Grasslands National Park East Block
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grasslands National Park East Block clear sky chart from 2005-11-06 to 2024-11-04.

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	7% 7% 3 7% 5% 5% 5% 4 6% 8% 42%	105532
	Night	6% 7% 3 8% 5% 5% 5% 4 7% 8% 42%	102127
Transparency	Day	40% 3 14% 28% 13% 2	105265
	Night	39% 3 15% 28% 13% 2	101982
Seeing	Day	12% 8% 30% 30% 16% 4	15900
	Night	11% 7% 33% 35% 12% 1	25340

By Month

Percentage Cloud Cover SW (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	4 12% 4 10% 7% 9% 8% 6% 8% 8% 26%	2219
	11th to 20th	Day	3 8% 3 11% 8% 7% 9% 8% 12% 12% 20%	2421
	21st to end	Day	4 10% 4 8% 6% 5% 6% 6% 7% 9% 35%	2794
February	1st to 10th	Day	8% 12% 5% 11% 8% 7% 7% 6% 7% 6% 24%	1690
	11th to 20th	Day	5% 13% 4 12% 8% 7% 5% 5% 8% 7% 25%	1665
	21st to end	Day	4 10% 6% 12% 8% 7% 8% 6% 6% 8% 26%	1485
March	1st to 10th	Day	4 9% 5% 13% 8% 6% 5% 4 6% 6% 34%	3130
	11th to 20th	Day	7% 9% 3 8% 4 4 5% 4 7% 9% 40%	3482
	21st to end	Day	7% 16% 5% 14% 7% 5% 5% 4 6% 8% 21%	4005
April	1st to 10th	Day	7% 10% 3 8% 6% 5% 5% 5% 7% 9% 35%	2280
	11th to 20th	Day	11% 15% 4 10% 6% 5% 5% 4 6% 7% 29%	2269
	21st to end	Day	13% 10% 4 7% 5% 4 4 3 5% 7% 37%	2583
May	1st to 10th	Day	16% 12% 3 7% 4 4 4 4 6% 6% 33%	4430
	11th to 20th	Day	9% 6% 3 7% 5% 6% 6% 5% 7% 9% 38%	4388
	21st to end	Day	15% 7% 2 7% 5% 6% 6% 4 6% 8% 34%	4825
June	1st to 10th	Day	14% 7% 2 7% 5% 4 5% 5% 5% 9% 37%	2779
	11th to 20th	Day	14% 7% 2 6% 5% 5% 4 4 7% 8% 38%	2850
	21st to end	Day	7% 5% 2 5% 5% 4 5% 5% 6% 8% 48%	2850
July	1st to 10th	Day	3 3 2 5% 4 6% 6% 6% 8% 9% 48%	4740
	11th to 20th	Day	3 2 1 3 3 3 4 5% 7% 9% 60%	4631
	21st to end	Day	3 3 1 3 3 3 3 3 6% 7% 65%	4802
August	1st to 10th	Day	7% 4 1 4 3 4 4 3 5% 8% 57%	4411
	11th to 20th	Day	5% 4 2 5% 4 4 4 4 5% 10% 55%	4103
	21st to end	Day	7% 4 1 4 3 3 3 3 5% 7% 59%	4319
September	1st to 10th	Day	9% 6% 2 5% 4 4 5% 4 5% 6% 50%	2280
	11th to 20th	Day	11% 7% 2 6% 4 4 5% 3 6% 7% 46%	2280
	21st to end	Day	12% 8% 2 4 4 3 4 3 6% 7% 49%	2024
October	1st to 10th	Day	7% 6% 2 5% 3 4 4 3 5% 7% 54%	3170
	11th to 20th	Day	5% 6% 2 6% 3 4 4 4 7% 11% 50%	3150
	21st to end	Day	4 8% 3 7% 4 4 4 4 6% 8% 47%	3105
November	1st to 10th	Day	9% 12% 5% 9% 6% 5% 4 3 6% 6% 37%	1548
	11th to 20th	Day	4 10% 5% 11% 8% 6% 4 4 6% 8% 36%	1520
	21st to end	Day	5% 9% 4 9% 6% 6% 7% 6% 6% 9% 33%	1520
December	1st to 10th	Day	5% 10% 4 12% 10% 7% 7% 5% 6% 8% 26%	1520
	11th to 20th	Day	2 10% 4 14% 8% 7% 7% 5% 8% 7% 29%	1822
	21st to end	Day	4 8% 3 9% 6% 5% 5% 4 7% 7% 43%	2442
January	1st to 10th	Night	2 7% 4 11% 6% 6% 7% 6% 8% 9% 33%	4726
	11th to 20th	Night	3 6% 4 13% 8% 7% 8% 6% 9% 8% 27%	4541
	21st to end	Night	2 9% 5% 10% 5% 6% 6% 5% 7% 9% 36%	4873
February	1st to 10th	Night	5% 13% 5% 9% 6% 7% 6% 5% 7% 9% 29%	2508
	11th to 20th	Night	4 10% 4 11% 8% 6% 6% 5% 8% 9% 29%	2402
	21st to end	Night	4 10% 4 8% 7% 6% 6% 5% 7% 8% 34%	1968
March	1st to 10th	Night	5% 7% 3 10% 7% 7% 6% 5% 7% 10% 34%	3734
	11th to 20th	Night	4 8% 2 9% 4 4 5% 4 8% 11% 41%	3495
	21st to end	Night	9% 13% 6% 14% 8% 7% 6% 4 5% 6% 23%	3642
April	1st to 10th	Night	7% 9% 3 7% 6% 5% 5% 5% 7% 8% 38%	1900
	11th to 20th	Night	11% 10% 4 8% 4 5% 4 4 7% 8% 35%	1857
	21st to end	Night	9% 10% 3 8% 6% 5% 4 4 6% 7% 39%	1596
May	1st to 10th	Night	14% 11% 3 7% 3 4 4 4 6% 6% 39%	2540
	11th to 20th	Night	9% 5% 2 6% 5% 5% 6% 4 8% 10% 40%	2517
	21st to end	Night	12% 6% 2 5% 4 5% 5% 5% 6% 9% 41%	2762
June	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 7% 8% 45%	1398
	11th to 20th	Night	12% 4 1 5% 4 5% 4 5% 7% 8% 45%	1330
	21st to end	Night	5% 4 1 4 3 4 4 3 6% 9% 56%	1330
July	1st to 10th	Night	4 3 1 5% 3 3 4 4 7% 9% 57%	2220
	11th to 20th	Night	5% 3 2 4 4 4 3 4 6% 9% 57%	2316
	21st to end	Night	2 2 1 3 3 3 5% 4 6% 9% 63%	2761
August	1st to 10th	Night	7% 5% 2 4 2 3 3 3 5% 7% 58%	2528
	11th to 20th	Night	4 3 2 4 3 3 4 4 6% 9% 57%	2720
	21st to end	Night	8% 4 1 5% 3 3 4 3 5% 6% 57%	3347
September	1st to 10th	Night	9% 4 1 4 3 4 3 2 5% 7% 58%	1900
	11th to 20th	Night	9% 5% 2 5% 3 4 3 3 7% 7% 52%	1900
	21st to end	Night	10% 5% 2 4 4 3 3 3 5% 8% 53%	2154
October	1st to 10th	Night	8% 5% 2 5% 4 3 4 3 6% 8% 52%	3800
	11th to 20th	Night	5% 4 1 5% 3 3 4 4 6% 10% 54%	3778
	21st to end	Night	5% 7% 2 7% 5% 5% 5% 4 7% 8% 44%	4422
November	1st to 10th	Night	10% 11% 3 8% 7% 5% 4 4 6% 7% 35%	2601
	11th to 20th	Night	4 9% 4 10% 7% 5% 5% 5% 7% 8% 36%	2660
	21st to end	Night	4 9% 5% 10% 6% 5% 5% 5% 6% 8% 37%	2660
December	1st to 10th	Night	5% 11% 4 11% 9% 7% 5% 6% 6% 7% 28%	2660
	11th to 20th	Night	4 10% 4 11% 7% 6% 6% 5% 8% 7% 32%	3367
	21st to end	Night	3 6% 2 7% 5% 4 4 4 5% 6% 52%	5214

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	56% 3 16% 20% 5%	2219
	11th to 20th	Day	53% 4 15% 18% 10% 1	2421
	21st to end	Day	44% 4 19% 27% 5%	2794
February	1st to 10th	Day	58% 4 10% 17% 10%	1690
	11th to 20th	Day	56% 4 14% 14% 11% 1	1674
	21st to end	Day	55% 4 14% 16% 10% 1	1485
March	1st to 10th	Day	51% 3 12% 28% 5%	3130
	11th to 20th	Day	42% 3 22% 26% 7%	3482
	21st to end	Day	62% 3 13% 16% 6%	4005
April	1st to 10th	Day	45% 4 16% 24% 11% 1	2280
	11th to 20th	Day	55% 3 13% 20% 9%	2272
	21st to end	Day	47% 2 15% 27% 9%	2583
May	1st to 10th	Day	52% 1 14% 22% 10% 1	4430
	11th to 20th	Day	43% 2 12% 30% 13%	4392
	21st to end	Day	48% 2 14% 27% 8%	4825
June	1st to 10th	Day	45% 3 17% 24% 10% 1	2779
	11th to 20th	Day	43% 3 16% 25% 11% 2	2850
	21st to end	Day	33% 4 16% 35% 11% 1	2850
July	1st to 10th	Day	28% 7% 19% 25% 18% 4	4666
	11th to 20th	Day	19% 4 14% 35% 22% 5%	4550
	21st to end	Day	18% 3 8% 32% 27% 12%	4651
August	1st to 10th	Day	27% 3 11% 35% 21% 3	4396
	11th to 20th	Day	25% 3 18% 35% 17% 2	4144
	21st to end	Day	26% 2 12% 39% 19% 2	4318
September	1st to 10th	Day	35% 3 10% 31% 17% 5%	2280
	11th to 20th	Day	37% 3 14% 29% 13% 3	2280
	21st to end	Day	36% 3 11% 30% 17% 3	2024
October	1st to 10th	Day	32% 2 11% 34% 18% 3	3170
	11th to 20th	Day	29% 2 14% 38% 13% 2	3150
	21st to end	Day	35% 3 15% 37% 10%	3105
November	1st to 10th	Day	49% 4 14% 22% 10% 1	1548
	11th to 20th	Day	47% 5% 18% 22% 8% 1	1520
	21st to end	Day	46% 6% 21% 21% 6% 1	1520
December	1st to 10th	Day	57% 5% 15% 18% 5% 1	1520
	11th to 20th	Day	52% 5% 22% 16% 5%	1820
	21st to end	Day	40% 3 9% 25% 16% 8%	2442
January	1st to 10th	Night	45% 4 22% 21% 7% 1	4726
	11th to 20th	Night	52% 3 13% 20% 12% 1	4541
	21st to end	Night	45% 4 20% 25% 5% 1	4873
February	1st to 10th	Night	51% 4 15% 21% 8% 1	2508
	11th to 20th	Night	50% 4 16% 18% 11% 1	2403
	21st to end	Night	46% 3 13% 25% 12%	1968
March	1st to 10th	Night	46% 2 14% 26% 12%	3734
	11th to 20th	Night	37% 3 25% 27% 7% 1	3495
	21st to end	Night	63% 2 13% 16% 6%	3642
April	1st to 10th	Night	42% 4 13% 27% 12% 1	1900
	11th to 20th	Night	46% 4 12% 24% 13% 2	1857
	21st to end	Night	45% 2 11% 29% 12%	1596
May	1st to 10th	Night	46% 2 9% 29% 13% 1	2540
	11th to 20th	Night	39% 3 13% 32% 13%	2518
	21st to end	Night	41% 2 15% 34% 7%	2762
June	1st to 10th	Night	36% 3 19% 27% 13% 2	1398
	11th to 20th	Night	35% 5% 15% 28% 15% 2	1330
	21st to end	Night	26% 3 20% 34% 14% 2	1330
July	1st to 10th	Night	22% 5% 24% 29% 17% 2	2185
	11th to 20th	Night	24% 3 13% 30% 23% 6%	2274
	21st to end	Night	18% 3 9% 33% 28% 8%	2674
August	1st to 10th	Night	26% 3 10% 34% 24% 3	2520
	11th to 20th	Night	23% 4 14% 38% 19% 2	2744
	21st to end	Night	28% 4 12% 35% 19% 3	3347
September	1st to 10th	Night	28% 2 11% 32% 21% 7%	1900
	11th to 20th	Night	32% 3 12% 29% 20% 4	1900
	21st to end	Night	30% 3 11% 32% 21% 2	2154
October	1st to 10th	Night	31% 3 10% 34% 21% 2	3800
	11th to 20th	Night	26% 2 13% 41% 17% 1	3778
	21st to end	Night	37% 4 12% 35% 12% 1	4422
November	1st to 10th	Night	49% 3 13% 23% 9% 2	2601
	11th to 20th	Night	45% 4 18% 24% 8% 1	2660
	21st to end	Night	45% 4 21% 22% 8%	2660
December	1st to 10th	Night	53% 5% 15% 19% 8% 1	2660
	11th to 20th	Night	48% 4 20% 21% 6% 1	3368
	21st to end	Night	33% 3 12% 26% 19% 7%	5214

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	9% 8% 36% 28% 17% 2	180
	11th to 20th	Day	11% 10% 34% 30% 13% 2	238
	21st to end	Day	9% 9% 36% 31% 13% 2	297
February	1st to 10th	Day	14% 10% 44% 24% 7% 1	270
	11th to 20th	Day	7% 8% 37% 33% 13% 2	270
	21st to end	Day	7% 6% 30% 37% 17% 3	225
March	1st to 10th	Day	16% 7% 32% 32% 13% 1	270
	11th to 20th	Day	11% 9% 37% 25% 14% 4	452
	21st to end	Day	12% 9% 31% 27% 16% 5%	506
April	1st to 10th	Day	12% 12% 35% 32% 8% 1	460
	11th to 20th	Day	19% 14% 32% 26% 7% 1	452
	21st to end	Day	19% 9% 28% 30% 13% 2	573
May	1st to 10th	Day	16% 7% 24% 31% 18% 4	653
	11th to 20th	Day	15% 6% 25% 28% 19% 8%	675
	21st to end	Day	18% 8% 25% 25% 18% 7%	734
June	1st to 10th	Day	15% 9% 27% 27% 18% 4	680
	11th to 20th	Day	18% 9% 35% 24% 11% 3	680
	21st to end	Day	10% 8% 25% 32% 21% 3	680
July	1st to 10th	Day	6% 4 25% 37% 25% 3	671
	11th to 20th	Day	6% 6% 28% 36% 19% 5%	668
	21st to end	Day	6% 6% 26% 34% 24% 6%	739
August	1st to 10th	Day	8% 6% 26% 32% 21% 7%	676
	11th to 20th	Day	10% 5% 23% 30% 25% 7%	528
	21st to end	Day	8% 7% 24% 38% 17% 5%	539
September	1st to 10th	Day	10% 9% 29% 33% 14% 4	490
	11th to 20th	Day	15% 9% 26% 32% 14% 3	490
	21st to end	Day	19% 7% 28% 30% 12% 4	348
October	1st to 10th	Day	18% 7% 25% 29% 18% 4	300
	11th to 20th	Day	9% 8% 37% 29% 14% 4	300
	21st to end	Day	15% 10% 42% 24% 8% 2	330
November	1st to 10th	Day	16% 12% 35% 29% 8% 1	279
	11th to 20th	Day	7% 13% 40% 26% 11% 1	270
	21st to end	Day	9% 9% 42% 28% 11% 1	270
December	1st to 10th	Day	9% 9% 40% 29% 10% 4	270
	11th to 20th	Day	5% 12% 43% 28% 10% 2	239
	21st to end	Day	10% 6% 34% 34% 13% 3	198
January	1st to 10th	Night	7% 10% 40% 31% 11% 1	950
	11th to 20th	Night	9% 12% 36% 32% 9% 1	950
	21st to end	Night	8% 11% 39% 32% 10% 1	1045
February	1st to 10th	Night	10% 8% 31% 35% 14% 1	798
	11th to 20th	Night	10% 8% 38% 35% 9% 1	744
	21st to end	Night	9% 6% 26% 37% 18% 4	628
March	1st to 10th	Night	11% 8% 36% 33% 11% 2	760
	11th to 20th	Night	10% 8% 37% 32% 11% 2	760
	21st to end	Night	12% 5% 31% 36% 13% 2	727
April	1st to 10th	Night	11% 9% 34% 38% 9%	570
	11th to 20th	Night	19% 7% 34% 32% 7%	563
	21st to end	Night	15% 7% 32% 36% 10%	570
May	1st to 10th	Night	15% 6% 24% 39% 15% 1	570
	11th to 20th	Night	11% 5% 29% 35% 19% 2	561
	21st to end	Night	18% 7% 23% 32% 18% 1	617
June	1st to 10th	Night	13% 7% 27% 37% 14% 2	448
	11th to 20th	Night	15% 6% 31% 37% 12% 1	380
	21st to end	Night	7% 4 30% 41% 17% 2	380
July	1st to 10th	Night	9% 4 28% 41% 18% 1	370
	11th to 20th	Night	6% 4 30% 46% 12% 1	425
	21st to end	Night	4 4 31% 44% 17% 1	624
August	1st to 10th	Night	7% 3 28% 44% 16% 2	567
	11th to 20th	Night	5% 4 23% 45% 21% 2	564
	21st to end	Night	10% 5% 27% 40% 17% 1	627
September	1st to 10th	Night	11% 3 26% 42% 17% 2	570
	11th to 20th	Night	12% 4 27% 39% 17% 1	570
	21st to end	Night	12% 4 30% 41% 11% 1	682
October	1st to 10th	Night	15% 7% 29% 34% 14% 1	760
	11th to 20th	Night	12% 9% 31% 34% 12% 1	760
	21st to end	Night	11% 10% 39% 32% 7%	1012
November	1st to 10th	Night	16% 9% 36% 32% 6% 1	943
	11th to 20th	Night	9% 11% 40% 29% 10% 1	950
	21st to end	Night	9% 10% 42% 30% 7% 1	950
December	1st to 10th	Night	11% 8% 35% 32% 13% 1	950
	11th to 20th	Night	9% 9% 39% 33% 9% 1	950
	21st to end	Night	12% 7% 37% 33% 10% 2	1045

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