Prairie Grass Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prairie Grass Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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% 4 8% 5% 4 4 4 6% 7% 35%	130718
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 36%	130993
Transparency	Day	47% 5% 11% 22% 13% 2	134299
	Night	47% 4 10% 22% 14% 2	135488
Seeing	Day	18% 8% 26% 25% 16% 6%	17427
	Night	19% 8% 28% 30% 13% 1	27396

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% 7% 14% 6% 4 4 4 5% 6% 22%	4691
	11th to 20th	Day	10% 13% 8% 15% 8% 8% 6% 5% 4 4 20%	4710
	21st to end	Day	18% 17% 6% 13% 5% 5% 4 4 4 4 20%	5233
February	1st to 10th	Day	16% 15% 6% 9% 8% 6% 4 3 4 5% 24%	2100
	11th to 20th	Day	15% 12% 4 11% 5% 5% 4 4 5% 6% 30%	2048
	21st to end	Day	15% 15% 5% 10% 4 4 4 4 4 5% 31%	1730
March	1st to 10th	Day	20% 15% 5% 8% 3 4 3 2 3 3 34%	4580
	11th to 20th	Day	11% 10% 3 6% 4 4 4 4 6% 7% 43%	4820
	21st to end	Day	17% 12% 5% 9% 5% 4 4 4 5% 7% 29%	5800
April	1st to 10th	Day	16% 11% 3 8% 5% 4 5% 4 5% 7% 33%	2400
	11th to 20th	Day	17% 9% 3 7% 5% 4 5% 3 5% 6% 36%	2374
	21st to end	Day	20% 9% 3 7% 4 4 4 3 6% 7% 33%	2555
May	1st to 10th	Day	19% 10% 3 7% 5% 5% 5% 4 6% 7% 29%	5862
	11th to 20th	Day	17% 7% 3 7% 5% 5% 5% 4 5% 7% 35%	5861
	21st to end	Day	13% 7% 2 7% 5% 5% 6% 4 6% 9% 37%	6753
June	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 5% 8% 42%	2800
	11th to 20th	Day	10% 4 2 6% 5% 5% 5% 4 6% 8% 45%	2800
	21st to end	Day	11% 6% 2 6% 5% 4 5% 4 6% 9% 41%	2800
July	1st to 10th	Day	11% 5% 2 5% 4 4 5% 5% 8% 11% 40%	6360
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 5% 8% 12% 39%	6360
	21st to end	Day	11% 7% 3 6% 5% 5% 5% 4 7% 9% 38%	6879
August	1st to 10th	Day	8% 4 2 5% 3 4 5% 4 6% 10% 48%	4234
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 4 7% 10% 43%	3953
	21st to end	Day	6% 4 2 5% 3 4 4 4 6% 10% 51%	4312
September	1st to 10th	Day	10% 6% 2 6% 4 4 4 3 5% 7% 49%	2395
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 6% 7% 50%	2200
	21st to end	Day	14% 6% 2 5% 3 4 4 4 6% 7% 45%	2200
October	1st to 10th	Day	13% 4 2 6% 4 4 3 3 6% 7% 47%	2200
	11th to 20th	Day	17% 7% 2 6% 3 4 4 4 5% 5% 43%	2015
	21st to end	Day	19% 9% 3 8% 4 4 5% 3 5% 6% 33%	1980
November	1st to 10th	Day	11% 8% 2 4 4 4 3 3 5% 7% 49%	1852
	11th to 20th	Day	18% 12% 4 6% 4 4 3 2 5% 5% 36%	1890
	21st to end	Day	18% 11% 4 10% 4 4 3 3 5% 6% 32%	1890
December	1st to 10th	Day	12% 12% 5% 8% 4 3 3 3 4 5% 41%	2923
	11th to 20th	Day	14% 14% 6% 13% 6% 4 5% 4 5% 6% 23%	3088
	21st to end	Day	27% 18% 7% 12% 5% 4 4 3 4 4 12%	4070
January	1st to 10th	Night	14% 15% 7% 13% 5% 5% 4 3 5% 5% 24%	8232
	11th to 20th	Night	10% 17% 7% 14% 7% 7% 6% 3 4 4 21%	8280
	21st to end	Night	17% 12% 5% 10% 6% 5% 5% 4 5% 5% 28%	8963
February	1st to 10th	Night	18% 14% 5% 10% 6% 4 4 3 4 5% 28%	2520
	11th to 20th	Night	14% 12% 5% 11% 5% 4 4 3 3 6% 32%	2458
	21st to end	Night	18% 13% 5% 9% 4 4 4 3 4 5% 33%	2076
March	1st to 10th	Night	18% 13% 4 7% 4 3 3 2 4 5% 37%	5493
	11th to 20th	Night	13% 7% 2 7% 4 4 4 3 5% 7% 43%	5294
	21st to end	Night	18% 9% 3 9% 5% 4 4 4 5% 6% 33%	5159
April	1st to 10th	Night	15% 9% 3 6% 5% 4 3 4 6% 7% 38%	2000
	11th to 20th	Night	17% 9% 2 6% 3 3 4 3 5% 8% 39%	1975
	21st to end	Night	16% 8% 4 6% 4 5% 4 4 6% 7% 36%	1845
May	1st to 10th	Night	19% 11% 4 7% 5% 4 4 3 5% 7% 30%	4033
	11th to 20th	Night	16% 6% 2 6% 4 3 4 4 6% 9% 39%	4032
	21st to end	Night	14% 8% 3 7% 4 4 4 4 5% 7% 39%	4099
June	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 5% 7% 49%	1600
	11th to 20th	Night	9% 6% 3 6% 5% 4 4 4 5% 8% 47%	1600
	21st to end	Night	9% 6% 2 6% 5% 5% 4 4 8% 9% 44%	1600
July	1st to 10th	Night	10% 5% 2 6% 4 4 5% 4 6% 10% 44%	3618
	11th to 20th	Night	8% 5% 2 6% 4 3 4 4 7% 10% 48%	3617
	21st to end	Night	11% 6% 3 8% 5% 5% 4 3 6% 8% 41%	3989
August	1st to 10th	Night	10% 4 2 5% 3 4 4 3 5% 8% 52%	2915
	11th to 20th	Night	7% 6% 2 7% 4 4 5% 4 6% 8% 48%	3091
	21st to end	Night	8% 4 2 6% 3 3 4 2 5% 7% 56%	3586
September	1st to 10th	Night	11% 5% 2 5% 3 4 3 3 5% 7% 51%	2000
	11th to 20th	Night	8% 4 2 3 3 3 3 2 5% 7% 59%	2185
	21st to end	Night	13% 6% 2 5% 3 4 4 3 5% 6% 49%	2200
October	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 4 6% 54%	2200
	11th to 20th	Night	17% 6% 2 5% 3 4 3 3 4 6% 46%	2381
	21st to end	Night	18% 9% 3 7% 4 4 4 3 5% 6% 38%	2860
November	1st to 10th	Night	13% 7% 3 5% 3 3 3 3 5% 5% 49%	2666
	11th to 20th	Night	14% 11% 3 7% 4 4 4 3 5% 7% 39%	2730
	21st to end	Night	19% 14% 4 8% 5% 4 3 3 4 5% 31%	2730
December	1st to 10th	Night	13% 10% 4 8% 5% 3 4 3 5% 5% 40%	4477
	11th to 20th	Night	16% 14% 6% 11% 5% 4 3 3 4 5% 28%	5339
	21st to end	Night	24% 19% 7% 10% 5% 4 4 3 4 5% 15%	7150

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	63% 4 11% 17% 5% 1	4692
	11th to 20th	Day	69% 2 8% 10% 9% 2	4710
	21st to end	Day	68% 2 8% 15% 6% 1	5233
February	1st to 10th	Day	64% 3 9% 15% 8% 1	2100
	11th to 20th	Day	55% 4 10% 21% 9% 1	2058
	21st to end	Day	56% 4 11% 19% 8% 2	1730
March	1st to 10th	Day	58% 3 5% 20% 12% 2	4580
	11th to 20th	Day	40% 4 9% 33% 12% 1	4820
	21st to end	Day	55% 5% 13% 18% 8% 1	5802
April	1st to 10th	Day	50% 5% 12% 19% 13%	2400
	11th to 20th	Day	48% 6% 11% 23% 10% 3	2374
	21st to end	Day	49% 6% 11% 20% 11% 3	2555
May	1st to 10th	Day	52% 6% 12% 19% 10%	5897
	11th to 20th	Day	47% 7% 10% 23% 11% 1	5865
	21st to end	Day	42% 6% 13% 23% 15% 1	6767
June	1st to 10th	Day	39% 5% 12% 25% 16% 3	2800
	11th to 20th	Day	36% 5% 12% 24% 20% 3	2800
	21st to end	Day	37% 8% 15% 20% 16% 3	2800
July	1st to 10th	Day	37% 5% 11% 28% 17% 2	6291
	11th to 20th	Day	37% 7% 17% 25% 13% 1	6285
	21st to end	Day	41% 7% 14% 23% 14% 2	6684
August	1st to 10th	Day	31% 7% 16% 29% 16% 1	4221
	11th to 20th	Day	35% 6% 13% 31% 15% 1	3992
	21st to end	Day	29% 5% 13% 31% 18% 4	4311
September	1st to 10th	Day	35% 5% 13% 26% 18% 4	2395
	11th to 20th	Day	32% 6% 11% 25% 24% 3	2200
	21st to end	Day	37% 6% 9% 24% 22% 2	2200
October	1st to 10th	Day	25% 3 10% 31% 29% 2	3590
	11th to 20th	Day	34% 5% 10% 25% 26% 1	3257
	21st to end	Day	44% 3 8% 24% 19% 1	3180
November	1st to 10th	Day	36% 4 13% 26% 17% 3	1852
	11th to 20th	Day	51% 2 10% 21% 12% 3	1890
	21st to end	Day	54% 5% 10% 21% 10% 1	1890
December	1st to 10th	Day	47% 3 11% 28% 11%	2923
	11th to 20th	Day	64% 4 10% 18% 4	3085
	21st to end	Day	76% 4 8% 9% 4	4070
January	1st to 10th	Night	62% 4 10% 17% 7%	8233
	11th to 20th	Night	68% 2 7% 11% 10% 2	8280
	21st to end	Night	59% 3 10% 20% 7% 1	8963
February	1st to 10th	Night	59% 4 11% 16% 8% 1	2520
	11th to 20th	Night	55% 3 9% 19% 11% 2	2459
	21st to end	Night	54% 4 11% 19% 10% 2	2076
March	1st to 10th	Night	52% 3 7% 24% 12% 2	5493
	11th to 20th	Night	40% 4 11% 26% 16% 2	5294
	21st to end	Night	50% 5% 11% 25% 7% 1	5159
April	1st to 10th	Night	45% 5% 12% 22% 15% 2	2000
	11th to 20th	Night	43% 5% 10% 22% 15% 4	1975
	21st to end	Night	46% 6% 11% 21% 13% 4	1845
May	1st to 10th	Night	54% 6% 8% 17% 12% 3	4060
	11th to 20th	Night	41% 7% 13% 25% 13% 1	4033
	21st to end	Night	43% 7% 11% 22% 14% 2	4107
June	1st to 10th	Night	33% 6% 9% 27% 20% 3	1600
	11th to 20th	Night	35% 7% 10% 24% 21% 4	1600
	21st to end	Night	35% 8% 14% 23% 16% 4	1600
July	1st to 10th	Night	36% 6% 11% 28% 16% 4	3580
	11th to 20th	Night	31% 6% 15% 28% 18% 1	3572
	21st to end	Night	40% 6% 14% 23% 15% 3	3874
August	1st to 10th	Night	30% 6% 16% 32% 14% 2	2904
	11th to 20th	Night	33% 6% 12% 29% 18% 1	3118
	21st to end	Night	28% 5% 11% 30% 22% 3	3586
September	1st to 10th	Night	33% 6% 11% 25% 20% 6%	2000
	11th to 20th	Night	26% 5% 10% 28% 28% 4	2185
	21st to end	Night	35% 6% 10% 23% 24% 3	2200
October	1st to 10th	Night	21% 4 6% 31% 37% 2	3590
	11th to 20th	Night	33% 3 8% 23% 32% 1	3878
	21st to end	Night	36% 3 8% 25% 24% 3	4610
November	1st to 10th	Night	37% 4 9% 26% 21% 3	2666
	11th to 20th	Night	45% 4 9% 23% 16% 3	2730
	21st to end	Night	57% 3 8% 22% 9%	2730
December	1st to 10th	Night	47% 3 11% 27% 11% 1	4477
	11th to 20th	Night	59% 3 11% 16% 9% 1	5341
	21st to end	Night	72% 5% 9% 10% 4	7150

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	23% 15% 34% 24% 4 1	330
	11th to 20th	Day	22% 22% 34% 17% 3 2	330
	21st to end	Day	28% 15% 33% 19% 5%	363
February	1st to 10th	Day	25% 12% 37% 21% 5%	330
	11th to 20th	Day	24% 18% 36% 14% 6% 1	330
	21st to end	Day	25% 14% 45% 14% 3	273
March	1st to 10th	Day	23% 19% 39% 15% 4	330
	11th to 20th	Day	29% 15% 32% 18% 6% 1	421
	21st to end	Day	32% 12% 33% 16% 6% 1	550
April	1st to 10th	Day	24% 13% 32% 22% 7% 2	500
	11th to 20th	Day	24% 11% 33% 20% 11% 2	492
	21st to end	Day	25% 12% 32% 19% 10% 2	500
May	1st to 10th	Day	21% 11% 31% 23% 12% 3	503
	11th to 20th	Day	21% 6% 27% 27% 14% 5%	527
	21st to end	Day	13% 4 30% 28% 16% 9%	711
June	1st to 10th	Day	12% 6% 26% 31% 19% 6%	730
	11th to 20th	Day	13% 3 26% 31% 21% 5%	730
	21st to end	Day	14% 4 27% 27% 19% 8%	730
July	1st to 10th	Day	12% 2 14% 29% 28% 15%	721
	11th to 20th	Day	9% 2 18% 35% 23% 14%	718
	21st to end	Day	11% 2 14% 32% 28% 12%	774
August	1st to 10th	Day	7% 3 12% 28% 32% 17%	526
	11th to 20th	Day	13% 2 14% 29% 29% 13%	519
	21st to end	Day	7% 1 14% 30% 34% 14%	583
September	1st to 10th	Day	13% 4 12% 29% 31% 11%	530
	11th to 20th	Day	10% 1 11% 29% 31% 18%	530
	21st to end	Day	15% 7% 20% 24% 24% 10%	530
October	1st to 10th	Day	13% 3 23% 33% 22% 7%	530
	11th to 20th	Day	15% 13% 31% 26% 13% 2	410
	21st to end	Day	29% 10% 37% 19% 5%	363
November	1st to 10th	Day	13% 15% 34% 23% 12% 3	330
	11th to 20th	Day	25% 17% 29% 19% 9% 1	330
	21st to end	Day	28% 11% 37% 20% 4	330
December	1st to 10th	Day	20% 17% 35% 21% 6% 1	330
	11th to 20th	Day	22% 14% 39% 20% 4 1	330
	21st to end	Day	31% 14% 33% 20% 2	363
January	1st to 10th	Night	22% 12% 38% 23% 4	1050
	11th to 20th	Night	24% 14% 37% 21% 4	1050
	21st to end	Night	23% 13% 36% 25% 3	1142
February	1st to 10th	Night	26% 10% 36% 26% 2	840
	11th to 20th	Night	21% 14% 38% 23% 4	823
	21st to end	Night	25% 12% 39% 22% 3	692
March	1st to 10th	Night	23% 13% 34% 25% 5%	840
	11th to 20th	Night	18% 12% 38% 26% 5% 1	829
	21st to end	Night	25% 9% 33% 26% 5% 1	753
April	1st to 10th	Night	19% 11% 38% 24% 7% 1	600
	11th to 20th	Night	23% 9% 33% 26% 8%	593
	21st to end	Night	22% 6% 38% 25% 8% 1	600
May	1st to 10th	Night	21% 5% 35% 29% 10%	600
	11th to 20th	Night	21% 4 24% 33% 16% 2	591
	21st to end	Night	11% 4 24% 38% 21% 2	650
June	1st to 10th	Night	12% 2 21% 45% 19%	600
	11th to 20th	Night	12% 3 20% 46% 19% 1	600
	21st to end	Night	11% 3 23% 39% 25% 1	600
July	1st to 10th	Night	10% 1 13% 41% 29% 7%	585
	11th to 20th	Night	7% 2 15% 41% 30% 5%	582
	21st to end	Night	10% 1 12% 43% 32% 2	657
August	1st to 10th	Night	8% 1 10% 44% 33% 5%	597
	11th to 20th	Night	7% 1 11% 42% 34% 6%	594
	21st to end	Night	9% 2 13% 33% 38% 5%	660
September	1st to 10th	Night	14% 2 13% 39% 28% 5%	600
	11th to 20th	Night	10% 3 12% 37% 34% 3	785
	21st to end	Night	14% 4 19% 35% 25% 3	800
October	1st to 10th	Night	11% 4 23% 39% 19% 4	800
	11th to 20th	Night	21% 9% 26% 31% 12% 1	800
	21st to end	Night	22% 11% 34% 25% 7%	880
November	1st to 10th	Night	17% 7% 37% 28% 10% 1	820
	11th to 20th	Night	20% 13% 33% 27% 7% 1	840
	21st to end	Night	26% 13% 30% 25% 7%	840
December	1st to 10th	Night	23% 12% 36% 24% 4	898
	11th to 20th	Night	25% 11% 33% 25% 6%	1050
	21st to end	Night	31% 12% 32% 23% 3	1155

Page generated at 2026-03-15 02:23:56. Version ac07

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