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 2025-03-27.

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% 7% 2 6% 4 4 4 4 6% 7% 41%	108881
	Night	15% 7% 2 6% 4 3 4 3 5% 6% 43%	110453
Transparency	Day	39% 7% 13% 23% 16% 3	112483
	Night	39% 6% 12% 23% 17% 4	115015
Seeing	Day	17% 11% 29% 27% 13% 4	15532
	Night	18% 10% 30% 29% 12% 1	24912

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	12% 7% 2 4 3 3 4 4 4 5% 52%	3597
	11th to 20th	Day	14% 7% 2 7% 4 5% 4 3 3 5% 46%	3632
	21st to end	Day	17% 10% 3 7% 4 4 3 3 3 4 42%	4466
February	1st to 10th	Day	14% 8% 3 6% 4 3 4 3 6% 7% 39%	1817
	11th to 20th	Day	17% 8% 3 5% 5% 3 4 4 4 6% 41%	1948
	21st to end	Day	16% 10% 3 6% 4 3 5% 4 5% 5% 40%	1650
March	1st to 10th	Day	25% 7% 2 5% 3 3 4 2 4 5% 40%	3216
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 7% 51%	3514
	21st to end	Day	20% 8% 2 5% 4 3 3 3 4 5% 43%	4164
April	1st to 10th	Day	17% 8% 2 6% 4 3 4 4 6% 7% 41%	2280
	11th to 20th	Day	13% 7% 2 6% 4 4 4 4 6% 7% 43%	2254
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 6% 7% 41%	2280
May	1st to 10th	Day	22% 10% 4 8% 5% 5% 4 3 5% 7% 27%	4032
	11th to 20th	Day	27% 8% 2 6% 4 5% 4 3 4 6% 31%	4326
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 10% 43%	4837
June	1st to 10th	Day	11% 7% 2 5% 4 5% 5% 5% 7% 9% 40%	2660
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 8% 9% 41%	2660
	21st to end	Day	9% 4 2 4 5% 5% 6% 6% 9% 10% 41%	2660
July	1st to 10th	Day	7% 5% 3 6% 4 5% 6% 6% 9% 12% 37%	4440
	11th to 20th	Day	11% 6% 2 5% 5% 5% 6% 5% 7% 10% 38%	4440
	21st to end	Day	10% 7% 3 8% 5% 6% 6% 5% 8% 10% 32%	4787
August	1st to 10th	Day	16% 6% 2 6% 4 5% 6% 5% 8% 10% 32%	4010
	11th to 20th	Day	11% 5% 3 7% 4 4 5% 4 7% 10% 39%	3730
	21st to end	Day	9% 5% 2 6% 3 4 4 5% 7% 9% 46%	4180
September	1st to 10th	Day	14% 6% 1 6% 3 4 4 4 6% 8% 44%	2280
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 5% 7% 48%	2216
	21st to end	Day	19% 8% 2 6% 4 3 5% 4 6% 6% 38%	2090
October	1st to 10th	Day	19% 6% 2 5% 4 4 4 3 4 6% 44%	1961
	11th to 20th	Day	16% 5% 2 5% 3 2 3 3 5% 6% 51%	1900
	21st to end	Day	15% 6% 2 5% 3 3 4 3 5% 5% 47%	1995
November	1st to 10th	Day	13% 5% 2 5% 4 3 3 2 4 7% 51%	1761
	11th to 20th	Day	15% 5% 2 5% 4 4 3 3 5% 6% 48%	1800
	21st to end	Day	14% 5% 2 5% 4 5% 4 3 4 7% 47%	1752
December	1st to 10th	Day	12% 5% 1 5% 3 3 5% 3 6% 8% 48%	2600
	11th to 20th	Day	20% 9% 2 4 3 3 3 2 3 4 46%	2986
	21st to end	Day	13% 7% 2 5% 3 3 4 4 7% 8% 43%	3960
January	1st to 10th	Night	13% 6% 3 5% 3 3 3 3 4 5% 52%	6349
	11th to 20th	Night	15% 7% 2 6% 3 3 3 3 4 4 51%	6251
	21st to end	Night	16% 8% 2 6% 4 3 3 3 5% 6% 44%	6490
February	1st to 10th	Night	17% 7% 2 6% 5% 3 4 4 5% 6% 42%	2581
	11th to 20th	Night	18% 7% 3 6% 4 4 4 3 5% 4 43%	2338
	21st to end	Night	18% 7% 2 6% 4 3 4 3 4 5% 44%	1980
March	1st to 10th	Night	21% 12% 3 6% 3 2 3 2 4 5% 38%	3844
	11th to 20th	Night	13% 6% 2 6% 4 3 4 3 4 6% 48%	3661
	21st to end	Night	18% 10% 3 5% 4 3 3 2 3 4 43%	3605
April	1st to 10th	Night	17% 9% 2 6% 5% 3 4 3 5% 6% 41%	1900
	11th to 20th	Night	13% 8% 2 6% 5% 5% 4 3 3 6% 44%	1875
	21st to end	Night	17% 6% 2 6% 4 3 4 4 6% 7% 41%	1900
May	1st to 10th	Night	19% 11% 4 10% 5% 4 4 3 5% 7% 28%	2908
	11th to 20th	Night	26% 8% 3 7% 4 4 4 3 5% 5% 31%	2595
	21st to end	Night	13% 9% 3 7% 4 4 4 4 5% 8% 39%	2738
June	1st to 10th	Night	15% 6% 2 5% 5% 4 5% 4 6% 7% 41%	1520
	11th to 20th	Night	10% 7% 3 6% 6% 4 5% 4 6% 8% 41%	1520
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 5% 8% 10% 41%	1520
July	1st to 10th	Night	7% 6% 4 9% 7% 6% 5% 4 8% 9% 35%	2520
	11th to 20th	Night	10% 7% 2 6% 5% 5% 4 4 6% 12% 38%	2520
	21st to end	Night	13% 9% 3 7% 5% 4 5% 4 6% 9% 33%	2762
August	1st to 10th	Night	18% 9% 3 6% 5% 4 5% 4 6% 9% 32%	2888
	11th to 20th	Night	10% 5% 3 7% 5% 4 5% 4 6% 8% 44%	3097
	21st to end	Night	8% 4 2 5% 4 4 4 4 6% 8% 52%	3476
September	1st to 10th	Night	14% 7% 2 7% 4 3 4 3 5% 7% 45%	1900
	11th to 20th	Night	12% 5% 1 4 3 3 3 4 5% 9% 50%	1962
	21st to end	Night	20% 8% 2 6% 4 3 4 3 6% 6% 38%	2090
October	1st to 10th	Night	18% 6% 1 6% 4 4 4 3 5% 6% 43%	2215
	11th to 20th	Night	15% 4 2 5% 2 3 3 3 5% 6% 52%	2280
	21st to end	Night	15% 6% 2 5% 3 3 4 3 4 6% 48%	2602
November	1st to 10th	Night	12% 5% 3 5% 3 3 3 3 5% 6% 52%	2537
	11th to 20th	Night	16% 8% 2 5% 4 3 4 2 4 5% 48%	2600
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 7% 47%	2645
December	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 5% 6% 51%	4580
	11th to 20th	Night	22% 6% 2 5% 3 3 3 3 5% 7% 42%	5208
	21st to end	Night	14% 8% 3 6% 4 4 4 4 5% 6% 42%	6996

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	34% 4 8% 25% 26% 3	3598
	11th to 20th	Day	41% 4 9% 23% 17% 7%	3662
	21st to end	Day	45% 5% 9% 25% 13% 3	4466
February	1st to 10th	Day	43% 5% 14% 24% 13% 1	1817
	11th to 20th	Day	45% 4 9% 24% 15% 3	1957
	21st to end	Day	44% 6% 10% 21% 14% 5%	1650
March	1st to 10th	Day	46% 4 9% 27% 11% 3	3219
	11th to 20th	Day	30% 6% 12% 30% 19% 3	3514
	21st to end	Day	44% 5% 8% 18% 20% 5%	4164
April	1st to 10th	Day	41% 8% 10% 20% 16% 6%	2280
	11th to 20th	Day	38% 6% 14% 22% 16% 4	2254
	21st to end	Day	41% 8% 11% 19% 17% 4	2280
May	1st to 10th	Day	55% 9% 11% 17% 8% 2	4046
	11th to 20th	Day	52% 9% 14% 17% 8% 1	4330
	21st to end	Day	34% 10% 17% 22% 16% 1	4837
June	1st to 10th	Day	37% 13% 14% 20% 14% 2	2660
	11th to 20th	Day	36% 12% 14% 24% 12% 2	2660
	21st to end	Day	33% 11% 15% 22% 18% 1	2660
July	1st to 10th	Day	35% 13% 20% 19% 11% 2	4370
	11th to 20th	Day	38% 10% 19% 23% 9% 1	4365
	21st to end	Day	43% 10% 14% 19% 12% 1	4634
August	1st to 10th	Day	43% 10% 19% 20% 8%	3997
	11th to 20th	Day	38% 8% 15% 27% 11% 1	3766
	21st to end	Day	31% 8% 13% 29% 17% 2	4180
September	1st to 10th	Day	36% 9% 13% 19% 20% 3	2280
	11th to 20th	Day	35% 5% 11% 23% 22% 3	2216
	21st to end	Day	45% 8% 10% 18% 17% 2	2090
October	1st to 10th	Day	35% 5% 9% 20% 25% 5%	3260
	11th to 20th	Day	25% 3 9% 24% 31% 8%	3140
	21st to end	Day	25% 4 13% 27% 24% 6%	3272
November	1st to 10th	Day	35% 3 14% 27% 18% 4	1761
	11th to 20th	Day	37% 4 12% 21% 23% 3	1800
	21st to end	Day	38% 4 13% 29% 14% 3	1752
December	1st to 10th	Day	33% 6% 19% 24% 16% 2	2600
	11th to 20th	Day	43% 4 10% 28% 12% 3	2986
	21st to end	Day	36% 7% 13% 23% 18% 2	3960
January	1st to 10th	Night	34% 4 7% 23% 23% 8%	6349
	11th to 20th	Night	38% 4 9% 24% 19% 7%	6296
	21st to end	Night	42% 4 10% 24% 18% 3	6490
February	1st to 10th	Night	44% 5% 13% 23% 14% 2	2581
	11th to 20th	Night	44% 4 9% 19% 17% 6%	2340
	21st to end	Night	43% 4 9% 21% 17% 6%	1980
March	1st to 10th	Night	48% 5% 9% 23% 13% 3	3854
	11th to 20th	Night	36% 6% 11% 26% 15% 5%	3662
	21st to end	Night	46% 4 7% 16% 22% 5%	3605
April	1st to 10th	Night	44% 5% 10% 19% 17% 5%	1900
	11th to 20th	Night	42% 5% 10% 20% 19% 4	1875
	21st to end	Night	41% 7% 11% 18% 20% 4	1900
May	1st to 10th	Night	54% 8% 9% 19% 9% 1	2914
	11th to 20th	Night	54% 8% 11% 17% 10% 1	2596
	21st to end	Night	40% 7% 17% 18% 15% 2	2739
June	1st to 10th	Night	41% 8% 12% 20% 18% 1	1520
	11th to 20th	Night	39% 9% 13% 25% 13% 2	1520
	21st to end	Night	35% 8% 15% 24% 15% 3	1520
July	1st to 10th	Night	44% 10% 16% 19% 9% 2	2480
	11th to 20th	Night	39% 8% 19% 25% 8% 1	2472
	21st to end	Night	47% 7% 16% 16% 13% 1	2674
August	1st to 10th	Night	48% 8% 18% 18% 8%	2877
	11th to 20th	Night	36% 8% 13% 31% 11% 1	3127
	21st to end	Night	30% 6% 11% 30% 21% 2	3476
September	1st to 10th	Night	40% 6% 13% 20% 18% 4	1900
	11th to 20th	Night	30% 7% 12% 25% 22% 4	1962
	21st to end	Night	45% 7% 10% 18% 18% 2	2090
October	1st to 10th	Night	37% 5% 11% 23% 21% 3	3696
	11th to 20th	Night	23% 4 12% 24% 30% 7%	3779
	21st to end	Night	25% 4 12% 30% 25% 5%	4275
November	1st to 10th	Night	32% 5% 11% 25% 20% 6%	2537
	11th to 20th	Night	39% 5% 9% 21% 21% 5%	2600
	21st to end	Night	36% 5% 13% 26% 19% 2	2645
December	1st to 10th	Night	35% 4 13% 28% 17% 3	4580
	11th to 20th	Night	42% 5% 12% 22% 15% 4	5208
	21st to end	Night	41% 6% 13% 20% 17% 3	6996

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	20% 21% 34% 20% 5% 2	200
	11th to 20th	Day	21% 27% 31% 19% 2	213
	21st to end	Day	22% 17% 38% 18% 4 2	330
February	1st to 10th	Day	21% 18% 42% 14% 4 1	317
	11th to 20th	Day	27% 20% 36% 16% 1	496
	21st to end	Day	22% 23% 34% 18% 4	414
March	1st to 10th	Day	21% 17% 36% 22% 4	500
	11th to 20th	Day	18% 19% 37% 18% 5% 2	500
	21st to end	Day	23% 17% 33% 22% 5%	530
April	1st to 10th	Day	21% 16% 38% 18% 6% 2	460
	11th to 20th	Day	16% 15% 35% 26% 7% 1	452
	21st to end	Day	23% 11% 34% 24% 7% 1	460
May	1st to 10th	Day	21% 13% 35% 21% 8% 2	463
	11th to 20th	Day	21% 8% 33% 29% 8% 1	487
	21st to end	Day	18% 3 20% 34% 20% 5%	529
June	1st to 10th	Day	12% 6% 29% 32% 18% 3	490
	11th to 20th	Day	12% 5% 28% 37% 14% 4	490
	21st to end	Day	12% 3 27% 36% 19% 3	490
July	1st to 10th	Day	10% 4 19% 34% 26% 7%	485
	11th to 20th	Day	6% 2 18% 40% 25% 9%	484
	21st to end	Day	10% 4 21% 39% 22% 5%	532
August	1st to 10th	Day	12% 3 19% 34% 25% 6%	486
	11th to 20th	Day	11% 4 16% 33% 27% 9%	479
	21st to end	Day	10% 6% 19% 34% 22% 9%	539
September	1st to 10th	Day	17% 6% 19% 27% 20% 11%	490
	11th to 20th	Day	12% 4 21% 33% 22% 8%	490
	21st to end	Day	25% 7% 26% 28% 10% 4	490
October	1st to 10th	Day	24% 7% 20% 28% 15% 7%	490
	11th to 20th	Day	17% 10% 27% 26% 17% 3	490
	21st to end	Day	18% 10% 31% 25% 13% 2	489
November	1st to 10th	Day	14% 11% 34% 23% 15% 3	395
	11th to 20th	Day	16% 17% 34% 24% 9%	400
	21st to end	Day	18% 16% 30% 24% 11% 1	352
December	1st to 10th	Day	21% 17% 41% 15% 7% 1	200
	11th to 20th	Day	24% 20% 34% 18% 4 1	200
	21st to end	Day	21% 17% 28% 23% 10% 1	220
January	1st to 10th	Night	20% 18% 37% 21% 4	800
	11th to 20th	Night	23% 16% 33% 24% 4	800
	21st to end	Night	18% 16% 41% 23% 2	880
February	1st to 10th	Night	21% 14% 38% 25% 3	800
	11th to 20th	Night	24% 18% 40% 17% 1	783
	21st to end	Night	21% 18% 36% 23% 3	660
March	1st to 10th	Night	20% 16% 36% 25% 4	800
	11th to 20th	Night	21% 13% 37% 22% 6% 1	800
	21st to end	Night	21% 10% 39% 23% 5% 1	863
April	1st to 10th	Night	21% 14% 38% 22% 5%	760
	11th to 20th	Night	15% 12% 33% 32% 7%	751
	21st to end	Night	18% 8% 37% 30% 7% 1	760
May	1st to 10th	Night	19% 8% 33% 30% 9%	618
	11th to 20th	Night	19% 7% 32% 32% 9%	435
	21st to end	Night	18% 3 19% 35% 22% 2	411
June	1st to 10th	Night	18% 6% 19% 39% 17% 1	380
	11th to 20th	Night	11% 3 20% 46% 20% 1	380
	21st to end	Night	9% 3 19% 50% 18% 1	380
July	1st to 10th	Night	9% 3 17% 43% 24% 3	370
	11th to 20th	Night	5% 2 18% 42% 30% 3	368
	21st to end	Night	9% 2 19% 44% 25% 1	427
August	1st to 10th	Night	11% 3 11% 44% 29% 3	604
	11th to 20th	Night	11% 3 16% 38% 29% 4	750
	21st to end	Night	13% 1 14% 39% 28% 5%	836
September	1st to 10th	Night	16% 4 18% 34% 23% 4	760
	11th to 20th	Night	14% 3 17% 33% 28% 5%	760
	21st to end	Night	25% 4 21% 30% 19% 1	760
October	1st to 10th	Night	21% 5% 24% 27% 19% 5%	760
	11th to 20th	Night	17% 10% 24% 32% 15% 1	760
	21st to end	Night	19% 8% 34% 28% 11% 1	836
November	1st to 10th	Night	14% 9% 35% 28% 13% 1	780
	11th to 20th	Night	19% 12% 33% 28% 8%	800
	21st to end	Night	14% 15% 35% 26% 8% 1	800
December	1st to 10th	Night	21% 16% 34% 21% 7% 1	800
	11th to 20th	Night	19% 13% 38% 24% 6%	800
	21st to end	Night	18% 12% 38% 28% 4	880

Page generated at 2025-03-24 09:04:25. Version ac07

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