Tye Preston Memorial Library
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tye Preston Memorial Library clear sky chart from 2005-11-06 to 2026-05-17.

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	14% 6% 2 5% 4 4 4 3 5% 7% 46%	132313
	Night	16% 6% 2 5% 3 3 3 3 4 6% 49%	130833
Transparency	Day	37% 4 9% 21% 18% 10%	136092
	Night	36% 4 10% 19% 19% 12%	135118
Seeing	Day	15% 8% 23% 29% 20% 4	16765
	Night	18% 7% 25% 34% 15% 1	26770

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	15% 9% 3 6% 2 3 3 2 3 4 51%	5289
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 4 4 54%	5310
	21st to end	Day	25% 8% 2 4 3 3 3 2 3 6% 41%	6270
February	1st to 10th	Day	20% 7% 2 5% 4 3 3 2 4 5% 45%	2100
	11th to 20th	Day	19% 7% 2 5% 3 4 3 3 4 6% 44%	2049
	21st to end	Day	19% 6% 2 4 2 3 3 2 4 5% 50%	1730
March	1st to 10th	Day	14% 7% 2 5% 3 3 3 3 4 5% 52%	4992
	11th to 20th	Day	17% 6% 2 5% 4 3 3 2 3 4 51%	5090
	21st to end	Day	18% 8% 2 5% 3 3 3 3 4 6% 46%	5583
April	1st to 10th	Day	15% 9% 2 7% 5% 4 4 3 5% 6% 42%	2310
	11th to 20th	Day	22% 7% 2 6% 3 3 4 3 5% 5% 41%	2286
	21st to end	Day	16% 8% 2 7% 5% 5% 4 3 5% 6% 41%	2667
May	1st to 10th	Day	23% 7% 2 6% 4 4 5% 4 5% 6% 34%	5992
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 6% 7% 47%	5941
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 5% 7% 9% 35%	6434
June	1st to 10th	Day	7% 4 2 4 4 4 5% 5% 7% 9% 47%	2600
	11th to 20th	Day	9% 5% 3 5% 5% 5% 6% 4 6% 9% 44%	2600
	21st to end	Day	8% 6% 2 7% 6% 6% 6% 6% 7% 10% 36%	2600
July	1st to 10th	Day	12% 7% 2 8% 5% 6% 6% 4 6% 8% 37%	5900
	11th to 20th	Day	5% 4 2 6% 5% 6% 7% 5% 8% 11% 40%	5900
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 5% 7% 10% 40%	6422
August	1st to 10th	Day	1 2 1 2 3 3 3 4 5% 9% 68%	4188
	11th to 20th	Day	3 3 1 4 3 4 4 4 8% 11% 56%	3846
	21st to end	Day	4 4 1 5% 5% 6% 5% 5% 7% 11% 47%	4126
September	1st to 10th	Day	9% 6% 3 8% 6% 5% 5% 3 6% 7% 43%	2200
	11th to 20th	Day	10% 7% 2 6% 5% 4 5% 4 7% 7% 42%	2200
	21st to end	Day	8% 4 1 6% 5% 4 5% 4 6% 7% 50%	2200
October	1st to 10th	Day	7% 4 2 4 4 4 4 3 5% 7% 56%	2200
	11th to 20th	Day	12% 5% 1 4 3 3 4 3 5% 5% 53%	2073
	21st to end	Day	12% 4 2 4 3 3 3 3 4 6% 57%	2200
November	1st to 10th	Day	16% 7% 2 6% 4 3 4 3 4 6% 46%	2058
	11th to 20th	Day	12% 6% 2 5% 4 4 3 3 5% 5% 51%	1969
	21st to end	Day	18% 7% 3 6% 4 3 4 3 3 5% 44%	1890
December	1st to 10th	Day	27% 8% 3 6% 3 3 2 2 3 5% 38%	3030
	11th to 20th	Day	18% 8% 2 5% 4 3 4 3 4 5% 43%	3481
	21st to end	Day	17% 5% 1 3 3 2 2 2 3 4 57%	4587
January	1st to 10th	Night	17% 7% 2 5% 3 2 3 2 3 5% 52%	7634
	11th to 20th	Night	11% 4 1 3 2 3 3 3 4 5% 63%	7680
	21st to end	Night	30% 6% 2 4 3 3 3 2 4 5% 40%	7986
February	1st to 10th	Night	20% 7% 2 5% 3 3 2 3 4 5% 48%	2520
	11th to 20th	Night	21% 7% 2 4 2 3 3 2 3 4 48%	2458
	21st to end	Night	20% 7% 2 4 4 2 3 3 4 4 46%	2076
March	1st to 10th	Night	18% 8% 2 5% 4 4 3 2 5% 5% 44%	5083
	11th to 20th	Night	18% 7% 2 5% 3 3 3 2 3 5% 49%	5090
	21st to end	Night	22% 9% 2 4 3 3 3 2 4 4 43%	5588
April	1st to 10th	Night	17% 7% 2 5% 4 4 4 3 4 7% 44%	2310
	11th to 20th	Night	22% 7% 2 5% 4 2 3 3 4 5% 43%	2283
	21st to end	Night	23% 7% 2 4 4 3 3 2 4 5% 43%	1948
May	1st to 10th	Night	27% 8% 3 5% 3 3 3 3 4 5% 34%	4143
	11th to 20th	Night	11% 4 2 6% 4 4 4 4 6% 8% 48%	4107
	21st to end	Night	17% 6% 2 7% 4 4 4 4 5% 9% 36%	4443
June	1st to 10th	Night	7% 4 1 4 3 3 3 3 6% 8% 58%	1800
	11th to 20th	Night	6% 4 2 4 4 4 4 3 5% 9% 55%	1800
	21st to end	Night	8% 4 2 5% 4 4 4 3 7% 10% 49%	1800
July	1st to 10th	Night	11% 7% 3 6% 3 4 5% 4 6% 10% 41%	4078
	11th to 20th	Night	4 4 2 6% 5% 5% 6% 5% 8% 11% 45%	4077
	21st to end	Night	7% 6% 3 7% 4 4 5% 4 6% 10% 44%	4449
August	1st to 10th	Night	1 1 1 2 2 2 2 2 4 9% 73%	2953
	11th to 20th	Night	3 2 1 4 3 3 4 4 7% 12% 57%	3213
	21st to end	Night	6% 3 2 4 3 4 4 4 7% 10% 54%	3774
September	1st to 10th	Night	8% 6% 2 7% 4 3 4 4 6% 8% 49%	2200
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 8% 47%	2200
	21st to end	Night	9% 5% 2 5% 4 3 4 3 6% 8% 51%	2200
October	1st to 10th	Night	7% 4 1 4 3 3 4 3 5% 7% 60%	2200
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 4 5% 56%	2305
	21st to end	Night	15% 5% 1 4 2 2 2 2 3 4 61%	2640
November	1st to 10th	Night	15% 7% 2 5% 3 3 2 2 3 4 54%	2460
	11th to 20th	Night	13% 5% 2 4 3 2 4 3 4 5% 55%	2646
	21st to end	Night	20% 5% 2 5% 3 3 3 3 5% 6% 47%	2730
December	1st to 10th	Night	25% 9% 3 5% 4 3 3 3 3 5% 38%	4380
	11th to 20th	Night	23% 9% 2 4 3 3 2 2 3 3 47%	4946
	21st to end	Night	19% 6% 2 4 2 2 2 2 3 4 55%	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	38% 3 5% 21% 23% 10%	5290
	11th to 20th	Day	35% 2 4 14% 24% 21%	5310
	21st to end	Day	47% 2 5% 13% 23% 11%	6270
February	1st to 10th	Day	44% 3 5% 12% 21% 16%	2100
	11th to 20th	Day	43% 3 7% 18% 18% 12%	2059
	21st to end	Day	39% 3 6% 16% 20% 15%	1730
March	1st to 10th	Day	37% 3 5% 20% 16% 21%	4992
	11th to 20th	Day	40% 2 4 8% 16% 29%	5090
	21st to end	Day	42% 3 7% 19% 20% 9%	5585
April	1st to 10th	Day	44% 4 7% 16% 17% 12%	2310
	11th to 20th	Day	46% 4 7% 15% 18% 11%	2286
	21st to end	Day	46% 4 6% 19% 16% 9%	2667
May	1st to 10th	Day	51% 4 9% 18% 15% 4	6030
	11th to 20th	Day	36% 5% 11% 19% 21% 8%	5945
	21st to end	Day	43% 6% 10% 27% 13% 1	6447
June	1st to 10th	Day	31% 7% 13% 28% 19% 3	2600
	11th to 20th	Day	36% 6% 11% 25% 18% 4	2600
	21st to end	Day	41% 8% 11% 25% 13% 3	2600
July	1st to 10th	Day	44% 7% 15% 28% 4 1	5836
	11th to 20th	Day	34% 8% 17% 29% 11% 1	5831
	21st to end	Day	36% 7% 16% 25% 14% 2	6240
August	1st to 10th	Day	15% 4 12% 42% 26% 1	4174
	11th to 20th	Day	21% 6% 23% 37% 11% 2	3882
	21st to end	Day	30% 9% 17% 25% 17% 2	4125
September	1st to 10th	Day	41% 5% 9% 20% 16% 9%	2200
	11th to 20th	Day	38% 7% 12% 20% 18% 5%	2200
	21st to end	Day	33% 5% 10% 22% 17% 13%	2200
October	1st to 10th	Day	17% 3 7% 12% 25% 35%	3590
	11th to 20th	Day	24% 4 7% 19% 25% 20%	3352
	21st to end	Day	28% 3 5% 18% 31% 15%	3538
November	1st to 10th	Day	40% 4 6% 18% 20% 12%	2058
	11th to 20th	Day	36% 3 7% 18% 21% 15%	1969
	21st to end	Day	44% 3 6% 15% 19% 13%	1890
December	1st to 10th	Day	51% 3 5% 17% 14% 9%	3030
	11th to 20th	Day	43% 4 7% 11% 21% 13%	3479
	21st to end	Day	33% 2 4 17% 27% 17%	4587
January	1st to 10th	Night	36% 3 5% 19% 23% 14%	7635
	11th to 20th	Night	26% 2 4 17% 30% 21%	7680
	21st to end	Night	48% 2 3 12% 21% 13%	7986
February	1st to 10th	Night	40% 3 7% 13% 20% 18%	2520
	11th to 20th	Night	42% 3 5% 17% 19% 15%	2458
	21st to end	Night	42% 3 5% 15% 19% 16%	2076
March	1st to 10th	Night	43% 3 5% 17% 16% 16%	5083
	11th to 20th	Night	40% 3 6% 11% 14% 26%	5090
	21st to end	Night	45% 3 6% 17% 18% 10%	5588
April	1st to 10th	Night	41% 4 9% 17% 16% 14%	2310
	11th to 20th	Night	45% 4 8% 13% 16% 14%	2283
	21st to end	Night	45% 4 8% 18% 15% 9%	1948
May	1st to 10th	Night	53% 4 8% 16% 14% 6%	4170
	11th to 20th	Night	34% 5% 11% 21% 20% 9%	4108
	21st to end	Night	43% 6% 13% 27% 10% 1	4452
June	1st to 10th	Night	25% 6% 16% 30% 21% 2	1800
	11th to 20th	Night	26% 6% 15% 30% 19% 5%	1800
	21st to end	Night	31% 7% 18% 27% 14% 3	1800
July	1st to 10th	Night	38% 9% 22% 26% 4 1	4035
	11th to 20th	Night	30% 9% 21% 29% 10%	4026
	21st to end	Night	35% 8% 20% 23% 11% 3	4320
August	1st to 10th	Night	11% 4 17% 48% 20%	2943
	11th to 20th	Night	19% 8% 31% 32% 8% 1	3243
	21st to end	Night	24% 10% 22% 26% 15% 2	3774
September	1st to 10th	Night	33% 7% 16% 20% 16% 8%	2200
	11th to 20th	Night	36% 7% 14% 23% 14% 6%	2200
	21st to end	Night	30% 6% 13% 21% 16% 13%	2200
October	1st to 10th	Night	17% 3 8% 15% 21% 36%	3590
	11th to 20th	Night	22% 3 9% 18% 29% 20%	3752
	21st to end	Night	27% 2 6% 19% 30% 16%	4252
November	1st to 10th	Night	36% 3 7% 18% 22% 14%	2460
	11th to 20th	Night	32% 4 8% 19% 20% 17%	2646
	21st to end	Night	40% 3 8% 17% 18% 14%	2730
December	1st to 10th	Night	51% 3 5% 15% 15% 11%	4380
	11th to 20th	Night	45% 3 5% 9% 21% 18%	4947
	21st to end	Night	36% 2 3 13% 29% 17%	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	23% 17% 34% 20% 5%	330
	11th to 20th	Day	28% 17% 28% 20% 7% 1	330
	21st to end	Day	24% 17% 35% 19% 5%	363
February	1st to 10th	Day	32% 12% 31% 19% 5%	330
	11th to 20th	Day	30% 17% 34% 16% 2 1	330
	21st to end	Day	26% 20% 39% 14% 1	273
March	1st to 10th	Day	27% 17% 40% 14% 2	330
	11th to 20th	Day	19% 21% 40% 15% 5%	330
	21st to end	Day	21% 14% 36% 22% 6% 1	363
April	1st to 10th	Day	21% 18% 37% 21% 3	330
	11th to 20th	Day	24% 18% 33% 22% 3	324
	21st to end	Day	21% 14% 38% 21% 6%	687
May	1st to 10th	Day	22% 12% 37% 25% 5%	753
	11th to 20th	Day	18% 10% 29% 31% 9% 1	756
	21st to end	Day	18% 7% 26% 34% 13% 1	789
June	1st to 10th	Day	9% 3 19% 40% 26% 4	730
	11th to 20th	Day	9% 2 20% 44% 23% 2	730
	21st to end	Day	9% 2 15% 38% 27% 9%	730
July	1st to 10th	Day	11% 2 10% 36% 33% 8%	721
	11th to 20th	Day	6% 1 9% 34% 42% 8%	718
	21st to end	Day	7% 1 8% 36% 41% 8%	794
August	1st to 10th	Day	3 1 5% 36% 43% 12%	706
	11th to 20th	Day	5% 1 3 41% 41% 9%	517
	21st to end	Day	5% 1 6% 29% 40% 19%	475
September	1st to 10th	Day	12% 2 11% 28% 36% 12%	330
	11th to 20th	Day	6% 8% 32% 42% 11%	330
	21st to end	Day	8% 1 11% 32% 35% 12%	330
October	1st to 10th	Day	5% 3 14% 39% 32% 7%	330
	11th to 20th	Day	11% 5% 21% 39% 19% 5%	330
	21st to end	Day	18% 11% 27% 26% 15% 4	363
November	1st to 10th	Day	19% 8% 29% 28% 14% 3	330
	11th to 20th	Day	18% 11% 30% 29% 11% 2	330
	21st to end	Day	25% 18% 30% 16% 9% 1	330
December	1st to 10th	Day	24% 9% 34% 24% 5% 4	330
	11th to 20th	Day	22% 15% 30% 25% 8%	330
	21st to end	Day	23% 8% 38% 24% 6% 1	363
January	1st to 10th	Night	22% 10% 28% 32% 8% 1	1050
	11th to 20th	Night	21% 12% 31% 29% 7%	1050
	21st to end	Night	25% 10% 28% 29% 7%	1008
February	1st to 10th	Night	27% 11% 29% 27% 6%	840
	11th to 20th	Night	28% 12% 32% 24% 4	824
	21st to end	Night	26% 12% 38% 21% 2	692
March	1st to 10th	Night	27% 12% 34% 24% 3	660
	11th to 20th	Night	24% 11% 33% 28% 5%	630
	21st to end	Night	22% 10% 38% 26% 4	693
April	1st to 10th	Night	20% 12% 37% 27% 4	630
	11th to 20th	Night	26% 12% 35% 24% 4	623
	21st to end	Night	25% 10% 36% 22% 6%	630
May	1st to 10th	Night	21% 10% 38% 26% 5%	630
	11th to 20th	Night	16% 9% 33% 32% 10%	609
	21st to end	Night	17% 6% 31% 34% 12%	650
June	1st to 10th	Night	8% 3 24% 43% 21% 1	600
	11th to 20th	Night	6% 3 31% 45% 14% 1	600
	21st to end	Night	9% 3 18% 43% 25% 3	600
July	1st to 10th	Night	11% 2 13% 43% 29% 3	585
	11th to 20th	Night	4 1 13% 50% 29% 2	582
	21st to end	Night	7% 1 10% 48% 30% 3	657
August	1st to 10th	Night	2 1 9% 49% 36% 4	597
	11th to 20th	Night	4 1 10% 49% 33% 3	594
	21st to end	Night	7% 1 6% 39% 42% 5%	660
September	1st to 10th	Night	9% 1 10% 38% 38% 5%	600
	11th to 20th	Night	11% 1 9% 39% 37% 3	600
	21st to end	Night	11% 1 13% 35% 37% 3	600
October	1st to 10th	Night	8% 2 11% 46% 29% 4	600
	11th to 20th	Night	16% 3 19% 39% 21% 2	705
	21st to end	Night	17% 8% 21% 33% 19% 2	880
November	1st to 10th	Night	17% 6% 18% 37% 20% 1	820
	11th to 20th	Night	15% 7% 26% 36% 15% 1	966
	21st to end	Night	22% 10% 26% 30% 11% 1	1050
December	1st to 10th	Night	23% 8% 28% 29% 11% 1	1050
	11th to 20th	Night	27% 8% 25% 31% 8%	1050
	21st to end	Night	23% 8% 28% 33% 8%	1155

Page generated at 2026-05-14 00:59:01. Version ac07

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