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 2025-01-22.

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%	105251
	Night	16% 6% 2 4 3 3 3 3 4 6% 49%	104234
Transparency	Day	37% 5% 9% 21% 19% 9%	109069
	Night	36% 4 10% 19% 19% 11%	108543
Seeing	Day	15% 8% 22% 30% 20% 5%	15040
	Night	17% 7% 25% 34% 16% 1	25081

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% 8% 2 5% 2 3 3 2 4 5% 50%	2937
	11th to 20th	Day	18% 6% 2 3 2 2 2 2 4 5% 55%	2940
	21st to end	Day	20% 7% 2 4 2 3 3 2 4 9% 43%	3382
February	1st to 10th	Day	20% 7% 2 5% 4 3 3 2 4 5% 44%	1900
	11th to 20th	Day	19% 7% 2 5% 3 4 3 3 4 6% 44%	1849
	21st to end	Day	20% 6% 2 4 2 3 3 2 4 5% 49%	1570
March	1st to 10th	Day	17% 9% 2 6% 3 3 3 3 4 5% 43%	3397
	11th to 20th	Day	25% 8% 4 7% 5% 4 4 3 4 4 33%	3480
	21st to end	Day	18% 7% 2 5% 3 3 3 3 5% 6% 46%	3814
April	1st to 10th	Day	15% 8% 2 6% 4 4 4 3 5% 6% 43%	2090
	11th to 20th	Day	22% 6% 2 5% 3 3 3 3 5% 5% 42%	2066
	21st to end	Day	15% 7% 2 6% 5% 5% 4 3 5% 6% 43%	2413
May	1st to 10th	Day	27% 8% 2 7% 5% 3 4 3 4 6% 31%	4082
	11th to 20th	Day	16% 6% 2 6% 4 4 4 3 5% 6% 43%	4082
	21st to end	Day	14% 5% 2 6% 5% 5% 5% 5% 7% 9% 38%	4477
June	1st to 10th	Day	8% 4 2 4 4 4 6% 5% 7% 9% 47%	2470
	11th to 20th	Day	9% 5% 2 6% 5% 4 6% 4 6% 8% 44%	2470
	21st to end	Day	9% 6% 2 7% 6% 6% 6% 6% 7% 10% 36%	2470
July	1st to 10th	Day	7% 5% 1 6% 5% 5% 6% 4 6% 8% 46%	4120
	11th to 20th	Day	5% 4 2 5% 5% 5% 5% 5% 8% 11% 46%	4120
	21st to end	Day	10% 6% 3 7% 6% 6% 5% 5% 7% 10% 36%	4465
August	1st to 10th	Day	1 2 1 2 3 3 3 4 4 9% 68%	4059
	11th to 20th	Day	4 3 1 4 3 4 4 3 7% 11% 56%	3726
	21st to end	Day	4 4 1 5% 5% 6% 5% 5% 7% 11% 46%	4000
September	1st to 10th	Day	9% 6% 2 8% 6% 5% 5% 3 6% 7% 42%	2090
	11th to 20th	Day	11% 7% 2 7% 5% 4 5% 4 7% 7% 40%	2090
	21st to end	Day	8% 4 2 6% 5% 4 5% 4 6% 7% 49%	2090
October	1st to 10th	Day	7% 4 2 4 4 4 4 3 5% 7% 54%	2090
	11th to 20th	Day	13% 5% 1 5% 3 3 4 3 5% 5% 51%	1970
	21st to end	Day	12% 4 2 4 3 3 3 3 4 6% 56%	2090
November	1st to 10th	Day	16% 7% 2 6% 4 3 4 3 4 6% 44%	1958
	11th to 20th	Day	12% 7% 2 5% 4 4 3 2 5% 5% 51%	1875
	21st to end	Day	18% 7% 3 6% 4 3 4 3 4 5% 45%	1800
December	1st to 10th	Day	28% 7% 3 6% 3 3 2 2 3 5% 37%	2940
	11th to 20th	Day	19% 8% 2 5% 4 3 4 3 4 5% 43%	3391
	21st to end	Day	17% 5% 1 3 3 2 2 2 3 4 57%	4488
January	1st to 10th	Night	20% 6% 1 4 2 2 2 2 3 5% 51%	4238
	11th to 20th	Night	14% 4 1 3 2 2 2 2 2 4 64%	4250
	21st to end	Night	23% 6% 2 5% 4 3 3 2 4 6% 42%	4322
February	1st to 10th	Night	20% 7% 2 5% 3 3 2 3 4 5% 46%	2280
	11th to 20th	Night	21% 7% 2 4 2 3 2 2 3 4 49%	2218
	21st to end	Night	21% 7% 2 4 4 3 3 3 4 5% 45%	1884
March	1st to 10th	Night	20% 9% 2 5% 4 4 3 3 5% 5% 40%	3458
	11th to 20th	Night	26% 10% 3 6% 4 4 3 3 4 5% 33%	3480
	21st to end	Night	20% 7% 2 5% 4 3 3 3 4 4 44%	3817
April	1st to 10th	Night	16% 7% 2 5% 4 3 3 3 4 7% 45%	2090
	11th to 20th	Night	21% 6% 2 5% 4 2 3 3 4 5% 44%	2063
	21st to end	Night	21% 6% 2 4 4 2 3 2 4 5% 46%	1762
May	1st to 10th	Night	30% 8% 2 5% 3 3 3 3 3 5% 35%	2823
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 5% 7% 48%	2823
	21st to end	Night	17% 5% 2 6% 3 3 4 4 5% 9% 43%	3090
June	1st to 10th	Night	7% 4 1 4 3 3 3 3 5% 8% 58%	1710
	11th to 20th	Night	5% 4 2 4 3 4 4 3 5% 9% 56%	1710
	21st to end	Night	9% 4 2 4 4 4 4 3 7% 9% 49%	1710
July	1st to 10th	Night	6% 4 2 4 2 3 4 4 6% 10% 54%	2850
	11th to 20th	Night	4 3 2 5% 4 5% 5% 5% 7% 10% 50%	2850
	21st to end	Night	9% 8% 3 7% 5% 4 5% 4 5% 10% 40%	3098
August	1st to 10th	Night	1 1 1 2 2 2 2 2 4 9% 73%	2862
	11th to 20th	Night	3 2 1 4 3 3 4 4 7% 12% 57%	3113
	21st to end	Night	6% 3 2 4 3 4 4 4 7% 10% 54%	3659
September	1st to 10th	Night	8% 5% 2 7% 4 3 4 4 6% 8% 49%	2090
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 5% 8% 46%	2090
	21st to end	Night	9% 5% 2 4 4 3 4 3 6% 8% 52%	2090
October	1st to 10th	Night	7% 4 1 4 4 3 4 3 5% 7% 60%	2090
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 4 5% 54%	2190
	21st to end	Night	16% 5% 1 3 2 2 2 2 3 4 61%	2508
November	1st to 10th	Night	15% 7% 2 5% 4 3 2 2 4 5% 52%	2340
	11th to 20th	Night	13% 5% 2 4 3 2 4 3 4 6% 54%	2520
	21st to end	Night	19% 5% 2 5% 3 3 3 3 5% 6% 47%	2600
December	1st to 10th	Night	25% 9% 3 5% 4 3 3 3 3 5% 37%	4250
	11th to 20th	Night	23% 9% 2 4 3 3 2 2 3 3 46%	4816
	21st to end	Night	19% 6% 2 4 2 2 2 2 3 4 55%	6490

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 2 17% 25% 14%	2938
	11th to 20th	Day	34% 2 5% 15% 26% 17%	2940
	21st to end	Day	41% 3 4 12% 29% 10%	3382
February	1st to 10th	Day	44% 3 5% 13% 21% 15%	1900
	11th to 20th	Day	43% 3 6% 17% 19% 13%	1859
	21st to end	Day	41% 3 6% 17% 18% 16%	1570
March	1st to 10th	Day	45% 3 6% 16% 20% 10%	3397
	11th to 20th	Day	57% 4 6% 9% 17% 8%	3480
	21st to end	Day	40% 3 9% 16% 21% 11%	3814
April	1st to 10th	Day	44% 4 8% 16% 16% 12%	2090
	11th to 20th	Day	45% 4 7% 16% 18% 11%	2066
	21st to end	Day	44% 4 5% 20% 18% 9%	2413
May	1st to 10th	Day	56% 4 8% 14% 14% 5%	4120
	11th to 20th	Day	42% 5% 8% 18% 20% 6%	4086
	21st to end	Day	40% 6% 11% 31% 11% 1	4489
June	1st to 10th	Day	31% 7% 13% 26% 19% 3	2470
	11th to 20th	Day	36% 6% 11% 25% 18% 4	2470
	21st to end	Day	41% 8% 10% 25% 13% 3	2470
July	1st to 10th	Day	33% 7% 16% 37% 6% 1	4056
	11th to 20th	Day	31% 7% 18% 29% 14% 1	4051
	21st to end	Day	39% 7% 17% 28% 8%	4325
August	1st to 10th	Day	15% 4 12% 42% 26% 1	4045
	11th to 20th	Day	21% 6% 23% 37% 11% 2	3762
	21st to end	Day	30% 9% 17% 24% 17% 2	3999
September	1st to 10th	Day	42% 6% 9% 19% 16% 9%	2090
	11th to 20th	Day	39% 8% 12% 19% 17% 5%	2090
	21st to end	Day	33% 5% 10% 22% 17% 13%	2090
October	1st to 10th	Day	18% 3 7% 12% 25% 36%	3480
	11th to 20th	Day	25% 4 8% 19% 24% 20%	3249
	21st to end	Day	28% 3 5% 18% 31% 14%	3428
November	1st to 10th	Day	42% 4 7% 18% 18% 11%	1958
	11th to 20th	Day	36% 3 8% 18% 20% 15%	1875
	21st to end	Day	43% 3 6% 15% 19% 14%	1800
December	1st to 10th	Day	51% 3 5% 18% 14% 9%	2940
	11th to 20th	Day	43% 4 7% 11% 20% 13%	3389
	21st to end	Day	33% 2 4 17% 27% 17%	4488
January	1st to 10th	Night	37% 3 4 15% 22% 19%	4238
	11th to 20th	Night	27% 2 4 17% 30% 20%	4250
	21st to end	Night	45% 3 4 12% 25% 12%	4322
February	1st to 10th	Night	41% 3 6% 13% 20% 17%	2280
	11th to 20th	Night	41% 3 5% 16% 20% 16%	2218
	21st to end	Night	43% 3 6% 15% 17% 16%	1884
March	1st to 10th	Night	48% 4 6% 15% 17% 9%	3458
	11th to 20th	Night	56% 4 8% 11% 12% 9%	3480
	21st to end	Night	43% 4 8% 15% 20% 10%	3817
April	1st to 10th	Night	40% 4 8% 18% 16% 14%	2090
	11th to 20th	Night	44% 4 8% 13% 17% 15%	2063
	21st to end	Night	43% 4 8% 19% 16% 10%	1762
May	1st to 10th	Night	53% 4 7% 16% 14% 6%	2850
	11th to 20th	Night	37% 5% 10% 22% 19% 7%	2824
	21st to end	Night	39% 5% 15% 29% 12% 1	3099
June	1st to 10th	Night	24% 6% 16% 29% 22% 3	1710
	11th to 20th	Night	25% 5% 15% 30% 19% 5%	1710
	21st to end	Night	31% 7% 17% 27% 15% 3	1710
July	1st to 10th	Night	25% 8% 24% 36% 5% 1	2805
	11th to 20th	Night	27% 10% 20% 29% 14% 1	2796
	21st to end	Night	39% 8% 23% 24% 6%	2999
August	1st to 10th	Night	11% 4 17% 47% 20%	2852
	11th to 20th	Night	19% 8% 31% 32% 9% 1	3143
	21st to end	Night	24% 10% 22% 26% 15% 2	3659
September	1st to 10th	Night	33% 7% 16% 21% 16% 8%	2090
	11th to 20th	Night	37% 7% 15% 22% 15% 6%	2090
	21st to end	Night	30% 6% 14% 21% 17% 13%	2090
October	1st to 10th	Night	17% 3 8% 15% 20% 36%	3480
	11th to 20th	Night	23% 3 9% 17% 29% 20%	3637
	21st to end	Night	27% 2 6% 19% 30% 16%	4120
November	1st to 10th	Night	38% 4 7% 18% 20% 14%	2340
	11th to 20th	Night	32% 5% 8% 18% 20% 17%	2520
	21st to end	Night	40% 3 8% 17% 19% 14%	2600
December	1st to 10th	Night	51% 3 5% 15% 14% 10%	4250
	11th to 20th	Night	45% 3 5% 9% 21% 17%	4817
	21st to end	Night	36% 2 3 13% 29% 17%	6490

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% 16% 35% 21% 4	300
	11th to 20th	Day	29% 16% 28% 20% 6% 1	300
	21st to end	Day	22% 15% 36% 21% 6%	300
February	1st to 10th	Day	34% 13% 30% 17% 5%	270
	11th to 20th	Day	30% 17% 32% 17% 3 1	270
	21st to end	Day	28% 22% 37% 12% 1	225
March	1st to 10th	Day	26% 16% 41% 14% 2	270
	11th to 20th	Day	20% 19% 42% 13% 6%	270
	21st to end	Day	23% 16% 36% 21% 4	297
April	1st to 10th	Day	21% 19% 37% 21% 2	270
	11th to 20th	Day	24% 18% 32% 22% 4	264
	21st to end	Day	20% 13% 38% 23% 7%	593
May	1st to 10th	Day	20% 11% 36% 26% 6%	653
	11th to 20th	Day	19% 9% 28% 32% 10% 1	675
	21st to end	Day	19% 7% 25% 35% 13% 1	734
June	1st to 10th	Day	9% 3 17% 40% 27% 4	680
	11th to 20th	Day	9% 2 18% 45% 24% 2	680
	21st to end	Day	9% 2 14% 38% 27% 9%	680
July	1st to 10th	Day	9% 2 10% 35% 34% 9%	671
	11th to 20th	Day	5% 1 8% 34% 45% 8%	668
	21st to end	Day	6% 1 7% 35% 42% 8%	739
August	1st to 10th	Day	3 1 5% 35% 43% 12%	657
	11th to 20th	Day	6% 1 3 40% 41% 9%	477
	21st to end	Day	5% 1 6% 29% 40% 19%	437
September	1st to 10th	Day	11% 2 10% 27% 37% 13%	300
	11th to 20th	Day	7% 8% 31% 41% 12%	300
	21st to end	Day	9% 1 10% 31% 36% 12%	300
October	1st to 10th	Day	6% 3 15% 39% 30% 7%	300
	11th to 20th	Day	12% 6% 20% 41% 17% 5%	300
	21st to end	Day	20% 10% 27% 25% 14% 4	330
November	1st to 10th	Day	20% 8% 29% 28% 12% 3	300
	11th to 20th	Day	19% 11% 30% 27% 11% 2	300
	21st to end	Day	25% 18% 30% 16% 9% 1	300
December	1st to 10th	Day	23% 9% 33% 25% 6% 4	300
	11th to 20th	Day	23% 15% 31% 24% 7%	300
	21st to end	Day	23% 9% 39% 23% 6% 1	330
January	1st to 10th	Night	22% 10% 28% 32% 8% 1	1000
	11th to 20th	Night	22% 11% 30% 29% 8%	1000
	21st to end	Night	24% 10% 28% 30% 8%	918
February	1st to 10th	Night	28% 11% 28% 26% 7%	760
	11th to 20th	Night	28% 12% 32% 24% 4	744
	21st to end	Night	27% 13% 39% 20% 2	628
March	1st to 10th	Night	25% 12% 35% 26% 4	598
	11th to 20th	Night	25% 10% 32% 28% 5%	570
	21st to end	Night	22% 10% 39% 26% 4	627
April	1st to 10th	Night	18% 12% 38% 27% 5%	570
	11th to 20th	Night	24% 10% 36% 25% 4	563
	21st to end	Night	22% 10% 37% 23% 7%	570
May	1st to 10th	Night	20% 9% 39% 26% 5%	570
	11th to 20th	Night	17% 7% 33% 33% 10% 1	561
	21st to end	Night	17% 6% 30% 35% 12%	617
June	1st to 10th	Night	7% 3 22% 45% 22% 1	570
	11th to 20th	Night	6% 2 30% 46% 15% 1	570
	21st to end	Night	9% 2 18% 42% 25% 3	570
July	1st to 10th	Night	10% 1 13% 43% 30% 3	555
	11th to 20th	Night	4 1 13% 50% 30% 3	552
	21st to end	Night	7% 1 9% 48% 31% 3	624
August	1st to 10th	Night	2 1 9% 49% 36% 4	567
	11th to 20th	Night	4 1 9% 49% 33% 3	564
	21st to end	Night	7% 1 6% 40% 41% 5%	627
September	1st to 10th	Night	9% 1 10% 38% 37% 5%	570
	11th to 20th	Night	12% 1 9% 38% 38% 3	570
	21st to end	Night	11% 1 13% 34% 38% 4	570
October	1st to 10th	Night	8% 2 12% 46% 29% 4	570
	11th to 20th	Night	17% 3 19% 39% 20% 3	670
	21st to end	Night	17% 8% 21% 32% 20% 2	836
November	1st to 10th	Night	18% 6% 18% 37% 20% 1	780
	11th to 20th	Night	15% 7% 26% 36% 15% 2	920
	21st to end	Night	21% 10% 26% 31% 12% 1	1000
December	1st to 10th	Night	23% 8% 27% 30% 11% 2	1000
	11th to 20th	Night	27% 8% 25% 31% 8%	1000
	21st to end	Night	23% 7% 29% 33% 8%	1100

Page generated at 2025-01-19 02:06:48. Version ac07

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