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 2024-01-15.

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	11% 5% 3 5% 5% 3 5% 5% 6% 8% 43%	74923
	Night	12% 5% 2 5% 5% 3 5% 5% 6% 8% 45%	74862
Transparency	Day	37% 8% 13% 18% 18% 6%	74646
	Night	36% 8% 12% 17% 19% 8%	74658
Seeing	Day	12% 8% 21% 24% 22% 13%	14100
	Night	13% 6% 23% 32% 22% 3	24215

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% 7% 3 5% 5% 3 3 4 5% 7% 48%	1706
	11th to 20th	Day	13% 7% 2 4 4 2 4 3 6% 7% 48%	1656
	21st to end	Day	17% 6% 3 5% 4 3 4 4 5% 6% 45%	1782
February	1st to 10th	Day	16% 7% 3 6% 4 2 4 5% 6% 7% 39%	1761
	11th to 20th	Day	17% 6% 2 4 4 3 4 3 5% 7% 45%	1748
	21st to end	Day	16% 7% 2 4 4 2 4 4 5% 7% 44%	1480
March	1st to 10th	Day	12% 5% 2 5% 4 2 4 4 4 6% 52%	1980
	11th to 20th	Day	14% 4 2 5% 5% 3 4 5% 5% 8% 44%	1980
	21st to end	Day	12% 6% 3 5% 5% 3 4 5% 5% 8% 45%	2178
April	1st to 10th	Day	9% 5% 3 4 4 3 4 5% 6% 6% 50%	1980
	11th to 20th	Day	12% 5% 2 5% 5% 2 5% 5% 4 7% 49%	2021
	21st to end	Day	6% 4 2 4 4 2 4 4 5% 8% 56%	2340
May	1st to 10th	Day	6% 4 2 4 4 3 5% 4 6% 8% 53%	2340
	11th to 20th	Day	9% 5% 2 6% 5% 3 6% 6% 6% 9% 42%	2301
	21st to end	Day	9% 6% 2 5% 6% 4 7% 5% 7% 9% 39%	2531
June	1st to 10th	Day	9% 6% 3 6% 6% 5% 7% 7% 9% 9% 34%	2340
	11th to 20th	Day	6% 3 2 4 5% 4 7% 7% 9% 12% 40%	2340
	21st to end	Day	4 6% 2 6% 7% 5% 7% 8% 9% 10% 35%	2340
July	1st to 10th	Day	7% 6% 3 5% 8% 6% 7% 7% 8% 11% 32%	2340
	11th to 20th	Day	7% 4 3 8% 7% 5% 8% 8% 9% 11% 29%	2340
	21st to end	Day	7% 5% 3 6% 7% 5% 7% 7% 10% 12% 30%	2547
August	1st to 10th	Day	7% 6% 3 7% 8% 5% 7% 8% 10% 12% 27%	2327
	11th to 20th	Day	10% 5% 3 7% 7% 5% 7% 8% 8% 11% 30%	2303
	21st to end	Day	12% 7% 4 7% 7% 5% 6% 7% 7% 10% 28%	2277
September	1st to 10th	Day	7% 5% 3 7% 8% 4 6% 8% 7% 8% 35%	1980
	11th to 20th	Day	12% 5% 3 5% 7% 4 4 5% 6% 8% 41%	1980
	21st to end	Day	15% 5% 3 7% 5% 4 5% 4 4 6% 42%	1980
October	1st to 10th	Day	12% 5% 2 5% 4 2 4 4 4 7% 50%	1958
	11th to 20th	Day	11% 4 2 3 2 2 4 3 4 7% 57%	1800
	21st to end	Day	9% 4 2 4 4 2 4 4 4 5% 57%	1980
November	1st to 10th	Day	11% 5% 2 5% 5% 2 4 4 5% 5% 52%	1857
	11th to 20th	Day	14% 6% 3 4 3 1 3 3 4 6% 54%	1753
	21st to end	Day	14% 5% 2 5% 4 2 4 4 5% 6% 47%	1710
December	1st to 10th	Day	14% 6% 3 6% 6% 3 5% 4 5% 7% 42%	1710
	11th to 20th	Day	16% 5% 2 4 4 2 3 3 4 8% 48%	2153
	21st to end	Day	18% 8% 3 6% 5% 2 4 3 4 6% 42%	3124
January	1st to 10th	Night	14% 6% 2 5% 4 3 4 5% 4 6% 46%	2459
	11th to 20th	Night	14% 6% 3 5% 5% 3 4 4 5% 6% 46%	2393
	21st to end	Night	18% 6% 3 5% 4 2 4 4 4 5% 46%	2574
February	1st to 10th	Night	15% 6% 3 5% 4 2 4 4 6% 6% 45%	2196
	11th to 20th	Night	17% 6% 2 5% 5% 2 3 3 4 5% 48%	2098
	21st to end	Night	13% 6% 3 5% 5% 3 5% 5% 5% 7% 44%	1774
March	1st to 10th	Night	10% 5% 2 4 5% 4 4 5% 5% 6% 51%	1980
	11th to 20th	Night	12% 5% 3 4 5% 2 5% 4 5% 7% 48%	1980
	21st to end	Night	9% 5% 3 5% 4 3 4 5% 5% 7% 49%	2178
April	1st to 10th	Night	10% 4 2 4 5% 3 4 5% 5% 7% 52%	1980
	11th to 20th	Night	10% 4 2 5% 5% 3 4 5% 4 8% 51%	1888
	21st to end	Night	7% 3 2 4 4 3 4 4 5% 9% 55%	1634
May	1st to 10th	Night	6% 4 2 4 4 2 4 5% 6% 8% 55%	1620
	11th to 20th	Night	8% 4 2 5% 6% 3 5% 7% 8% 8% 43%	1592
	21st to end	Night	9% 5% 2 5% 6% 3 5% 6% 7% 9% 43%	1744
June	1st to 10th	Night	9% 5% 3 5% 7% 5% 7% 6% 8% 11% 35%	1620
	11th to 20th	Night	6% 5% 2 5% 5% 4 7% 7% 7% 10% 42%	1620
	21st to end	Night	6% 4 2 4 6% 5% 8% 8% 9% 12% 35%	1620
July	1st to 10th	Night	7% 6% 3 6% 7% 5% 6% 7% 7% 13% 33%	1620
	11th to 20th	Night	6% 7% 3 8% 6% 3 6% 8% 10% 13% 31%	1620
	21st to end	Night	6% 5% 2 5% 7% 4 7% 8% 8% 12% 36%	1771
August	1st to 10th	Night	6% 4 3 7% 7% 5% 7% 9% 10% 13% 28%	1611
	11th to 20th	Night	10% 5% 3 6% 7% 4 6% 7% 8% 13% 31%	1598
	21st to end	Night	10% 7% 3 6% 7% 5% 6% 7% 8% 10% 31%	2056
September	1st to 10th	Night	9% 5% 3 5% 5% 4 7% 6% 7% 11% 37%	1980
	11th to 20th	Night	10% 7% 3 8% 5% 4 4 4 6% 9% 41%	1980
	21st to end	Night	12% 7% 3 5% 6% 4 5% 5% 6% 7% 41%	1980
October	1st to 10th	Night	12% 6% 2 4 4 2 4 4 5% 6% 50%	2002
	11th to 20th	Night	14% 4 2 4 3 2 3 3 4 5% 58%	2160
	21st to end	Night	9% 4 2 5% 5% 3 4 4 4 6% 55%	2376
November	1st to 10th	Night	11% 5% 2 5% 4 3 4 4 4 5% 54%	2221
	11th to 20th	Night	14% 6% 2 4 4 2 3 4 4 6% 51%	2423
	21st to end	Night	13% 5% 2 5% 4 3 4 4 4 6% 50%	2470
December	1st to 10th	Night	15% 5% 2 4 5% 3 5% 4 6% 7% 45%	2470
	11th to 20th	Night	17% 6% 2 4 3 2 3 3 5% 5% 50%	3042
	21st to end	Night	17% 8% 3 6% 6% 3 4 4 6% 6% 38%	4532

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	36% 5% 10% 18% 26% 5%	1707
	11th to 20th	Day	36% 6% 8% 18% 23% 8%	1656
	21st to end	Day	41% 4 6% 17% 24% 8%	1782
February	1st to 10th	Day	42% 5% 10% 18% 18% 7%	1761
	11th to 20th	Day	39% 5% 7% 17% 25% 7%	1758
	21st to end	Day	39% 5% 8% 18% 18% 11%	1480
March	1st to 10th	Day	34% 4 9% 17% 22% 14%	1980
	11th to 20th	Day	38% 7% 12% 18% 17% 9%	1980
	21st to end	Day	38% 5% 11% 18% 21% 7%	2178
April	1st to 10th	Day	33% 6% 11% 19% 21% 10%	1980
	11th to 20th	Day	34% 6% 9% 19% 20% 11%	2021
	21st to end	Day	26% 6% 11% 25% 25% 6%	2340
May	1st to 10th	Day	28% 8% 12% 23% 24% 6%	2340
	11th to 20th	Day	36% 10% 15% 20% 16% 2	2305
	21st to end	Day	38% 11% 12% 21% 16% 3	2531
June	1st to 10th	Day	41% 12% 16% 17% 12% 1	2340
	11th to 20th	Day	32% 11% 20% 23% 13% 1	2340
	21st to end	Day	37% 13% 21% 16% 12% 2	2340
July	1st to 10th	Day	40% 15% 19% 16% 7% 3	2275
	11th to 20th	Day	41% 14% 25% 14% 5% 2	2271
	21st to end	Day	39% 15% 21% 19% 5% 1	2405
August	1st to 10th	Day	42% 16% 19% 18% 4	2314
	11th to 20th	Day	43% 16% 18% 16% 7%	2303
	21st to end	Day	47% 13% 18% 13% 7% 1	2277
September	1st to 10th	Day	41% 14% 15% 14% 12% 5%	1980
	11th to 20th	Day	39% 10% 12% 16% 19% 5%	1980
	21st to end	Day	42% 6% 11% 17% 18% 6%	1980
October	1st to 10th	Day	35% 5% 10% 19% 21% 11%	1958
	11th to 20th	Day	27% 5% 9% 23% 26% 10%	1800
	21st to end	Day	28% 5% 7% 15% 29% 16%	1980
November	1st to 10th	Day	34% 4 7% 16% 24% 15%	1857
	11th to 20th	Day	32% 4 7% 19% 24% 14%	1753
	21st to end	Day	37% 5% 9% 18% 21% 9%	1710
December	1st to 10th	Day	41% 6% 11% 15% 19% 7%	1710
	11th to 20th	Day	37% 4 7% 17% 27% 8%	2150
	21st to end	Day	44% 6% 7% 15% 24% 5%	3124
January	1st to 10th	Night	38% 6% 8% 18% 22% 8%	2459
	11th to 20th	Night	38% 5% 7% 16% 24% 8%	2393
	21st to end	Night	41% 5% 5% 14% 24% 11%	2574
February	1st to 10th	Night	39% 5% 9% 17% 19% 10%	2196
	11th to 20th	Night	39% 5% 7% 16% 22% 11%	2099
	21st to end	Night	39% 7% 9% 15% 17% 14%	1774
March	1st to 10th	Night	33% 6% 8% 17% 23% 12%	1980
	11th to 20th	Night	34% 7% 13% 17% 18% 11%	1980
	21st to end	Night	34% 7% 12% 18% 20% 10%	2178
April	1st to 10th	Night	31% 7% 12% 19% 20% 12%	1980
	11th to 20th	Night	32% 5% 9% 20% 21% 12%	1888
	21st to end	Night	26% 7% 13% 25% 21% 8%	1634
May	1st to 10th	Night	26% 6% 15% 23% 23% 8%	1620
	11th to 20th	Night	34% 11% 18% 18% 16% 3	1593
	21st to end	Night	35% 10% 13% 21% 18% 3	1745
June	1st to 10th	Night	39% 11% 20% 19% 10%	1620
	11th to 20th	Night	33% 12% 19% 24% 11% 1	1620
	21st to end	Night	35% 15% 22% 15% 10% 2	1620
July	1st to 10th	Night	40% 15% 19% 16% 7% 3	1575
	11th to 20th	Night	38% 16% 24% 16% 6%	1566
	21st to end	Night	34% 16% 22% 22% 4 1	1672
August	1st to 10th	Night	38% 20% 20% 17% 5%	1600
	11th to 20th	Night	40% 15% 22% 17% 6%	1598
	21st to end	Night	45% 13% 18% 16% 7% 1	2056
September	1st to 10th	Night	38% 12% 18% 16% 11% 5%	1980
	11th to 20th	Night	39% 10% 15% 16% 16% 5%	1980
	21st to end	Night	40% 9% 14% 15% 17% 4	1980
October	1st to 10th	Night	34% 6% 11% 17% 22% 9%	2002
	11th to 20th	Night	30% 4 9% 20% 27% 11%	2160
	21st to end	Night	30% 5% 7% 17% 25% 16%	2376
November	1st to 10th	Night	31% 5% 9% 14% 25% 16%	2221
	11th to 20th	Night	34% 4 8% 16% 23% 15%	2423
	21st to end	Night	36% 5% 7% 19% 22% 10%	2470
December	1st to 10th	Night	38% 7% 12% 17% 21% 7%	2470
	11th to 20th	Night	37% 5% 8% 15% 26% 9%	3044
	21st to end	Night	45% 6% 7% 16% 22% 4	4532

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	13% 18% 46% 19% 4	270
	11th to 20th	Day	15% 18% 33% 24% 8% 2	252
	21st to end	Day	21% 23% 40% 11% 4	264
February	1st to 10th	Day	17% 19% 39% 18% 7%	240
	11th to 20th	Day	26% 14% 36% 21% 3	240
	21st to end	Day	17% 15% 41% 19% 7% 2	198
March	1st to 10th	Day	14% 15% 29% 28% 13% 1	420
	11th to 20th	Day	14% 13% 37% 25% 10% 1	420
	21st to end	Day	17% 11% 35% 26% 9% 1	462
April	1st to 10th	Day	15% 14% 28% 26% 15% 2	420
	11th to 20th	Day	13% 13% 28% 19% 22% 6%	412
	21st to end	Day	7% 10% 23% 33% 19% 8%	420
May	1st to 10th	Day	7% 12% 26% 29% 17% 10%	423
	11th to 20th	Day	8% 4 16% 30% 25% 17%	447
	21st to end	Day	14% 4 12% 22% 29% 20%	485
June	1st to 10th	Day	11% 2 13% 24% 30% 21%	450
	11th to 20th	Day	6% 5% 12% 19% 28% 30%	450
	21st to end	Day	6% 3 9% 28% 31% 24%	450
July	1st to 10th	Day	7% 2 4 19% 37% 31%	445
	11th to 20th	Day	6% 3 4 17% 34% 37%	444
	21st to end	Day	5% 2 6% 25% 35% 27%	488
August	1st to 10th	Day	10% 2 10% 22% 31% 25%	446
	11th to 20th	Day	8% 3 5% 17% 33% 33%	439
	21st to end	Day	14% 2 9% 23% 29% 24%	495
September	1st to 10th	Day	7% 3 10% 25% 35% 20%	450
	11th to 20th	Day	13% 3 13% 26% 31% 14%	450
	21st to end	Day	16% 2 11% 28% 29% 14%	450
October	1st to 10th	Day	14% 4 20% 30% 21% 11%	450
	11th to 20th	Day	10% 4 21% 30% 23% 12%	450
	21st to end	Day	13% 5% 22% 29% 26% 5%	495
November	1st to 10th	Day	15% 6% 26% 26% 20% 6%	455
	11th to 20th	Day	19% 9% 36% 25% 11% 1	313
	21st to end	Day	11% 16% 26% 30% 15% 1	270
December	1st to 10th	Day	20% 13% 34% 21% 11% 2	270
	11th to 20th	Day	22% 14% 33% 21% 9% 1	270
	21st to end	Day	24% 11% 33% 22% 8% 2	297
January	1st to 10th	Night	17% 11% 38% 27% 6% 1	760
	11th to 20th	Night	15% 12% 32% 31% 10%	737
	21st to end	Night	20% 11% 39% 26% 5%	792
February	1st to 10th	Night	18% 11% 31% 34% 7%	720
	11th to 20th	Night	19% 12% 36% 30% 3	703
	21st to end	Night	15% 10% 39% 32% 4	592
March	1st to 10th	Night	14% 9% 34% 38% 6%	720
	11th to 20th	Night	14% 11% 34% 31% 10%	720
	21st to end	Night	12% 11% 36% 30% 11%	792
April	1st to 10th	Night	12% 10% 34% 32% 12% 1	720
	11th to 20th	Night	11% 10% 31% 32% 16%	711
	21st to end	Night	8% 5% 27% 43% 16% 1	554
May	1st to 10th	Night	7% 6% 28% 43% 16% 1	540
	11th to 20th	Night	8% 4 22% 40% 23% 4	531
	21st to end	Night	11% 2 12% 36% 35% 4	584
June	1st to 10th	Night	11% 3 11% 39% 32% 5%	540
	11th to 20th	Night	9% 2 11% 35% 32% 10%	540
	21st to end	Night	7% 1 10% 34% 39% 9%	540
July	1st to 10th	Night	9% 1 7% 27% 46% 9%	525
	11th to 20th	Night	8% 2 5% 26% 51% 9%	522
	21st to end	Night	8% 1 7% 33% 45% 7%	591
August	1st to 10th	Night	7% 1 7% 28% 47% 10%	537
	11th to 20th	Night	11% 1 7% 26% 47% 7%	534
	21st to end	Night	10% 2 9% 25% 43% 10%	702
September	1st to 10th	Night	10% 2 10% 25% 45% 8%	720
	11th to 20th	Night	13% 2 11% 32% 40% 3	720
	21st to end	Night	15% 2 13% 31% 34% 4	720
October	1st to 10th	Night	14% 3 18% 37% 24% 5%	720
	11th to 20th	Night	15% 4 20% 33% 25% 3	720
	21st to end	Night	11% 5% 23% 35% 25% 2	792
November	1st to 10th	Night	14% 6% 20% 34% 23% 3	740
	11th to 20th	Night	16% 9% 26% 33% 15% 1	760
	21st to end	Night	16% 7% 26% 32% 18%	760
December	1st to 10th	Night	17% 8% 29% 33% 12% 1	760
	11th to 20th	Night	18% 8% 29% 30% 13% 1	760
	21st to end	Night	22% 11% 27% 30% 10%	836

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