T. Spence Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the T. Spence Memorial Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	11% 6% 2 6% 4 4 5% 4 6% 9% 43%	131054
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	129788
Transparency	Day	36% 8% 13% 18% 17% 7%	134765
	Night	36% 8% 12% 17% 19% 9%	134147
Seeing	Day	12% 8% 22% 25% 21% 12%	16904
	Night	13% 7% 24% 32% 21% 3	27014

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	12% 5% 2 5% 4 4 4 3 5% 7% 50%	5258
	11th to 20th	Day	14% 5% 1 4 3 2 2 2 5% 7% 56%	5280
	21st to end	Day	14% 8% 2 7% 4 4 4 3 5% 6% 43%	5780
February	1st to 10th	Day	14% 7% 2 6% 4 3 4 4 7% 7% 41%	2055
	11th to 20th	Day	18% 7% 2 4 3 4 3 3 5% 7% 44%	1998
	21st to end	Day	15% 7% 2 5% 4 3 4 3 5% 6% 46%	1651
March	1st to 10th	Day	19% 8% 3 7% 4 4 4 3 5% 6% 38%	4935
	11th to 20th	Day	7% 3 1 5% 3 4 4 4 5% 8% 55%	4980
	21st to end	Day	14% 6% 3 7% 4 4 4 3 4 5% 46%	5465
April	1st to 10th	Day	9% 6% 2 5% 3 4 4 4 6% 6% 50%	2200
	11th to 20th	Day	11% 5% 2 5% 4 3 4 4 5% 7% 50%	2249
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 8% 54%	2600
May	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 6% 8% 43%	5862
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 4 6% 9% 36%	5861
	21st to end	Day	8% 5% 2 6% 5% 4 5% 4 6% 10% 45%	6433
June	1st to 10th	Day	9% 6% 3 7% 6% 7% 7% 5% 8% 9% 34%	2600
	11th to 20th	Day	5% 4 2 5% 5% 6% 7% 5% 9% 11% 41%	2600
	21st to end	Day	4 6% 2 7% 6% 7% 6% 6% 9% 10% 36%	2600
July	1st to 10th	Day	11% 7% 3 8% 6% 7% 7% 6% 9% 11% 23%	5900
	11th to 20th	Day	3 3 2 5% 5% 6% 8% 6% 10% 16% 37%	5900
	21st to end	Day	6% 5% 3 8% 6% 7% 7% 6% 10% 13% 28%	6424
August	1st to 10th	Day	16% 6% 3 8% 6% 7% 7% 6% 9% 11% 23%	4234
	11th to 20th	Day	7% 4 2 6% 5% 6% 6% 6% 8% 12% 40%	4174
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 11% 40%	4109
September	1st to 10th	Day	7% 5% 3 8% 7% 6% 5% 6% 7% 8% 37%	2200
	11th to 20th	Day	12% 5% 3 6% 6% 4 5% 4 6% 9% 40%	2200
	21st to end	Day	15% 6% 2 7% 5% 5% 4 3 5% 6% 42%	2200
October	1st to 10th	Day	12% 6% 2 6% 4 4 4 3 5% 7% 48%	2174
	11th to 20th	Day	10% 4 2 4 2 3 3 2 4 6% 60%	2000
	21st to end	Day	9% 4 1 4 3 3 4 3 4 5% 58%	2200
November	1st to 10th	Day	12% 6% 2 6% 4 3 4 2 5% 5% 52%	2057
	11th to 20th	Day	13% 6% 2 4 2 2 2 2 4 6% 56%	1937
	21st to end	Day	13% 6% 2 6% 4 3 4 3 5% 7% 47%	1890
December	1st to 10th	Day	10% 6% 2 7% 5% 3 4 3 4 6% 49%	3020
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 7% 49%	3463
	21st to end	Day	19% 8% 2 6% 4 3 4 3 4 6% 43%	4565
January	1st to 10th	Night	10% 5% 2 5% 3 4 4 3 5% 7% 51%	7635
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 5% 6% 48%	7680
	21st to end	Night	17% 8% 3 7% 4 3 3 3 4 6% 42%	8401
February	1st to 10th	Night	13% 6% 2 6% 3 4 4 4 6% 6% 46%	2562
	11th to 20th	Night	17% 7% 2 5% 5% 2 3 3 5% 5% 46%	2399
	21st to end	Night	11% 6% 2 5% 5% 4 5% 4 5% 6% 46%	1977
March	1st to 10th	Night	19% 8% 3 6% 3 4 4 3 5% 5% 40%	4918
	11th to 20th	Night	9% 4 2 4 3 4 4 4 6% 7% 53%	4980
	21st to end	Night	14% 6% 2 5% 4 3 4 4 5% 6% 49%	5467
April	1st to 10th	Night	10% 4 2 5% 5% 4 4 4 6% 7% 50%	2200
	11th to 20th	Night	10% 4 2 5% 4 3 4 4 5% 7% 52%	2100
	21st to end	Night	7% 3 2 4 4 4 5% 3 5% 9% 54%	1815
May	1st to 10th	Night	8% 7% 2 7% 5% 5% 4 4 7% 10% 40%	4033
	11th to 20th	Night	16% 5% 2 5% 4 4 5% 4 8% 10% 38%	4032
	21st to end	Night	8% 5% 2 5% 4 4 5% 4 7% 9% 47%	4421
June	1st to 10th	Night	8% 6% 3 7% 5% 6% 6% 5% 8% 11% 35%	1800
	11th to 20th	Night	6% 5% 2 6% 5% 5% 7% 6% 7% 11% 41%	1800
	21st to end	Night	6% 5% 2 5% 6% 7% 7% 6% 10% 12% 35%	1800
July	1st to 10th	Night	12% 9% 3 9% 7% 6% 7% 5% 7% 10% 24%	4058
	11th to 20th	Night	4 5% 2 7% 6% 6% 7% 6% 10% 15% 32%	4057
	21st to end	Night	9% 7% 3 9% 6% 6% 7% 5% 9% 12% 27%	4427
August	1st to 10th	Night	16% 7% 3 8% 6% 6% 6% 6% 8% 11% 23%	2915
	11th to 20th	Night	8% 5% 2 7% 5% 5% 6% 5% 9% 11% 37%	2875
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 7% 11% 45%	3736
September	1st to 10th	Night	10% 6% 3 6% 5% 5% 7% 4 7% 11% 38%	2200
	11th to 20th	Night	10% 7% 3 8% 5% 4 4 4 7% 8% 40%	2200
	21st to end	Night	13% 7% 3 6% 5% 5% 5% 4 6% 7% 41%	2200
October	1st to 10th	Night	12% 7% 2 5% 3 4 4 3 5% 6% 49%	2225
	11th to 20th	Night	13% 4 1 5% 2 3 2 2 4 5% 60%	2400
	21st to end	Night	9% 5% 1 5% 4 4 3 3 4 5% 57%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 4 4 4 5% 54%	2461
	11th to 20th	Night	13% 7% 2 4 3 3 2 3 4 6% 52%	2679
	21st to end	Night	12% 5% 2 5% 3 4 4 3 5% 6% 51%	2730
December	1st to 10th	Night	13% 5% 2 5% 3 3 4 2 4 5% 55%	4380
	11th to 20th	Night	15% 6% 3 6% 3 4 3 2 4 5% 47%	4952
	21st to end	Night	18% 7% 2 6% 3 4 3 3 5% 5% 43%	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	33% 5% 9% 19% 27% 8%	5259
	11th to 20th	Day	29% 4 5% 16% 30% 16%	5280
	21st to end	Day	41% 5% 6% 18% 19% 11%	5780
February	1st to 10th	Day	40% 5% 9% 19% 21% 7%	2055
	11th to 20th	Day	40% 5% 7% 17% 25% 7%	2008
	21st to end	Day	38% 5% 8% 18% 20% 11%	1651
March	1st to 10th	Day	45% 5% 8% 11% 21% 10%	4935
	11th to 20th	Day	26% 7% 10% 16% 21% 21%	4980
	21st to end	Day	41% 4 7% 18% 20% 11%	5465
April	1st to 10th	Day	33% 6% 11% 20% 20% 10%	2200
	11th to 20th	Day	33% 6% 9% 19% 21% 12%	2249
	21st to end	Day	27% 7% 12% 25% 25% 5%	2600
May	1st to 10th	Day	35% 9% 16% 24% 14% 2	5897
	11th to 20th	Day	42% 9% 17% 20% 11% 1	5865
	21st to end	Day	32% 10% 16% 20% 20% 1	6447
June	1st to 10th	Day	41% 12% 16% 18% 13% 1	2600
	11th to 20th	Day	31% 11% 20% 23% 14% 1	2600
	21st to end	Day	36% 13% 21% 17% 12% 2	2600
July	1st to 10th	Day	47% 16% 20% 14% 3 1	5835
	11th to 20th	Day	28% 17% 34% 18% 2 1	5831
	21st to end	Day	40% 17% 22% 17% 3	6240
August	1st to 10th	Day	49% 14% 19% 15% 2	4221
	11th to 20th	Day	32% 13% 18% 25% 11% 1	4213
	21st to end	Day	33% 11% 18% 29% 8% 1	4109
September	1st to 10th	Day	40% 13% 15% 15% 12% 5%	2200
	11th to 20th	Day	39% 10% 12% 17% 18% 5%	2200
	21st to end	Day	42% 6% 12% 18% 17% 6%	2200
October	1st to 10th	Day	36% 5% 7% 13% 27% 12%	3538
	11th to 20th	Day	16% 3 5% 16% 28% 33%	3240
	21st to end	Day	22% 4 6% 17% 35% 16%	3538
November	1st to 10th	Day	34% 5% 8% 15% 24% 15%	2057
	11th to 20th	Day	31% 4 7% 19% 25% 14%	1937
	21st to end	Day	36% 5% 9% 18% 22% 9%	1890
December	1st to 10th	Day	35% 5% 8% 16% 26% 9%	3020
	11th to 20th	Day	35% 5% 8% 24% 21% 7%	3460
	21st to end	Day	42% 5% 7% 20% 23% 4	4565
January	1st to 10th	Night	31% 6% 9% 20% 26% 8%	7636
	11th to 20th	Night	35% 5% 6% 14% 28% 13%	7680
	21st to end	Night	44% 5% 5% 13% 23% 9%	8401
February	1st to 10th	Night	38% 5% 8% 17% 22% 10%	2562
	11th to 20th	Night	41% 5% 8% 15% 22% 10%	2400
	21st to end	Night	37% 6% 8% 15% 18% 15%	1977
March	1st to 10th	Night	43% 6% 8% 14% 18% 10%	4918
	11th to 20th	Night	27% 6% 9% 13% 25% 19%	4980
	21st to end	Night	35% 6% 8% 16% 21% 14%	5467
April	1st to 10th	Night	31% 7% 12% 19% 19% 12%	2200
	11th to 20th	Night	30% 5% 9% 20% 22% 13%	2100
	21st to end	Night	26% 7% 14% 26% 20% 7%	1815
May	1st to 10th	Night	34% 9% 16% 24% 13% 3	4060
	11th to 20th	Night	38% 10% 19% 21% 11% 1	4033
	21st to end	Night	30% 9% 15% 24% 19% 3	4429
June	1st to 10th	Night	38% 12% 20% 19% 11% 1	1800
	11th to 20th	Night	33% 12% 19% 24% 11% 1	1800
	21st to end	Night	34% 15% 23% 16% 11% 2	1800
July	1st to 10th	Night	52% 14% 19% 12% 3 1	4015
	11th to 20th	Night	34% 17% 32% 14% 2	4006
	21st to end	Night	45% 16% 19% 16% 4	4297
August	1st to 10th	Night	50% 15% 20% 13% 3	2904
	11th to 20th	Night	34% 13% 21% 24% 6% 1	2902
	21st to end	Night	30% 10% 20% 32% 7% 1	3736
September	1st to 10th	Night	38% 11% 17% 17% 11% 5%	2200
	11th to 20th	Night	40% 10% 14% 17% 15% 4	2200
	21st to end	Night	40% 8% 15% 17% 16% 5%	2200
October	1st to 10th	Night	35% 7% 11% 13% 23% 11%	3641
	11th to 20th	Night	18% 2 5% 12% 26% 37%	3899
	21st to end	Night	21% 4 7% 17% 32% 19%	4252
November	1st to 10th	Night	31% 5% 9% 14% 24% 16%	2461
	11th to 20th	Night	33% 4 7% 16% 25% 14%	2679
	21st to end	Night	34% 5% 8% 19% 23% 11%	2730
December	1st to 10th	Night	32% 4 7% 16% 31% 8%	4380
	11th to 20th	Night	39% 5% 10% 17% 19% 9%	4954
	21st to end	Night	42% 5% 6% 19% 25% 3	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	14% 16% 46% 20% 4	330
	11th to 20th	Day	16% 21% 33% 21% 8% 2	330
	21st to end	Day	21% 23% 40% 12% 4	363
February	1st to 10th	Day	15% 18% 42% 18% 6%	330
	11th to 20th	Day	28% 16% 34% 20% 3	315
	21st to end	Day	16% 18% 38% 20% 6% 2	250
March	1st to 10th	Day	16% 17% 27% 27% 12% 1	500
	11th to 20th	Day	13% 14% 37% 27% 9% 1	500
	21st to end	Day	17% 13% 36% 25% 8% 1	550
April	1st to 10th	Day	14% 15% 30% 26% 14% 2	500
	11th to 20th	Day	12% 12% 27% 24% 20% 5%	492
	21st to end	Day	6% 9% 24% 33% 20% 8%	500
May	1st to 10th	Day	7% 11% 26% 29% 17% 10%	503
	11th to 20th	Day	10% 5% 19% 29% 22% 15%	527
	21st to end	Day	13% 5% 14% 24% 27% 17%	573
June	1st to 10th	Day	10% 3 14% 25% 29% 19%	530
	11th to 20th	Day	5% 4 13% 22% 29% 28%	530
	21st to end	Day	5% 3 8% 26% 32% 25%	530
July	1st to 10th	Day	7% 3 5% 20% 34% 31%	525
	11th to 20th	Day	5% 2 3 16% 37% 36%	524
	21st to end	Day	5% 2 6% 24% 36% 27%	576
August	1st to 10th	Day	12% 3 12% 21% 29% 22%	526
	11th to 20th	Day	8% 3 7% 18% 33% 33%	519
	21st to end	Day	14% 2 9% 25% 28% 23%	583
September	1st to 10th	Day	8% 3 11% 26% 33% 18%	530
	11th to 20th	Day	13% 3 13% 27% 31% 12%	530
	21st to end	Day	17% 3 11% 28% 29% 12%	530
October	1st to 10th	Day	15% 5% 21% 29% 20% 10%	530
	11th to 20th	Day	10% 5% 22% 31% 23% 10%	530
	21st to end	Day	13% 5% 22% 30% 24% 5%	583
November	1st to 10th	Day	16% 7% 25% 26% 20% 6%	535
	11th to 20th	Day	16% 9% 37% 24% 12% 2	377
	21st to end	Day	11% 17% 29% 28% 14% 1	330
December	1st to 10th	Day	21% 13% 36% 20% 9% 2	330
	11th to 20th	Day	20% 15% 31% 22% 11% 1	330
	21st to end	Day	24% 10% 32% 25% 7% 2	363
January	1st to 10th	Night	16% 12% 38% 27% 5%	840
	11th to 20th	Night	15% 12% 34% 29% 9%	840
	21st to end	Night	20% 11% 39% 25% 4	924
February	1st to 10th	Night	16% 10% 32% 34% 6%	840
	11th to 20th	Night	19% 13% 37% 28% 3	804
	21st to end	Night	14% 12% 39% 32% 4	660
March	1st to 10th	Night	15% 10% 33% 36% 6%	800
	11th to 20th	Night	14% 11% 35% 31% 9%	800
	21st to end	Night	13% 10% 37% 29% 10%	880
April	1st to 10th	Night	11% 10% 36% 32% 11% 1	800
	11th to 20th	Night	11% 10% 31% 33% 15%	791
	21st to end	Night	8% 5% 27% 44% 17%	615
May	1st to 10th	Night	7% 6% 28% 44% 15% 1	600
	11th to 20th	Night	9% 4 23% 40% 21% 3	591
	21st to end	Night	10% 3 13% 38% 32% 4	650
June	1st to 10th	Night	10% 3 12% 39% 31% 5%	600
	11th to 20th	Night	8% 2 13% 35% 32% 9%	600
	21st to end	Night	7% 1 10% 35% 39% 9%	600
July	1st to 10th	Night	9% 2 7% 28% 46% 9%	585
	11th to 20th	Night	7% 2 4 24% 52% 11%	582
	21st to end	Night	7% 2 7% 32% 45% 7%	657
August	1st to 10th	Night	10% 1 8% 27% 45% 9%	597
	11th to 20th	Night	10% 2 8% 26% 47% 8%	594
	21st to end	Night	11% 3 9% 27% 41% 10%	780
September	1st to 10th	Night	10% 3 11% 27% 42% 8%	800
	11th to 20th	Night	13% 2 11% 33% 39% 3	800
	21st to end	Night	16% 2 14% 31% 33% 4	800
October	1st to 10th	Night	14% 4 18% 37% 23% 4	800
	11th to 20th	Night	14% 4 22% 33% 25% 3	800
	21st to end	Night	11% 5% 23% 35% 25% 2	880
November	1st to 10th	Night	13% 7% 21% 34% 22% 3	820
	11th to 20th	Night	15% 9% 26% 34% 15% 1	840
	21st to end	Night	15% 8% 27% 32% 17%	840
December	1st to 10th	Night	17% 8% 30% 32% 11% 1	840
	11th to 20th	Night	17% 9% 30% 30% 13% 1	840
	21st to end	Night	20% 11% 28% 31% 10%	924

Page generated at 2026-02-13 01:42:14. Version ac07

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