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 2026-08-15.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	143363
	Night	16% 10% 4 8% 5% 4 4 3 5% 7% 34%	138418
Transparency	Day	49% 4 10% 22% 13% 2	147008
	Night	50% 4 9% 21% 14% 2	142845
Seeing	Day	23% 14% 25% 22% 12% 5%	17358
	Night	21% 11% 24% 28% 14% 2	27472

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	18% 20% 8% 13% 6% 4 3 2 3 5% 18%	5258
	11th to 20th	Day	14% 17% 8% 15% 7% 4 4 2 3 4 22%	5280
	21st to end	Day	17% 16% 4 13% 7% 5% 4 4 5% 5% 21%	5780
February	1st to 10th	Day	20% 15% 5% 8% 6% 3 3 3 4 6% 28%	1890
	11th to 20th	Day	22% 15% 4 10% 4 4 3 3 4 5% 25%	1951
	21st to end	Day	21% 14% 4 8% 4 3 3 3 5% 5% 30%	1730
March	1st to 10th	Day	17% 15% 4 8% 4 4 3 3 5% 5% 32%	5013
	11th to 20th	Day	14% 7% 3 6% 4 4 4 4 6% 8% 42%	5090
	21st to end	Day	18% 9% 3 8% 5% 4 5% 4 6% 6% 31%	5586
April	1st to 10th	Day	16% 10% 3 6% 5% 4 5% 3 5% 7% 35%	2331
	11th to 20th	Day	16% 9% 2 6% 5% 4 3 3 5% 8% 40%	2591
	21st to end	Day	19% 9% 3 6% 4 4 4 3 6% 6% 36%	2730
May	1st to 10th	Day	17% 9% 3 8% 5% 5% 5% 5% 7% 8% 28%	7616
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	7626
	21st to end	Day	18% 8% 3 7% 5% 5% 5% 4 6% 8% 31%	8391
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 7% 8% 39%	2730
	11th to 20th	Day	10% 5% 2 7% 6% 6% 6% 5% 7% 9% 36%	2730
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 5% 8% 10% 37%	2730
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 5% 6% 8% 40%	7680
	11th to 20th	Day	12% 7% 3 6% 5% 5% 6% 5% 7% 10% 34%	7680
	21st to end	Day	15% 7% 3 8% 5% 5% 6% 4 7% 8% 31%	8340
August	1st to 10th	Day	10% 7% 3 8% 5% 6% 7% 5% 8% 10% 32%	4364
	11th to 20th	Day	11% 6% 3 7% 5% 6% 6% 6% 7% 9% 35%	4226
	21st to end	Day	6% 4 1 5% 4 5% 5% 4 6% 8% 50%	4519
September	1st to 10th	Day	14% 5% 2 5% 3 4 4 4 6% 7% 46%	2266
	11th to 20th	Day	12% 4 2 7% 4 5% 4 3 5% 8% 45%	2200
	21st to end	Day	20% 8% 3 6% 4 4 4 4 5% 6% 38%	2200
October	1st to 10th	Day	15% 6% 3 5% 3 3 3 4 5% 8% 46%	2073
	11th to 20th	Day	16% 7% 2 6% 4 4 4 2 5% 7% 44%	2000
	21st to end	Day	20% 7% 3 6% 4 4 4 3 4 6% 40%	2200
November	1st to 10th	Day	13% 7% 2 5% 3 3 4 4 4 6% 48%	1939
	11th to 20th	Day	15% 13% 3 6% 5% 3 3 3 6% 6% 36%	1890
	21st to end	Day	17% 13% 4 7% 5% 4 3 3 4 6% 33%	1890
December	1st to 10th	Day	20% 13% 4 14% 4 5% 3 3 3 4 26%	2815
	11th to 20th	Day	24% 14% 5% 8% 4 4 3 3 4 5% 25%	3463
	21st to end	Day	19% 16% 5% 10% 6% 5% 5% 4 5% 6% 20%	4565
January	1st to 10th	Night	17% 18% 6% 15% 5% 4 4 3 4 5% 19%	7635
	11th to 20th	Night	17% 15% 7% 16% 6% 4 4 2 4 4 22%	7680
	21st to end	Night	18% 12% 4 11% 7% 5% 4 3 4 5% 28%	8401
February	1st to 10th	Night	19% 14% 4 9% 5% 4 4 4 4 5% 29%	2730
	11th to 20th	Night	22% 12% 4 10% 5% 5% 4 3 4 5% 26%	2553
	21st to end	Night	21% 15% 4 8% 4 4 3 3 5% 4 28%	2076
March	1st to 10th	Night	20% 12% 3 6% 4 4 3 3 4 6% 36%	5058
	11th to 20th	Night	12% 9% 3 6% 4 4 4 3 5% 8% 42%	5090
	21st to end	Night	16% 9% 4 8% 6% 5% 5% 3 5% 6% 34%	5588
April	1st to 10th	Night	16% 11% 3 6% 5% 4 4 4 5% 7% 36%	2289
	11th to 20th	Night	14% 7% 3 6% 5% 4 5% 4 6% 8% 38%	1995
	21st to end	Night	15% 8% 3 7% 5% 4 4 4 5% 7% 38%	1890
May	1st to 10th	Night	20% 10% 3 9% 5% 5% 5% 3 5% 7% 28%	5237
	11th to 20th	Night	13% 8% 3 7% 5% 5% 5% 4 6% 9% 35%	5234
	21st to end	Night	21% 10% 4 8% 4 4 4 3 5% 7% 29%	5761
June	1st to 10th	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 43%	1890
	11th to 20th	Night	9% 6% 2 8% 5% 5% 6% 4 7% 9% 40%	1890
	21st to end	Night	11% 5% 2 5% 4 5% 5% 4 6% 9% 44%	1890
July	1st to 10th	Night	11% 7% 3 7% 5% 4 5% 4 6% 9% 40%	5278
	11th to 20th	Night	14% 8% 3 7% 5% 5% 5% 4 6% 10% 35%	5277
	21st to end	Night	12% 8% 3 7% 5% 4 5% 5% 7% 9% 36%	5736
August	1st to 10th	Night	11% 8% 3 8% 5% 6% 5% 5% 7% 9% 33%	3005
	11th to 20th	Night	12% 6% 3 8% 4 4 4 3 5% 8% 42%	2911
	21st to end	Night	7% 3 2 5% 4 4 5% 3 7% 10% 51%	3345
September	1st to 10th	Night	11% 6% 2 6% 4 4 5% 3 5% 7% 48%	2130
	11th to 20th	Night	10% 4 1 5% 4 4 4 3 5% 8% 52%	2200
	21st to end	Night	16% 7% 2 7% 4 5% 4 4 4 7% 41%	2200
October	1st to 10th	Night	11% 5% 2 5% 4 3 4 4 5% 7% 49%	2326
	11th to 20th	Night	15% 6% 2 6% 3 4 4 3 5% 6% 47%	2400
	21st to end	Night	21% 6% 2 6% 4 4 4 3 4 6% 41%	2640
November	1st to 10th	Night	13% 7% 2 6% 4 3 4 4 5% 5% 48%	2576
	11th to 20th	Night	19% 10% 3 6% 4 4 4 4 5% 6% 35%	2730
	21st to end	Night	18% 11% 4 7% 4 4 4 3 4 6% 35%	2730
December	1st to 10th	Night	18% 13% 4 13% 4 4 4 3 4 5% 28%	4462
	11th to 20th	Night	19% 16% 7% 9% 5% 4 4 2 4 5% 26%	4952
	21st to end	Night	17% 17% 5% 10% 6% 5% 5% 3 4 6% 23%	6633

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	71% 2 10% 13% 4	5259
	11th to 20th	Day	68% 2 5% 14% 9% 1	5280
	21st to end	Day	66% 2 7% 18% 6% 1	5780
February	1st to 10th	Day	60% 3 9% 19% 9% 1	1890
	11th to 20th	Day	62% 3 8% 16% 9% 1	1960
	21st to end	Day	57% 4 8% 20% 10% 1	1730
March	1st to 10th	Day	54% 3 6% 22% 11% 4	5013
	11th to 20th	Day	40% 3 7% 27% 19% 3	5090
	21st to end	Day	53% 4 7% 21% 13% 2	5586
April	1st to 10th	Day	49% 5% 10% 20% 13% 3	2331
	11th to 20th	Day	43% 4 11% 21% 17% 4	2591
	21st to end	Day	48% 5% 12% 22% 12% 2	2730
May	1st to 10th	Day	53% 4 10% 22% 9% 2	7651
	11th to 20th	Day	48% 5% 10% 24% 11% 1	7630
	21st to end	Day	50% 5% 10% 17% 13% 5%	8405
June	1st to 10th	Day	41% 6% 13% 22% 17% 1	2730
	11th to 20th	Day	41% 7% 15% 21% 14% 1	2730
	21st to end	Day	39% 8% 14% 24% 14% 2	2730
July	1st to 10th	Day	41% 5% 12% 25% 15% 2	7615
	11th to 20th	Day	44% 4 11% 28% 11% 1	7611
	21st to end	Day	49% 3 11% 19% 16% 1	8156
August	1st to 10th	Day	45% 6% 13% 26% 9%	4351
	11th to 20th	Day	43% 6% 9% 28% 14% 1	4265
	21st to end	Day	31% 5% 13% 30% 20% 2	4519
September	1st to 10th	Day	36% 6% 9% 21% 23% 5%	2266
	11th to 20th	Day	37% 5% 9% 26% 21% 2	2200
	21st to end	Day	47% 4 8% 20% 19% 2	2200
October	1st to 10th	Day	30% 3 8% 24% 33% 3	3372
	11th to 20th	Day	34% 2 10% 26% 21% 6%	3240
	21st to end	Day	33% 2 9% 31% 21% 5%	3538
November	1st to 10th	Day	38% 4 8% 28% 19% 4	1939
	11th to 20th	Day	48% 3 8% 21% 18% 3	1890
	21st to end	Day	53% 3 9% 20% 14%	1890
December	1st to 10th	Day	64% 2 6% 21% 5%	2815
	11th to 20th	Day	62% 2 10% 17% 8%	3460
	21st to end	Day	64% 4 10% 17% 5%	4565
January	1st to 10th	Night	69% 2 9% 15% 5%	7636
	11th to 20th	Night	68% 3 5% 13% 9% 2	7680
	21st to end	Night	61% 2 9% 19% 8% 1	8401
February	1st to 10th	Night	58% 4 9% 19% 10% 1	2730
	11th to 20th	Night	63% 3 7% 17% 10% 1	2555
	21st to end	Night	59% 3 8% 17% 10% 3	2076
March	1st to 10th	Night	51% 3 6% 24% 14% 2	5058
	11th to 20th	Night	41% 4 7% 26% 21% 3	5090
	21st to end	Night	52% 3 8% 21% 14% 2	5588
April	1st to 10th	Night	49% 4 9% 19% 16% 3	2289
	11th to 20th	Night	43% 5% 11% 21% 15% 5%	1995
	21st to end	Night	46% 4 9% 21% 17% 2	1890
May	1st to 10th	Night	57% 4 10% 16% 11% 2	5264
	11th to 20th	Night	46% 7% 12% 23% 12% 1	5235
	21st to end	Night	54% 5% 10% 16% 12% 4	5771
June	1st to 10th	Night	36% 7% 13% 24% 18% 2	1890
	11th to 20th	Night	38% 9% 13% 20% 18% 2	1890
	21st to end	Night	36% 8% 15% 23% 17% 2	1890
July	1st to 10th	Night	42% 6% 9% 24% 16% 2	5235
	11th to 20th	Night	46% 5% 13% 25% 10% 1	5226
	21st to end	Night	45% 5% 11% 20% 17% 2	5606
August	1st to 10th	Night	47% 5% 14% 23% 9% 1	2994
	11th to 20th	Night	41% 4 9% 24% 21% 1	2938
	21st to end	Night	30% 4 12% 30% 21% 2	3345
September	1st to 10th	Night	35% 6% 13% 19% 20% 7%	2130
	11th to 20th	Night	31% 5% 11% 26% 25% 3	2200
	21st to end	Night	43% 4 9% 22% 20% 1	2200
October	1st to 10th	Night	30% 4 7% 25% 28% 5%	3807
	11th to 20th	Night	33% 2 9% 24% 24% 9%	3899
	21st to end	Night	33% 2 6% 28% 25% 6%	4252
November	1st to 10th	Night	37% 4 10% 22% 22% 5%	2576
	11th to 20th	Night	49% 5% 6% 19% 17% 3	2730
	21st to end	Night	52% 3 9% 21% 14% 1	2730
December	1st to 10th	Night	61% 2 9% 21% 6% 1	4462
	11th to 20th	Night	63% 2 10% 17% 7% 1	4954
	21st to end	Night	63% 4 9% 16% 8%	6633

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	34% 24% 27% 11% 3 1	330
	11th to 20th	Day	37% 29% 28% 6% 1	330
	21st to end	Day	33% 29% 28% 8% 2	363
February	1st to 10th	Day	33% 21% 34% 8% 3 1	330
	11th to 20th	Day	46% 23% 23% 6% 1	330
	21st to end	Day	37% 25% 30% 8%	273
March	1st to 10th	Day	31% 22% 27% 15% 4	521
	11th to 20th	Day	30% 23% 29% 14% 4 1	540
	21st to end	Day	32% 18% 33% 13% 4	594
April	1st to 10th	Day	29% 20% 26% 17% 7%	540
	11th to 20th	Day	27% 18% 28% 16% 6% 3	532
	21st to end	Day	29% 16% 28% 19% 7% 2	540
May	1st to 10th	Day	27% 19% 27% 17% 7% 3	543
	11th to 20th	Day	22% 13% 31% 19% 11% 4	567
	21st to end	Day	19% 10% 24% 26% 15% 7%	617
June	1st to 10th	Day	14% 10% 25% 28% 17% 6%	570
	11th to 20th	Day	13% 10% 25% 30% 16% 5%	570
	21st to end	Day	11% 9% 29% 26% 18% 7%	570
July	1st to 10th	Day	14% 7% 18% 26% 19% 16%	565
	11th to 20th	Day	9% 5% 21% 36% 20% 9%	564
	21st to end	Day	18% 6% 18% 29% 20% 9%	620
August	1st to 10th	Day	14% 7% 20% 31% 19% 9%	566
	11th to 20th	Day	14% 6% 19% 30% 20% 11%	535
	21st to end	Day	10% 6% 16% 32% 25% 10%	583
September	1st to 10th	Day	15% 5% 17% 28% 25% 10%	530
	11th to 20th	Day	11% 3 12% 32% 28% 12%	530
	21st to end	Day	24% 9% 23% 25% 15% 4	530
October	1st to 10th	Day	15% 7% 25% 27% 21% 5%	530
	11th to 20th	Day	19% 13% 28% 24% 12% 3	530
	21st to end	Day	28% 14% 27% 23% 7% 2	583
November	1st to 10th	Day	22% 14% 31% 23% 9% 2	417
	11th to 20th	Day	32% 18% 27% 18% 5% 1	330
	21st to end	Day	29% 22% 29% 17% 3 1	330
December	1st to 10th	Day	36% 18% 27% 13% 4	262
	11th to 20th	Day	30% 21% 33% 12% 4	330
	21st to end	Day	35% 23% 25% 12% 4	363
January	1st to 10th	Night	29% 18% 29% 19% 4	840
	11th to 20th	Night	33% 20% 26% 19% 2	840
	21st to end	Night	28% 18% 31% 20% 2	924
February	1st to 10th	Night	28% 15% 29% 26% 2	840
	11th to 20th	Night	32% 20% 29% 18% 1	823
	21st to end	Night	32% 16% 31% 18% 2	692
March	1st to 10th	Night	27% 16% 29% 24% 4	840
	11th to 20th	Night	27% 16% 29% 20% 7%	840
	21st to end	Night	24% 17% 32% 22% 5%	924
April	1st to 10th	Night	24% 17% 33% 21% 6%	840
	11th to 20th	Night	19% 15% 32% 25% 10%	751
	21st to end	Night	18% 11% 33% 29% 8%	630
May	1st to 10th	Night	20% 10% 31% 27% 12%	630
	11th to 20th	Night	19% 9% 27% 31% 14% 1	621
	21st to end	Night	17% 5% 23% 29% 23% 3	683
June	1st to 10th	Night	12% 3 22% 37% 24% 2	630
	11th to 20th	Night	12% 4 20% 37% 25% 3	630
	21st to end	Night	14% 4 18% 40% 22% 3	630
July	1st to 10th	Night	12% 4 14% 33% 31% 6%	615
	11th to 20th	Night	10% 4 14% 35% 33% 4	612
	21st to end	Night	12% 3 14% 36% 30% 5%	690
August	1st to 10th	Night	11% 4 11% 41% 30% 3	627
	11th to 20th	Night	11% 4 13% 36% 30% 5%	606
	21st to end	Night	12% 2 14% 36% 31% 5%	730
September	1st to 10th	Night	14% 3 14% 34% 30% 5%	800
	11th to 20th	Night	12% 2 14% 34% 34% 4	800
	21st to end	Night	17% 8% 21% 31% 21% 3	800
October	1st to 10th	Night	13% 5% 21% 33% 25% 3	800
	11th to 20th	Night	18% 9% 28% 29% 15% 2	800
	21st to end	Night	24% 11% 23% 30% 11% 1	880
November	1st to 10th	Night	16% 10% 27% 32% 14%	820
	11th to 20th	Night	26% 15% 26% 25% 8% 1	840
	21st to end	Night	26% 12% 24% 29% 7% 1	840
December	1st to 10th	Night	32% 16% 26% 22% 4	840
	11th to 20th	Night	25% 17% 28% 24% 5%	840
	21st to end	Night	29% 15% 27% 27% 2	924

Page generated at 2026-08-12 03:37:18. Version ac07

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