Harold E. Taylor Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Harold E. Taylor Observatory clear sky chart from 2005-11-06 to 2024-02-15.

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	14% 6% 3 5% 5% 3 5% 5% 6% 8% 42%	78297
	Night	15% 7% 3 5% 5% 3 4 4 5% 7% 43%	80769
Transparency	Day	39% 8% 13% 22% 15% 3	78003
	Night	41% 6% 12% 22% 16% 3	80597
Seeing	Day	17% 10% 28% 27% 14% 4	14066
	Night	18% 9% 30% 29% 12% 1	23464

By Month

Percentage Cloud Cover NE (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	14% 9% 2 5% 4 2 3 4 4 6% 46%	2517
	11th to 20th	Day	15% 8% 2 6% 6% 3 5% 4 3 6% 41%	2554
	21st to end	Day	21% 11% 4 6% 5% 3 4 3 3 4 36%	3124
February	1st to 10th	Day	15% 8% 4 6% 5% 3 4 4 6% 7% 40%	1726
	11th to 20th	Day	18% 6% 4 5% 5% 3 4 4 4 5% 42%	1788
	21st to end	Day	17% 10% 3 6% 4 2 5% 5% 4 5% 39%	1480
March	1st to 10th	Day	14% 5% 2 5% 4 3 5% 4 4 6% 49%	1797
	11th to 20th	Day	16% 6% 2 4 4 3 4 4 5% 6% 44%	1934
	21st to end	Day	19% 7% 3 5% 5% 3 5% 4 5% 6% 40%	2332
April	1st to 10th	Day	16% 7% 3 5% 5% 2 3 4 6% 7% 41%	2160
	11th to 20th	Day	14% 6% 2 4 4 3 4 5% 6% 7% 44%	2134
	21st to end	Day	16% 6% 2 5% 5% 3 5% 5% 6% 7% 40%	2160
May	1st to 10th	Day	17% 6% 3 5% 5% 3 5% 5% 5% 8% 38%	2294
	11th to 20th	Day	18% 7% 2 4 6% 3 5% 5% 5% 7% 38%	2434
	21st to end	Day	13% 6% 2 4 4 3 4 4 6% 10% 43%	2725
June	1st to 10th	Day	12% 6% 3 4 5% 3 5% 6% 7% 10% 40%	2520
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 5% 8% 9% 40%	2520
	21st to end	Day	9% 4 2 4 5% 4 6% 7% 9% 9% 42%	2520
July	1st to 10th	Day	9% 5% 3 5% 5% 3 6% 6% 8% 10% 40%	2520
	11th to 20th	Day	6% 4 2 4 5% 4 6% 6% 8% 12% 44%	2520
	21st to end	Day	9% 5% 2 5% 6% 4 6% 7% 9% 9% 37%	2730
August	1st to 10th	Day	9% 5% 3 5% 5% 3 6% 6% 9% 10% 40%	2274
	11th to 20th	Day	12% 4 3 5% 5% 4 5% 5% 7% 10% 40%	2126
	21st to end	Day	11% 5% 2 5% 5% 3 5% 6% 7% 10% 41%	2376
September	1st to 10th	Day	14% 5% 2 5% 4 3 4 5% 6% 8% 44%	2160
	11th to 20th	Day	11% 5% 3 5% 5% 2 4 3 5% 7% 48%	2100
	21st to end	Day	18% 7% 3 5% 5% 3 5% 5% 5% 7% 39%	1980
October	1st to 10th	Day	19% 6% 2 4 5% 3 4 4 5% 5% 43%	1858
	11th to 20th	Day	17% 5% 2 4 4 2 3 3 5% 6% 49%	1800
	21st to end	Day	16% 7% 2 5% 4 3 4 4 5% 5% 45%	1890
November	1st to 10th	Day	14% 5% 2 4 5% 2 4 3 4 7% 50%	1671
	11th to 20th	Day	15% 5% 2 5% 4 3 3 5% 4 7% 48%	1710
	21st to end	Day	14% 5% 2 5% 4 4 4 4 4 7% 47%	1665
December	1st to 10th	Day	17% 7% 2 5% 4 2 4 5% 5% 7% 40%	1520
	11th to 20th	Day	17% 8% 3 4 4 2 4 3 4 5% 46%	1906
	21st to end	Day	15% 7% 2 4 4 3 5% 5% 7% 8% 39%	2772
January	1st to 10th	Night	16% 6% 3 4 4 2 4 4 4 6% 47%	4429
	11th to 20th	Night	19% 8% 2 5% 4 2 3 3 4 4 45%	4404
	21st to end	Night	20% 10% 3 5% 5% 3 4 4 4 5% 38%	4532
February	1st to 10th	Night	17% 7% 2 5% 5% 2 4 4 5% 6% 42%	2452
	11th to 20th	Night	18% 7% 3 5% 5% 2 5% 4 5% 5% 42%	2153
	21st to end	Night	18% 6% 3 5% 5% 2 4 4 4 5% 44%	1776
March	1st to 10th	Night	15% 5% 2 5% 3 2 3 3 5% 6% 49%	2150
	11th to 20th	Night	18% 7% 3 5% 5% 2 4 4 3 6% 43%	2021
	21st to end	Night	18% 8% 4 5% 5% 3 4 4 4 5% 42%	2022
April	1st to 10th	Night	17% 8% 3 5% 5% 3 4 4 4 6% 41%	1800
	11th to 20th	Night	13% 7% 3 6% 6% 3 4 4 3 6% 45%	1775
	21st to end	Night	18% 6% 3 5% 5% 2 3 5% 5% 8% 40%	1800
May	1st to 10th	Night	17% 9% 3 6% 5% 3 5% 4 6% 8% 33%	1666
	11th to 20th	Night	18% 6% 3 7% 5% 3 5% 4 5% 7% 38%	1473
	21st to end	Night	16% 9% 4 6% 5% 3 5% 5% 5% 8% 36%	1550
June	1st to 10th	Night	15% 6% 2 5% 6% 3 5% 5% 5% 7% 40%	1440
	11th to 20th	Night	11% 6% 3 5% 7% 3 5% 5% 6% 8% 40%	1440
	21st to end	Night	8% 4 3 5% 6% 3 5% 6% 8% 10% 42%	1440
July	1st to 10th	Night	8% 5% 4 7% 8% 3 5% 6% 7% 10% 38%	1440
	11th to 20th	Night	5% 6% 3 5% 6% 5% 5% 6% 6% 12% 42%	1440
	21st to end	Night	7% 4 3 6% 5% 4 5% 6% 7% 10% 41%	1584
August	1st to 10th	Night	8% 6% 3 4 6% 4 6% 6% 7% 8% 43%	1646
	11th to 20th	Night	9% 5% 2 6% 6% 4 5% 5% 6% 9% 42%	1774
	21st to end	Night	10% 5% 2 5% 6% 3 5% 5% 7% 8% 43%	1980
September	1st to 10th	Night	15% 6% 2 5% 5% 2 4 5% 5% 7% 44%	1800
	11th to 20th	Night	11% 5% 1 3 4 2 4 5% 5% 9% 51%	1858
	21st to end	Night	19% 7% 3 5% 4 2 4 4 6% 6% 40%	1980
October	1st to 10th	Night	19% 5% 1 5% 5% 3 4 4 4 7% 43%	2098
	11th to 20th	Night	16% 4 2 4 3 2 2 4 4 6% 50%	2160
	21st to end	Night	16% 7% 2 5% 4 3 4 4 4 6% 46%	2465
November	1st to 10th	Night	13% 4 3 4 4 2 3 4 5% 6% 52%	2407
	11th to 20th	Night	16% 7% 2 4 4 3 3 3 4 5% 47%	2470
	21st to end	Night	13% 5% 2 4 4 3 5% 4 5% 7% 48%	2512
December	1st to 10th	Night	18% 8% 3 5% 4 2 4 4 5% 5% 42%	2660
	11th to 20th	Night	17% 5% 2 5% 5% 3 3 4 6% 8% 42%	3288
	21st to end	Night	14% 8% 3 6% 5% 3 5% 5% 6% 7% 38%	4884

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% 5% 11% 27% 16% 4	2518
	11th to 20th	Day	44% 5% 11% 22% 13% 4	2554
	21st to end	Day	51% 6% 10% 20% 11% 2	3124
February	1st to 10th	Day	42% 6% 14% 24% 13% 1	1726
	11th to 20th	Day	44% 4 9% 25% 14% 3	1797
	21st to end	Day	46% 7% 9% 20% 13% 5%	1480
March	1st to 10th	Day	38% 5% 12% 26% 16% 3	1800
	11th to 20th	Day	40% 6% 13% 21% 16% 4	1934
	21st to end	Day	45% 6% 9% 20% 16% 4	2332
April	1st to 10th	Day	41% 8% 10% 20% 16% 6%	2160
	11th to 20th	Day	38% 6% 14% 22% 16% 4	2134
	21st to end	Day	42% 8% 12% 18% 17% 5%	2160
May	1st to 10th	Day	44% 7% 12% 21% 13% 3	2294
	11th to 20th	Day	44% 8% 13% 20% 13% 2	2438
	21st to end	Day	38% 9% 17% 23% 12% 2	2725
June	1st to 10th	Day	37% 12% 14% 21% 13% 2	2520
	11th to 20th	Day	38% 12% 14% 22% 12% 2	2520
	21st to end	Day	34% 12% 14% 22% 18% 1	2520
July	1st to 10th	Day	36% 11% 17% 20% 14% 2	2450
	11th to 20th	Day	30% 11% 17% 28% 12% 2	2445
	21st to end	Day	36% 12% 15% 23% 13% 2	2577
August	1st to 10th	Day	35% 11% 17% 22% 14% 1	2261
	11th to 20th	Day	37% 10% 17% 20% 14% 2	2126
	21st to end	Day	36% 10% 15% 20% 15% 3	2376
September	1st to 10th	Day	36% 9% 13% 20% 18% 3	2160
	11th to 20th	Day	35% 5% 11% 22% 22% 4	2100
	21st to end	Day	43% 8% 10% 18% 18% 3	1980
October	1st to 10th	Day	42% 6% 12% 21% 17% 2	1858
	11th to 20th	Day	36% 4 11% 25% 22% 3	1800
	21st to end	Day	40% 6% 10% 23% 18% 3	1890
November	1st to 10th	Day	36% 3 13% 27% 17% 4	1671
	11th to 20th	Day	38% 4 12% 20% 23% 3	1710
	21st to end	Day	38% 5% 12% 29% 14% 3	1665
December	1st to 10th	Day	42% 6% 16% 21% 13% 1	1520
	11th to 20th	Day	41% 6% 10% 27% 14% 2	1906
	21st to end	Day	40% 9% 14% 23% 13% 1	2772
January	1st to 10th	Night	38% 5% 9% 29% 15% 4	4429
	11th to 20th	Night	43% 5% 11% 24% 15% 3	4404
	21st to end	Night	48% 5% 8% 24% 13% 2	4532
February	1st to 10th	Night	43% 5% 13% 23% 14% 2	2452
	11th to 20th	Night	45% 4 9% 19% 17% 6%	2155
	21st to end	Night	43% 4 9% 21% 17% 5%	1776
March	1st to 10th	Night	36% 5% 12% 24% 19% 4	2160
	11th to 20th	Night	43% 6% 9% 21% 17% 5%	2022
	21st to end	Night	46% 4 8% 18% 17% 6%	2022
April	1st to 10th	Night	44% 5% 10% 19% 17% 5%	1800
	11th to 20th	Night	41% 5% 10% 20% 20% 4	1775
	21st to end	Night	42% 7% 10% 17% 19% 5%	1800
May	1st to 10th	Night	48% 7% 10% 18% 15% 2	1666
	11th to 20th	Night	46% 7% 11% 17% 17% 3	1474
	21st to end	Night	46% 8% 15% 18% 12% 1	1551
June	1st to 10th	Night	42% 8% 12% 19% 18% 1	1440
	11th to 20th	Night	40% 9% 13% 23% 13% 2	1440
	21st to end	Night	35% 9% 15% 24% 15% 2	1440
July	1st to 10th	Night	41% 11% 16% 20% 10% 3	1400
	11th to 20th	Night	34% 9% 17% 24% 13% 1	1392
	21st to end	Night	34% 9% 16% 25% 14% 1	1496
August	1st to 10th	Night	36% 10% 17% 22% 14% 1	1635
	11th to 20th	Night	36% 10% 16% 21% 15% 2	1774
	21st to end	Night	37% 9% 15% 20% 17% 3	1980
September	1st to 10th	Night	40% 6% 13% 20% 17% 4	1800
	11th to 20th	Night	30% 7% 13% 24% 22% 4	1858
	21st to end	Night	43% 7% 9% 19% 19% 2	1980
October	1st to 10th	Night	42% 7% 13% 20% 17% 2	2098
	11th to 20th	Night	35% 5% 13% 27% 17% 3	2160
	21st to end	Night	39% 5% 10% 24% 19% 2	2465
November	1st to 10th	Night	33% 5% 12% 26% 19% 5%	2407
	11th to 20th	Night	40% 5% 9% 20% 21% 5%	2470
	21st to end	Night	35% 5% 13% 26% 18% 2	2512
December	1st to 10th	Night	45% 4 12% 23% 14% 2	2660
	11th to 20th	Night	39% 6% 13% 20% 18% 3	3288
	21st to end	Night	42% 8% 15% 24% 10% 1	4884

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	17% 20% 35% 22% 5% 2	180
	11th to 20th	Day	18% 28% 30% 21% 3	191
	21st to end	Day	21% 16% 38% 20% 4 2	297
February	1st to 10th	Day	20% 16% 43% 15% 5% 1	286
	11th to 20th	Day	24% 19% 39% 16% 2	432
	21st to end	Day	23% 21% 35% 18% 3	346
March	1st to 10th	Day	17% 16% 40% 24% 4	420
	11th to 20th	Day	18% 17% 39% 19% 4 2	420
	21st to end	Day	23% 17% 31% 24% 5%	462
April	1st to 10th	Day	20% 15% 39% 18% 6% 2	420
	11th to 20th	Day	16% 13% 35% 27% 8% 1	412
	21st to end	Day	25% 11% 34% 23% 6% 1	420
May	1st to 10th	Day	21% 12% 36% 21% 8% 2	423
	11th to 20th	Day	21% 7% 34% 28% 9% 1	447
	21st to end	Day	19% 3 20% 32% 21% 4	485
June	1st to 10th	Day	13% 6% 28% 32% 18% 4	450
	11th to 20th	Day	13% 5% 28% 37% 13% 4	450
	21st to end	Day	12% 2 25% 37% 20% 4	450
July	1st to 10th	Day	11% 4 18% 33% 27% 8%	445
	11th to 20th	Day	5% 2 16% 42% 26% 9%	444
	21st to end	Day	10% 3 19% 40% 22% 5%	488
August	1st to 10th	Day	12% 2 17% 35% 27% 7%	446
	11th to 20th	Day	13% 3 13% 35% 28% 8%	439
	21st to end	Day	10% 5% 18% 35% 24% 9%	495
September	1st to 10th	Day	17% 5% 19% 26% 22% 12%	450
	11th to 20th	Day	12% 4 20% 32% 23% 8%	450
	21st to end	Day	22% 6% 26% 30% 11% 5%	450
October	1st to 10th	Day	25% 6% 19% 28% 16% 6%	450
	11th to 20th	Day	18% 9% 25% 27% 18% 3	450
	21st to end	Day	19% 10% 32% 25% 12% 2	450
November	1st to 10th	Day	15% 10% 32% 24% 16% 3	365
	11th to 20th	Day	16% 17% 34% 24% 9%	370
	21st to end	Day	17% 14% 31% 26% 11% 1	325
December	1st to 10th	Day	21% 16% 40% 15% 7% 1	180
	11th to 20th	Day	23% 19% 34% 19% 4 1	180
	21st to end	Day	22% 17% 28% 23% 9% 1	198
January	1st to 10th	Night	19% 17% 38% 22% 4	760
	11th to 20th	Night	23% 15% 33% 25% 4	760
	21st to end	Night	18% 15% 41% 24% 2	836
February	1st to 10th	Night	21% 14% 37% 26% 3	760
	11th to 20th	Night	22% 18% 42% 17% 1	722
	21st to end	Night	21% 18% 36% 23% 2	592
March	1st to 10th	Night	19% 14% 38% 26% 3	720
	11th to 20th	Night	22% 12% 37% 23% 6% 2	720
	21st to end	Night	21% 8% 39% 25% 6% 1	792
April	1st to 10th	Night	21% 14% 38% 22% 6%	720
	11th to 20th	Night	16% 11% 33% 33% 8%	711
	21st to end	Night	19% 8% 37% 29% 6% 1	720
May	1st to 10th	Night	19% 8% 33% 30% 10%	586
	11th to 20th	Night	19% 7% 33% 32% 9%	412
	21st to end	Night	20% 3 18% 35% 22% 2	389
June	1st to 10th	Night	18% 5% 19% 38% 18% 1	360
	11th to 20th	Night	11% 3 19% 46% 20% 1	360
	21st to end	Night	9% 2 18% 51% 19% 1	360
July	1st to 10th	Night	9% 3 17% 43% 24% 3	350
	11th to 20th	Night	4 3 17% 42% 31% 3	348
	21st to end	Night	9% 1 19% 44% 26% 1	404
August	1st to 10th	Night	9% 3 10% 44% 30% 3	572
	11th to 20th	Night	11% 3 15% 38% 30% 4	710
	21st to end	Night	13% 2 14% 38% 28% 5%	792
September	1st to 10th	Night	17% 4 16% 34% 24% 5%	720
	11th to 20th	Night	13% 3 18% 33% 27% 6%	720
	21st to end	Night	23% 4 21% 31% 20% 1	720
October	1st to 10th	Night	21% 4 24% 27% 19% 5%	720
	11th to 20th	Night	18% 9% 24% 34% 15% 1	720
	21st to end	Night	20% 8% 34% 26% 10% 1	792
November	1st to 10th	Night	15% 9% 34% 27% 13% 1	740
	11th to 20th	Night	19% 12% 32% 28% 8%	760
	21st to end	Night	14% 14% 35% 27% 9% 1	760
December	1st to 10th	Night	21% 15% 34% 21% 8% 1	760
	11th to 20th	Night	19% 14% 39% 23% 6%	760
	21st to end	Night	17% 13% 38% 28% 4	836

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