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 2024-12-06.

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% 36%	101354
	Night	15% 10% 4 8% 4 4 4 3 5% 6% 38%	98400
Transparency	Day	45% 5% 12% 23% 14% 2	104980
	Night	46% 5% 10% 22% 15% 2	102916
Seeing	Day	18% 8% 26% 25% 17% 7%	15744
	Night	19% 8% 28% 30% 13% 1	25593

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	15% 14% 9% 14% 5% 5% 4 4 5% 4 20%	2527
	11th to 20th	Day	15% 14% 8% 14% 6% 5% 5% 8% 5% 4 15%	2530
	21st to end	Day	27% 22% 7% 10% 4 3 3 2 2 4 16%	2806
February	1st to 10th	Day	16% 15% 5% 9% 8% 6% 4 2 4 5% 25%	1900
	11th to 20th	Day	16% 12% 3 10% 5% 5% 4 4 5% 6% 30%	1848
	21st to end	Day	16% 16% 5% 9% 4 4 4 4 4 5% 29%	1570
March	1st to 10th	Day	20% 19% 4 7% 3 4 3 2 4 4 29%	3120
	11th to 20th	Day	13% 8% 3 6% 4 4 4 4 6% 7% 42%	3318
	21st to end	Day	16% 11% 5% 9% 5% 4 4 4 5% 7% 29%	4054
April	1st to 10th	Day	15% 11% 3 8% 5% 5% 5% 4 5% 7% 33%	2280
	11th to 20th	Day	17% 9% 3 6% 4 4 5% 3 5% 7% 36%	2254
	21st to end	Day	20% 9% 3 7% 4 4 4 3 6% 7% 33%	2427
May	1st to 10th	Day	18% 10% 3 7% 5% 4 5% 4 6% 8% 28%	4082
	11th to 20th	Day	18% 7% 3 6% 4 4 4 4 5% 8% 36%	4081
	21st to end	Day	10% 6% 2 6% 5% 4 5% 4 6% 9% 42%	4698
June	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	2660
	11th to 20th	Day	9% 4 2 6% 5% 5% 5% 4 6% 8% 47%	2660
	21st to end	Day	11% 6% 2 6% 5% 4 5% 5% 6% 9% 41%	2660
July	1st to 10th	Day	14% 6% 3 6% 5% 4 5% 4 7% 10% 36%	4440
	11th to 20th	Day	7% 4 2 5% 4 4 4 5% 7% 11% 46%	4440
	21st to end	Day	10% 6% 3 6% 5% 5% 5% 4 7% 9% 40%	4782
August	1st to 10th	Day	8% 4 2 5% 3 4 5% 4 7% 10% 48%	4104
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 4 7% 10% 43%	3830
	21st to end	Day	6% 5% 2 5% 3 4 4 4 6% 10% 51%	4180
September	1st to 10th	Day	11% 7% 2 6% 4 4 4 3 5% 7% 48%	2275
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 6% 7% 49%	2090
	21st to end	Day	14% 6% 2 5% 3 4 4 3 6% 7% 45%	2090
October	1st to 10th	Day	13% 5% 2 6% 4 4 3 3 6% 7% 47%	2090
	11th to 20th	Day	17% 7% 2 6% 3 4 4 4 5% 5% 42%	1914
	21st to end	Day	19% 9% 3 8% 4 4 5% 3 5% 6% 33%	1881
November	1st to 10th	Day	12% 7% 2 4 4 4 3 3 5% 7% 49%	1762
	11th to 20th	Day	18% 12% 4 7% 4 4 3 2 5% 5% 36%	1800
	21st to end	Day	18% 11% 4 9% 4 4 4 3 5% 6% 32%	1800
December	1st to 10th	Day	17% 16% 5% 9% 5% 4 3 3 5% 5% 28%	1700
	11th to 20th	Day	14% 14% 6% 13% 5% 3 4 3 5% 6% 26%	1918
	21st to end	Day	22% 21% 8% 13% 4 3 4 2 4 4 14%	2783
January	1st to 10th	Night	17% 17% 9% 11% 4 4 3 3 4 4 23%	4428
	11th to 20th	Night	13% 20% 8% 14% 6% 6% 5% 3 4 3 18%	4440
	21st to end	Night	25% 18% 7% 10% 4 3 3 2 3 4 21%	4845
February	1st to 10th	Night	19% 14% 5% 10% 6% 4 4 3 4 5% 27%	2280
	11th to 20th	Night	14% 12% 5% 11% 5% 4 4 3 4 6% 33%	2218
	21st to end	Night	19% 13% 5% 9% 4 4 4 3 4 5% 31%	1884
March	1st to 10th	Night	19% 14% 4 7% 4 3 3 2 4 6% 34%	3733
	11th to 20th	Night	12% 7% 3 6% 4 4 4 3 5% 7% 45%	3641
	21st to end	Night	19% 9% 3 9% 4 3 4 3 5% 6% 34%	3607
April	1st to 10th	Night	14% 9% 3 6% 5% 4 3 4 6% 7% 38%	1900
	11th to 20th	Night	18% 9% 2 5% 3 3 4 3 5% 8% 39%	1875
	21st to end	Night	17% 8% 4 6% 5% 5% 4 3 6% 6% 36%	1753
May	1st to 10th	Night	17% 10% 3 8% 5% 4 4 4 6% 8% 30%	2813
	11th to 20th	Night	20% 7% 2 6% 4 3 4 4 6% 7% 38%	2812
	21st to end	Night	10% 5% 2 7% 4 4 5% 4 6% 8% 44%	2848
June	1st to 10th	Night	8% 6% 3 6% 4 4 5% 4 5% 7% 50%	1520
	11th to 20th	Night	8% 7% 2 5% 4 4 5% 4 5% 8% 48%	1520
	21st to end	Night	9% 6% 2 6% 5% 5% 4 4 8% 8% 43%	1520
July	1st to 10th	Night	10% 4 2 6% 4 4 5% 4 6% 10% 44%	2530
	11th to 20th	Night	6% 4 1 4 3 2 4 4 7% 8% 56%	2530
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 6% 8% 43%	2779
August	1st to 10th	Night	10% 4 2 5% 3 4 4 3 5% 8% 52%	2825
	11th to 20th	Night	7% 6% 2 7% 4 4 5% 4 6% 8% 47%	2995
	21st to end	Night	8% 4 2 6% 3 3 4 2 5% 7% 55%	3476
September	1st to 10th	Night	11% 5% 2 5% 3 4 3 4 5% 7% 51%	1900
	11th to 20th	Night	9% 4 1 3 3 3 3 2 5% 8% 58%	2076
	21st to end	Night	13% 6% 1 5% 3 4 4 3 5% 6% 49%	2090
October	1st to 10th	Night	10% 5% 2 6% 3 2 4 3 5% 6% 54%	2090
	11th to 20th	Night	17% 6% 2 5% 3 4 3 3 4 5% 47%	2262
	21st to end	Night	18% 9% 2 6% 4 4 4 3 5% 6% 38%	2717
November	1st to 10th	Night	13% 7% 3 5% 3 3 3 3 5% 5% 49%	2536
	11th to 20th	Night	14% 11% 3 7% 4 4 4 3 5% 6% 39%	2600
	21st to end	Night	18% 14% 4 8% 5% 4 3 3 4 5% 31%	2600
December	1st to 10th	Night	16% 13% 5% 9% 6% 4 4 3 4 6% 30%	2594
	11th to 20th	Night	16% 15% 5% 10% 5% 4 3 3 4 5% 31%	3279
	21st to end	Night	22% 19% 7% 10% 5% 5% 4 3 4 4 16%	4884

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	66% 4 7% 17% 6%	2528
	11th to 20th	Day	70% 4 9% 10% 7%	2530
	21st to end	Day	75% 2 6% 12% 3 1	2806
February	1st to 10th	Day	64% 3 9% 14% 8% 2	1900
	11th to 20th	Day	54% 5% 10% 22% 9% 1	1858
	21st to end	Day	57% 4 11% 19% 7% 2	1570
March	1st to 10th	Day	61% 4 7% 18% 10% 1	3120
	11th to 20th	Day	41% 5% 10% 29% 14% 1	3318
	21st to end	Day	54% 6% 15% 22% 3	4054
April	1st to 10th	Day	50% 6% 12% 19% 13%	2280
	11th to 20th	Day	47% 6% 12% 23% 10% 3	2254
	21st to end	Day	50% 6% 11% 19% 11% 3	2427
May	1st to 10th	Day	51% 7% 13% 22% 7%	4120
	11th to 20th	Day	46% 8% 12% 26% 9% 1	4085
	21st to end	Day	36% 7% 12% 22% 20% 2	4711
June	1st to 10th	Day	39% 6% 12% 24% 16% 3	2660
	11th to 20th	Day	34% 5% 12% 25% 21% 3	2660
	21st to end	Day	38% 8% 15% 19% 16% 4	2660
July	1st to 10th	Day	43% 6% 11% 22% 15% 2	4371
	11th to 20th	Day	30% 6% 16% 29% 18% 1	4365
	21st to end	Day	39% 7% 15% 18% 17% 3	4632
August	1st to 10th	Day	31% 7% 17% 29% 16% 1	4091
	11th to 20th	Day	34% 6% 12% 31% 15% 1	3869
	21st to end	Day	29% 5% 14% 30% 18% 4	4179
September	1st to 10th	Day	36% 5% 13% 25% 17% 5%	2275
	11th to 20th	Day	33% 6% 11% 24% 23% 3	2090
	21st to end	Day	37% 6% 9% 24% 21% 2	2090
October	1st to 10th	Day	25% 3 10% 31% 29% 2	3480
	11th to 20th	Day	34% 5% 10% 24% 26% 1	3156
	21st to end	Day	44% 3 8% 24% 19% 1	3081
November	1st to 10th	Day	36% 4 13% 26% 17% 3	1762
	11th to 20th	Day	52% 2 10% 21% 12% 3	1800
	21st to end	Day	53% 5% 10% 21% 10% 1	1800
December	1st to 10th	Day	59% 4 10% 18% 8%	1700
	11th to 20th	Day	61% 4 7% 24% 4 1	1915
	21st to end	Day	75% 4 8% 9% 4	2783
January	1st to 10th	Night	64% 5% 9% 15% 6%	4428
	11th to 20th	Night	72% 3 7% 12% 6%	4440
	21st to end	Night	69% 3 8% 14% 6% 1	4845
February	1st to 10th	Night	60% 4 12% 15% 9% 1	2280
	11th to 20th	Night	54% 4 9% 19% 12% 2	2219
	21st to end	Night	56% 4 10% 18% 9% 2	1884
March	1st to 10th	Night	53% 4 9% 23% 10% 2	3733
	11th to 20th	Night	38% 5% 10% 26% 18% 2	3641
	21st to end	Night	50% 6% 12% 25% 6% 1	3607
April	1st to 10th	Night	44% 6% 12% 22% 15% 2	1900
	11th to 20th	Night	43% 5% 10% 22% 15% 5%	1875
	21st to end	Night	47% 6% 11% 20% 13% 4	1753
May	1st to 10th	Night	51% 8% 10% 21% 9% 1	2840
	11th to 20th	Night	43% 7% 14% 25% 9% 1	2813
	21st to end	Night	34% 9% 11% 23% 19% 4	2856
June	1st to 10th	Night	33% 7% 10% 28% 20% 3	1520
	11th to 20th	Night	34% 7% 10% 24% 22% 4	1520
	21st to end	Night	35% 8% 13% 23% 15% 5%	1520
July	1st to 10th	Night	36% 6% 12% 26% 15% 5%	2490
	11th to 20th	Night	24% 6% 13% 31% 26% 1	2482
	21st to end	Night	38% 6% 14% 20% 17% 4	2691
August	1st to 10th	Night	30% 6% 17% 31% 14% 2	2814
	11th to 20th	Night	33% 6% 12% 29% 18% 1	3022
	21st to end	Night	29% 5% 12% 30% 21% 3	3476
September	1st to 10th	Night	33% 6% 12% 24% 19% 6%	1900
	11th to 20th	Night	26% 5% 10% 28% 27% 4	2076
	21st to end	Night	35% 6% 10% 24% 23% 3	2090
October	1st to 10th	Night	20% 4 6% 31% 37% 2	3480
	11th to 20th	Night	33% 3 8% 23% 33% 1	3759
	21st to end	Night	36% 4 8% 25% 24% 3	4467
November	1st to 10th	Night	37% 5% 9% 26% 21% 3	2536
	11th to 20th	Night	46% 4 9% 23% 16% 3	2600
	21st to end	Night	57% 3 8% 22% 9%	2600
December	1st to 10th	Night	56% 4 10% 20% 9% 1	2594
	11th to 20th	Night	56% 4 9% 20% 11% 1	3281
	21st to end	Night	71% 5% 9% 10% 5%	4884

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	24% 13% 34% 23% 4 1	270
	11th to 20th	Day	24% 17% 34% 20% 3 2	270
	21st to end	Day	32% 11% 31% 20% 6%	297
February	1st to 10th	Day	27% 11% 37% 20% 5%	270
	11th to 20th	Day	25% 17% 34% 15% 7% 1	270
	21st to end	Day	27% 13% 43% 14% 3	225
March	1st to 10th	Day	22% 19% 38% 16% 5%	270
	11th to 20th	Day	28% 13% 33% 19% 6% 1	363
	21st to end	Day	32% 11% 32% 17% 7% 1	506
April	1st to 10th	Day	22% 12% 33% 22% 8% 2	460
	11th to 20th	Day	24% 10% 32% 19% 11% 3	452
	21st to end	Day	26% 12% 32% 18% 9% 3	460
May	1st to 10th	Day	21% 10% 31% 23% 11% 3	463
	11th to 20th	Day	22% 5% 26% 28% 14% 5%	487
	21st to end	Day	12% 4 29% 28% 18% 10%	660
June	1st to 10th	Day	12% 5% 26% 31% 20% 6%	680
	11th to 20th	Day	12% 3 26% 31% 21% 6%	680
	21st to end	Day	15% 4 29% 26% 18% 9%	680
July	1st to 10th	Day	13% 1 15% 28% 28% 15%	671
	11th to 20th	Day	9% 1 18% 35% 22% 15%	668
	21st to end	Day	11% 2 14% 33% 28% 12%	720
August	1st to 10th	Day	8% 3 13% 29% 29% 18%	486
	11th to 20th	Day	13% 2 15% 31% 27% 13%	479
	21st to end	Day	7% 1 13% 30% 35% 14%	539
September	1st to 10th	Day	14% 3 12% 30% 31% 10%	490
	11th to 20th	Day	11% 1 11% 30% 30% 17%	490
	21st to end	Day	16% 7% 21% 24% 23% 9%	490
October	1st to 10th	Day	14% 3 23% 32% 21% 7%	490
	11th to 20th	Day	15% 14% 33% 24% 12% 2	376
	21st to end	Day	31% 8% 37% 18% 5%	330
November	1st to 10th	Day	14% 13% 33% 25% 12% 4	300
	11th to 20th	Day	24% 16% 31% 18% 10% 1	300
	21st to end	Day	27% 12% 37% 21% 4	300
December	1st to 10th	Day	22% 18% 33% 20% 7% 1	285
	11th to 20th	Day	22% 13% 38% 21% 5% 1	270
	21st to end	Day	30% 15% 32% 20% 3	297
January	1st to 10th	Night	23% 13% 37% 23% 5%	950
	11th to 20th	Night	25% 12% 36% 22% 4	950
	21st to end	Night	24% 12% 34% 27% 3	1034
February	1st to 10th	Night	27% 9% 35% 26% 2	760
	11th to 20th	Night	21% 14% 36% 25% 4	743
	21st to end	Night	27% 11% 37% 21% 3	628
March	1st to 10th	Night	22% 13% 33% 26% 6%	760
	11th to 20th	Night	18% 11% 39% 26% 5% 1	760
	21st to end	Night	26% 8% 33% 27% 5% 1	716
April	1st to 10th	Night	18% 11% 38% 25% 7% 1	570
	11th to 20th	Night	23% 9% 33% 26% 9%	563
	21st to end	Night	22% 6% 39% 24% 8% 1	570
May	1st to 10th	Night	21% 5% 35% 29% 9%	570
	11th to 20th	Night	22% 3 23% 34% 17% 2	561
	21st to end	Night	11% 3 24% 38% 22% 2	617
June	1st to 10th	Night	12% 2 21% 46% 19%	570
	11th to 20th	Night	11% 2 20% 46% 20% 1	570
	21st to end	Night	11% 2 23% 38% 25% 1	570
July	1st to 10th	Night	9% 1 13% 40% 29% 7%	555
	11th to 20th	Night	7% 2 15% 40% 31% 6%	552
	21st to end	Night	10% 1 11% 44% 32% 2	624
August	1st to 10th	Night	8% 1 10% 44% 32% 5%	567
	11th to 20th	Night	7% 1 11% 42% 34% 6%	564
	21st to end	Night	9% 2 13% 32% 39% 5%	627
September	1st to 10th	Night	14% 2 12% 39% 27% 5%	570
	11th to 20th	Night	11% 3 13% 37% 33% 3	746
	21st to end	Night	14% 4 19% 35% 25% 2	760
October	1st to 10th	Night	11% 4 23% 38% 19% 5%	760
	11th to 20th	Night	21% 9% 27% 29% 12% 1	760
	21st to end	Night	23% 11% 34% 25% 7%	836
November	1st to 10th	Night	17% 7% 36% 28% 10% 1	780
	11th to 20th	Night	21% 13% 32% 27% 7% 1	800
	21st to end	Night	25% 13% 30% 25% 7%	800
December	1st to 10th	Night	23% 11% 36% 24% 5%	835
	11th to 20th	Night	26% 9% 33% 26% 7%	950
	21st to end	Night	30% 12% 31% 24% 3	1045

Page generated at 2024-12-03 06:45:09. Version ac07

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