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 2025-07-21.

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 8% 5% 5% 5% 4 6% 8% 41%	124091
	Night	6% 7% 3 9% 6% 5% 5% 4 7% 8% 40%	123842
Transparency	Day	41% 3 14% 27% 13% 2	123825
	Night	41% 4 15% 26% 13% 2	123697
Seeing	Day	12% 8% 30% 30% 16% 4	16869
	Night	11% 8% 33% 35% 12% 1	26303

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% 5% 13% 9% 9% 8% 5% 7% 6% 21%	3179
	11th to 20th	Day	2 6% 4 13% 7% 6% 8% 8% 10% 10% 25%	3479
	21st to end	Day	3 8% 4 10% 6% 5% 6% 5% 6% 8% 38%	4004
February	1st to 10th	Day	8% 12% 5% 11% 8% 7% 7% 6% 7% 6% 24%	1779
	11th to 20th	Day	4 12% 4 12% 8% 7% 5% 5% 8% 7% 26%	1755
	21st to end	Day	4 10% 6% 12% 7% 7% 8% 5% 6% 8% 27%	1561
March	1st to 10th	Day	3 7% 4 13% 7% 6% 4 3 5% 7% 40%	4500
	11th to 20th	Day	5% 8% 4 9% 5% 5% 5% 5% 8% 10% 36%	4992
	21st to end	Day	6% 14% 5% 14% 8% 6% 5% 4 6% 8% 23%	5744
April	1st to 10th	Day	6% 10% 4 8% 6% 5% 4 5% 7% 9% 36%	2400
	11th to 20th	Day	11% 14% 4 10% 6% 5% 5% 4 6% 7% 29%	2390
	21st to end	Day	12% 10% 4 7% 5% 4 4 3 5% 7% 38%	2719
May	1st to 10th	Day	13% 9% 2 6% 4 4 4 3 5% 7% 42%	6340
	11th to 20th	Day	15% 9% 3 8% 5% 5% 6% 4 7% 8% 30%	6298
	21st to end	Day	12% 6% 2 6% 4 5% 5% 4 6% 8% 40%	6925
June	1st to 10th	Day	13% 7% 2 7% 5% 4 5% 5% 5% 9% 38%	2925
	11th to 20th	Day	14% 7% 2 6% 5% 5% 4 4 7% 8% 37%	3000
	21st to end	Day	7% 5% 2 5% 4 4 5% 5% 6% 8% 48%	3000
July	1st to 10th	Day	3 3 2 5% 4 6% 6% 6% 8% 9% 48%	4890
	11th to 20th	Day	3 2 1 3 3 4 4 5% 7% 9% 59%	4778
	21st to end	Day	3 3 1 3 3 3 3 3 6% 7% 65%	4803
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% 11% 5% 9% 6% 5% 4 4 6% 6% 36%	1604
	11th to 20th	Day	4 11% 5% 11% 8% 6% 5% 4 6% 8% 35%	1600
	21st to end	Day	5% 9% 4 10% 7% 6% 7% 5% 6% 9% 32%	1600
December	1st to 10th	Day	3 9% 3 14% 9% 7% 7% 4 6% 7% 30%	2620
	11th to 20th	Day	5% 11% 6% 16% 7% 6% 6% 4 7% 7% 25%	2866
	21st to end	Day	4 7% 3 11% 8% 6% 5% 4 7% 8% 39%	3498
January	1st to 10th	Night	2 9% 4 13% 9% 7% 7% 6% 8% 9% 27%	6766
	11th to 20th	Night	2 6% 4 14% 8% 7% 8% 6% 9% 9% 28%	6490
	21st to end	Night	2 7% 4 11% 7% 6% 6% 5% 7% 9% 37%	6974
February	1st to 10th	Night	5% 13% 5% 9% 6% 7% 6% 5% 7% 10% 28%	2640
	11th to 20th	Night	4 10% 4 11% 8% 6% 6% 4 8% 9% 31%	2532
	21st to end	Night	4 10% 4 8% 7% 6% 6% 5% 7% 8% 35%	2068
March	1st to 10th	Night	5% 7% 2 8% 6% 6% 5% 4 7% 11% 40%	5374
	11th to 20th	Night	5% 8% 3 9% 5% 4 5% 4 7% 11% 39%	5005
	21st to end	Night	8% 12% 6% 15% 8% 7% 6% 4 6% 7% 22%	5219
April	1st to 10th	Night	6% 9% 3 7% 6% 5% 5% 5% 7% 8% 38%	2000
	11th to 20th	Night	11% 10% 4 8% 4 5% 5% 4 7% 8% 35%	1956
	21st to end	Night	9% 9% 3 8% 6% 5% 4 4 6% 7% 38%	1680
May	1st to 10th	Night	10% 8% 2 6% 3 3 4 4 6% 7% 47%	3640
	11th to 20th	Night	16% 8% 3 7% 5% 5% 6% 4 7% 8% 32%	3617
	21st to end	Night	10% 6% 2 5% 4 4 4 4 5% 8% 46%	3972
June	1st to 10th	Night	9% 5% 2 7% 4 4 4 4 7% 8% 45%	1471
	11th to 20th	Night	13% 4 1 5% 4 5% 4 5% 7% 8% 43%	1400
	21st to end	Night	6% 4 1 4 3 4 4 3 6% 9% 56%	1400
July	1st to 10th	Night	4 3 1 5% 3 3 4 4 7% 9% 56%	2290
	11th to 20th	Night	5% 3 2 4 4 4 3 4 6% 9% 57%	2389
	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%	2692
	11th to 20th	Night	5% 9% 4 10% 7% 5% 5% 5% 7% 8% 36%	2800
	21st to end	Night	4 10% 5% 10% 6% 6% 6% 5% 6% 7% 35%	2800
December	1st to 10th	Night	5% 11% 5% 15% 8% 7% 6% 5% 6% 7% 24%	4570
	11th to 20th	Night	4 11% 5% 14% 7% 7% 7% 5% 7% 7% 27%	5329
	21st to end	Night	3 8% 3 10% 7% 4 4 4 6% 6% 46%	7458

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	62% 4 13% 17% 5%	3179
	11th to 20th	Day	49% 4 14% 18% 14%	3479
	21st to end	Day	43% 5% 24% 25% 3	4004
February	1st to 10th	Day	58% 4 10% 17% 11%	1779
	11th to 20th	Day	54% 4 14% 13% 13% 1	1764
	21st to end	Day	54% 3 14% 18% 9% 1	1561
March	1st to 10th	Day	45% 3 16% 26% 10%	4500
	11th to 20th	Day	42% 4 18% 28% 8%	4992
	21st to end	Day	59% 5% 15% 16% 5%	5744
April	1st to 10th	Day	44% 4 15% 25% 11% 1	2400
	11th to 20th	Day	55% 3 12% 20% 8%	2393
	21st to end	Day	47% 2 14% 27% 9%	2719
May	1st to 10th	Day	43% 1 11% 26% 15% 2	6340
	11th to 20th	Day	52% 3 12% 24% 10%	6302
	21st to end	Day	43% 3 13% 28% 13%	6926
June	1st to 10th	Day	44% 3 16% 25% 11% 1	2925
	11th to 20th	Day	44% 3 16% 25% 11% 1	3000
	21st to end	Day	33% 4 16% 36% 11% 1	3000
July	1st to 10th	Day	28% 7% 19% 25% 18% 4	4816
	11th to 20th	Day	20% 4 14% 35% 22% 5%	4697
	21st to end	Day	18% 3 8% 33% 27% 12%	4652
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	48% 4 14% 23% 10% 1	1604
	11th to 20th	Day	47% 5% 17% 22% 8% 1	1600
	21st to end	Day	47% 6% 21% 20% 6% 1	1600
December	1st to 10th	Day	52% 6% 22% 16% 3	2620
	11th to 20th	Day	57% 4 22% 13% 3	2864
	21st to end	Day	43% 3 9% 27% 13% 5%	3498
January	1st to 10th	Night	51% 5% 17% 19% 8% 1	6766
	11th to 20th	Night	50% 4 14% 16% 16% 1	6490
	21st to end	Night	43% 5% 22% 25% 4	6974
February	1st to 10th	Night	50% 4 15% 20% 10% 1	2640
	11th to 20th	Night	48% 4 15% 18% 13% 2	2533
	21st to end	Night	45% 3 14% 25% 12%	2068
March	1st to 10th	Night	38% 3 18% 25% 16%	5374
	11th to 20th	Night	39% 3 20% 27% 10%	5005
	21st to end	Night	61% 3 14% 15% 7%	5219
April	1st to 10th	Night	42% 5% 13% 27% 12% 1	2000
	11th to 20th	Night	46% 4 12% 24% 13% 1	1956
	21st to end	Night	45% 3 11% 29% 12%	1680
May	1st to 10th	Night	36% 2 8% 33% 18% 1	3640
	11th to 20th	Night	49% 4 13% 24% 9%	3618
	21st to end	Night	38% 3 14% 32% 12% 2	3972
June	1st to 10th	Night	36% 3 18% 26% 14% 3	1471
	11th to 20th	Night	36% 5% 15% 28% 14% 2	1400
	21st to end	Night	26% 3 20% 34% 15% 2	1400
July	1st to 10th	Night	23% 5% 23% 29% 18% 2	2255
	11th to 20th	Night	24% 4 14% 30% 23% 6%	2347
	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% 4 13% 23% 9% 2	2692
	11th to 20th	Night	46% 4 17% 24% 9% 1	2800
	21st to end	Night	47% 4 20% 21% 7%	2800
December	1st to 10th	Night	58% 6% 17% 14% 5% 1	4570
	11th to 20th	Night	55% 5% 20% 16% 4	5330
	21st to end	Night	39% 3 11% 25% 17% 5%	7458

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	10% 9% 36% 28% 16% 2	200
	11th to 20th	Day	9% 11% 38% 28% 12% 2	265
	21st to end	Day	9% 10% 38% 30% 12% 2	330
February	1st to 10th	Day	14% 10% 45% 24% 7% 1	300
	11th to 20th	Day	7% 7% 37% 34% 13% 3	300
	21st to end	Day	7% 7% 33% 35% 16% 3	249
March	1st to 10th	Day	15% 7% 33% 31% 13% 1	300
	11th to 20th	Day	10% 10% 37% 25% 14% 4	492
	21st to end	Day	12% 10% 32% 27% 14% 5%	550
April	1st to 10th	Day	11% 12% 34% 32% 9% 1	500
	11th to 20th	Day	20% 15% 33% 25% 7% 1	492
	21st to end	Day	17% 9% 28% 31% 13% 2	619
May	1st to 10th	Day	15% 7% 24% 31% 18% 5%	703
	11th to 20th	Day	17% 7% 25% 26% 18% 7%	725
	21st to end	Day	17% 8% 24% 26% 18% 6%	789
June	1st to 10th	Day	15% 10% 26% 27% 18% 4	730
	11th to 20th	Day	18% 10% 35% 24% 11% 3	730
	21st to end	Day	11% 8% 25% 33% 20% 3	730
July	1st to 10th	Day	6% 4 26% 36% 24% 3	721
	11th to 20th	Day	7% 6% 28% 36% 19% 4	718
	21st to end	Day	6% 6% 26% 34% 24% 6%	740
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	15% 12% 35% 29% 8% 1	300
	11th to 20th	Day	8% 14% 41% 25% 10% 1	300
	21st to end	Day	9% 9% 41% 28% 11% 1	300
December	1st to 10th	Day	8% 11% 42% 27% 9% 3	300
	11th to 20th	Day	5% 12% 43% 29% 9% 2	265
	21st to end	Day	10% 5% 35% 34% 13% 3	220
January	1st to 10th	Night	7% 9% 40% 32% 11% 1	1000
	11th to 20th	Night	8% 13% 37% 32% 9% 1	1000
	21st to end	Night	7% 11% 40% 31% 9% 1	1100
February	1st to 10th	Night	10% 9% 32% 34% 14% 1	840
	11th to 20th	Night	10% 7% 38% 36% 9% 1	784
	21st to end	Night	9% 7% 27% 37% 17% 3	660
March	1st to 10th	Night	11% 8% 35% 33% 12% 2	800
	11th to 20th	Night	11% 8% 37% 32% 11% 2	800
	21st to end	Night	12% 6% 32% 36% 12% 2	765
April	1st to 10th	Night	11% 8% 33% 39% 9%	600
	11th to 20th	Night	18% 7% 35% 32% 7%	593
	21st to end	Night	15% 7% 32% 36% 10%	600
May	1st to 10th	Night	14% 7% 24% 40% 14% 1	600
	11th to 20th	Night	11% 6% 29% 34% 19% 2	591
	21st to end	Night	17% 8% 24% 32% 17% 1	650
June	1st to 10th	Night	12% 7% 27% 37% 14% 2	471
	11th to 20th	Night	15% 6% 31% 37% 11% 1	400
	21st to end	Night	7% 4 31% 40% 17% 2	400
July	1st to 10th	Night	8% 4 28% 42% 17% 1	390
	11th to 20th	Night	6% 5% 30% 46% 12% 1	448
	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% 10% 36% 32% 6% 1	975
	11th to 20th	Night	9% 11% 41% 28% 10% 1	1000
	21st to end	Night	9% 11% 42% 30% 7% 1	1000
December	1st to 10th	Night	11% 9% 35% 32% 12% 1	1000
	11th to 20th	Night	9% 9% 39% 33% 10% 1	1000
	21st to end	Night	11% 7% 37% 33% 10% 2	1100

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