Nick Drew Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nick Drew Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	12% 7% 3 7% 4 4 4 4 6% 8% 41%	126731
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	123550
Transparency	Day	40% 5% 11% 25% 17% 2	130294
	Night	41% 5% 10% 24% 18% 3	128064
Seeing	Day	14% 9% 24% 26% 19% 7%	17007
	Night	16% 8% 27% 32% 15% 2	26774

By Month

Percentage Cloud Cover SE (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% 10% 6% 13% 5% 4 4 3 5% 6% 29%	3617
	11th to 20th	Day	10% 9% 3 8% 7% 8% 4 4 5% 6% 36%	3782
	21st to end	Day	18% 15% 5% 7% 4 3 3 2 4 4 34%	4488
February	1st to 10th	Day	16% 11% 4 7% 4 3 4 4 5% 6% 35%	1815
	11th to 20th	Day	16% 10% 3 7% 5% 5% 5% 4 5% 6% 35%	1948
	21st to end	Day	15% 11% 3 7% 5% 3 4 4 4 6% 39%	1650
March	1st to 10th	Day	16% 11% 2 5% 3 3 3 2 3 4 47%	4480
	11th to 20th	Day	8% 8% 2 4 3 3 4 4 6% 8% 49%	4932
	21st to end	Day	15% 8% 3 8% 5% 5% 5% 3 6% 8% 34%	5826
April	1st to 10th	Day	13% 7% 2 7% 5% 5% 5% 4 5% 8% 40%	2400
	11th to 20th	Day	17% 7% 3 7% 5% 4 3 3 4 5% 42%	2374
	21st to end	Day	16% 7% 3 7% 4 4 4 3 5% 7% 38%	2400
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 8% 11% 29%	5671
	11th to 20th	Day	15% 8% 3 7% 4 4 5% 4 6% 8% 37%	6169
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 5% 6% 8% 40%	6935
June	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 4 5% 9% 47%	2800
	11th to 20th	Day	8% 4 2 5% 4 5% 5% 4 6% 9% 47%	2800
	21st to end	Day	5% 5% 2 6% 5% 4 6% 5% 8% 10% 45%	2800
July	1st to 10th	Day	9% 4 2 6% 4 4 4 4 7% 10% 45%	6360
	11th to 20th	Day	5% 5% 3 8% 5% 6% 7% 6% 8% 11% 38%	6360
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	6746
August	1st to 10th	Day	5% 4 2 4 4 4 5% 4 6% 10% 51%	4154
	11th to 20th	Day	11% 5% 2 5% 4 5% 5% 5% 6% 10% 41%	3849
	21st to end	Day	5% 5% 1 5% 3 5% 4 4 7% 10% 52%	4312
September	1st to 10th	Day	12% 5% 3 4 4 3 3 2 5% 8% 50%	2400
	11th to 20th	Day	7% 5% 3 5% 4 4 4 3 5% 9% 51%	2345
	21st to end	Day	8% 5% 2 5% 4 4 5% 3 5% 7% 51%	2200
October	1st to 10th	Day	10% 4 1 5% 4 3 4 3 5% 6% 56%	2045
	11th to 20th	Day	12% 5% 2 5% 4 4 4 2 5% 6% 51%	2000
	21st to end	Day	18% 7% 2 6% 4 3 3 3 4 6% 46%	2133
November	1st to 10th	Day	11% 6% 2 4 3 3 4 3 4 7% 53%	1780
	11th to 20th	Day	14% 8% 2 5% 4 3 3 3 4 7% 46%	1800
	21st to end	Day	15% 11% 4 8% 4 4 3 2 5% 6% 40%	1760
December	1st to 10th	Day	9% 10% 3 6% 4 3 3 3 4 6% 50%	2610
	11th to 20th	Day	14% 7% 4 10% 5% 5% 5% 3 5% 5% 36%	3008
	21st to end	Day	26% 14% 5% 9% 5% 4 4 2 4 4 22%	3982
January	1st to 10th	Night	15% 11% 5% 11% 5% 4 4 3 5% 5% 32%	6348
	11th to 20th	Night	11% 11% 5% 12% 6% 5% 6% 3 3 4 34%	6194
	21st to end	Night	19% 12% 4 9% 4 3 4 2 4 5% 33%	6490
February	1st to 10th	Night	15% 11% 5% 8% 4 5% 5% 3 5% 7% 33%	2583
	11th to 20th	Night	15% 8% 3 9% 4 3 4 3 5% 5% 40%	2338
	21st to end	Night	16% 9% 3 7% 5% 3 3 3 4 5% 41%	1980
March	1st to 10th	Night	18% 12% 3 6% 4 3 3 3 4 4 41%	5373
	11th to 20th	Night	11% 8% 2 5% 3 3 3 3 4 7% 51%	5072
	21st to end	Night	17% 9% 3 8% 5% 4 5% 3 5% 6% 35%	5101
April	1st to 10th	Night	13% 7% 2 7% 5% 4 3 3 4 7% 45%	2000
	11th to 20th	Night	14% 7% 3 5% 4 3 4 3 5% 6% 45%	1975
	21st to end	Night	18% 9% 3 6% 5% 4 3 3 5% 6% 40%	2000
May	1st to 10th	Night	17% 8% 3 7% 5% 4 5% 4 6% 9% 32%	4224
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 7% 8% 42%	3736
	21st to end	Night	12% 7% 2 8% 4 5% 5% 4 6% 9% 38%	3942
June	1st to 10th	Night	7% 5% 2 5% 4 4 5% 4 6% 9% 49%	1600
	11th to 20th	Night	7% 4 1 5% 4 4 5% 4 6% 8% 53%	1600
	21st to end	Night	5% 4 2 5% 4 4 5% 4 7% 12% 50%	1600
July	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 6% 9% 48%	3618
	11th to 20th	Night	5% 4 2 6% 5% 5% 6% 4 7% 11% 45%	3617
	21st to end	Night	6% 5% 2 7% 5% 6% 6% 6% 8% 11% 39%	4120
August	1st to 10th	Night	4 4 2 4 3 3 4 5% 6% 10% 55%	2989
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 3 6% 7% 49%	3200
	21st to end	Night	5% 3 1 3 3 4 4 4 6% 10% 57%	3586
September	1st to 10th	Night	10% 4 2 5% 4 3 4 3 6% 7% 54%	2000
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 5% 7% 52%	2035
	21st to end	Night	9% 5% 2 5% 4 4 4 4 5% 6% 53%	2200
October	1st to 10th	Night	8% 4 2 4 3 3 4 3 4 7% 58%	2351
	11th to 20th	Night	13% 6% 2 4 2 2 3 2 4 7% 53%	2400
	21st to end	Night	18% 7% 2 6% 3 3 4 3 4 5% 45%	2705
November	1st to 10th	Night	10% 6% 2 4 3 3 4 3 5% 6% 54%	2563
	11th to 20th	Night	13% 7% 2 6% 4 4 3 3 4 7% 48%	2600
	21st to end	Night	18% 8% 3 5% 4 3 3 4 4 5% 43%	2635
December	1st to 10th	Night	12% 8% 3 7% 4 4 4 3 4 5% 46%	4580
	11th to 20th	Night	16% 7% 3 7% 4 4 4 3 5% 7% 41%	5199
	21st to end	Night	27% 16% 4 9% 5% 4 4 3 4 4 20%	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	56% 4 9% 20% 9% 1	3618
	11th to 20th	Day	48% 5% 12% 18% 14% 3	3782
	21st to end	Day	55% 3 7% 18% 15% 4	4488
February	1st to 10th	Day	49% 5% 13% 23% 10% 1	1815
	11th to 20th	Day	49% 5% 9% 20% 14% 3	1958
	21st to end	Day	47% 4 9% 22% 15% 3	1650
March	1st to 10th	Day	43% 3 5% 23% 24% 3	4480
	11th to 20th	Day	33% 4 10% 27% 21% 4	4932
	21st to end	Day	46% 6% 14% 25% 9% 1	5828
April	1st to 10th	Day	42% 5% 11% 25% 16% 1	2400
	11th to 20th	Day	45% 4 9% 24% 14% 3	2374
	21st to end	Day	45% 6% 12% 23% 12% 1	2400
May	1st to 10th	Day	47% 7% 14% 22% 10%	5706
	11th to 20th	Day	45% 6% 11% 24% 13% 1	6173
	21st to end	Day	41% 6% 10% 23% 19% 1	6949
June	1st to 10th	Day	32% 7% 14% 28% 16% 3	2800
	11th to 20th	Day	31% 8% 12% 24% 23% 3	2800
	21st to end	Day	31% 8% 17% 23% 17% 4	2800
July	1st to 10th	Day	34% 6% 10% 30% 18% 2	6291
	11th to 20th	Day	37% 8% 15% 25% 14% 1	6285
	21st to end	Day	36% 7% 17% 29% 10% 1	6553
August	1st to 10th	Day	27% 7% 14% 38% 15%	4140
	11th to 20th	Day	36% 7% 13% 25% 17%	3885
	21st to end	Day	27% 6% 15% 34% 15% 3	4311
September	1st to 10th	Day	33% 5% 11% 26% 21% 4	2400
	11th to 20th	Day	32% 5% 9% 26% 26% 3	2345
	21st to end	Day	31% 5% 9% 26% 25% 3	2200
October	1st to 10th	Day	19% 3 6% 20% 45% 8%	3331
	11th to 20th	Day	24% 3 8% 21% 37% 8%	3240
	21st to end	Day	33% 3 7% 27% 27% 3	3423
November	1st to 10th	Day	32% 4 12% 27% 21% 5%	1780
	11th to 20th	Day	40% 4 9% 23% 18% 6%	1800
	21st to end	Day	46% 4 9% 26% 13%	1760
December	1st to 10th	Day	37% 4 13% 30% 15%	2610
	11th to 20th	Day	50% 4 10% 23% 12% 2	3005
	21st to end	Day	67% 4 11% 12% 6% 1	3982
January	1st to 10th	Night	54% 4 10% 23% 7% 1	6348
	11th to 20th	Night	54% 3 7% 20% 12% 3	6194
	21st to end	Night	55% 3 7% 18% 15% 1	6490
February	1st to 10th	Night	52% 5% 9% 21% 12% 1	2583
	11th to 20th	Night	46% 4 10% 20% 17% 2	2339
	21st to end	Night	46% 4 10% 22% 15% 3	1980
March	1st to 10th	Night	47% 4 7% 22% 20% 2	5373
	11th to 20th	Night	34% 4 9% 24% 22% 5%	5072
	21st to end	Night	49% 5% 9% 25% 11% 1	5101
April	1st to 10th	Night	39% 4 10% 27% 16% 3	2000
	11th to 20th	Night	39% 5% 9% 24% 19% 4	1975
	21st to end	Night	46% 6% 8% 23% 16% 2	2000
May	1st to 10th	Night	47% 7% 14% 19% 11% 1	4254
	11th to 20th	Night	38% 6% 13% 26% 15% 2	3737
	21st to end	Night	41% 7% 12% 23% 16% 2	3950
June	1st to 10th	Night	30% 7% 14% 26% 19% 3	1600
	11th to 20th	Night	27% 6% 11% 26% 25% 4	1600
	21st to end	Night	25% 7% 16% 27% 18% 7%	1600
July	1st to 10th	Night	33% 5% 10% 29% 21% 2	3580
	11th to 20th	Night	31% 8% 15% 30% 14% 2	3572
	21st to end	Night	36% 7% 20% 26% 11% 1	4002
August	1st to 10th	Night	23% 7% 16% 36% 17% 1	2979
	11th to 20th	Night	34% 6% 12% 26% 22% 1	3230
	21st to end	Night	22% 6% 14% 35% 20% 3	3586
September	1st to 10th	Night	30% 5% 8% 27% 23% 6%	2000
	11th to 20th	Night	30% 5% 8% 27% 24% 5%	2035
	21st to end	Night	31% 5% 9% 21% 28% 5%	2200
October	1st to 10th	Night	17% 3 6% 19% 47% 8%	3845
	11th to 20th	Night	23% 2 8% 23% 38% 6%	3899
	21st to end	Night	32% 4 8% 25% 25% 6%	4365
November	1st to 10th	Night	31% 5% 11% 25% 23% 6%	2563
	11th to 20th	Night	37% 4 9% 24% 22% 4	2600
	21st to end	Night	43% 4 10% 25% 17% 1	2635
December	1st to 10th	Night	41% 3 10% 29% 15% 1	4580
	11th to 20th	Night	43% 4 13% 24% 16%	5201
	21st to end	Night	67% 4 10% 12% 5% 1	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	21% 16% 35% 20% 8%	300
	11th to 20th	Day	25% 17% 36% 18% 4	300
	21st to end	Day	19% 15% 44% 17% 5%	330
February	1st to 10th	Day	27% 9% 42% 18% 4	300
	11th to 20th	Day	24% 19% 33% 20% 3 1	300
	21st to end	Day	20% 17% 42% 19% 1 1	249
March	1st to 10th	Day	18% 16% 40% 22% 4	300
	11th to 20th	Day	25% 18% 35% 15% 7% 1	300
	21st to end	Day	26% 15% 33% 20% 6%	487
April	1st to 10th	Day	18% 14% 35% 23% 9% 2	500
	11th to 20th	Day	20% 15% 29% 23% 12% 2	492
	21st to end	Day	20% 11% 35% 22% 11% 1	500
May	1st to 10th	Day	17% 10% 30% 29% 11% 3	503
	11th to 20th	Day	16% 8% 27% 26% 17% 6%	653
	21st to end	Day	11% 6% 27% 31% 17% 8%	789
June	1st to 10th	Day	11% 6% 19% 30% 25% 10%	730
	11th to 20th	Day	9% 3 18% 35% 26% 10%	730
	21st to end	Day	5% 6% 23% 33% 24% 8%	730
July	1st to 10th	Day	10% 2 13% 25% 30% 19%	721
	11th to 20th	Day	4 4 11% 37% 31% 13%	718
	21st to end	Day	8% 2 13% 31% 32% 13%	794
August	1st to 10th	Day	8% 2 12% 34% 32% 12%	685
	11th to 20th	Day	10% 3 12% 29% 31% 15%	519
	21st to end	Day	8% 2 10% 27% 36% 17%	583
September	1st to 10th	Day	11% 4 12% 27% 32% 14%	530
	11th to 20th	Day	9% 3 10% 26% 36% 17%	530
	21st to end	Day	10% 9% 18% 25% 25% 13%	530
October	1st to 10th	Day	10% 4 21% 36% 21% 9%	375
	11th to 20th	Day	11% 12% 35% 24% 14% 3	330
	21st to end	Day	28% 14% 30% 20% 7%	363
November	1st to 10th	Day	14% 7% 33% 29% 12% 5%	306
	11th to 20th	Day	17% 18% 34% 19% 10% 2	300
	21st to end	Day	24% 14% 26% 28% 8% 1	300
December	1st to 10th	Day	13% 15% 42% 20% 6% 5%	300
	11th to 20th	Day	20% 15% 34% 24% 6%	300
	21st to end	Day	27% 15% 36% 14% 7%	330
January	1st to 10th	Night	18% 14% 37% 27% 5%	1000
	11th to 20th	Night	21% 14% 37% 23% 5%	1000
	21st to end	Night	21% 11% 39% 26% 2	1100
February	1st to 10th	Night	21% 14% 36% 27% 2	983
	11th to 20th	Night	20% 15% 38% 24% 2	783
	21st to end	Night	22% 13% 38% 25% 2	660
March	1st to 10th	Night	22% 13% 37% 25% 3	800
	11th to 20th	Night	22% 10% 34% 29% 6%	640
	21st to end	Night	23% 11% 31% 28% 6%	660
April	1st to 10th	Night	17% 11% 36% 30% 6%	600
	11th to 20th	Night	17% 9% 34% 28% 11% 1	593
	21st to end	Night	21% 8% 35% 28% 8% 1	600
May	1st to 10th	Night	18% 6% 30% 35% 10% 1	600
	11th to 20th	Night	16% 4 24% 37% 17% 2	591
	21st to end	Night	9% 3 23% 41% 20% 3	650
June	1st to 10th	Night	9% 3 18% 44% 27%	600
	11th to 20th	Night	9% 1 17% 47% 24% 3	600
	21st to end	Night	7% 2 19% 41% 30% 2	600
July	1st to 10th	Night	11% 1 7% 35% 39% 7%	585
	11th to 20th	Night	4 2 10% 42% 37% 4	582
	21st to end	Night	8% 1 11% 40% 37% 4	657
August	1st to 10th	Night	9% 1 8% 45% 34% 4	597
	11th to 20th	Night	8% 2 9% 41% 36% 5%	594
	21st to end	Night	6% 1 9% 38% 40% 6%	660
September	1st to 10th	Night	11% 2 10% 39% 31% 7%	600
	11th to 20th	Night	8% 1 12% 38% 36% 4	635
	21st to end	Night	11% 5% 20% 32% 27% 5%	800
October	1st to 10th	Night	9% 3 23% 37% 24% 4	800
	11th to 20th	Night	16% 9% 27% 33% 15% 1	800
	21st to end	Night	21% 10% 32% 31% 6%	880
November	1st to 10th	Night	13% 7% 34% 34% 11% 1	789
	11th to 20th	Night	17% 11% 35% 29% 8%	800
	21st to end	Night	23% 10% 33% 27% 7%	835
December	1st to 10th	Night	20% 13% 33% 29% 5% 1	1000
	11th to 20th	Night	23% 9% 33% 30% 6%	1000
	21st to end	Night	28% 14% 33% 23% 3	1100

Page generated at 2025-10-31 01:02:38. Version ac07

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