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 2024-01-13.

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	11% 8% 3 6% 5% 3 5% 5% 5% 7% 42%	74574
	Night	12% 9% 4 7% 6% 3 5% 5% 5% 7% 38%	75161
Transparency	Day	42% 6% 12% 21% 15% 4	74276
	Night	45% 7% 12% 19% 13% 4	74977
Seeing	Day	14% 8% 19% 27% 22% 9%	13608
	Night	17% 6% 15% 36% 23% 3	22906

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	22% 19% 5% 9% 7% 4 4 5% 5% 5% 15%	1519
	11th to 20th	Day	20% 16% 8% 15% 8% 4 4 4 4 4 13%	1456
	21st to end	Day	17% 17% 7% 12% 7% 4 5% 4 3 6% 17%	1584
February	1st to 10th	Day	18% 14% 5% 8% 7% 4 5% 4 5% 8% 23%	1453
	11th to 20th	Day	13% 13% 5% 8% 7% 4 6% 5% 6% 7% 28%	1671
	21st to end	Day	12% 14% 6% 7% 5% 3 6% 6% 5% 6% 29%	1480
March	1st to 10th	Day	14% 11% 5% 7% 6% 5% 6% 6% 6% 7% 26%	1800
	11th to 20th	Day	13% 10% 4 7% 7% 4 6% 5% 6% 8% 30%	1838
	21st to end	Day	14% 13% 5% 8% 7% 4 5% 5% 5% 7% 25%	2366
April	1st to 10th	Day	12% 8% 4 8% 7% 4 6% 6% 8% 8% 28%	2160
	11th to 20th	Day	10% 7% 3 7% 7% 4 5% 6% 6% 8% 37%	2181
	21st to end	Day	9% 7% 4 7% 6% 4 6% 6% 6% 7% 39%	2340
May	1st to 10th	Day	6% 5% 2 5% 6% 3 6% 6% 7% 9% 46%	2485
	11th to 20th	Day	10% 6% 3 6% 6% 4 6% 5% 6% 9% 39%	2477
	21st to end	Day	9% 7% 4 6% 6% 4 6% 5% 6% 9% 38%	2723
June	1st to 10th	Day	14% 8% 3 6% 5% 3 4 4 6% 7% 41%	2672
	11th to 20th	Day	12% 6% 2 5% 5% 3 5% 6% 6% 8% 42%	2700
	21st to end	Day	6% 4 2 4 4 3 4 4 5% 6% 59%	2683
July	1st to 10th	Day	2 2 2 4 3 2 4 4 4 5% 67%	2520
	11th to 20th	Day	3 2 1 2 3 2 3 4 4 6% 71%	2520
	21st to end	Day	2 1 1 1 2 1 2 3 3 4 81%	2741
August	1st to 10th	Day	3 2 1 3 3 2 3 3 4 7% 70%	2507
	11th to 20th	Day	3 2 1 2 2 1 3 4 5% 7% 69%	2386
	21st to end	Day	4 4 1 4 5% 3 3 4 5% 6% 61%	2411
September	1st to 10th	Day	4 4 1 3 3 2 3 3 4 6% 67%	2160
	11th to 20th	Day	5% 4 2 4 4 3 4 5% 5% 8% 55%	2070
	21st to end	Day	7% 5% 2 5% 5% 2 5% 5% 6% 8% 49%	1980
October	1st to 10th	Day	10% 7% 3 6% 6% 3 4 4 4 7% 47%	1893
	11th to 20th	Day	12% 6% 2 5% 4 3 5% 5% 7% 7% 44%	1706
	21st to end	Day	16% 10% 3 7% 5% 4 4 5% 4 8% 35%	1782
November	1st to 10th	Day	21% 9% 4 8% 7% 3 5% 4 5% 6% 29%	1674
	11th to 20th	Day	17% 12% 5% 9% 7% 4 5% 5% 4 5% 28%	1600
	21st to end	Day	15% 17% 6% 11% 7% 5% 5% 5% 4 6% 21%	1418
December	1st to 10th	Day	17% 13% 6% 12% 8% 4 6% 5% 5% 7% 18%	1330
	11th to 20th	Day	22% 17% 8% 13% 9% 4 5% 4 3 3 10%	1682
	21st to end	Day	16% 19% 6% 10% 8% 5% 6% 5% 5% 5% 15%	2606
January	1st to 10th	Night	24% 16% 5% 9% 8% 3 5% 4 4 6% 17%	2646
	11th to 20th	Night	19% 14% 7% 11% 7% 3 4 5% 4 6% 18%	2548
	21st to end	Night	16% 15% 6% 11% 7% 5% 5% 5% 5% 7% 20%	2772
February	1st to 10th	Night	15% 15% 5% 8% 6% 4 4 5% 5% 7% 27%	2506
	11th to 20th	Night	12% 12% 4 8% 7% 3 5% 5% 5% 7% 31%	2176
	21st to end	Night	10% 11% 4 8% 6% 4 4 6% 5% 8% 34%	1776
March	1st to 10th	Night	14% 10% 4 7% 6% 3 6% 5% 6% 6% 33%	2160
	11th to 20th	Night	12% 10% 3 6% 6% 4 6% 5% 5% 8% 34%	2120
	21st to end	Night	12% 8% 4 8% 8% 4 5% 6% 6% 9% 29%	1986
April	1st to 10th	Night	10% 7% 4 7% 6% 4 6% 7% 6% 9% 35%	1800
	11th to 20th	Night	9% 8% 4 7% 6% 4 5% 5% 5% 6% 42%	1728
	21st to end	Night	8% 8% 3 6% 6% 4 5% 7% 6% 8% 40%	1620
May	1st to 10th	Night	6% 5% 3 5% 7% 4 4 5% 7% 9% 45%	1472
	11th to 20th	Night	8% 6% 2 6% 6% 3 5% 5% 7% 9% 43%	1416
	21st to end	Night	8% 6% 2 6% 5% 3 5% 8% 7% 9% 41%	1552
June	1st to 10th	Night	10% 5% 3 5% 6% 5% 5% 5% 5% 8% 44%	1283
	11th to 20th	Night	8% 6% 3 5% 6% 4 6% 6% 5% 8% 43%	1260
	21st to end	Night	5% 4 2 4 5% 2 5% 5% 5% 8% 56%	1271
July	1st to 10th	Night	2 2 1 2 4 2 3 3 4 8% 68%	1440
	11th to 20th	Night	2 2 1 2 3 1 3 2 4 6% 73%	1440
	21st to end	Night	1 1 2 2 1 2 2 3 5% 81%	1576
August	1st to 10th	Night	3 3 1 3 2 1 2 3 4 7% 72%	1432
	11th to 20th	Night	3 3 1 2 2 2 3 4 4 6% 70%	1514
	21st to end	Night	3 2 1 2 3 2 4 5% 6% 7% 65%	1943
September	1st to 10th	Night	4 2 1 2 3 2 4 4 5% 7% 68%	1800
	11th to 20th	Night	6% 3 2 5% 5% 2 4 4 5% 8% 56%	1890
	21st to end	Night	8% 5% 1 4 5% 3 4 4 5% 8% 53%	1980
October	1st to 10th	Night	10% 5% 2 4 5% 4 4 5% 4 8% 49%	2065
	11th to 20th	Night	10% 6% 2 6% 5% 3 4 5% 5% 7% 48%	2254
	21st to end	Night	14% 7% 2 5% 5% 3 5% 5% 5% 7% 42%	2574
November	1st to 10th	Night	18% 10% 4 6% 6% 3 4 5% 6% 8% 29%	2405
	11th to 20th	Night	15% 11% 4 8% 7% 4 7% 5% 6% 6% 28%	2579
	21st to end	Night	17% 13% 4 8% 8% 4 5% 5% 5% 7% 25%	2740
December	1st to 10th	Night	15% 12% 6% 11% 7% 3 5% 5% 6% 7% 24%	2850
	11th to 20th	Night	20% 17% 6% 12% 8% 4 6% 5% 5% 5% 14%	3510
	21st to end	Night	18% 15% 6% 11% 8% 5% 6% 6% 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% 9% 9% 8% 3	1519
	11th to 20th	Day	73% 8% 10% 7% 2	1456
	21st to end	Day	68% 8% 10% 11% 3 1	1584
February	1st to 10th	Day	61% 9% 14% 10% 5% 1	1453
	11th to 20th	Day	56% 9% 14% 14% 7%	1680
	21st to end	Day	55% 9% 10% 17% 8%	1480
March	1st to 10th	Day	55% 8% 16% 15% 6% 1	1800
	11th to 20th	Day	52% 5% 16% 20% 6%	1838
	21st to end	Day	58% 6% 18% 13% 3 1	2366
April	1st to 10th	Day	52% 7% 15% 19% 6% 1	2160
	11th to 20th	Day	45% 6% 17% 19% 12% 1	2181
	21st to end	Day	43% 7% 16% 22% 10% 2	2340
May	1st to 10th	Day	33% 6% 16% 29% 15% 1	2485
	11th to 20th	Day	40% 7% 17% 25% 12%	2480
	21st to end	Day	41% 7% 16% 25% 10% 1	2724
June	1st to 10th	Day	42% 6% 11% 24% 15% 2	2672
	11th to 20th	Day	38% 7% 12% 26% 12% 3	2700
	21st to end	Day	26% 5% 11% 31% 21% 6%	2683
July	1st to 10th	Day	20% 4 8% 26% 28% 14%	2451
	11th to 20th	Day	16% 4 8% 20% 36% 17%	2445
	21st to end	Day	9% 3 4 21% 45% 18%	2589
August	1st to 10th	Day	16% 3 8% 24% 33% 15%	2490
	11th to 20th	Day	15% 4 10% 29% 34% 9%	2386
	21st to end	Day	23% 4 9% 28% 31% 5%	2411
September	1st to 10th	Day	19% 5% 11% 29% 27% 10%	2160
	11th to 20th	Day	27% 6% 13% 26% 24% 5%	2070
	21st to end	Day	32% 8% 17% 28% 12% 4	1980
October	1st to 10th	Day	38% 6% 13% 29% 13% 1	1893
	11th to 20th	Day	38% 7% 16% 28% 10% 1	1706
	21st to end	Day	49% 8% 15% 20% 8%	1782
November	1st to 10th	Day	56% 8% 13% 17% 4 1	1674
	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	69% 7% 11% 10% 2 1	2646
	11th to 20th	Night	66% 8% 14% 10% 2	2548
	21st to end	Night	63% 9% 13% 11% 4 1	2772
February	1st to 10th	Night	56% 9% 14% 13% 7% 1	2506
	11th to 20th	Night	52% 7% 19% 14% 8%	2178
	21st to end	Night	49% 7% 14% 20% 10% 1	1776
March	1st to 10th	Night	51% 7% 14% 19% 8% 1	2160
	11th to 20th	Night	49% 6% 13% 21% 11% 1	2121
	21st to end	Night	52% 6% 15% 20% 7% 1	1986
April	1st to 10th	Night	44% 7% 12% 25% 11% 1	1800
	11th to 20th	Night	43% 5% 10% 23% 17% 2	1728
	21st to end	Night	40% 7% 12% 23% 16% 2	1620
May	1st to 10th	Night	32% 7% 12% 30% 16% 3	1472
	11th to 20th	Night	36% 6% 13% 27% 16% 3	1416
	21st to end	Night	36% 7% 16% 28% 13% 1	1552
June	1st to 10th	Night	37% 7% 8% 24% 21% 4	1283
	11th to 20th	Night	37% 8% 11% 22% 17% 5%	1260
	21st to end	Night	25% 6% 8% 28% 25% 8%	1271
July	1st to 10th	Night	16% 3 6% 21% 34% 21%	1400
	11th to 20th	Night	13% 3 6% 20% 30% 27%	1392
	21st to end	Night	9% 1 3 15% 38% 33%	1488
August	1st to 10th	Night	14% 3 5% 16% 37% 25%	1424
	11th to 20th	Night	15% 3 6% 23% 35% 17%	1514
	21st to end	Night	16% 5% 9% 24% 33% 13%	1943
September	1st to 10th	Night	17% 4 8% 27% 30% 14%	1800
	11th to 20th	Night	27% 5% 10% 23% 25% 10%	1890
	21st to end	Night	29% 4 14% 30% 15% 7%	1980
October	1st to 10th	Night	35% 4 10% 30% 19% 1	2065
	11th to 20th	Night	36% 6% 15% 27% 16% 1	2254
	21st to end	Night	41% 7% 13% 26% 11% 1	2574
November	1st to 10th	Night	53% 8% 16% 17% 6% 1	2405
	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% 23% 16% 8%	180
	11th to 20th	Day	30% 9% 11% 24% 15% 10%	164
	21st to end	Day	27% 6% 15% 34% 16% 2	176
February	1st to 10th	Day	25% 12% 21% 26% 14% 2	173
	11th to 20th	Day	22% 15% 17% 27% 15% 3	379
	21st to end	Day	16% 14% 24% 24% 18% 4	346
March	1st to 10th	Day	19% 15% 23% 23% 15% 4	420
	11th to 20th	Day	18% 10% 21% 23% 23% 5%	420
	21st to end	Day	18% 15% 23% 27% 13% 3	462
April	1st to 10th	Day	17% 15% 21% 25% 15% 7%	420
	11th to 20th	Day	14% 18% 26% 18% 15% 8%	412
	21st to end	Day	11% 13% 21% 30% 18% 8%	420
May	1st to 10th	Day	10% 8% 24% 30% 22% 5%	423
	11th to 20th	Day	16% 6% 18% 26% 23% 12%	448
	21st to end	Day	12% 8% 15% 27% 25% 13%	485
June	1st to 10th	Day	15% 6% 20% 29% 22% 7%	602
	11th to 20th	Day	13% 7% 28% 31% 16% 6%	630
	21st to end	Day	6% 5% 18% 28% 31% 12%	613
July	1st to 10th	Day	4 4 19% 35% 32% 7%	446
	11th to 20th	Day	4 6% 21% 32% 24% 12%	444
	21st to end	Day	2 3 16% 30% 33% 16%	487
August	1st to 10th	Day	6% 4 13% 29% 29% 20%	447
	11th to 20th	Day	3 2 14% 26% 34% 22%	437
	21st to end	Day	7% 4 19% 26% 26% 18%	495
September	1st to 10th	Day	9% 4 18% 27% 27% 16%	450
	11th to 20th	Day	8% 7% 18% 26% 26% 15%	450
	21st to end	Day	10% 7% 22% 29% 24% 8%	450
October	1st to 10th	Day	14% 7% 22% 25% 23% 9%	450
	11th to 20th	Day	16% 8% 16% 22% 27% 10%	356
	21st to end	Day	24% 9% 15% 32% 13% 6%	297
November	1st to 10th	Day	22% 9% 16% 30% 20% 3	270
	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% 14% 31% 18% 4	728
	21st to end	Night	23% 7% 13% 35% 21% 2	792
February	1st to 10th	Night	23% 9% 16% 36% 15% 2	720
	11th to 20th	Night	20% 7% 19% 34% 19% 2	704
	21st to end	Night	16% 9% 19% 34% 19% 3	592
March	1st to 10th	Night	20% 7% 17% 37% 18% 2	720
	11th to 20th	Night	18% 10% 17% 31% 21% 2	720
	21st to end	Night	17% 9% 20% 39% 14% 2	792
April	1st to 10th	Night	14% 5% 19% 37% 22% 3	720
	11th to 20th	Night	13% 7% 23% 39% 15% 2	660
	21st to end	Night	10% 8% 22% 37% 20% 3	540
May	1st to 10th	Night	8% 7% 19% 37% 25% 4	392
	11th to 20th	Night	11% 6% 13% 31% 32% 7%	354
	21st to end	Night	11% 5% 16% 33% 30% 4	389
June	1st to 10th	Night	13% 3 16% 40% 24% 5%	360
	11th to 20th	Night	12% 5% 21% 36% 24% 3	360
	21st to end	Night	6% 4 12% 43% 31% 4	360
July	1st to 10th	Night	3 2 11% 41% 40% 3	350
	11th to 20th	Night	3 2 16% 44% 34% 2	348
	21st to end	Night	1 1 9% 42% 43% 5%	394
August	1st to 10th	Night	4 1 9% 36% 44% 5%	358
	11th to 20th	Night	3 1 9% 36% 41% 10%	446
	21st to end	Night	4 3 11% 40% 38% 5%	755
September	1st to 10th	Night	5% 3 11% 42% 34% 6%	720
	11th to 20th	Night	7% 2 12% 32% 38% 8%	720
	21st to end	Night	10% 2 16% 37% 29% 6%	720
October	1st to 10th	Night	13% 3 12% 40% 28% 4	720
	11th to 20th	Night	13% 7% 15% 34% 27% 4	720
	21st to end	Night	19% 4 19% 35% 19% 3	792
November	1st to 10th	Night	25% 6% 14% 38% 17% 1	740
	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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