Sprague
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sprague clear sky chart from 2005-11-06 to 2026-06-16.

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	12% 9% 3 9% 5% 5% 5% 4 6% 7% 35%	137978
	Night	10% 10% 4 10% 6% 5% 5% 4 6% 7% 32%	134331
Transparency	Day	32% 34% 9% 17% 7%	141411
	Night	35% 34% 8% 16% 7%	138996
Seeing	Day	17% 13% 34% 24% 10% 2	18436
	Night	16% 9% 33% 30% 11% 1	28397

By Month

Percentage Cloud Cover NE (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	6% 14% 7% 16% 7% 5% 7% 4 5% 5% 23%	4124
	11th to 20th	Day	4 11% 7% 15% 9% 9% 10% 8% 9% 6% 13%	4112
	21st to end	Day	4 13% 8% 13% 7% 8% 7% 5% 7% 8% 19%	5161
February	1st to 10th	Day	8% 14% 5% 11% 8% 7% 8% 5% 5% 7% 23%	1890
	11th to 20th	Day	7% 11% 4 9% 6% 8% 7% 5% 8% 8% 28%	1844
	21st to end	Day	5% 11% 4 12% 10% 7% 6% 4 6% 7% 28%	1578
March	1st to 10th	Day	10% 10% 3 8% 8% 5% 4 3 6% 7% 36%	4859
	11th to 20th	Day	10% 8% 4 12% 7% 5% 5% 5% 7% 8% 30%	5090
	21st to end	Day	14% 14% 4 9% 5% 4 4 3 4 6% 32%	5585
April	1st to 10th	Day	12% 12% 4 7% 5% 5% 4 3 6% 7% 34%	2516
	11th to 20th	Day	14% 12% 3 8% 6% 4 5% 4 6% 8% 31%	2702
	21st to end	Day	19% 9% 3 8% 5% 5% 4 4 5% 6% 31%	2730
May	1st to 10th	Day	12% 9% 3 6% 4 4 4 3 5% 6% 43%	8020
	11th to 20th	Day	16% 9% 3 6% 4 5% 5% 4 5% 7% 37%	8237
	21st to end	Day	13% 7% 2 6% 4 4 4 4 5% 7% 45%	9683
June	1st to 10th	Day	13% 7% 2 7% 5% 5% 5% 5% 7% 8% 37%	3150
	11th to 20th	Day	15% 6% 3 6% 5% 5% 5% 4 7% 8% 36%	3076
	21st to end	Day	14% 7% 2 6% 5% 5% 6% 5% 8% 9% 35%	3000
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 43%	6799
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 6% 8% 44%	6800
	21st to end	Day	11% 6% 3 6% 5% 5% 5% 5% 7% 8% 41%	7374
August	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 5% 6% 9% 44%	4250
	11th to 20th	Day	12% 5% 2 7% 4 4 5% 5% 6% 9% 41%	4171
	21st to end	Day	14% 5% 2 5% 4 4 4 4 5% 9% 44%	4675
September	1st to 10th	Day	11% 5% 3 6% 5% 4 5% 3 5% 8% 46%	2440
	11th to 20th	Day	16% 7% 2 6% 4 5% 5% 4 6% 8% 38%	2250
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 8% 40%	2185
October	1st to 10th	Day	17% 11% 3 8% 4 5% 4 4 4 6% 34%	2000
	11th to 20th	Day	15% 12% 5% 8% 4 5% 4 4 6% 6% 32%	2000
	21st to end	Day	16% 15% 4 11% 5% 4 4 3 5% 6% 26%	2015
November	1st to 10th	Day	14% 16% 4 9% 6% 4 4 5% 5% 5% 27%	1647
	11th to 20th	Day	12% 15% 6% 15% 8% 5% 4 4 4 4 25%	1680
	21st to end	Day	11% 14% 6% 15% 7% 6% 4 5% 5% 6% 21%	1680
December	1st to 10th	Day	13% 15% 4 16% 10% 8% 5% 4 5% 7% 14%	2371
	11th to 20th	Day	11% 17% 6% 12% 5% 4 5% 4 6% 6% 23%	2709
	21st to end	Day	12% 17% 7% 21% 8% 6% 4 2 4 4 15%	3575
January	1st to 10th	Night	5% 13% 6% 17% 9% 7% 7% 4 6% 6% 20%	8800
	11th to 20th	Night	5% 13% 6% 14% 9% 8% 9% 8% 9% 5% 13%	8803
	21st to end	Night	7% 14% 6% 12% 6% 5% 9% 5% 8% 8% 20%	9034
February	1st to 10th	Night	8% 11% 5% 11% 8% 6% 6% 5% 7% 8% 25%	2730
	11th to 20th	Night	7% 10% 3 7% 5% 6% 7% 5% 7% 8% 34%	2663
	21st to end	Night	5% 10% 5% 13% 9% 6% 6% 4 6% 7% 29%	2247
March	1st to 10th	Night	9% 10% 4 10% 8% 6% 5% 4 6% 7% 31%	5210
	11th to 20th	Night	6% 10% 5% 11% 7% 5% 6% 4 6% 8% 32%	5089
	21st to end	Night	15% 14% 5% 10% 5% 4 3 3 4 5% 31%	5588
April	1st to 10th	Night	14% 9% 3 8% 5% 5% 5% 3 6% 6% 38%	2101
	11th to 20th	Night	15% 10% 4 8% 5% 4 4 4 5% 5% 37%	1869
	21st to end	Night	15% 9% 3 7% 6% 5% 4 3 5% 6% 36%	1890
May	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 5% 7% 49%	4893
	11th to 20th	Night	18% 7% 2 5% 4 4 4 3 5% 6% 42%	4640
	21st to end	Night	13% 6% 3 5% 3 4 4 3 5% 6% 48%	4526
June	1st to 10th	Night	13% 5% 2 5% 5% 4 5% 4 6% 7% 43%	1470
	11th to 20th	Night	14% 5% 2 6% 4 4 4 4 6% 9% 43%	1435
	21st to end	Night	10% 7% 2 7% 4 4 5% 4 7% 7% 45%	1400
July	1st to 10th	Night	10% 4 2 5% 4 4 4 3 7% 8% 50%	3180
	11th to 20th	Night	7% 4 2 4 3 3 4 4 6% 10% 54%	3180
	21st to end	Night	9% 4 2 5% 4 4 5% 5% 7% 9% 48%	3498
August	1st to 10th	Night	5% 3 2 6% 4 5% 6% 4 7% 10% 48%	2893
	11th to 20th	Night	12% 4 2 3 3 3 4 4 6% 9% 51%	2888
	21st to end	Night	11% 5% 2 7% 4 4 4 3 6% 8% 47%	3234
September	1st to 10th	Night	10% 6% 2 4 4 4 3 4 5% 7% 53%	1960
	11th to 20th	Night	14% 6% 1 5% 4 3 4 3 6% 8% 46%	2146
	21st to end	Night	17% 6% 2 5% 4 3 4 3 5% 8% 44%	2200
October	1st to 10th	Night	15% 8% 2 6% 4 4 4 4 5% 8% 39%	2395
	11th to 20th	Night	16% 11% 3 7% 4 4 5% 3 6% 8% 34%	2400
	21st to end	Night	16% 12% 4 10% 5% 5% 5% 3 5% 6% 29%	2821
November	1st to 10th	Night	13% 12% 5% 11% 5% 4 5% 4 5% 6% 29%	2871
	11th to 20th	Night	12% 14% 4 13% 6% 4 6% 5% 5% 5% 25%	2940
	21st to end	Night	11% 14% 5% 13% 7% 5% 5% 5% 6% 7% 23%	2940
December	1st to 10th	Night	8% 17% 6% 17% 12% 7% 5% 3 5% 5% 14%	5034
	11th to 20th	Night	8% 15% 5% 14% 8% 6% 5% 6% 7% 6% 20%	5718
	21st to end	Night	11% 14% 7% 19% 9% 6% 4 3 4 5% 16%	7645

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	52% 21% 4 15% 8%	4125
	11th to 20th	Day	55% 20% 7% 13% 5%	4140
	21st to end	Day	49% 22% 7% 17% 5%	5161
February	1st to 10th	Day	27% 59% 6% 5% 3	1890
	11th to 20th	Day	22% 58% 6% 11% 4	1853
	21st to end	Day	22% 59% 5% 8% 6%	1578
March	1st to 10th	Day	33% 28% 13% 13% 10% 3	4861
	11th to 20th	Day	38% 28% 11% 13% 10%	5090
	21st to end	Day	40% 27% 4 17% 11% 1	5585
April	1st to 10th	Day	20% 59% 4 9% 6%	2516
	11th to 20th	Day	20% 59% 5% 10% 4 1	2702
	21st to end	Day	21% 58% 4 11% 4 1	2730
May	1st to 10th	Day	31% 22% 7% 25% 14%	8031
	11th to 20th	Day	39% 22% 6% 20% 11% 2	8241
	21st to end	Day	30% 22% 6% 26% 15% 1	9683
June	1st to 10th	Day	17% 59% 7% 12% 5%	3150
	11th to 20th	Day	19% 60% 6% 10% 4	3076
	21st to end	Day	19% 61% 8% 10% 3	3000
July	1st to 10th	Day	28% 26% 19% 23% 4	6726
	11th to 20th	Day	29% 25% 17% 25% 4	6719
	21st to end	Day	33% 25% 15% 23% 4	7161
August	1st to 10th	Day	24% 36% 12% 20% 8%	4235
	11th to 20th	Day	26% 36% 16% 20% 2	4210
	21st to end	Day	28% 35% 17% 18% 2	4674
September	1st to 10th	Day	16% 57% 8% 15% 4	2440
	11th to 20th	Day	20% 58% 9% 11% 2	2250
	21st to end	Day	21% 57% 6% 13% 3	2185
October	1st to 10th	Day	24% 35% 10% 22% 9%	3260
	11th to 20th	Day	24% 36% 9% 16% 15%	3240
	21st to end	Day	33% 35% 10% 17% 5%	3236
November	1st to 10th	Day	30% 53% 5% 9% 2	1647
	11th to 20th	Day	27% 55% 7% 8% 2	1680
	21st to end	Day	32% 54% 5% 7% 3	1680
December	1st to 10th	Day	50% 36% 7% 5% 2	2371
	11th to 20th	Day	41% 30% 8% 12% 8%	2710
	21st to end	Day	56% 25% 4 13% 3	3575
January	1st to 10th	Night	54% 20% 8% 10% 8%	8800
	11th to 20th	Night	56% 21% 4 15% 4	8850
	21st to end	Night	50% 22% 7% 16% 6%	9034
February	1st to 10th	Night	25% 59% 7% 6% 3	2730
	11th to 20th	Night	21% 58% 6% 11% 3	2664
	21st to end	Night	22% 58% 3 11% 5%	2247
March	1st to 10th	Night	36% 30% 11% 13% 8% 2	5221
	11th to 20th	Night	37% 29% 11% 14% 9%	5090
	21st to end	Night	44% 27% 3 16% 9%	5588
April	1st to 10th	Night	19% 59% 4 9% 8% 1	2101
	11th to 20th	Night	20% 59% 3 10% 7% 1	1869
	21st to end	Night	20% 58% 3 14% 3 1	1890
May	1st to 10th	Night	27% 22% 7% 29% 14%	4902
	11th to 20th	Night	35% 22% 7% 19% 17%	4641
	21st to end	Night	29% 22% 6% 25% 16% 3	4526
June	1st to 10th	Night	16% 59% 7% 11% 6%	1470
	11th to 20th	Night	16% 61% 5% 11% 7% 1	1435
	21st to end	Night	16% 61% 9% 12% 3	1400
July	1st to 10th	Night	28% 26% 19% 22% 3 2	3145
	11th to 20th	Night	22% 25% 17% 31% 4	3138
	21st to end	Night	27% 25% 20% 24% 4	3397
August	1st to 10th	Night	23% 35% 12% 25% 5%	2884
	11th to 20th	Night	23% 35% 19% 20% 3	2915
	21st to end	Night	27% 35% 15% 19% 4	3234
September	1st to 10th	Night	14% 58% 8% 15% 4	1960
	11th to 20th	Night	17% 58% 11% 11% 3	2146
	21st to end	Night	20% 57% 7% 11% 5%	2200
October	1st to 10th	Night	24% 36% 5% 27% 8%	3915
	11th to 20th	Night	25% 36% 8% 25% 6%	3899
	21st to end	Night	35% 34% 6% 20% 6%	4559
November	1st to 10th	Night	28% 53% 7% 9% 2 1	2871
	11th to 20th	Night	25% 55% 8% 9% 3	2940
	21st to end	Night	32% 54% 6% 6% 3	2940
December	1st to 10th	Night	52% 34% 5% 7% 2	5034
	11th to 20th	Night	42% 31% 7% 9% 11%	5716
	21st to end	Night	54% 27% 5% 11% 3	7645

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	15% 15% 39% 22% 8% 2	330
	11th to 20th	Day	15% 12% 38% 28% 7%	330
	21st to end	Day	15% 12% 40% 23% 9% 1	363
February	1st to 10th	Day	18% 15% 38% 22% 6% 1	330
	11th to 20th	Day	11% 19% 38% 20% 8% 3	330
	21st to end	Day	15% 17% 41% 19% 7%	273
March	1st to 10th	Day	17% 15% 40% 21% 5% 1	330
	11th to 20th	Day	19% 17% 32% 22% 9% 2	330
	21st to end	Day	19% 14% 41% 18% 7% 1	363
April	1st to 10th	Day	16% 16% 37% 21% 9% 1	536
	11th to 20th	Day	21% 18% 33% 21% 7% 1	740
	21st to end	Day	23% 12% 31% 26% 8% 1	750
May	1st to 10th	Day	20% 15% 34% 23% 6% 2	753
	11th to 20th	Day	22% 14% 33% 22% 8% 2	775
	21st to end	Day	19% 11% 31% 27% 10% 2	844
June	1st to 10th	Day	17% 11% 33% 24% 13% 2	780
	11th to 20th	Day	18% 12% 35% 24% 9% 2	756
	21st to end	Day	18% 9% 31% 26% 11% 4	730
July	1st to 10th	Day	10% 8% 30% 29% 16% 6%	721
	11th to 20th	Day	13% 10% 28% 30% 15% 4	718
	21st to end	Day	8% 7% 31% 34% 15% 6%	794
August	1st to 10th	Day	9% 9% 29% 30% 18% 4	726
	11th to 20th	Day	12% 7% 29% 29% 17% 6%	715
	21st to end	Day	15% 8% 33% 25% 16% 3	803
September	1st to 10th	Day	13% 11% 35% 26% 14% 2	570
	11th to 20th	Day	22% 12% 36% 22% 7% 1	380
	21st to end	Day	25% 13% 37% 18% 5% 2	330
October	1st to 10th	Day	23% 15% 30% 24% 8% 1	330
	11th to 20th	Day	20% 16% 37% 23% 4	330
	21st to end	Day	28% 17% 32% 18% 4	363
November	1st to 10th	Day	25% 15% 34% 21% 5% 1	330
	11th to 20th	Day	22% 20% 34% 20% 4	330
	21st to end	Day	20% 20% 35% 23% 2 1	330
December	1st to 10th	Day	20% 16% 34% 23% 6% 1	330
	11th to 20th	Day	20% 15% 45% 17% 3	330
	21st to end	Day	19% 11% 37% 22% 9% 2	363
January	1st to 10th	Night	11% 13% 35% 29% 11% 2	1050
	11th to 20th	Night	15% 9% 37% 30% 9% 1	1050
	21st to end	Night	15% 12% 38% 28% 7% 1	1155
February	1st to 10th	Night	14% 10% 34% 32% 9% 1	1050
	11th to 20th	Night	12% 10% 36% 29% 11% 2	1029
	21st to end	Night	11% 10% 40% 28% 10% 2	863
March	1st to 10th	Night	15% 12% 34% 28% 10% 1	723
	11th to 20th	Night	12% 11% 42% 27% 8% 1	630
	21st to end	Night	20% 10% 30% 27% 10% 2	693
April	1st to 10th	Night	19% 8% 32% 31% 9% 1	630
	11th to 20th	Night	21% 9% 33% 28% 9%	623
	21st to end	Night	18% 7% 31% 34% 9%	630
May	1st to 10th	Night	18% 10% 32% 32% 9% 1	630
	11th to 20th	Night	20% 10% 31% 29% 10% 1	621
	21st to end	Night	19% 6% 31% 34% 11%	683
June	1st to 10th	Night	16% 6% 30% 35% 12%	630
	11th to 20th	Night	15% 7% 32% 35% 11%	615
	21st to end	Night	11% 7% 26% 37% 18% 1	600
July	1st to 10th	Night	10% 2 26% 38% 22% 1	585
	11th to 20th	Night	8% 5% 28% 40% 17% 2	582
	21st to end	Night	8% 3 23% 46% 19% 2	657
August	1st to 10th	Night	9% 3 24% 42% 21% 1	597
	11th to 20th	Night	13% 5% 24% 34% 22% 3	594
	21st to end	Night	13% 6% 26% 35% 19% 1	660
September	1st to 10th	Night	13% 5% 31% 34% 15% 2	600
	11th to 20th	Night	16% 6% 34% 29% 14% 1	600
	21st to end	Night	20% 7% 32% 28% 12% 1	600
October	1st to 10th	Night	19% 11% 37% 26% 7%	795
	11th to 20th	Night	22% 9% 35% 26% 7% 1	800
	21st to end	Night	24% 11% 31% 26% 7% 1	1042
November	1st to 10th	Night	20% 11% 36% 24% 9% 1	1025
	11th to 20th	Night	19% 12% 34% 26% 7% 1	1050
	21st to end	Night	18% 13% 40% 21% 6% 1	1050
December	1st to 10th	Night	15% 9% 37% 29% 9% 1	1050
	11th to 20th	Night	18% 12% 38% 24% 6% 1	1050
	21st to end	Night	20% 10% 34% 26% 8% 1	1155

Page generated at 2026-06-13 01:31:43. Version ac07

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