Spencer
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spencer clear sky chart from 2005-11-06 to 2025-04-11.

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	15% 10% 3 8% 5% 4 4 4 5% 7% 34%	113860
	Night	16% 10% 4 8% 5% 4 4 3 5% 6% 34%	114327
Transparency	Day	48% 4 9% 22% 14% 2	117549
	Night	49% 4 9% 21% 14% 2	118777
Seeing	Day	23% 14% 25% 22% 12% 5%	15517
	Night	21% 11% 24% 28% 14% 2	25764

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	16% 21% 9% 15% 7% 4 3 2 3 4 17%	4056
	11th to 20th	Day	16% 20% 10% 11% 7% 4 4 2 3 3 20%	4060
	21st to end	Day	18% 17% 4 10% 6% 4 3 3 4 5% 25%	4466
February	1st to 10th	Day	20% 15% 5% 8% 5% 3 3 3 4 5% 28%	1800
	11th to 20th	Day	22% 15% 4 10% 4 4 3 3 4 5% 25%	1856
	21st to end	Day	22% 14% 4 8% 4 3 3 3 5% 5% 30%	1650
March	1st to 10th	Day	16% 15% 4 8% 4 4 3 3 5% 5% 33%	4904
	11th to 20th	Day	13% 7% 3 5% 4 4 4 3 6% 8% 42%	4980
	21st to end	Day	18% 9% 3 8% 5% 4 5% 4 6% 6% 31%	5465
April	1st to 10th	Day	16% 11% 4 7% 5% 4 5% 4 5% 6% 33%	2220
	11th to 20th	Day	17% 9% 2 6% 5% 4 3 3 5% 8% 40%	2341
	21st to end	Day	20% 9% 3 6% 4 4 4 3 5% 7% 36%	2470
May	1st to 10th	Day	21% 11% 3 8% 5% 5% 5% 4 6% 7% 26%	4082
	11th to 20th	Day	19% 8% 3 8% 4 4 4 4 6% 8% 32%	4081
	21st to end	Day	10% 6% 2 6% 5% 6% 6% 4 7% 10% 37%	4476
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 7% 8% 39%	2470
	11th to 20th	Day	10% 5% 2 7% 6% 6% 6% 5% 7% 9% 37%	2470
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 10% 37%	2470
July	1st to 10th	Day	9% 5% 2 6% 5% 6% 6% 5% 6% 9% 41%	4120
	11th to 20th	Day	10% 6% 2 5% 5% 5% 5% 4 7% 9% 41%	4120
	21st to end	Day	18% 8% 3 8% 5% 5% 6% 4 6% 7% 30%	4466
August	1st to 10th	Day	10% 7% 3 8% 5% 6% 7% 5% 8% 10% 32%	4104
	11th to 20th	Day	11% 6% 3 7% 5% 6% 6% 5% 7% 9% 35%	4044
	21st to end	Day	6% 4 1 5% 4 5% 5% 4 6% 8% 50%	4381
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 6% 7% 46%	2153
	11th to 20th	Day	12% 4 2 7% 5% 5% 4 3 6% 8% 45%	2090
	21st to end	Day	20% 8% 3 6% 4 4 4 4 4 6% 39%	2090
October	1st to 10th	Day	15% 6% 3 5% 3 3 3 4 6% 8% 45%	1970
	11th to 20th	Day	16% 7% 2 6% 4 4 4 2 5% 6% 43%	1900
	21st to end	Day	20% 7% 3 6% 4 4 4 2 4 6% 41%	2090
November	1st to 10th	Day	13% 7% 2 5% 3 3 3 4 4 6% 49%	1845
	11th to 20th	Day	15% 13% 3 6% 5% 4 3 4 6% 7% 36%	1800
	21st to end	Day	17% 13% 4 7% 5% 4 3 3 4 5% 34%	1800
December	1st to 10th	Day	20% 13% 4 14% 4 5% 3 3 3 4 26%	2731
	11th to 20th	Day	24% 14% 5% 8% 4 4 3 3 4 5% 25%	3373
	21st to end	Day	19% 15% 5% 10% 6% 5% 5% 4 5% 6% 20%	4466
January	1st to 10th	Night	16% 20% 6% 17% 5% 3 3 2 3 4 19%	5889
	11th to 20th	Night	18% 16% 6% 15% 7% 5% 4 2 3 4 21%	5900
	21st to end	Night	20% 12% 5% 9% 6% 4 3 2 4 4 30%	6490
February	1st to 10th	Night	20% 14% 4 8% 5% 4 4 3 4 5% 28%	2600
	11th to 20th	Night	22% 12% 4 10% 5% 5% 4 3 4 5% 26%	2429
	21st to end	Night	21% 15% 3 8% 4 4 3 3 6% 4 28%	1980
March	1st to 10th	Night	20% 12% 3 6% 4 4 3 3 4 6% 36%	4947
	11th to 20th	Night	12% 9% 3 6% 4 4 4 3 5% 8% 42%	4980
	21st to end	Night	16% 9% 4 8% 6% 5% 5% 3 5% 6% 34%	5467
April	1st to 10th	Night	17% 11% 3 6% 6% 4 3 3 5% 6% 35%	2180
	11th to 20th	Night	14% 7% 3 6% 5% 4 4 4 6% 8% 39%	1810
	21st to end	Night	16% 7% 3 7% 5% 4 4 4 5% 6% 39%	1710
May	1st to 10th	Night	25% 11% 3 9% 5% 5% 4 3 5% 6% 24%	2813
	11th to 20th	Night	19% 10% 3 7% 5% 4 4 3 5% 7% 34%	2812
	21st to end	Night	13% 7% 3 6% 4 4 5% 4 7% 8% 39%	3079
June	1st to 10th	Night	10% 6% 2 6% 4 5% 5% 4 6% 10% 43%	1710
	11th to 20th	Night	8% 5% 2 7% 5% 5% 6% 5% 7% 9% 41%	1710
	21st to end	Night	11% 5% 2 5% 5% 5% 4 4 7% 9% 44%	1710
July	1st to 10th	Night	10% 5% 2 7% 5% 5% 5% 4 8% 9% 40%	2840
	11th to 20th	Night	10% 7% 3 6% 5% 5% 4 4 6% 9% 42%	2840
	21st to end	Night	12% 9% 3 8% 6% 4 4 5% 6% 7% 36%	3087
August	1st to 10th	Night	11% 8% 3 8% 5% 5% 5% 5% 7% 9% 33%	2825
	11th to 20th	Night	12% 6% 2 8% 4 4 5% 3 6% 8% 42%	2785
	21st to end	Night	7% 3 2 5% 4 4 5% 3 7% 10% 51%	3242
September	1st to 10th	Night	11% 6% 2 5% 4 4 5% 3 6% 7% 47%	2023
	11th to 20th	Night	11% 4 1 4 4 4 4 3 5% 8% 52%	2090
	21st to end	Night	16% 6% 2 6% 4 4 4 4 4 7% 42%	2090
October	1st to 10th	Night	12% 5% 2 6% 4 4 4 4 5% 7% 48%	2209
	11th to 20th	Night	15% 6% 2 7% 3 4 4 3 4 6% 46%	2280
	21st to end	Night	21% 5% 2 6% 4 4 4 3 4 6% 41%	2508
November	1st to 10th	Night	13% 6% 2 6% 4 3 4 4 5% 5% 48%	2450
	11th to 20th	Night	19% 10% 3 6% 4 4 4 4 5% 6% 35%	2600
	21st to end	Night	18% 11% 3 7% 4 4 4 3 4 6% 36%	2600
December	1st to 10th	Night	18% 13% 4 13% 4 4 4 3 4 5% 29%	4330
	11th to 20th	Night	19% 16% 7% 9% 5% 4 4 2 4 5% 26%	4822
	21st to end	Night	17% 17% 5% 10% 6% 5% 5% 3 4 6% 23%	6490

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	73% 3 8% 11% 4	4057
	11th to 20th	Day	70% 2 5% 14% 8%	4060
	21st to end	Day	63% 3 7% 19% 8% 1	4466
February	1st to 10th	Day	60% 3 10% 18% 8% 1	1800
	11th to 20th	Day	63% 3 8% 16% 9% 1	1865
	21st to end	Day	57% 4 8% 19% 10% 1	1650
March	1st to 10th	Day	54% 3 6% 22% 11% 4	4904
	11th to 20th	Day	40% 3 6% 28% 19% 3	4980
	21st to end	Day	53% 4 7% 21% 13% 2	5465
April	1st to 10th	Day	50% 5% 10% 18% 13% 3	2220
	11th to 20th	Day	43% 4 10% 21% 18% 4	2341
	21st to end	Day	48% 5% 12% 21% 12% 3	2470
May	1st to 10th	Day	56% 6% 9% 19% 9% 1	4120
	11th to 20th	Day	49% 7% 10% 19% 13% 1	4085
	21st to end	Day	39% 8% 15% 21% 13% 4	4489
June	1st to 10th	Day	41% 6% 12% 23% 16% 1	2470
	11th to 20th	Day	41% 7% 14% 22% 15% 2	2470
	21st to end	Day	39% 8% 14% 23% 14% 2	2470
July	1st to 10th	Day	39% 6% 13% 26% 13% 3	4055
	11th to 20th	Day	39% 5% 10% 30% 14% 2	4051
	21st to end	Day	52% 5% 10% 17% 15% 2	4324
August	1st to 10th	Day	45% 6% 13% 26% 9%	4091
	11th to 20th	Day	43% 6% 9% 28% 14% 1	4083
	21st to end	Day	31% 5% 13% 29% 20% 2	4381
September	1st to 10th	Day	37% 6% 9% 21% 22% 5%	2153
	11th to 20th	Day	37% 5% 9% 26% 20% 2	2090
	21st to end	Day	46% 5% 8% 20% 19% 2	2090
October	1st to 10th	Day	30% 3 8% 24% 32% 3	3269
	11th to 20th	Day	35% 2 10% 26% 21% 6%	3140
	21st to end	Day	32% 2 9% 32% 21% 5%	3428
November	1st to 10th	Day	37% 4 7% 29% 19% 4	1845
	11th to 20th	Day	49% 3 7% 20% 18% 3	1800
	21st to end	Day	53% 4 8% 20% 15% 1	1800
December	1st to 10th	Day	64% 3 6% 21% 6%	2731
	11th to 20th	Day	62% 3 10% 17% 8%	3370
	21st to end	Day	64% 4 10% 17% 5%	4466
January	1st to 10th	Night	70% 3 8% 14% 5% 1	5889
	11th to 20th	Night	69% 3 6% 13% 6% 2	5900
	21st to end	Night	60% 2 8% 20% 9% 1	6490
February	1st to 10th	Night	58% 4 9% 18% 10% 1	2600
	11th to 20th	Night	63% 3 7% 16% 11% 1	2431
	21st to end	Night	59% 3 8% 16% 10% 3	1980
March	1st to 10th	Night	50% 3 6% 24% 14% 2	4947
	11th to 20th	Night	41% 4 7% 26% 21% 3	4980
	21st to end	Night	52% 3 7% 21% 15% 2	5467
April	1st to 10th	Night	50% 4 8% 19% 15% 3	2180
	11th to 20th	Night	42% 5% 11% 20% 16% 6%	1810
	21st to end	Night	46% 4 10% 20% 18% 2	1710
May	1st to 10th	Night	60% 6% 8% 15% 10% 2	2840
	11th to 20th	Night	48% 8% 9% 23% 11% 2	2813
	21st to end	Night	40% 8% 13% 22% 12% 4	3087
June	1st to 10th	Night	36% 7% 13% 25% 17% 2	1710
	11th to 20th	Night	37% 10% 13% 20% 18% 2	1710
	21st to end	Night	35% 8% 15% 24% 16% 2	1710
July	1st to 10th	Night	39% 6% 12% 23% 16% 4	2795
	11th to 20th	Night	39% 6% 11% 29% 14% 2	2786
	21st to end	Night	46% 7% 9% 19% 15% 3	2988
August	1st to 10th	Night	47% 5% 14% 23% 9% 1	2814
	11th to 20th	Night	41% 4 9% 23% 22% 1	2812
	21st to end	Night	30% 4 12% 31% 21% 2	3242
September	1st to 10th	Night	35% 6% 13% 20% 19% 7%	2023
	11th to 20th	Night	32% 5% 11% 25% 24% 3	2090
	21st to end	Night	42% 4 9% 23% 21% 1	2090
October	1st to 10th	Night	31% 4 8% 25% 27% 5%	3690
	11th to 20th	Night	33% 2 9% 24% 23% 9%	3779
	21st to end	Night	33% 2 6% 28% 25% 6%	4120
November	1st to 10th	Night	37% 5% 10% 22% 22% 5%	2450
	11th to 20th	Night	49% 5% 6% 19% 17% 4	2600
	21st to end	Night	51% 3 9% 21% 14% 2	2600
December	1st to 10th	Night	60% 2 9% 22% 6% 1	4330
	11th to 20th	Night	63% 2 10% 17% 7% 1	4824
	21st to end	Night	63% 4 9% 16% 8%	6490

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	33% 25% 27% 11% 3 1	300
	11th to 20th	Day	36% 28% 29% 7% 1	300
	21st to end	Day	32% 29% 28% 9% 2	330
February	1st to 10th	Day	34% 22% 33% 7% 3 1	300
	11th to 20th	Day	47% 23% 23% 6% 1	300
	21st to end	Day	37% 25% 29% 8%	249
March	1st to 10th	Day	30% 21% 27% 16% 5%	482
	11th to 20th	Day	28% 23% 29% 14% 5% 1	500
	21st to end	Day	33% 18% 32% 13% 4	550
April	1st to 10th	Day	31% 20% 25% 17% 7%	500
	11th to 20th	Day	28% 16% 29% 17% 7% 4	452
	21st to end	Day	30% 15% 28% 17% 7% 2	460
May	1st to 10th	Day	28% 16% 27% 18% 8% 4	463
	11th to 20th	Day	24% 11% 30% 19% 13% 4	487
	21st to end	Day	14% 10% 23% 29% 16% 8%	529
June	1st to 10th	Day	15% 9% 26% 28% 17% 6%	490
	11th to 20th	Day	13% 8% 24% 31% 17% 6%	490
	21st to end	Day	11% 10% 31% 26% 16% 6%	490
July	1st to 10th	Day	14% 7% 20% 24% 18% 16%	485
	11th to 20th	Day	8% 5% 23% 36% 19% 9%	484
	21st to end	Day	18% 5% 18% 29% 20% 10%	532
August	1st to 10th	Day	13% 7% 19% 30% 21% 11%	486
	11th to 20th	Day	13% 6% 20% 30% 19% 11%	479
	21st to end	Day	11% 5% 15% 32% 26% 11%	539
September	1st to 10th	Day	15% 5% 18% 28% 25% 10%	490
	11th to 20th	Day	11% 3 13% 32% 28% 12%	490
	21st to end	Day	24% 10% 23% 24% 15% 5%	490
October	1st to 10th	Day	16% 7% 27% 26% 20% 4	490
	11th to 20th	Day	20% 13% 27% 24% 12% 3	490
	21st to end	Day	26% 14% 27% 24% 8% 2	539
November	1st to 10th	Day	21% 13% 31% 23% 10% 2	383
	11th to 20th	Day	30% 16% 27% 19% 6% 1	300
	21st to end	Day	26% 23% 30% 17% 3 1	300
December	1st to 10th	Day	36% 18% 26% 15% 5%	238
	11th to 20th	Day	30% 20% 33% 13% 4	300
	21st to end	Day	34% 22% 26% 13% 4	330
January	1st to 10th	Night	29% 19% 29% 19% 4	800
	11th to 20th	Night	32% 19% 26% 20% 2	800
	21st to end	Night	28% 18% 31% 21% 3	880
February	1st to 10th	Night	28% 15% 29% 26% 2	800
	11th to 20th	Night	33% 20% 28% 17% 1	783
	21st to end	Night	32% 16% 31% 18% 2	660
March	1st to 10th	Night	27% 16% 29% 25% 4	800
	11th to 20th	Night	27% 16% 29% 20% 8% 1	800
	21st to end	Night	24% 17% 31% 22% 5%	880
April	1st to 10th	Night	25% 16% 32% 21% 6%	800
	11th to 20th	Night	19% 14% 31% 25% 11%	682
	21st to end	Night	18% 11% 34% 29% 8%	570
May	1st to 10th	Night	20% 9% 31% 27% 13%	570
	11th to 20th	Night	20% 8% 26% 31% 14% 1	561
	21st to end	Night	14% 4 23% 30% 25% 4	617
June	1st to 10th	Night	11% 3 22% 38% 24% 2	570
	11th to 20th	Night	12% 4 19% 36% 27% 3	570
	21st to end	Night	14% 4 19% 40% 21% 2	570
July	1st to 10th	Night	12% 4 15% 33% 30% 5%	555
	11th to 20th	Night	9% 3 15% 36% 33% 4	552
	21st to end	Night	12% 3 14% 36% 30% 5%	624
August	1st to 10th	Night	11% 4 10% 39% 33% 3	567
	11th to 20th	Night	11% 4 13% 38% 30% 5%	564
	21st to end	Night	12% 2 13% 35% 32% 6%	693
September	1st to 10th	Night	14% 3 14% 34% 30% 6%	760
	11th to 20th	Night	13% 2 15% 34% 33% 4	760
	21st to end	Night	17% 8% 21% 31% 21% 3	760
October	1st to 10th	Night	13% 5% 22% 32% 25% 3	760
	11th to 20th	Night	19% 9% 28% 28% 15% 2	760
	21st to end	Night	24% 10% 22% 31% 12% 1	836
November	1st to 10th	Night	16% 10% 26% 32% 15% 1	780
	11th to 20th	Night	25% 14% 26% 25% 8% 1	800
	21st to end	Night	25% 12% 24% 30% 7% 1	800
December	1st to 10th	Night	32% 15% 26% 22% 5%	800
	11th to 20th	Night	25% 17% 28% 24% 5%	800
	21st to end	Night	29% 14% 27% 28% 2	880

Page generated at 2025-04-08 09:17:23. Version ac07

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