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 2025-02-21.

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	12% 10% 4 9% 6% 5% 5% 4 6% 7% 33%	108313
	Night	11% 11% 4 10% 6% 5% 5% 4 6% 7% 31%	110449
Transparency	Day	29% 43% 9% 14% 5%	111795
	Night	33% 41% 7% 14% 5%	115137
Seeing	Day	17% 12% 33% 25% 10% 2	16543
	Night	16% 9% 33% 30% 11% 1	26655

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	5% 14% 7% 14% 7% 6% 7% 5% 6% 6% 24%	3178
	11th to 20th	Day	4 10% 6% 17% 9% 8% 8% 8% 10% 7% 13%	3152
	21st to end	Day	6% 17% 10% 16% 8% 7% 5% 4 5% 7% 17%	3979
February	1st to 10th	Day	8% 13% 5% 11% 8% 7% 8% 5% 5% 7% 23%	1800
	11th to 20th	Day	6% 10% 4 9% 6% 7% 7% 6% 8% 8% 28%	1754
	21st to end	Day	6% 11% 4 11% 9% 6% 5% 4 6% 8% 29%	1432
March	1st to 10th	Day	13% 11% 3 9% 10% 5% 5% 3 5% 7% 30%	3305
	11th to 20th	Day	7% 8% 4 14% 7% 4 4 5% 6% 8% 33%	3480
	21st to end	Day	13% 17% 4 8% 5% 4 4 3 4 6% 32%	3814
April	1st to 10th	Day	11% 11% 4 7% 5% 5% 4 4 6% 7% 36%	2276
	11th to 20th	Day	13% 12% 3 8% 6% 4 5% 4 6% 8% 31%	2442
	21st to end	Day	19% 10% 3 8% 5% 5% 4 3 5% 6% 31%	2470
May	1st to 10th	Day	18% 9% 3 6% 4 4 4 4 5% 6% 35%	4319
	11th to 20th	Day	18% 8% 3 7% 5% 5% 6% 4 6% 6% 33%	4414
	21st to end	Day	20% 10% 2 7% 5% 4 5% 3 5% 6% 34%	5173
June	1st to 10th	Day	13% 7% 2 7% 6% 5% 5% 5% 7% 8% 37%	2850
	11th to 20th	Day	16% 6% 3 6% 5% 5% 5% 4 7% 7% 36%	2850
	21st to end	Day	14% 7% 2 6% 5% 5% 6% 5% 8% 9% 35%	2850
July	1st to 10th	Day	8% 4 2 5% 5% 5% 6% 5% 7% 10% 42%	4750
	11th to 20th	Day	8% 4 1 5% 4 4 5% 4 7% 9% 49%	4750
	21st to end	Day	9% 5% 2 6% 5% 5% 4 5% 7% 8% 45%	5132
August	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 5% 6% 9% 44%	4119
	11th to 20th	Day	12% 5% 2 7% 4 4 5% 5% 6% 9% 41%	4041
	21st to end	Day	14% 5% 2 5% 4 4 4 4 5% 9% 43%	4532
September	1st to 10th	Day	11% 5% 3 5% 5% 4 4 3 5% 7% 47%	2318
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 6% 9% 39%	2138
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 8% 39%	2076
October	1st to 10th	Day	17% 11% 3 8% 4 5% 4 4 4 6% 34%	1900
	11th to 20th	Day	14% 12% 5% 8% 4 5% 4 4 6% 6% 32%	1900
	21st to end	Day	15% 15% 4 12% 5% 4 4 3 5% 6% 26%	1914
November	1st to 10th	Day	14% 16% 4 8% 6% 4 4 5% 6% 5% 27%	1567
	11th to 20th	Day	12% 15% 6% 15% 8% 5% 4 4 4 4 24%	1600
	21st to end	Day	11% 14% 6% 15% 7% 6% 4 5% 5% 5% 21%	1600
December	1st to 10th	Day	13% 15% 4 16% 10% 8% 5% 4 5% 7% 13%	2301
	11th to 20th	Day	11% 17% 6% 12% 5% 4 5% 4 6% 6% 23%	2639
	21st to end	Day	12% 17% 7% 21% 8% 6% 4 2 4 4 15%	3498
January	1st to 10th	Night	4 13% 6% 15% 9% 8% 7% 5% 6% 6% 20%	6787
	11th to 20th	Night	5% 13% 6% 13% 9% 9% 9% 7% 10% 5% 13%	6753
	21st to end	Night	9% 18% 8% 14% 7% 5% 5% 4 6% 7% 17%	6980
February	1st to 10th	Night	8% 11% 5% 10% 8% 6% 6% 5% 7% 8% 26%	2600
	11th to 20th	Night	7% 10% 3 7% 5% 7% 7% 5% 7% 9% 34%	2533
	21st to end	Night	6% 10% 5% 13% 9% 6% 5% 4 6% 7% 30%	2040
March	1st to 10th	Night	12% 10% 3 10% 7% 5% 5% 4 5% 7% 32%	3540
	11th to 20th	Night	5% 9% 4 10% 6% 5% 6% 4 6% 9% 35%	3479
	21st to end	Night	14% 14% 5% 10% 5% 5% 4 3 4 5% 30%	3817
April	1st to 10th	Night	13% 9% 3 7% 5% 5% 5% 3 6% 6% 38%	1901
	11th to 20th	Night	15% 11% 4 8% 4 4 4 4 5% 5% 37%	1689
	21st to end	Night	16% 9% 3 7% 6% 5% 4 3 5% 6% 36%	1710
May	1st to 10th	Night	20% 8% 3 6% 4 4 3 3 5% 6% 38%	2631
	11th to 20th	Night	19% 7% 2 6% 4 5% 4 4 6% 6% 37%	2489
	21st to end	Night	18% 8% 3 6% 3 4 4 3 4 5% 43%	2414
June	1st to 10th	Night	14% 4 3 5% 5% 4 5% 4 6% 7% 43%	1330
	11th to 20th	Night	14% 4 2 6% 4 5% 5% 4 6% 9% 43%	1330
	21st to end	Night	10% 6% 2 6% 4 3 5% 3 7% 7% 45%	1330
July	1st to 10th	Night	9% 3 1 4 3 3 4 4 7% 9% 52%	2220
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 10% 59%	2220
	21st to end	Night	8% 3 1 4 3 4 5% 4 7% 9% 52%	2430
August	1st to 10th	Night	5% 3 2 6% 4 4 6% 4 7% 10% 48%	2804
	11th to 20th	Night	12% 4 2 3 3 3 4 4 6% 9% 52%	2798
	21st to end	Night	11% 5% 2 7% 3 4 4 3 6% 8% 47%	3135
September	1st to 10th	Night	10% 6% 1 4 4 4 3 4 5% 7% 53%	1862
	11th to 20th	Night	13% 6% 1 5% 4 3 4 4 5% 8% 46%	2038
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 8% 43%	2090
October	1st to 10th	Night	15% 8% 2 6% 4 4 4 4 5% 8% 39%	2275
	11th to 20th	Night	16% 11% 3 7% 4 4 5% 3 6% 8% 34%	2280
	21st to end	Night	16% 12% 4 9% 5% 5% 5% 4 6% 6% 29%	2680
November	1st to 10th	Night	14% 13% 5% 11% 5% 4 5% 4 6% 6% 28%	2731
	11th to 20th	Night	12% 14% 5% 14% 6% 4 6% 5% 5% 5% 24%	2800
	21st to end	Night	12% 14% 5% 12% 7% 5% 5% 5% 6% 7% 23%	2800
December	1st to 10th	Night	8% 17% 6% 17% 12% 7% 5% 4 5% 5% 14%	4885
	11th to 20th	Night	8% 15% 5% 14% 8% 6% 5% 6% 7% 6% 20%	5568
	21st to end	Night	11% 14% 8% 19% 9% 6% 4 3 5% 5% 16%	7480

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	46% 27% 4 13% 9%	3179
	11th to 20th	Day	48% 26% 7% 15% 4	3180
	21st to end	Day	51% 28% 9% 10% 3	3979
February	1st to 10th	Day	25% 62% 6% 5% 3	1800
	11th to 20th	Day	21% 61% 6% 10% 3	1763
	21st to end	Day	17% 65% 4 7% 6%	1432
March	1st to 10th	Day	32% 40% 8% 14% 6%	3307
	11th to 20th	Day	30% 40% 10% 12% 8%	3480
	21st to end	Day	34% 40% 4 15% 7% 1	3814
April	1st to 10th	Day	15% 65% 4 9% 6%	2276
	11th to 20th	Day	16% 66% 4 9% 4 1	2442
	21st to end	Day	18% 65% 4 9% 3 1	2470
May	1st to 10th	Day	29% 42% 6% 17% 6%	4330
	11th to 20th	Day	34% 41% 7% 12% 7%	4418
	21st to end	Day	33% 40% 7% 17% 3	5173
June	1st to 10th	Day	14% 65% 6% 10% 5%	2850
	11th to 20th	Day	18% 65% 5% 8% 4	2850
	21st to end	Day	17% 64% 7% 9% 2	2850
July	1st to 10th	Day	25% 37% 16% 19% 3	4676
	11th to 20th	Day	18% 35% 17% 24% 5%	4669
	21st to end	Day	25% 35% 12% 24% 3	4969
August	1st to 10th	Day	23% 37% 12% 20% 8%	4104
	11th to 20th	Day	26% 37% 16% 19% 2	4080
	21st to end	Day	28% 36% 16% 17% 2	4531
September	1st to 10th	Day	14% 60% 7% 14% 4	2318
	11th to 20th	Day	17% 60% 9% 12% 2	2138
	21st to end	Day	21% 60% 6% 11% 3	2076
October	1st to 10th	Day	23% 36% 10% 21% 9%	3160
	11th to 20th	Day	23% 37% 9% 17% 15%	3140
	21st to end	Day	32% 36% 10% 17% 5%	3135
November	1st to 10th	Day	28% 56% 6% 8% 2	1567
	11th to 20th	Day	27% 58% 7% 7% 2	1600
	21st to end	Day	30% 56% 5% 7% 2	1600
December	1st to 10th	Day	49% 37% 8% 5% 1	2301
	11th to 20th	Day	40% 31% 8% 13% 8%	2640
	21st to end	Day	55% 26% 3 12% 3	3498
January	1st to 10th	Night	50% 27% 6% 8% 9%	6787
	11th to 20th	Night	51% 27% 4 15% 4	6800
	21st to end	Night	52% 28% 7% 9% 4	6980
February	1st to 10th	Night	23% 62% 6% 6% 3	2600
	11th to 20th	Night	19% 61% 6% 11% 3	2534
	21st to end	Night	18% 64% 3 10% 5%	2040
March	1st to 10th	Night	29% 42% 8% 14% 7%	3551
	11th to 20th	Night	26% 42% 12% 13% 7%	3480
	21st to end	Night	37% 40% 3 13% 7% 1	3817
April	1st to 10th	Night	15% 65% 4 8% 8%	1901
	11th to 20th	Night	17% 65% 2 8% 7% 1	1689
	21st to end	Night	17% 65% 3 12% 2 1	1710
May	1st to 10th	Night	30% 40% 5% 17% 7%	2639
	11th to 20th	Night	30% 40% 7% 15% 8%	2490
	21st to end	Night	28% 40% 6% 21% 5%	2414
June	1st to 10th	Night	14% 65% 6% 10% 5%	1330
	11th to 20th	Night	15% 65% 4 9% 7% 1	1330
	21st to end	Night	14% 64% 8% 11% 2	1330
July	1st to 10th	Night	21% 37% 18% 20% 3	2185
	11th to 20th	Night	14% 35% 15% 32% 4 1	2178
	21st to end	Night	20% 35% 18% 24% 3	2353
August	1st to 10th	Night	22% 36% 11% 25% 5%	2795
	11th to 20th	Night	22% 37% 19% 19% 3	2825
	21st to end	Night	27% 36% 16% 18% 4	3135
September	1st to 10th	Night	13% 61% 7% 14% 4	1862
	11th to 20th	Night	15% 60% 10% 11% 3	2038
	21st to end	Night	20% 60% 6% 10% 4	2090
October	1st to 10th	Night	23% 37% 5% 26% 8%	3795
	11th to 20th	Night	24% 37% 8% 25% 6%	3779
	21st to end	Night	33% 35% 6% 20% 6%	4418
November	1st to 10th	Night	26% 56% 7% 9% 2	2731
	11th to 20th	Night	25% 58% 7% 8% 2	2800
	21st to end	Night	30% 57% 5% 5% 3	2800
December	1st to 10th	Night	52% 35% 5% 6% 1	4885
	11th to 20th	Night	41% 32% 7% 9% 11%	5566
	21st to end	Night	53% 27% 5% 11% 3	7480

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	14% 14% 38% 24% 9% 1	300
	11th to 20th	Day	14% 11% 37% 30% 8%	300
	21st to end	Day	16% 11% 39% 23% 9% 1	330
February	1st to 10th	Day	17% 14% 39% 23% 7% 1	300
	11th to 20th	Day	10% 20% 40% 20% 8% 3	300
	21st to end	Day	14% 17% 41% 20% 8%	225
March	1st to 10th	Day	16% 11% 41% 24% 6% 1	270
	11th to 20th	Day	16% 16% 34% 21% 11% 2	270
	21st to end	Day	16% 14% 40% 21% 8% 1	297
April	1st to 10th	Day	14% 15% 37% 23% 11% 1	456
	11th to 20th	Day	20% 17% 31% 22% 8% 1	640
	21st to end	Day	22% 12% 32% 26% 7% 1	650
May	1st to 10th	Day	21% 15% 34% 23% 6% 2	653
	11th to 20th	Day	20% 12% 33% 24% 9% 2	675
	21st to end	Day	21% 12% 30% 26% 9% 1	734
June	1st to 10th	Day	16% 12% 32% 24% 13% 3	680
	11th to 20th	Day	18% 12% 35% 23% 9% 2	680
	21st to end	Day	17% 9% 30% 27% 12% 5%	680
July	1st to 10th	Day	10% 8% 29% 30% 17% 7%	671
	11th to 20th	Day	12% 10% 27% 31% 16% 4	668
	21st to end	Day	8% 7% 29% 34% 15% 6%	739
August	1st to 10th	Day	8% 9% 30% 30% 18% 4	676
	11th to 20th	Day	12% 7% 28% 30% 17% 6%	665
	21st to end	Day	15% 7% 34% 25% 16% 3	748
September	1st to 10th	Day	13% 11% 35% 26% 14% 1	528
	11th to 20th	Day	19% 13% 37% 23% 7% 1	348
	21st to end	Day	27% 13% 36% 19% 4 1	300
October	1st to 10th	Day	24% 15% 28% 23% 9% 1	300
	11th to 20th	Day	18% 15% 38% 24% 4	300
	21st to end	Day	27% 18% 31% 18% 5%	330
November	1st to 10th	Day	23% 14% 34% 22% 6% 1	300
	11th to 20th	Day	22% 20% 34% 20% 4	300
	21st to end	Day	21% 21% 33% 22% 2 1	300
December	1st to 10th	Day	19% 15% 35% 24% 7% 1	300
	11th to 20th	Day	19% 15% 45% 18% 3	300
	21st to end	Day	18% 10% 37% 22% 10% 3	330
January	1st to 10th	Night	11% 12% 35% 29% 12% 2	1000
	11th to 20th	Night	15% 8% 36% 31% 9% 1	1000
	21st to end	Night	15% 11% 38% 28% 7% 1	1100
February	1st to 10th	Night	14% 9% 35% 33% 10% 1	1000
	11th to 20th	Night	12% 10% 36% 28% 11% 3	979
	21st to end	Night	10% 9% 40% 28% 10% 2	784
March	1st to 10th	Night	15% 10% 33% 28% 11% 1	655
	11th to 20th	Night	11% 10% 42% 27% 9% 1	570
	21st to end	Night	20% 10% 28% 29% 11% 2	627
April	1st to 10th	Night	17% 8% 32% 32% 10% 1	570
	11th to 20th	Night	20% 9% 33% 29% 9% 1	563
	21st to end	Night	17% 7% 32% 35% 9%	570
May	1st to 10th	Night	19% 9% 32% 31% 8% 1	570
	11th to 20th	Night	20% 9% 30% 30% 10% 1	561
	21st to end	Night	20% 6% 31% 33% 10%	617
June	1st to 10th	Night	16% 6% 29% 36% 13% 1	570
	11th to 20th	Night	15% 7% 32% 33% 12%	570
	21st to end	Night	11% 7% 25% 38% 19% 1	570
July	1st to 10th	Night	10% 2 26% 39% 22% 1	555
	11th to 20th	Night	7% 6% 27% 40% 18% 2	552
	21st to end	Night	8% 3 23% 44% 19% 2	624
August	1st to 10th	Night	8% 3 25% 42% 20% 1	567
	11th to 20th	Night	12% 5% 23% 34% 23% 3	564
	21st to end	Night	13% 6% 26% 36% 19% 1	627
September	1st to 10th	Night	13% 5% 31% 34% 15% 2	570
	11th to 20th	Night	15% 6% 34% 29% 15% 1	570
	21st to end	Night	20% 6% 33% 28% 12% 1	570
October	1st to 10th	Night	20% 10% 36% 27% 7%	755
	11th to 20th	Night	21% 9% 36% 26% 7% 1	760
	21st to end	Night	23% 11% 32% 26% 7% 1	990
November	1st to 10th	Night	20% 11% 36% 24% 9% 1	975
	11th to 20th	Night	20% 11% 34% 27% 7% 1	1000
	21st to end	Night	19% 12% 40% 22% 7% 1	1000
December	1st to 10th	Night	15% 9% 37% 29% 10% 1	1000
	11th to 20th	Night	18% 12% 38% 24% 6% 1	1000
	21st to end	Night	20% 10% 34% 27% 9% 1	1100

Page generated at 2025-02-18 02:26:31. Version ac07

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