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 2024-11-30.

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% 5% 5% 5% 4 6% 9% 42%	101301
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 43%	98241
Transparency	Day	36% 9% 13% 18% 17% 7%	105056
	Night	36% 8% 12% 17% 18% 9%	102625
Seeing	Day	12% 8% 21% 25% 21% 13%	15320
	Night	13% 6% 23% 32% 22% 3	25371

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% 2 6% 4 4 4 3 5% 7% 45%	2836
	11th to 20th	Day	12% 6% 2 4 3 3 3 2 6% 7% 52%	2840
	21st to end	Day	14% 8% 3 7% 4 3 4 3 4 6% 44%	3124
February	1st to 10th	Day	15% 8% 2 6% 4 4 4 4 6% 7% 40%	1859
	11th to 20th	Day	17% 7% 2 4 3 4 3 3 5% 7% 44%	1848
	21st to end	Day	16% 7% 2 5% 4 3 4 4 5% 7% 44%	1571
March	1st to 10th	Day	20% 9% 3 8% 4 4 4 3 5% 5% 35%	3435
	11th to 20th	Day	8% 4 1 5% 4 4 4 4 5% 9% 52%	3480
	21st to end	Day	17% 7% 3 6% 4 4 4 3 4 6% 43%	3815
April	1st to 10th	Day	9% 6% 2 5% 3 4 4 4 6% 6% 50%	2090
	11th to 20th	Day	12% 6% 2 5% 4 3 4 4 5% 7% 49%	2135
	21st to end	Day	6% 5% 2 4 4 3 4 4 6% 7% 55%	2470
May	1st to 10th	Day	7% 5% 2 6% 5% 4 5% 4 6% 8% 47%	4082
	11th to 20th	Day	15% 7% 2 7% 4 5% 5% 4 5% 8% 39%	4081
	21st to end	Day	6% 4 1 5% 5% 4 5% 4 7% 11% 49%	4476
June	1st to 10th	Day	9% 6% 3 7% 6% 7% 7% 5% 9% 9% 34%	2470
	11th to 20th	Day	6% 4 2 5% 4 5% 7% 5% 9% 11% 41%	2470
	21st to end	Day	4 6% 2 7% 6% 7% 6% 6% 9% 10% 36%	2470
July	1st to 10th	Day	12% 7% 3 8% 6% 7% 6% 6% 9% 11% 24%	4120
	11th to 20th	Day	4 3 2 7% 6% 7% 8% 6% 10% 15% 32%	4120
	21st to end	Day	7% 7% 3 9% 7% 7% 8% 6% 11% 13% 22%	4466
August	1st to 10th	Day	15% 5% 3 8% 6% 7% 7% 6% 9% 11% 24%	4104
	11th to 20th	Day	7% 4 2 6% 5% 6% 6% 6% 8% 12% 40%	4044
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	3983
September	1st to 10th	Day	8% 6% 3 8% 7% 6% 6% 6% 7% 8% 35%	2090
	11th to 20th	Day	13% 5% 3 6% 6% 5% 5% 4 6% 8% 40%	2090
	21st to end	Day	15% 5% 2 7% 4 5% 4 3 4 6% 43%	2090
October	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 49%	2066
	11th to 20th	Day	10% 4 2 4 2 3 3 3 4 6% 59%	1900
	21st to end	Day	8% 4 2 5% 3 3 4 3 4 5% 58%	2090
November	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 5% 5% 50%	1957
	11th to 20th	Day	14% 6% 2 5% 3 2 3 2 4 6% 54%	1845
	21st to end	Day	14% 6% 2 6% 4 3 4 3 5% 6% 47%	1797
December	1st to 10th	Day	14% 6% 3 7% 5% 4 4 3 5% 7% 42%	1710
	11th to 20th	Day	16% 6% 2 5% 3 3 3 2 4 7% 48%	2153
	21st to end	Day	18% 9% 3 6% 4 3 4 3 3 6% 42%	3124
January	1st to 10th	Night	16% 6% 2 5% 4 4 4 4 5% 7% 44%	4109
	11th to 20th	Night	9% 5% 2 6% 5% 5% 5% 4 6% 8% 45%	4120
	21st to end	Night	16% 8% 2 7% 4 4 4 4 4 5% 43%	4532
February	1st to 10th	Night	14% 6% 2 6% 4 4 3 4 6% 6% 45%	2318
	11th to 20th	Night	17% 7% 2 5% 5% 2 3 2 5% 5% 47%	2218
	21st to end	Night	12% 7% 3 6% 5% 4 5% 4 5% 6% 45%	1881
March	1st to 10th	Night	21% 9% 3 7% 4 3 3 3 4 5% 37%	3418
	11th to 20th	Night	11% 5% 2 4 3 4 4 4 5% 7% 51%	3480
	21st to end	Night	17% 6% 2 5% 4 3 3 4 5% 6% 44%	3817
April	1st to 10th	Night	10% 4 2 5% 5% 4 3 4 5% 6% 51%	2090
	11th to 20th	Night	10% 4 2 5% 4 3 4 4 5% 7% 51%	1994
	21st to end	Night	7% 3 2 4 4 4 4 3 5% 9% 55%	1724
May	1st to 10th	Night	7% 6% 2 6% 5% 4 4 4 7% 10% 45%	2813
	11th to 20th	Night	12% 5% 2 6% 5% 4 5% 4 6% 8% 42%	2812
	21st to end	Night	7% 3 2 5% 4 3 4 4 7% 8% 53%	3079
June	1st to 10th	Night	8% 6% 3 7% 6% 6% 6% 5% 8% 11% 35%	1710
	11th to 20th	Night	6% 5% 2 6% 5% 5% 6% 6% 7% 10% 42%	1710
	21st to end	Night	6% 5% 2 5% 6% 7% 7% 6% 10% 11% 35%	1710
July	1st to 10th	Night	14% 9% 3 8% 7% 6% 6% 5% 7% 11% 26%	2840
	11th to 20th	Night	5% 6% 2 8% 6% 6% 7% 6% 11% 13% 29%	2840
	21st to end	Night	9% 8% 3 9% 7% 6% 7% 6% 9% 12% 24%	3087
August	1st to 10th	Night	16% 6% 3 8% 6% 6% 6% 7% 8% 11% 24%	2825
	11th to 20th	Night	8% 5% 2 7% 5% 5% 6% 5% 9% 11% 38%	2785
	21st to end	Night	6% 5% 2 5% 4 4 5% 4 7% 11% 46%	3622
September	1st to 10th	Night	10% 6% 3 6% 5% 6% 7% 4 7% 11% 37%	2090
	11th to 20th	Night	11% 7% 3 8% 5% 4 4 3 7% 8% 39%	2090
	21st to end	Night	12% 7% 3 6% 5% 5% 5% 4 6% 6% 41%	2090
October	1st to 10th	Night	12% 7% 2 5% 3 4 4 3 5% 6% 50%	2114
	11th to 20th	Night	13% 4 1 4 2 3 3 2 4 5% 60%	2280
	21st to end	Night	8% 4 1 6% 4 4 3 3 4 6% 57%	2508
November	1st to 10th	Night	11% 5% 2 6% 3 4 4 4 5% 5% 52%	2341
	11th to 20th	Night	14% 7% 2 4 4 3 3 3 5% 6% 51%	2551
	21st to end	Night	13% 6% 2 5% 3 4 4 3 4 6% 51%	2599
December	1st to 10th	Night	15% 5% 2 5% 4 3 5% 3 6% 7% 45%	2470
	11th to 20th	Night	17% 6% 2 4 3 3 3 2 5% 5% 49%	3042
	21st to end	Night	17% 8% 3 7% 5% 4 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% 6% 8% 17% 22% 10%	2837
	11th to 20th	Day	31% 5% 7% 17% 28% 12%	2840
	21st to end	Day	40% 6% 6% 15% 22% 12%	3124
February	1st to 10th	Day	42% 5% 9% 18% 19% 7%	1859
	11th to 20th	Day	39% 5% 7% 16% 25% 7%	1858
	21st to end	Day	39% 5% 8% 17% 19% 11%	1571
March	1st to 10th	Day	48% 7% 9% 13% 15% 8%	3435
	11th to 20th	Day	28% 8% 11% 18% 23% 13%	3480
	21st to end	Day	43% 5% 8% 19% 19% 7%	3815
April	1st to 10th	Day	33% 6% 12% 19% 21% 10%	2090
	11th to 20th	Day	34% 6% 9% 19% 20% 12%	2135
	21st to end	Day	27% 6% 11% 25% 26% 5%	2470
May	1st to 10th	Day	31% 10% 14% 26% 16% 3	4120
	11th to 20th	Day	41% 8% 11% 23% 15% 1	4085
	21st to end	Day	26% 11% 18% 21% 21% 2	4489
June	1st to 10th	Day	41% 12% 16% 17% 13% 1	2470
	11th to 20th	Day	31% 11% 19% 24% 14% 1	2470
	21st to end	Day	37% 12% 21% 16% 12% 2	2470
July	1st to 10th	Day	47% 15% 19% 14% 4 2	4055
	11th to 20th	Day	34% 18% 28% 16% 3 1	4051
	21st to end	Day	45% 19% 20% 12% 3	4324
August	1st to 10th	Day	48% 14% 20% 15% 3	4091
	11th to 20th	Day	32% 13% 18% 25% 12% 1	4083
	21st to end	Day	33% 11% 19% 29% 8% 1	3983
September	1st to 10th	Day	42% 13% 15% 13% 11% 5%	2090
	11th to 20th	Day	40% 10% 12% 16% 18% 5%	2090
	21st to end	Day	41% 6% 11% 18% 18% 6%	2090
October	1st to 10th	Day	35% 5% 8% 13% 27% 12%	3430
	11th to 20th	Day	16% 3 5% 16% 27% 34%	3140
	21st to end	Day	21% 4 6% 17% 35% 16%	3428
November	1st to 10th	Day	35% 5% 8% 16% 23% 14%	1957
	11th to 20th	Day	33% 4 6% 19% 24% 14%	1845
	21st to end	Day	37% 5% 9% 18% 22% 9%	1797
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	37% 6% 8% 16% 21% 12%	4109
	11th to 20th	Night	33% 7% 8% 18% 25% 9%	4120
	21st to end	Night	43% 7% 5% 10% 25% 11%	4532
February	1st to 10th	Night	39% 5% 9% 17% 20% 10%	2318
	11th to 20th	Night	40% 5% 7% 15% 22% 11%	2219
	21st to end	Night	38% 7% 9% 15% 17% 14%	1881
March	1st to 10th	Night	48% 7% 10% 13% 14% 7%	3418
	11th to 20th	Night	30% 7% 10% 13% 26% 13%	3480
	21st to end	Night	39% 7% 9% 16% 18% 10%	3817
April	1st to 10th	Night	32% 7% 12% 18% 20% 12%	2090
	11th to 20th	Night	31% 5% 10% 20% 21% 13%	1994
	21st to end	Night	26% 7% 14% 25% 21% 7%	1724
May	1st to 10th	Night	29% 10% 16% 25% 16% 4	2840
	11th to 20th	Night	36% 10% 15% 23% 16% 2	2813
	21st to end	Night	25% 9% 15% 26% 21% 4	3087
June	1st to 10th	Night	39% 12% 20% 18% 10% 1	1710
	11th to 20th	Night	33% 12% 18% 25% 11% 1	1710
	21st to end	Night	35% 15% 22% 15% 10% 2	1710
July	1st to 10th	Night	51% 13% 19% 12% 4 1	2795
	11th to 20th	Night	38% 17% 26% 16% 3	2786
	21st to end	Night	48% 18% 18% 13% 3 1	2988
August	1st to 10th	Night	48% 15% 21% 13% 3	2814
	11th to 20th	Night	34% 13% 21% 25% 7% 1	2812
	21st to end	Night	29% 10% 21% 32% 7% 1	3622
September	1st to 10th	Night	39% 11% 18% 15% 11% 6%	2090
	11th to 20th	Night	41% 10% 14% 16% 15% 4	2090
	21st to end	Night	40% 9% 15% 16% 17% 5%	2090
October	1st to 10th	Night	35% 7% 10% 12% 24% 12%	3530
	11th to 20th	Night	18% 2 5% 12% 25% 37%	3779
	21st to end	Night	21% 4 7% 17% 32% 20%	4120
November	1st to 10th	Night	32% 6% 10% 14% 24% 15%	2341
	11th to 20th	Night	35% 4 7% 15% 24% 14%	2551
	21st to end	Night	35% 5% 7% 19% 23% 11%	2599
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	14% 20% 34% 23% 8% 2	270
	21st to end	Day	21% 23% 39% 12% 4	297
February	1st to 10th	Day	17% 20% 38% 19% 7%	270
	11th to 20th	Day	26% 14% 34% 22% 3	270
	21st to end	Day	17% 19% 38% 19% 6% 1	226
March	1st to 10th	Day	15% 16% 28% 28% 13% 1	460
	11th to 20th	Day	13% 13% 38% 26% 9% 1	460
	21st to end	Day	17% 12% 35% 25% 9% 1	506
April	1st to 10th	Day	14% 16% 30% 25% 14% 2	460
	11th to 20th	Day	13% 12% 28% 21% 20% 5%	452
	21st to end	Day	7% 10% 24% 34% 18% 8%	460
May	1st to 10th	Day	7% 12% 26% 27% 17% 11%	463
	11th to 20th	Day	10% 4 18% 29% 23% 16%	487
	21st to end	Day	13% 3 13% 24% 28% 19%	529
June	1st to 10th	Day	10% 2 13% 26% 30% 20%	490
	11th to 20th	Day	5% 4 13% 21% 27% 28%	490
	21st to end	Day	6% 3 9% 28% 31% 23%	490
July	1st to 10th	Day	7% 3 5% 19% 35% 31%	485
	11th to 20th	Day	6% 2 4 17% 35% 37%	484
	21st to end	Day	5% 2 6% 26% 35% 26%	532
August	1st to 10th	Day	10% 3 11% 21% 31% 24%	486
	11th to 20th	Day	8% 3 7% 18% 33% 32%	479
	21st to end	Day	13% 1 9% 24% 29% 24%	539
September	1st to 10th	Day	8% 3 11% 25% 33% 20%	490
	11th to 20th	Day	14% 4 13% 26% 31% 13%	490
	21st to end	Day	16% 3 12% 28% 29% 13%	490
October	1st to 10th	Day	15% 4 21% 29% 20% 11%	490
	11th to 20th	Day	9% 4 23% 32% 21% 11%	490
	21st to end	Day	12% 5% 22% 31% 26% 5%	539
November	1st to 10th	Day	17% 7% 25% 25% 20% 6%	495
	11th to 20th	Day	18% 9% 36% 24% 11% 2	345
	21st to end	Day	11% 17% 27% 29% 15% 1	299
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%	760
	21st to end	Night	20% 11% 39% 25% 5%	836
February	1st to 10th	Night	17% 10% 31% 35% 6%	760
	11th to 20th	Night	19% 12% 36% 29% 3	743
	21st to end	Night	15% 12% 38% 32% 3	628
March	1st to 10th	Night	15% 9% 33% 37% 6%	760
	11th to 20th	Night	14% 11% 34% 31% 9%	760
	21st to end	Night	13% 11% 36% 29% 11%	836
April	1st to 10th	Night	11% 10% 35% 32% 11% 1	760
	11th to 20th	Night	11% 10% 31% 32% 15%	751
	21st to end	Night	8% 5% 27% 44% 16% 1	584
May	1st to 10th	Night	6% 5% 28% 43% 16% 1	570
	11th to 20th	Night	8% 4 22% 40% 22% 3	561
	21st to end	Night	10% 2 12% 38% 34% 4	617
June	1st to 10th	Night	11% 3 11% 39% 31% 5%	570
	11th to 20th	Night	8% 2 13% 35% 32% 10%	570
	21st to end	Night	7% 1 11% 34% 39% 8%	570
July	1st to 10th	Night	9% 1 7% 27% 46% 9%	555
	11th to 20th	Night	7% 2 5% 25% 51% 10%	552
	21st to end	Night	8% 2 7% 33% 45% 7%	624
August	1st to 10th	Night	8% 1 7% 28% 46% 10%	567
	11th to 20th	Night	10% 1 8% 27% 46% 8%	564
	21st to end	Night	10% 2 9% 27% 42% 10%	741
September	1st to 10th	Night	10% 3 10% 26% 43% 8%	760
	11th to 20th	Night	14% 2 11% 31% 39% 3	760
	21st to end	Night	15% 2 14% 31% 33% 4	760
October	1st to 10th	Night	14% 3 17% 37% 24% 5%	760
	11th to 20th	Night	14% 4 22% 34% 24% 3	760
	21st to end	Night	10% 4 22% 35% 26% 2	836
November	1st to 10th	Night	14% 7% 20% 33% 23% 3	780
	11th to 20th	Night	16% 9% 26% 32% 15% 1	800
	21st to end	Night	16% 8% 26% 32% 18%	800
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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