Taylor Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Taylor Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	7% 4 1 4 3 3 3 3 4 6% 63%	124264
	Night	8% 6% 2 5% 3 3 3 3 4 6% 57%	123771
Transparency	Day	23% 4 7% 19% 25% 22%	124023
	Night	27% 5% 8% 17% 23% 20%	123600
Seeing	Day	12% 11% 24% 28% 19% 6%	16199
	Night	12% 7% 17% 30% 27% 6%	25418

By Month

Percentage Cloud Cover SW (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% 9% 2 6% 5% 5% 4 4 5% 8% 41%	3841
	11th to 20th	Day	16% 8% 3 5% 3 3 2 2 3 4 51%	4100
	21st to end	Day	18% 7% 3 5% 3 3 3 3 4 6% 47%	4510
February	1st to 10th	Day	15% 9% 2 6% 4 3 4 4 5% 5% 44%	1800
	11th to 20th	Day	12% 9% 2 6% 5% 4 4 4 6% 6% 42%	1775
	21st to end	Day	16% 7% 2 5% 4 4 3 4 5% 7% 43%	1650
March	1st to 10th	Day	16% 13% 3 7% 4 4 3 3 4 5% 37%	4942
	11th to 20th	Day	10% 7% 2 6% 5% 4 4 4 5% 7% 46%	5000
	21st to end	Day	12% 9% 2 7% 5% 5% 5% 4 6% 8% 35%	5517
April	1st to 10th	Day	11% 9% 2 6% 5% 5% 3 3 5% 9% 42%	2400
	11th to 20th	Day	8% 6% 2 4 5% 4 4 4 5% 7% 53%	2369
	21st to end	Day	4 5% 2 5% 4 3 3 4 6% 8% 57%	2400
May	1st to 10th	Day	3 2 1 2 2 1 2 2 4 6% 75%	5868
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 5% 78%	6005
	21st to end	Day	2 1 2 2 2 2 2 4 6% 78%	6902
June	1st to 10th	Day	4 4 1 4 3 3 3 3 5% 8% 63%	2800
	11th to 20th	Day	1 1 2 2 2 2 2 5% 6% 77%	2800
	21st to end	Day	2 1 1 2 1 1 2 3 4 84%	2800
July	1st to 10th	Day	1 1 1 1 1 1 1 2 91%	4560
	11th to 20th	Day	1 1 1 1 1 1 2 4 87%	4309
	21st to end	Day	1 1 1 1 1 2 3 90%	4456
August	1st to 10th	Day	1 1 1 1 2 2 3 5% 6% 78%	4091
	11th to 20th	Day	1 1 1 1 1 2 4 89%	3943
	21st to end	Day	1 1 2 1 1 1 1 2 4 86%	4180
September	1st to 10th	Day	1 1 1 1 2 1 3 4 85%	2280
	11th to 20th	Day	2 2 1 2 3 2 2 2 4 6% 73%	2264
	21st to end	Day	2 1 2 2 1 2 2 4 5% 79%	2005
October	1st to 10th	Day	3 2 1 2 1 2 2 2 3 6% 76%	3180
	11th to 20th	Day	6% 2 1 3 2 2 2 2 4 7% 69%	3161
	21st to end	Day	6% 2 1 3 4 4 5% 4 6% 7% 58%	3448
November	1st to 10th	Day	9% 4 2 4 4 3 4 3 5% 8% 54%	1924
	11th to 20th	Day	10% 7% 2 5% 5% 4 4 4 6% 7% 47%	1732
	21st to end	Day	10% 6% 2 5% 4 3 4 3 6% 6% 51%	1600
December	1st to 10th	Day	8% 5% 2 4 3 3 3 4 6% 10% 52%	2620
	11th to 20th	Day	22% 12% 3 7% 4 4 5% 4 6% 7% 27%	3028
	21st to end	Day	18% 10% 3 7% 6% 6% 6% 5% 7% 7% 26%	4004
January	1st to 10th	Night	11% 9% 2 6% 4 4 5% 4 6% 9% 39%	6111
	11th to 20th	Night	14% 9% 2 7% 4 3 3 2 3 4 50%	5880
	21st to end	Night	17% 9% 3 6% 3 3 3 2 3 5% 45%	6468
February	1st to 10th	Night	12% 7% 2 5% 5% 4 4 4 5% 5% 46%	2600
	11th to 20th	Night	12% 8% 3 7% 4 4 5% 4 5% 7% 42%	2510
	21st to end	Night	11% 7% 1 5% 5% 3 4 3 5% 6% 49%	1980
March	1st to 10th	Night	14% 16% 5% 8% 4 4 3 3 4 6% 34%	4949
	11th to 20th	Night	12% 9% 3 5% 3 4 4 3 5% 6% 47%	5000
	21st to end	Night	11% 13% 4 8% 5% 5% 5% 4 5% 6% 34%	5456
April	1st to 10th	Night	8% 7% 2 7% 4 4 4 3 5% 7% 48%	2000
	11th to 20th	Night	8% 4 1 6% 4 4 4 4 5% 8% 53%	1979
	21st to end	Night	4 4 2 3 4 3 3 3 5% 8% 61%	2000
May	1st to 10th	Night	3 4 1 3 2 2 2 2 3 5% 73%	4110
	11th to 20th	Night	2 2 1 3 3 3 2 2 3 4 75%	3899
	21st to end	Night	1 1 1 2 1 1 2 2 3 4 82%	3972
June	1st to 10th	Night	4 4 1 4 3 3 2 3 5% 8% 63%	1600
	11th to 20th	Night	1 1 1 1 2 2 2 3 4 82%	1600
	21st to end	Night	2 1 1 3 2 2 2 2 3 4 81%	1600
July	1st to 10th	Night	1 1 1 2 2 92%	2620
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 86%	2874
	21st to end	Night	1 1 1 1 2 3 89%	3109
August	1st to 10th	Night	2 1 1 2 2 2 2 3 5% 79%	2848
	11th to 20th	Night	1 1 1 1 2 2 3 88%	2898
	21st to end	Night	1 1 2 1 1 1 1 2 3 85%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 4 86%	1900
	11th to 20th	Night	2 2 1 4 2 2 2 2 5% 6% 72%	1912
	21st to end	Night	2 1 2 1 2 3 3 5% 5% 77%	2174
October	1st to 10th	Night	2 2 1 3 2 2 2 2 3 4 78%	3800
	11th to 20th	Night	3 2 2 4 3 3 4 2 5% 5% 67%	3777
	21st to end	Night	6% 3 1 4 3 3 5% 4 6% 7% 59%	4121
November	1st to 10th	Night	8% 7% 1 5% 4 4 4 4 6% 7% 50%	2373
	11th to 20th	Night	10% 5% 2 5% 5% 4 5% 4 7% 7% 47%	2665
	21st to end	Night	8% 6% 2 6% 5% 4 4 3 5% 6% 53%	2800
December	1st to 10th	Night	10% 5% 2 4 3 3 4 3 5% 7% 54%	4560
	11th to 20th	Night	23% 10% 3 7% 5% 5% 4 5% 6% 8% 25%	5176
	21st to end	Night	20% 10% 4 10% 5% 6% 5% 3 4 5% 29%	6952

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	41% 7% 12% 20% 12% 8%	3841
	11th to 20th	Day	38% 4 6% 10% 25% 17%	4100
	21st to end	Day	40% 3 6% 13% 14% 24%	4510
February	1st to 10th	Day	42% 5% 7% 18% 19% 10%	1800
	11th to 20th	Day	41% 5% 8% 19% 19% 8%	1784
	21st to end	Day	39% 6% 9% 22% 16% 7%	1650
March	1st to 10th	Day	49% 5% 6% 18% 17% 4	4942
	11th to 20th	Day	36% 7% 11% 25% 13% 8%	5000
	21st to end	Day	44% 9% 13% 22% 11% 2	5517
April	1st to 10th	Day	40% 5% 11% 23% 16% 4	2400
	11th to 20th	Day	30% 6% 13% 23% 21% 8%	2372
	21st to end	Day	23% 7% 13% 29% 22% 6%	2400
May	1st to 10th	Day	12% 3 9% 25% 32% 19%	5868
	11th to 20th	Day	10% 3 6% 29% 39% 12%	6008
	21st to end	Day	9% 3 7% 35% 41% 5%	6904
June	1st to 10th	Day	20% 4 11% 29% 28% 8%	2800
	11th to 20th	Day	9% 3 9% 23% 35% 21%	2800
	21st to end	Day	8% 2 3 16% 37% 34%	2800
July	1st to 10th	Day	3 1 2 7% 19% 68%	4491
	11th to 20th	Day	5% 2 3 6% 19% 65%	4237
	21st to end	Day	4 1 1 7% 25% 62%	4316
August	1st to 10th	Day	8% 2 3 11% 25% 51%	4076
	11th to 20th	Day	3 1 2 12% 43% 39%	3982
	21st to end	Day	5% 2 4 12% 39% 38%	4179
September	1st to 10th	Day	5% 2 2 9% 30% 51%	2280
	11th to 20th	Day	12% 4 5% 25% 37% 16%	2264
	21st to end	Day	10% 3 5% 20% 33% 28%	2005
October	1st to 10th	Day	13% 2 3 12% 25% 44%	3180
	11th to 20th	Day	16% 4 5% 25% 31% 19%	3161
	21st to end	Day	23% 6% 9% 20% 30% 13%	3448
November	1st to 10th	Day	28% 7% 7% 26% 19% 13%	1924
	11th to 20th	Day	35% 5% 11% 21% 19% 9%	1732
	21st to end	Day	32% 6% 10% 18% 18% 16%	1600
December	1st to 10th	Day	28% 5% 6% 20% 30% 12%	2620
	11th to 20th	Day	55% 6% 7% 15% 15% 3	3028
	21st to end	Day	52% 8% 10% 17% 10% 3	4004
January	1st to 10th	Night	38% 7% 19% 17% 14% 5%	6111
	11th to 20th	Night	39% 4 6% 14% 23% 14%	5880
	21st to end	Night	41% 5% 6% 9% 18% 22%	6468
February	1st to 10th	Night	39% 6% 7% 17% 20% 12%	2600
	11th to 20th	Night	38% 6% 8% 20% 19% 8%	2512
	21st to end	Night	32% 6% 12% 24% 20% 6%	1980
March	1st to 10th	Night	46% 8% 9% 16% 15% 4	4949
	11th to 20th	Night	33% 8% 8% 24% 19% 8%	5000
	21st to end	Night	43% 10% 13% 19% 12% 2	5456
April	1st to 10th	Night	34% 7% 13% 23% 18% 5%	2000
	11th to 20th	Night	27% 7% 13% 25% 19% 9%	1979
	21st to end	Night	21% 6% 12% 31% 25% 6%	2000
May	1st to 10th	Night	14% 4 4 24% 34% 20%	4107
	11th to 20th	Night	13% 3 6% 26% 34% 18%	3900
	21st to end	Night	7% 3 8% 31% 43% 8%	3972
June	1st to 10th	Night	19% 5% 10% 24% 33% 9%	1600
	11th to 20th	Night	8% 3 8% 21% 37% 24%	1600
	21st to end	Night	10% 3 3 13% 34% 37%	1600
July	1st to 10th	Night	3 1 1 6% 18% 71%	2580
	11th to 20th	Night	5% 1 3 6% 16% 69%	2823
	21st to end	Night	3 1 1 6% 23% 64%	3011
August	1st to 10th	Night	9% 2 2 7% 25% 55%	2839
	11th to 20th	Night	4 1 2 7% 42% 44%	2925
	21st to end	Night	7% 2 3 11% 36% 41%	3498
September	1st to 10th	Night	3 1 3 10% 27% 55%	1900
	11th to 20th	Night	13% 3 5% 19% 37% 24%	1912
	21st to end	Night	8% 4 4 22% 28% 34%	2174
October	1st to 10th	Night	13% 2 4 11% 22% 48%	3800
	11th to 20th	Night	15% 5% 8% 24% 28% 20%	3777
	21st to end	Night	21% 6% 7% 22% 25% 20%	4121
November	1st to 10th	Night	30% 7% 10% 21% 18% 14%	2373
	11th to 20th	Night	31% 6% 15% 19% 20% 10%	2665
	21st to end	Night	33% 5% 8% 18% 22% 15%	2800
December	1st to 10th	Night	28% 5% 5% 21% 26% 15%	4560
	11th to 20th	Night	52% 7% 7% 14% 16% 4	5176
	21st to end	Night	55% 6% 10% 16% 10% 3	6952

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	34% 15% 20% 16% 13% 1	285
	11th to 20th	Day	24% 14% 24% 22% 14% 2	400
	21st to end	Day	19% 13% 24% 23% 16% 4	440
February	1st to 10th	Day	26% 13% 23% 20% 16% 2	400
	11th to 20th	Day	26% 19% 21% 21% 11% 1	405
	21st to end	Day	20% 17% 22% 28% 12% 1	414
March	1st to 10th	Day	29% 19% 23% 19% 8% 2	500
	11th to 20th	Day	26% 15% 26% 20% 11% 3	500
	21st to end	Day	23% 18% 31% 20% 8% 1	550
April	1st to 10th	Day	20% 18% 30% 20% 10% 3	500
	11th to 20th	Day	16% 20% 28% 20% 13% 3	492
	21st to end	Day	11% 18% 32% 21% 13% 5%	500
May	1st to 10th	Day	12% 17% 29% 25% 13% 4	503
	11th to 20th	Day	12% 14% 31% 22% 14% 6%	528
	21st to end	Day	8% 12% 25% 29% 19% 6%	573
June	1st to 10th	Day	7% 13% 27% 29% 16% 7%	530
	11th to 20th	Day	6% 12% 33% 28% 17% 4	530
	21st to end	Day	2 6% 27% 33% 23% 8%	530
July	1st to 10th	Day	2 3 22% 41% 25% 7%	526
	11th to 20th	Day	1 1 18% 41% 29% 11%	524
	21st to end	Day	2 12% 45% 27% 14%	532
August	1st to 10th	Day	1 2 21% 39% 27% 9%	487
	11th to 20th	Day	2 3 22% 36% 25% 11%	477
	21st to end	Day	2 3 16% 40% 28% 11%	539
September	1st to 10th	Day	1 3 20% 41% 28% 8%	490
	11th to 20th	Day	5% 6% 23% 34% 25% 7%	490
	21st to end	Day	3 8% 22% 35% 24% 8%	490
October	1st to 10th	Day	7% 9% 24% 28% 24% 8%	490
	11th to 20th	Day	10% 7% 19% 26% 28% 10%	490
	21st to end	Day	12% 10% 22% 25% 23% 7%	539
November	1st to 10th	Day	13% 12% 20% 25% 24% 7%	460
	11th to 20th	Day	14% 9% 18% 25% 22% 13%	265
	21st to end	Day	16% 12% 24% 22% 18% 9%	200
December	1st to 10th	Day	22% 12% 19% 21% 16% 11%	200
	11th to 20th	Day	29% 13% 19% 19% 13% 8%	200
	21st to end	Day	26% 13% 22% 23% 13% 3	220
January	1st to 10th	Night	22% 10% 16% 28% 20% 5%	800
	11th to 20th	Night	22% 7% 17% 32% 16% 5%	800
	21st to end	Night	19% 7% 18% 29% 22% 5%	880
February	1st to 10th	Night	22% 9% 15% 32% 20% 3	800
	11th to 20th	Night	19% 10% 21% 28% 19% 4	784
	21st to end	Night	18% 12% 19% 31% 17% 2	660
March	1st to 10th	Night	24% 11% 19% 27% 17% 3	800
	11th to 20th	Night	21% 9% 20% 27% 19% 4	800
	21st to end	Night	21% 11% 25% 26% 14% 3	880
April	1st to 10th	Night	16% 12% 26% 25% 19% 3	800
	11th to 20th	Night	12% 14% 23% 27% 19% 4	791
	21st to end	Night	8% 14% 27% 26% 21% 4	800
May	1st to 10th	Night	8% 10% 27% 27% 24% 5%	610
	11th to 20th	Night	6% 7% 24% 30% 31% 3	521
	21st to end	Night	6% 10% 20% 30% 31% 3	433
June	1st to 10th	Night	7% 10% 21% 34% 26% 3	400
	11th to 20th	Night	3 8% 24% 35% 29% 2	400
	21st to end	Night	3 3 14% 40% 37% 4	400
July	1st to 10th	Night	2 3 14% 32% 43% 6%	390
	11th to 20th	Night	1 1 9% 29% 47% 13%	535
	21st to end	Night	1 9% 25% 51% 14%	624
August	1st to 10th	Night	2 1 9% 31% 42% 16%	567
	11th to 20th	Night	1 14% 30% 42% 13%	611
	21st to end	Night	1 2 8% 30% 49% 10%	836
September	1st to 10th	Night	1 3 8% 34% 45% 9%	760
	11th to 20th	Night	4 4 13% 34% 35% 10%	760
	21st to end	Night	3 5% 15% 29% 37% 11%	760
October	1st to 10th	Night	6% 4 15% 31% 35% 9%	760
	11th to 20th	Night	7% 5% 13% 28% 39% 8%	760
	21st to end	Night	10% 6% 12% 30% 30% 12%	836
November	1st to 10th	Night	13% 7% 15% 27% 31% 8%	780
	11th to 20th	Night	13% 6% 18% 35% 24% 5%	800
	21st to end	Night	13% 9% 17% 32% 22% 7%	800
December	1st to 10th	Night	22% 7% 15% 29% 20% 7%	800
	11th to 20th	Night	27% 9% 15% 28% 17% 5%	800
	21st to end	Night	25% 10% 19% 29% 14% 3	880

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