% 1	950
	21st to end	Night	32% 9% 14% 32% 12% 2	960

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