Frosty Drew Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Frosty Drew Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	18% 7% 2 6% 4 4 4 3 5% 7% 39%	130513
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	130678
Transparency	Day	30% 36% 7% 14% 10% 2	134072
	Night	30% 35% 7% 14% 12% 2	135206
Seeing	Day	24% 17% 33% 18% 7% 2	16756
	Night	23% 13% 31% 24% 8% 1	25823

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% 8% 3 6% 4 4 4 3 5% 6% 45%	4661
	11th to 20th	Day	16% 8% 2 6% 3 3 3 3 4 4 47%	4650
	21st to end	Day	16% 9% 2 5% 3 3 3 2 4 5% 47%	5124
February	1st to 10th	Day	18% 7% 2 6% 5% 5% 4 3 6% 6% 39%	2035
	11th to 20th	Day	15% 7% 3 6% 5% 4 5% 4 5% 6% 41%	2048
	21st to end	Day	20% 9% 3 5% 4 3 3 3 4 7% 38%	1730
March	1st to 10th	Day	22% 10% 2 6% 3 3 2 2 3 6% 40%	4496
	11th to 20th	Day	13% 9% 2 5% 4 4 4 4 5% 7% 43%	4743
	21st to end	Day	23% 9% 2 5% 4 4 4 3 5% 6% 34%	5822
April	1st to 10th	Day	20% 7% 3 5% 4 4 4 3 5% 6% 39%	2400
	11th to 20th	Day	19% 7% 2 6% 4 4 4 3 4 7% 41%	2374
	21st to end	Day	20% 6% 2 6% 4 4 4 4 5% 7% 38%	2575
May	1st to 10th	Day	29% 10% 3 6% 4 3 4 3 5% 8% 26%	5886
	11th to 20th	Day	21% 9% 3 7% 5% 5% 4 3 4 6% 33%	5896
	21st to end	Day	21% 7% 3 7% 4 4 5% 4 5% 7% 33%	6949
June	1st to 10th	Day	19% 6% 2 6% 4 5% 4 4 6% 9% 35%	2800
	11th to 20th	Day	16% 7% 3 5% 4 4 5% 5% 6% 8% 37%	2800
	21st to end	Day	15% 6% 2 5% 4 4 5% 4 6% 10% 40%	2800
July	1st to 10th	Day	19% 8% 2 6% 4 4 4 3 6% 9% 34%	6360
	11th to 20th	Day	15% 7% 2 7% 4 4 5% 4 7% 11% 34%	6360
	21st to end	Day	18% 6% 2 6% 4 4 4 4 5% 7% 39%	6926
August	1st to 10th	Day	18% 8% 3 8% 5% 4 5% 4 5% 8% 33%	4245
	11th to 20th	Day	13% 6% 2 6% 4 4 6% 4 6% 8% 40%	4019
	21st to end	Day	7% 3 2 5% 4 5% 6% 5% 8% 10% 45%	4312
September	1st to 10th	Day	16% 6% 2 6% 3 3 5% 4 4 6% 45%	2400
	11th to 20th	Day	12% 6% 2 4 3 4 3 3 4 7% 52%	2204
	21st to end	Day	19% 7% 2 5% 4 4 5% 3 5% 6% 40%	2200
October	1st to 10th	Day	20% 7% 2 4 4 3 3 3 4 5% 45%	2200
	11th to 20th	Day	16% 5% 2 5% 3 3 4 3 5% 7% 47%	1980
	21st to end	Day	20% 5% 2 6% 4 4 3 2 5% 5% 45%	1980
November	1st to 10th	Day	15% 6% 2 6% 4 4 3 3 5% 4 49%	1851
	11th to 20th	Day	16% 5% 2 4 4 3 3 4 5% 5% 49%	1890
	21st to end	Day	18% 7% 3 5% 3 3 4 3 5% 5% 44%	1890
December	1st to 10th	Day	14% 7% 3 6% 3 3 4 3 4 6% 48%	2793
	11th to 20th	Day	21% 7% 2 4 3 2 2 2 4 5% 48%	3066
	21st to end	Day	21% 10% 3 6% 4 3 3 2 3 4 40%	4048
January	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 4 5% 50%	8234
	11th to 20th	Night	16% 9% 3 7% 4 4 3 3 4 5% 43%	8235
	21st to end	Night	15% 9% 3 6% 4 4 5% 4 5% 6% 40%	9057
February	1st to 10th	Night	15% 10% 3 7% 5% 4 3 3 4 6% 41%	2580
	11th to 20th	Night	19% 7% 2 5% 4 3 3 3 4 6% 42%	2458
	21st to end	Night	19% 9% 2 6% 3 5% 4 4 4 5% 40%	2076
March	1st to 10th	Night	23% 9% 1 4 3 3 4 3 5% 5% 40%	5389
	11th to 20th	Night	17% 10% 3 6% 4 3 4 3 4 5% 40%	5215
	21st to end	Night	22% 10% 2 6% 4 4 4 4 5% 5% 35%	5107
April	1st to 10th	Night	20% 9% 3 5% 4 4 4 3 4 6% 38%	2000
	11th to 20th	Night	19% 8% 2 6% 4 4 4 3 5% 6% 39%	1975
	21st to end	Night	22% 7% 2 6% 5% 3 3 3 4 6% 39%	1825
May	1st to 10th	Night	28% 11% 4 7% 4 4 4 3 4 6% 26%	4054
	11th to 20th	Night	21% 11% 3 8% 4 4 4 3 6% 7% 28%	4033
	21st to end	Night	22% 9% 3 7% 5% 5% 5% 4 5% 6% 29%	3933
June	1st to 10th	Night	19% 8% 2 7% 4 4 5% 4 5% 7% 36%	1600
	11th to 20th	Night	21% 7% 2 5% 4 4 3 3 6% 9% 37%	1600
	21st to end	Night	16% 8% 2 6% 4 4 3 4 6% 9% 38%	1600
July	1st to 10th	Night	22% 10% 4 8% 4 4 4 4 5% 7% 29%	3600
	11th to 20th	Night	21% 12% 3 6% 4 4 3 3 6% 8% 30%	3600
	21st to end	Night	16% 8% 2 6% 4 5% 4 4 5% 7% 40%	3926
August	1st to 10th	Night	24% 8% 2 7% 5% 4 5% 4 7% 6% 28%	2871
	11th to 20th	Night	9% 7% 3 7% 5% 5% 6% 5% 8% 9% 36%	3026
	21st to end	Night	9% 4 1 5% 3 4 3 3 6% 9% 53%	3586
September	1st to 10th	Night	15% 7% 2 7% 4 5% 4 4 5% 6% 41%	2000
	11th to 20th	Night	14% 6% 2 5% 3 2 3 2 4 6% 52%	2195
	21st to end	Night	21% 7% 3 6% 5% 5% 4 3 5% 4 38%	2200
October	1st to 10th	Night	18% 8% 2 5% 4 4 3 3 4 6% 42%	2200
	11th to 20th	Night	15% 5% 3 5% 3 3 3 4 5% 6% 48%	2416
	21st to end	Night	19% 7% 2 5% 3 4 3 3 5% 7% 42%	2860
November	1st to 10th	Night	13% 6% 2 6% 4 4 3 3 5% 6% 48%	2667
	11th to 20th	Night	17% 7% 2 5% 3 3 4 3 4 5% 47%	2730
	21st to end	Night	15% 6% 2 6% 4 3 4 3 4 7% 46%	2730
December	1st to 10th	Night	17% 7% 2 6% 3 3 3 2 3 4 49%	4602
	11th to 20th	Night	25% 8% 2 4 3 3 3 2 3 4 42%	5348
	21st to end	Night	18% 10% 3 6% 4 3 3 3 3 4 43%	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	33% 21% 6% 20% 17% 3	4662
	11th to 20th	Day	32% 21% 6% 18% 15% 8%	4680
	21st to end	Day	34% 21% 7% 18% 17% 1	5124
February	1st to 10th	Day	19% 59% 6% 8% 6% 1	2035
	11th to 20th	Day	20% 57% 3 9% 8% 2	2057
	21st to end	Day	18% 59% 6% 9% 6% 1	1730
March	1st to 10th	Day	37% 29% 6% 9% 9% 10%	4499
	11th to 20th	Day	29% 28% 8% 24% 9% 1	4743
	21st to end	Day	42% 29% 4 10% 11% 5%	5822
April	1st to 10th	Day	21% 61% 4 6% 6% 2	2400
	11th to 20th	Day	17% 62% 3 8% 9% 1	2374
	21st to end	Day	19% 62% 3 8% 6% 2	2575
May	1st to 10th	Day	45% 29% 12% 9% 4	5897
	11th to 20th	Day	42% 28% 11% 12% 7% 1	5900
	21st to end	Day	39% 28% 7% 17% 9%	6949
June	1st to 10th	Day	18% 63% 4 8% 7% 1	2800
	11th to 20th	Day	17% 62% 4 10% 6% 1	2800
	21st to end	Day	15% 61% 4 10% 8% 1	2800
July	1st to 10th	Day	38% 27% 9% 14% 10% 1	6290
	11th to 20th	Day	36% 26% 14% 18% 6% 1	6285
	21st to end	Day	34% 25% 7% 20% 12% 1	6728
August	1st to 10th	Day	36% 35% 10% 13% 5%	4231
	11th to 20th	Day	28% 37% 10% 21% 5%	4058
	21st to end	Day	19% 37% 10% 19% 13% 1	4312
September	1st to 10th	Day	17% 57% 4 13% 8% 1	2400
	11th to 20th	Day	16% 58% 4 10% 11% 1	2204
	21st to end	Day	22% 57% 4 8% 8%	2200
October	1st to 10th	Day	24% 36% 5% 16% 15% 5%	3590
	11th to 20th	Day	17% 35% 4 16% 25% 3	3209
	21st to end	Day	18% 35% 9% 20% 16% 2	3180
November	1st to 10th	Day	21% 53% 6% 9% 10% 1	1851
	11th to 20th	Day	16% 54% 4 10% 13% 3	1890
	21st to end	Day	18% 54% 4 15% 7% 2	1890
December	1st to 10th	Day	25% 34% 8% 21% 11%	2793
	11th to 20th	Day	28% 30% 8% 13% 16% 6%	3066
	21st to end	Day	38% 26% 3 17% 11% 4	4048
January	1st to 10th	Night	31% 20% 7% 25% 16% 3	8234
	11th to 20th	Night	36% 20% 6% 17% 15% 5%	8280
	21st to end	Night	39% 22% 6% 14% 15% 4	9057
February	1st to 10th	Night	19% 59% 5% 10% 6% 1	2580
	11th to 20th	Night	19% 57% 3 8% 11% 2	2460
	21st to end	Night	20% 59% 3 8% 7% 2	2076
March	1st to 10th	Night	37% 29% 6% 11% 10% 7%	5399
	11th to 20th	Night	34% 28% 12% 16% 8% 2	5216
	21st to end	Night	40% 28% 3 8% 17% 4	5107
April	1st to 10th	Night	21% 61% 3 7% 5% 2	2000
	11th to 20th	Night	17% 62% 5% 6% 8% 1	1975
	21st to end	Night	20% 61% 3 8% 7% 1	1825
May	1st to 10th	Night	46% 29% 11% 9% 5%	4060
	11th to 20th	Night	43% 29% 11% 11% 4 2	4034
	21st to end	Night	42% 28% 8% 16% 6%	3934
June	1st to 10th	Night	17% 62% 2 9% 8% 1	1600
	11th to 20th	Night	17% 63% 3 11% 6% 1	1600
	21st to end	Night	15% 60% 5% 12% 7% 1	1600
July	1st to 10th	Night	41% 27% 7% 16% 9%	3560
	11th to 20th	Night	41% 26% 11% 15% 6%	3552
	21st to end	Night	34% 25% 9% 20% 11% 1	3809
August	1st to 10th	Night	39% 37% 7% 11% 5% 1	2861
	11th to 20th	Night	30% 37% 9% 20% 4	3053
	21st to end	Night	14% 36% 11% 22% 16%	3586
September	1st to 10th	Night	18% 57% 5% 13% 7% 1	2000
	11th to 20th	Night	16% 57% 5% 9% 12% 1	2195
	21st to end	Night	25% 57% 3 8% 8%	2200
October	1st to 10th	Night	25% 36% 6% 18% 13% 2	3590
	11th to 20th	Night	12% 35% 4 16% 28% 4	3926
	21st to end	Night	18% 35% 7% 21% 17% 2	4610
November	1st to 10th	Night	19% 54% 6% 10% 10% 2	2667
	11th to 20th	Night	18% 54% 3 9% 12% 4	2730
	21st to end	Night	17% 54% 6% 15% 7% 2	2730
December	1st to 10th	Night	27% 34% 7% 20% 12% 1	4602
	11th to 20th	Night	34% 30% 7% 10% 17% 2	5348
	21st to end	Night	36% 26% 5% 15% 16% 4	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	27% 28% 29% 15% 1	220
	11th to 20th	Day	28% 33% 29% 7% 3	220
	21st to end	Day	29% 27% 33% 7% 3	242
February	1st to 10th	Day	27% 24% 36% 12% 1	487
	11th to 20th	Day	31% 26% 32% 9% 1	536
	21st to end	Day	28% 23% 34% 11% 3	446
March	1st to 10th	Day	29% 24% 30% 15% 2	508
	11th to 20th	Day	26% 26% 33% 12% 3 1	500
	21st to end	Day	35% 19% 30% 12% 3 1	550
April	1st to 10th	Day	33% 24% 26% 11% 5%	500
	11th to 20th	Day	30% 24% 30% 12% 3	492
	21st to end	Day	30% 18% 34% 12% 4 2	500
May	1st to 10th	Day	33% 20% 31% 12% 3	503
	11th to 20th	Day	24% 15% 44% 13% 3 1	527
	21st to end	Day	22% 11% 34% 26% 5% 2	573
June	1st to 10th	Day	20% 15% 36% 21% 7% 1	530
	11th to 20th	Day	20% 15% 37% 22% 6% 1	530
	21st to end	Day	19% 12% 37% 25% 6% 2	530
July	1st to 10th	Day	20% 7% 33% 27% 11% 1	525
	11th to 20th	Day	16% 8% 33% 29% 12% 2	524
	21st to end	Day	15% 12% 39% 25% 9% 1	576
August	1st to 10th	Day	15% 10% 33% 28% 10% 3	526
	11th to 20th	Day	17% 8% 32% 27% 11% 4	519
	21st to end	Day	11% 10% 35% 26% 13% 5%	583
September	1st to 10th	Day	20% 11% 28% 24% 14% 3	530
	11th to 20th	Day	15% 11% 31% 26% 12% 6%	530
	21st to end	Day	23% 9% 36% 20% 8% 3	530
October	1st to 10th	Day	24% 12% 28% 21% 11% 4	530
	11th to 20th	Day	22% 17% 31% 19% 10% 1	486
	21st to end	Day	27% 14% 32% 19% 7% 2	462
November	1st to 10th	Day	19% 19% 32% 21% 8% 1	425
	11th to 20th	Day	23% 25% 29% 17% 6%	430
	21st to end	Day	27% 20% 29% 17% 7% 1	430
December	1st to 10th	Day	25% 23% 33% 16% 4	294
	11th to 20th	Day	34% 25% 30% 7% 4	220
	21st to end	Day	29% 22% 32% 12% 4 2	242
January	1st to 10th	Night	22% 21% 35% 19% 3	840
	11th to 20th	Night	30% 19% 32% 16% 3	840
	21st to end	Night	23% 23% 32% 20% 2	924
February	1st to 10th	Night	22% 20% 35% 22% 2	840
	11th to 20th	Night	27% 20% 38% 13% 2	823
	21st to end	Night	26% 22% 33% 17% 2	692
March	1st to 10th	Night	25% 19% 34% 18% 3	809
	11th to 20th	Night	24% 17% 37% 20% 3 1	800
	21st to end	Night	26% 17% 35% 18% 3 1	880
April	1st to 10th	Night	27% 15% 33% 21% 3	800
	11th to 20th	Night	24% 15% 36% 20% 4	791
	21st to end	Night	24% 13% 36% 22% 4	625
May	1st to 10th	Night	28% 10% 32% 23% 6% 1	600
	11th to 20th	Night	22% 14% 38% 21% 4	592
	21st to end	Night	23% 6% 28% 34% 8% 1	440
June	1st to 10th	Night	20% 5% 29% 36% 10%	400
	11th to 20th	Night	24% 6% 28% 34% 10%	400
	21st to end	Night	21% 3 27% 40% 10%	400
July	1st to 10th	Night	20% 4 22% 36% 16% 1	390
	11th to 20th	Night	19% 3 20% 42% 15% 2	388
	21st to end	Night	16% 5% 23% 44% 12% 1	438
August	1st to 10th	Night	15% 4 29% 39% 13% 1	566
	11th to 20th	Night	15% 4 26% 36% 18% 2	681
	21st to end	Night	14% 5% 22% 37% 18% 3	880
September	1st to 10th	Night	18% 4 29% 31% 16% 2	800
	11th to 20th	Night	17% 5% 25% 32% 17% 4	800
	21st to end	Night	24% 6% 27% 27% 14% 2	800
October	1st to 10th	Night	22% 10% 28% 25% 14% 2	800
	11th to 20th	Night	19% 12% 33% 24% 10% 2	800
	21st to end	Night	24% 12% 34% 22% 8%	880
November	1st to 10th	Night	18% 14% 33% 23% 11% 1	820
	11th to 20th	Night	22% 19% 33% 19% 6%	840
	21st to end	Night	22% 19% 30% 22% 6% 1	840
December	1st to 10th	Night	27% 17% 31% 20% 4 1	840
	11th to 20th	Night	26% 19% 32% 18% 4	840
	21st to end	Night	25% 19% 35% 18% 3	924

Page generated at 2026-02-28 01:40:23. Version ac07

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