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 2026-07-31.

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	15% 7% 2 6% 4 4 4 4 6% 8% 41%	138212
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	134815
Transparency	Day	39% 7% 12% 23% 16% 3	141767
	Night	40% 5% 11% 23% 17% 4	139348
Seeing	Day	18% 11% 29% 26% 12% 3	17337
	Night	18% 10% 30% 29% 12% 1	26530

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 5% 4 4 4 4 4 5% 48%	4661
	11th to 20th	Day	14% 7% 2 7% 4 4 3 3 3 4 49%	4726
	21st to end	Day	15% 8% 3 6% 4 4 4 3 4 5% 45%	5780
February	1st to 10th	Day	14% 8% 3 6% 4 4 4 3 6% 7% 40%	1908
	11th to 20th	Day	17% 8% 3 5% 5% 4 4 4 4 6% 40%	2048
	21st to end	Day	17% 10% 3 6% 3 3 5% 4 5% 5% 40%	1730
March	1st to 10th	Day	20% 6% 2 4 2 3 3 3 4 5% 47%	4576
	11th to 20th	Day	12% 8% 2 5% 4 4 4 3 5% 6% 46%	4985
	21st to end	Day	16% 7% 2 5% 4 4 4 3 5% 6% 44%	6000
April	1st to 10th	Day	17% 8% 2 5% 4 3 3 4 6% 7% 41%	2520
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 6% 7% 42%	2494
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 6% 7% 41%	2520
May	1st to 10th	Day	20% 8% 3 6% 4 5% 4 4 5% 7% 34%	7496
	11th to 20th	Day	21% 7% 2 5% 4 4 4 3 5% 6% 39%	8094
	21st to end	Day	23% 8% 2 6% 4 4 4 4 5% 7% 34%	9060
June	1st to 10th	Day	11% 7% 2 5% 4 5% 5% 4 7% 9% 40%	2940
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 8% 9% 41%	2940
	21st to end	Day	10% 5% 2 4 5% 4 6% 6% 9% 9% 41%	2940
July	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 5% 8% 12% 39%	6500
	11th to 20th	Day	11% 6% 3 7% 6% 6% 7% 5% 8% 11% 31%	6500
	21st to end	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 10% 39%	7025
August	1st to 10th	Day	16% 6% 2 6% 4 5% 5% 5% 8% 10% 33%	4137
	11th to 20th	Day	12% 5% 3 7% 4 4 5% 4 7% 11% 39%	3850
	21st to end	Day	9% 5% 2 6% 3 4 4 5% 7% 9% 46%	4312
September	1st to 10th	Day	14% 6% 1 6% 3 4 4 4 5% 8% 45%	2400
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 5% 7% 48%	2332
	21st to end	Day	19% 8% 3 6% 5% 4 5% 4 6% 6% 37%	2200
October	1st to 10th	Day	18% 6% 2 5% 4 4 3 3 5% 6% 45%	2064
	11th to 20th	Day	17% 5% 2 5% 4 2 3 3 5% 6% 50%	2000
	21st to end	Day	15% 6% 2 5% 3 3 4 3 5% 6% 47%	2100
November	1st to 10th	Day	13% 5% 2 5% 4 3 3 2 3 7% 52%	1851
	11th to 20th	Day	14% 5% 2 5% 4 3 3 4 5% 7% 48%	1890
	21st to end	Day	14% 6% 2 5% 4 4 4 3 4 7% 46%	1839
December	1st to 10th	Day	12% 5% 2 5% 3 3 4 3 6% 8% 48%	2680
	11th to 20th	Day	20% 8% 2 4 3 3 3 2 3 5% 46%	3066
	21st to end	Day	13% 7% 2 5% 3 3 4 4 7% 8% 43%	4048
January	1st to 10th	Night	12% 7% 3 5% 3 3 4 3 4 6% 49%	8234
	11th to 20th	Night	16% 7% 2 6% 3 3 3 2 4 4 51%	8157
	21st to end	Night	14% 8% 2 6% 4 3 3 3 4 6% 47%	8401
February	1st to 10th	Night	17% 8% 2 6% 5% 3 4 4 5% 6% 42%	2710
	11th to 20th	Night	18% 7% 3 6% 4 4 4 3 5% 4 42%	2458
	21st to end	Night	17% 7% 2 6% 4 3 4 3 5% 5% 44%	2076
March	1st to 10th	Night	18% 10% 2 5% 3 2 3 2 4 5% 45%	5484
	11th to 20th	Night	13% 12% 3 5% 3 3 4 3 4 5% 46%	5189
	21st to end	Night	16% 8% 3 5% 4 4 4 3 4 5% 44%	5172
April	1st to 10th	Night	17% 9% 2 6% 5% 3 3 3 4 6% 41%	2100
	11th to 20th	Night	13% 8% 2 6% 5% 5% 4 3 4 7% 44%	2075
	21st to end	Night	16% 6% 2 7% 4 3 4 4 6% 7% 42%	2100
May	1st to 10th	Night	17% 9% 3 8% 4 4 5% 4 6% 9% 33%	5389
	11th to 20th	Night	19% 7% 2 7% 4 4 4 4 6% 7% 38%	4821
	21st to end	Night	26% 9% 3 7% 4 4 4 3 4 6% 32%	5114
June	1st to 10th	Night	14% 6% 2 5% 5% 4 5% 4 5% 7% 42%	1680
	11th to 20th	Night	11% 7% 3 6% 6% 4 5% 4 6% 8% 40%	1680
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 8% 10% 40%	1680
July	1st to 10th	Night	9% 7% 3 9% 6% 6% 5% 5% 7% 9% 34%	3680
	11th to 20th	Night	12% 9% 3 8% 6% 5% 5% 5% 6% 11% 30%	3680
	21st to end	Night	10% 8% 3 7% 4 4 5% 4 7% 9% 38%	4043
August	1st to 10th	Night	18% 9% 3 6% 5% 4 5% 4 6% 9% 32%	2980
	11th to 20th	Night	11% 6% 3 7% 5% 4 4 4 6% 8% 44%	3197
	21st to end	Night	8% 4 2 5% 4 4 4 4 6% 7% 52%	3586
September	1st to 10th	Night	14% 6% 2 7% 4 3 4 3 5% 7% 45%	2000
	11th to 20th	Night	12% 5% 1 4 3 3 3 4 5% 8% 50%	2066
	21st to end	Night	20% 8% 2 6% 4 3 4 3 6% 6% 38%	2200
October	1st to 10th	Night	17% 5% 1 6% 4 4 4 4 5% 6% 45%	2332
	11th to 20th	Night	16% 4 2 5% 2 3 3 3 4 6% 51%	2400
	21st to end	Night	15% 6% 2 6% 3 3 4 3 4 6% 48%	2739
November	1st to 10th	Night	11% 5% 3 5% 3 3 4 3 5% 6% 53%	2667
	11th to 20th	Night	15% 8% 2 5% 4 3 3 2 4 5% 48%	2730
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 7% 47%	2777
December	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 5% 7% 50%	4720
	11th to 20th	Night	22% 6% 2 4 3 3 3 3 5% 7% 42%	5348
	21st to end	Night	14% 8% 3 6% 4 4 4 3 5% 6% 42%	7150

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	37% 4 8% 26% 22% 3	4662
	11th to 20th	Day	40% 3 7% 22% 20% 7%	4756
	21st to end	Day	41% 4 9% 30% 13% 3	5780
February	1st to 10th	Day	42% 5% 14% 24% 14% 2	1908
	11th to 20th	Day	45% 4 9% 24% 15% 3	2057
	21st to end	Day	45% 6% 9% 21% 14% 5%	1730
March	1st to 10th	Day	39% 3 9% 24% 17% 7%	4579
	11th to 20th	Day	36% 5% 11% 26% 17% 5%	4985
	21st to end	Day	40% 5% 10% 17% 21% 7%	6000
April	1st to 10th	Day	41% 8% 10% 19% 16% 6%	2520
	11th to 20th	Day	39% 6% 14% 22% 16% 3	2494
	21st to end	Day	41% 7% 11% 20% 17% 4	2520
May	1st to 10th	Day	49% 7% 14% 18% 12% 1	7507
	11th to 20th	Day	44% 6% 11% 25% 13% 1	8098
	21st to end	Day	48% 7% 11% 20% 12% 1	9061
June	1st to 10th	Day	36% 12% 13% 21% 16% 2	2940
	11th to 20th	Day	36% 11% 14% 24% 12% 3	2940
	21st to end	Day	34% 11% 14% 21% 18% 1	2940
July	1st to 10th	Day	35% 12% 19% 20% 12% 1	6430
	11th to 20th	Day	43% 11% 18% 20% 7% 1	6425
	21st to end	Day	37% 9% 12% 23% 18% 1	6827
August	1st to 10th	Day	42% 9% 19% 21% 9%	4124
	11th to 20th	Day	38% 8% 15% 27% 11% 1	3886
	21st to end	Day	31% 8% 13% 29% 17% 2	4312
September	1st to 10th	Day	36% 9% 13% 21% 20% 3	2400
	11th to 20th	Day	35% 5% 11% 24% 21% 3	2332
	21st to end	Day	46% 8% 11% 17% 16% 2	2200
October	1st to 10th	Day	35% 5% 9% 21% 26% 5%	3363
	11th to 20th	Day	26% 3 9% 24% 30% 8%	3240
	21st to end	Day	25% 4 13% 27% 24% 6%	3377
November	1st to 10th	Day	35% 3 13% 27% 18% 4	1851
	11th to 20th	Day	37% 4 12% 21% 23% 3	1890
	21st to end	Day	38% 4 12% 28% 14% 3	1839
December	1st to 10th	Day	33% 5% 19% 24% 16% 2	2680
	11th to 20th	Day	42% 4 10% 28% 12% 3	3066
	21st to end	Day	37% 7% 13% 23% 18% 2	4048
January	1st to 10th	Night	36% 4 11% 24% 18% 6%	8234
	11th to 20th	Night	38% 3 8% 22% 20% 8%	8202
	21st to end	Night	39% 3 9% 26% 19% 3	8401
February	1st to 10th	Night	44% 5% 12% 23% 14% 2	2710
	11th to 20th	Night	45% 4 9% 19% 17% 5%	2460
	21st to end	Night	43% 4 9% 22% 17% 5%	2076
March	1st to 10th	Night	42% 4 9% 22% 13% 10%	5494
	11th to 20th	Night	40% 5% 11% 24% 14% 6%	5190
	21st to end	Night	43% 5% 8% 16% 23% 5%	5172
April	1st to 10th	Night	45% 5% 10% 19% 17% 5%	2100
	11th to 20th	Night	42% 5% 10% 20% 19% 4	2075
	21st to end	Night	40% 6% 11% 18% 20% 5%	2100
May	1st to 10th	Night	47% 6% 13% 21% 10% 2	5395
	11th to 20th	Night	46% 6% 10% 23% 13% 1	4822
	21st to end	Night	52% 5% 11% 18% 13% 1	5115
June	1st to 10th	Night	40% 7% 12% 20% 19% 2	1680
	11th to 20th	Night	40% 9% 13% 23% 13% 2	1680
	21st to end	Night	36% 8% 15% 23% 15% 2	1680
July	1st to 10th	Night	45% 9% 13% 20% 11% 1	3640
	11th to 20th	Night	47% 9% 17% 19% 6% 1	3632
	21st to end	Night	40% 7% 15% 21% 16% 1	3926
August	1st to 10th	Night	48% 7% 17% 18% 8%	2969
	11th to 20th	Night	37% 7% 13% 31% 11% 1	3227
	21st to end	Night	30% 6% 11% 29% 21% 2	3586
September	1st to 10th	Night	39% 6% 12% 21% 18% 4	2000
	11th to 20th	Night	31% 7% 12% 25% 21% 3	2066
	21st to end	Night	45% 7% 10% 18% 17% 2	2200
October	1st to 10th	Night	36% 5% 11% 23% 21% 3	3813
	11th to 20th	Night	24% 4 12% 24% 29% 7%	3899
	21st to end	Night	25% 4 12% 30% 24% 5%	4412
November	1st to 10th	Night	32% 5% 11% 25% 20% 6%	2667
	11th to 20th	Night	39% 5% 9% 20% 21% 5%	2730
	21st to end	Night	36% 5% 12% 25% 19% 2	2777
December	1st to 10th	Night	35% 4 13% 28% 17% 3	4720
	11th to 20th	Night	42% 4 12% 22% 16% 4	5348
	21st to end	Night	41% 6% 13% 20% 17% 3	7150

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% 35% 19% 4 1	220
	11th to 20th	Day	23% 27% 30% 18% 2	234
	21st to end	Day	23% 19% 36% 17% 4 1	363
February	1st to 10th	Day	21% 18% 41% 14% 4 1	348
	11th to 20th	Day	27% 20% 37% 15% 1	536
	21st to end	Day	23% 23% 33% 17% 4	446
March	1st to 10th	Day	23% 17% 35% 21% 4	540
	11th to 20th	Day	20% 19% 37% 17% 4 2	540
	21st to end	Day	23% 19% 33% 21% 4	594
April	1st to 10th	Day	22% 16% 38% 16% 5% 2	540
	11th to 20th	Day	17% 17% 35% 24% 7% 1	532
	21st to end	Day	23% 11% 34% 24% 6% 1	540
May	1st to 10th	Day	23% 14% 35% 19% 7% 2	543
	11th to 20th	Day	21% 9% 34% 27% 8% 2	567
	21st to end	Day	23% 5% 20% 30% 18% 4	617
June	1st to 10th	Day	13% 7% 29% 31% 16% 3	570
	11th to 20th	Day	14% 8% 28% 35% 12% 4	570
	21st to end	Day	13% 4 27% 34% 19% 3	570
July	1st to 10th	Day	11% 4 21% 34% 23% 7%	565
	11th to 20th	Day	6% 3 20% 39% 24% 8%	564
	21st to end	Day	10% 5% 22% 38% 20% 5%	616
August	1st to 10th	Day	12% 3 21% 34% 24% 6%	526
	11th to 20th	Day	12% 4 17% 33% 25% 9%	519
	21st to end	Day	10% 6% 20% 34% 22% 9%	583
September	1st to 10th	Day	17% 6% 22% 25% 20% 10%	530
	11th to 20th	Day	13% 4 22% 32% 21% 7%	530
	21st to end	Day	24% 7% 26% 29% 9% 4	530
October	1st to 10th	Day	23% 6% 21% 27% 15% 7%	530
	11th to 20th	Day	19% 11% 26% 25% 16% 3	530
	21st to end	Day	18% 12% 32% 24% 13% 2	528
November	1st to 10th	Day	15% 13% 33% 23% 14% 3	425
	11th to 20th	Day	18% 17% 33% 23% 8%	430
	21st to end	Day	20% 15% 31% 23% 10% 1	379
December	1st to 10th	Day	22% 17% 39% 15% 6% 1	220
	11th to 20th	Day	24% 20% 34% 17% 4	220
	21st to end	Day	22% 18% 29% 21% 9% 1	242
January	1st to 10th	Night	20% 18% 37% 21% 4	840
	11th to 20th	Night	23% 18% 33% 23% 4	840
	21st to end	Night	19% 16% 41% 22% 2	924
February	1st to 10th	Night	21% 13% 38% 25% 2	840
	11th to 20th	Night	24% 18% 40% 17% 1	823
	21st to end	Night	21% 18% 36% 23% 3	692
March	1st to 10th	Night	20% 16% 35% 25% 4	840
	11th to 20th	Night	21% 13% 37% 22% 5% 1	840
	21st to end	Night	21% 10% 40% 23% 5% 1	924
April	1st to 10th	Night	21% 14% 38% 22% 5%	840
	11th to 20th	Night	16% 12% 34% 31% 7%	831
	21st to end	Night	18% 8% 37% 30% 7%	840
May	1st to 10th	Night	18% 10% 33% 29% 9%	682
	11th to 20th	Night	18% 8% 34% 31% 8%	481
	21st to end	Night	21% 4 20% 33% 20% 2	455
June	1st to 10th	Night	17% 6% 22% 38% 17% 1	420
	11th to 20th	Night	13% 4 20% 45% 18%	420
	21st to end	Night	10% 3 21% 48% 17% 1	420
July	1st to 10th	Night	10% 3 17% 44% 23% 3	410
	11th to 20th	Night	6% 2 19% 41% 29% 3	408
	21st to end	Night	9% 2 19% 43% 25% 1	470
August	1st to 10th	Night	11% 3 11% 43% 29% 3	636
	11th to 20th	Night	12% 3 15% 37% 29% 4	790
	21st to end	Night	13% 1 15% 39% 27% 4	880
September	1st to 10th	Night	16% 5% 18% 34% 23% 4	800
	11th to 20th	Night	14% 3 17% 34% 27% 5%	800
	21st to end	Night	25% 4 20% 31% 19% 1	800
October	1st to 10th	Night	20% 5% 24% 26% 20% 5%	800
	11th to 20th	Night	18% 11% 24% 32% 15% 1	800
	21st to end	Night	20% 8% 34% 27% 10% 1	880
November	1st to 10th	Night	14% 10% 34% 28% 13% 1	820
	11th to 20th	Night	20% 14% 33% 26% 8%	840
	21st to end	Night	14% 16% 34% 27% 8% 1	840
December	1st to 10th	Night	20% 17% 34% 21% 7% 1	840
	11th to 20th	Night	20% 14% 38% 23% 5%	840
	21st to end	Night	18% 14% 37% 27% 4	924

Page generated at 2026-07-28 03:25:25. Version ac07

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