Driftwood Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Driftwood Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	13% 6% 2 5% 4 4 4 3 5% 7% 46%	132315
	Night	15% 6% 2 5% 3 3 3 3 4 6% 49%	130841
Transparency	Day	36% 6% 10% 20% 18% 10%	136081
	Night	35% 5% 10% 19% 19% 12%	135138
Seeing	Day	15% 8% 23% 29% 20% 5%	16790
	Night	17% 7% 25% 34% 15% 1	26790

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% 8% 3 6% 3 2 2 2 4 5% 51%	5289
	11th to 20th	Day	15% 5% 2 4 3 3 3 3 4 5% 54%	5310
	21st to end	Day	25% 7% 2 6% 3 3 3 2 4 6% 40%	6206
February	1st to 10th	Day	21% 6% 2 6% 4 3 3 2 4 4 45%	2100
	11th to 20th	Day	19% 7% 2 5% 3 3 3 3 4 7% 44%	2049
	21st to end	Day	19% 5% 2 5% 3 3 3 2 3 5% 50%	1730
March	1st to 10th	Day	13% 6% 2 6% 4 3 3 3 5% 6% 51%	5014
	11th to 20th	Day	15% 7% 2 5% 4 3 3 2 4 5% 51%	5090
	21st to end	Day	17% 7% 2 6% 3 3 4 3 4 6% 46%	5583
April	1st to 10th	Day	16% 8% 2 6% 4 4 4 4 5% 6% 42%	2310
	11th to 20th	Day	20% 7% 3 6% 4 4 4 3 4 5% 40%	2286
	21st to end	Day	16% 7% 3 6% 5% 5% 4 3 4 7% 41%	2669
May	1st to 10th	Day	22% 8% 2 6% 4 4 4 4 5% 7% 33%	5992
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 46%	5941
	21st to end	Day	14% 6% 3 7% 5% 5% 6% 5% 7% 9% 34%	6434
June	1st to 10th	Day	7% 4 2 5% 4 4 5% 4 7% 10% 48%	2600
	11th to 20th	Day	9% 5% 2 5% 5% 5% 5% 5% 6% 9% 45%	2600
	21st to end	Day	8% 6% 2 6% 5% 7% 7% 5% 7% 10% 37%	2600
July	1st to 10th	Day	11% 7% 3 7% 6% 5% 5% 4 6% 9% 38%	5900
	11th to 20th	Day	5% 4 2 5% 5% 5% 6% 5% 9% 11% 41%	5900
	21st to end	Day	8% 6% 2 6% 4 5% 6% 5% 8% 10% 40%	6422
August	1st to 10th	Day	1 2 1 3 2 2 3 3 5% 9% 70%	4234
	11th to 20th	Day	4 2 1 3 2 4 4 4 7% 10% 58%	3859
	21st to end	Day	4 3 1 5% 4 6% 6% 5% 8% 11% 47%	4116
September	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 4 6% 6% 44%	2200
	11th to 20th	Day	11% 7% 2 6% 4 4 5% 4 7% 8% 42%	2200
	21st to end	Day	7% 4 2 5% 4 4 5% 5% 6% 8% 51%	2200
October	1st to 10th	Day	6% 5% 2 5% 4 4 4 3 5% 8% 56%	2200
	11th to 20th	Day	12% 4 2 4 3 4 3 3 5% 5% 53%	2056
	21st to end	Day	11% 4 1 5% 3 3 3 3 5% 5% 58%	2200
November	1st to 10th	Day	13% 8% 2 5% 4 3 4 3 5% 5% 48%	2058
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 4 5% 52%	1979
	21st to end	Day	18% 7% 2 6% 5% 4 2 3 4 5% 45%	1890
December	1st to 10th	Day	25% 8% 3 6% 3 3 3 2 3 5% 39%	3030
	11th to 20th	Day	17% 7% 2 6% 4 4 4 3 5% 5% 43%	3481
	21st to end	Day	15% 5% 2 4 3 3 3 2 3 4 57%	4587
January	1st to 10th	Night	16% 7% 2 5% 2 2 3 2 3 5% 53%	7634
	11th to 20th	Night	10% 3 1 3 2 2 3 3 4 4 64%	7680
	21st to end	Night	29% 6% 2 5% 3 3 3 2 3 5% 40%	8063
February	1st to 10th	Night	18% 8% 2 5% 3 2 3 3 4 4 48%	2520
	11th to 20th	Night	20% 7% 2 5% 3 3 2 2 3 4 49%	2458
	21st to end	Night	20% 7% 3 4 3 2 3 3 4 5% 46%	2076
March	1st to 10th	Night	17% 8% 2 4 4 4 3 3 5% 6% 44%	5060
	11th to 20th	Night	17% 7% 2 5% 3 3 3 2 3 5% 49%	5090
	21st to end	Night	20% 9% 2 5% 3 3 3 3 4 5% 43%	5588
April	1st to 10th	Night	15% 6% 2 6% 4 3 4 2 5% 7% 45%	2310
	11th to 20th	Night	21% 7% 2 5% 4 3 3 3 5% 5% 43%	2283
	21st to end	Night	20% 8% 2 5% 3 3 3 2 3 6% 43%	1944
May	1st to 10th	Night	26% 8% 3 6% 3 3 4 3 4 5% 34%	4143
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 5% 8% 49%	4107
	21st to end	Night	15% 8% 2 7% 4 4 5% 4 6% 9% 36%	4443
June	1st to 10th	Night	7% 4 1 4 3 3 4 3 5% 9% 59%	1800
	11th to 20th	Night	6% 3 2 5% 5% 4 4 3 5% 9% 55%	1800
	21st to end	Night	7% 5% 1 4 3 3 4 4 8% 10% 50%	1800
July	1st to 10th	Night	10% 6% 2 6% 4 4 5% 4 6% 10% 42%	4078
	11th to 20th	Night	4 4 2 5% 4 4 5% 4 7% 12% 49%	4077
	21st to end	Night	8% 7% 2 6% 4 4 4 4 7% 10% 45%	4449
August	1st to 10th	Night	1 1 1 2 2 2 2 2 4 9% 74%	2926
	11th to 20th	Night	3 3 1 3 3 3 4 4 7% 12% 58%	3182
	21st to end	Night	5% 4 1 4 3 3 4 4 7% 11% 54%	3784
September	1st to 10th	Night	8% 5% 2 5% 4 4 4 3 5% 8% 52%	2200
	11th to 20th	Night	10% 6% 2 6% 4 5% 4 4 5% 7% 47%	2200
	21st to end	Night	8% 4 2 6% 3 3 4 3 6% 7% 53%	2200
October	1st to 10th	Night	6% 4 1 4 3 3 3 3 4 7% 61%	2200
	11th to 20th	Night	13% 5% 2 4 3 2 4 2 4 6% 55%	2321
	21st to end	Night	15% 5% 2 3 2 2 2 2 3 4 60%	2640
November	1st to 10th	Night	14% 7% 1 5% 3 2 3 2 4 5% 54%	2460
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 4 6% 56%	2636
	21st to end	Night	18% 6% 2 5% 3 3 4 2 5% 6% 46%	2730
December	1st to 10th	Night	22% 8% 3 6% 3 3 3 2 4 4 40%	4380
	11th to 20th	Night	21% 9% 2 5% 3 2 2 2 2 4 47%	4946
	21st to end	Night	18% 6% 2 4 2 2 2 2 3 4 56%	6633

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	38% 2 6% 22% 23% 9%	5290
	11th to 20th	Day	34% 2 4 14% 25% 20%	5310
	21st to end	Day	47% 2 5% 12% 23% 11%	6206
February	1st to 10th	Day	43% 4 6% 11% 20% 16%	2100
	11th to 20th	Day	42% 4 7% 17% 18% 11%	2059
	21st to end	Day	39% 3 6% 17% 20% 15%	1730
March	1st to 10th	Day	36% 3 6% 18% 17% 20%	5014
	11th to 20th	Day	39% 4 5% 8% 16% 28%	5090
	21st to end	Day	41% 4 8% 18% 20% 9%	5585
April	1st to 10th	Day	44% 5% 7% 15% 16% 12%	2310
	11th to 20th	Day	45% 5% 7% 13% 17% 11%	2286
	21st to end	Day	44% 5% 7% 18% 17% 9%	2669
May	1st to 10th	Day	48% 6% 9% 18% 14% 4	6030
	11th to 20th	Day	36% 6% 11% 19% 22% 7%	5945
	21st to end	Day	42% 8% 11% 25% 13% 2	6447
June	1st to 10th	Day	29% 8% 13% 27% 19% 3	2600
	11th to 20th	Day	34% 7% 11% 25% 17% 5%	2600
	21st to end	Day	39% 10% 12% 23% 15% 3	2600
July	1st to 10th	Day	41% 9% 16% 28% 5% 1	5836
	11th to 20th	Day	31% 10% 18% 28% 12% 1	5831
	21st to end	Day	34% 9% 16% 25% 14% 3	6240
August	1st to 10th	Day	13% 5% 12% 43% 27% 1	4220
	11th to 20th	Day	19% 8% 22% 37% 11% 2	3895
	21st to end	Day	27% 11% 19% 25% 17% 2	4115
September	1st to 10th	Day	39% 8% 8% 20% 16% 9%	2200
	11th to 20th	Day	38% 8% 12% 19% 18% 5%	2200
	21st to end	Day	30% 9% 10% 20% 17% 13%	2200
October	1st to 10th	Day	17% 4 7% 12% 24% 36%	3590
	11th to 20th	Day	22% 4 9% 18% 26% 20%	3322
	21st to end	Day	26% 4 7% 18% 30% 15%	3538
November	1st to 10th	Day	37% 6% 8% 17% 20% 12%	2058
	11th to 20th	Day	34% 4 9% 17% 21% 15%	1979
	21st to end	Day	43% 4 5% 15% 19% 13%	1890
December	1st to 10th	Day	49% 3 6% 17% 16% 9%	3030
	11th to 20th	Day	41% 7% 6% 11% 22% 13%	3479
	21st to end	Day	33% 3 5% 16% 27% 17%	4587
January	1st to 10th	Night	36% 3 6% 18% 23% 14%	7635
	11th to 20th	Night	25% 2 4 17% 29% 22%	7680
	21st to end	Night	48% 3 4 11% 21% 14%	8063
February	1st to 10th	Night	40% 4 6% 12% 20% 18%	2520
	11th to 20th	Night	41% 3 7% 16% 19% 14%	2458
	21st to end	Night	41% 4 6% 15% 19% 16%	2076
March	1st to 10th	Night	41% 4 6% 16% 17% 16%	5060
	11th to 20th	Night	39% 4 7% 10% 14% 25%	5090
	21st to end	Night	43% 5% 8% 17% 18% 10%	5588
April	1st to 10th	Night	40% 5% 9% 17% 15% 14%	2310
	11th to 20th	Night	44% 5% 8% 13% 16% 14%	2283
	21st to end	Night	44% 5% 10% 18% 15% 9%	1944
May	1st to 10th	Night	51% 5% 8% 17% 13% 5%	4170
	11th to 20th	Night	34% 6% 11% 21% 19% 9%	4108
	21st to end	Night	41% 8% 13% 26% 10% 2	4452
June	1st to 10th	Night	23% 6% 15% 30% 22% 3	1800
	11th to 20th	Night	25% 7% 15% 29% 20% 4	1800
	21st to end	Night	28% 10% 18% 27% 15% 3	1800
July	1st to 10th	Night	37% 10% 22% 26% 4	4035
	11th to 20th	Night	26% 9% 23% 30% 11%	4026
	21st to end	Night	33% 9% 20% 24% 12% 3	4320
August	1st to 10th	Night	9% 6% 16% 46% 23% 1	2916
	11th to 20th	Night	19% 9% 31% 31% 9% 2	3211
	21st to end	Night	23% 12% 21% 27% 15% 2	3784
September	1st to 10th	Night	32% 6% 17% 21% 16% 8%	2200
	11th to 20th	Night	34% 8% 14% 24% 14% 6%	2200
	21st to end	Night	29% 7% 14% 21% 17% 12%	2200
October	1st to 10th	Night	16% 4 8% 15% 20% 36%	3590
	11th to 20th	Night	21% 4 10% 17% 29% 20%	3781
	21st to end	Night	25% 4 6% 19% 30% 16%	4252
November	1st to 10th	Night	34% 5% 8% 17% 22% 14%	2460
	11th to 20th	Night	31% 5% 8% 18% 21% 17%	2636
	21st to end	Night	40% 4 8% 16% 18% 15%	2730
December	1st to 10th	Night	49% 4 5% 16% 16% 10%	4380
	11th to 20th	Night	44% 3 6% 8% 22% 17%	4947
	21st to end	Night	35% 3 4 14% 28% 16%	6633

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	22% 16% 35% 22% 5%	330
	11th to 20th	Day	25% 18% 27% 22% 7% 1	330
	21st to end	Day	22% 16% 38% 20% 4	363
February	1st to 10th	Day	29% 13% 31% 22% 5%	330
	11th to 20th	Day	27% 18% 35% 16% 4	330
	21st to end	Day	26% 16% 42% 15% 2	273
March	1st to 10th	Day	25% 18% 38% 16% 2	330
	11th to 20th	Day	18% 19% 42% 15% 5% 1	330
	21st to end	Day	20% 13% 35% 24% 7% 1	363
April	1st to 10th	Day	24% 15% 37% 20% 4	330
	11th to 20th	Day	21% 19% 33% 21% 6%	324
	21st to end	Day	21% 12% 39% 21% 6%	689
May	1st to 10th	Day	20% 11% 36% 26% 6%	753
	11th to 20th	Day	18% 10% 30% 31% 10% 2	756
	21st to end	Day	17% 8% 25% 32% 15% 2	789
June	1st to 10th	Day	9% 2 16% 40% 26% 6%	730
	11th to 20th	Day	9% 2 17% 43% 25% 3	730
	21st to end	Day	9% 4 11% 38% 29% 9%	730
July	1st to 10th	Day	10% 3 11% 37% 30% 10%	721
	11th to 20th	Day	6% 2 8% 34% 43% 8%	718
	21st to end	Day	6% 2 6% 33% 43% 9%	794
August	1st to 10th	Day	3 1 6% 32% 45% 14%	726
	11th to 20th	Day	6% 1 5% 37% 42% 10%	530
	21st to end	Day	6% 1 4 32% 36% 21%	465
September	1st to 10th	Day	9% 2 7% 31% 36% 15%	330
	11th to 20th	Day	8% 1 7% 27% 45% 13%	330
	21st to end	Day	10% 1 10% 30% 37% 12%	330
October	1st to 10th	Day	3 2 15% 36% 32% 10%	330
	11th to 20th	Day	12% 3 21% 38% 20% 6%	330
	21st to end	Day	17% 13% 26% 26% 14% 4	363
November	1st to 10th	Day	18% 9% 26% 31% 12% 4	330
	11th to 20th	Day	17% 11% 26% 33% 12% 2	330
	21st to end	Day	21% 16% 36% 15% 10% 2	330
December	1st to 10th	Day	21% 11% 35% 24% 6% 4	330
	11th to 20th	Day	20% 17% 28% 24% 11%	330
	21st to end	Day	19% 13% 36% 24% 7% 1	363
January	1st to 10th	Night	21% 10% 29% 31% 8% 1	1050
	11th to 20th	Night	20% 12% 30% 31% 7%	1050
	21st to end	Night	25% 10% 28% 30% 6%	1034
February	1st to 10th	Night	26% 11% 30% 27% 5%	840
	11th to 20th	Night	27% 11% 33% 25% 3	824
	21st to end	Night	24% 14% 39% 21% 2	692
March	1st to 10th	Night	25% 12% 36% 24% 3	648
	11th to 20th	Night	22% 11% 33% 29% 5%	630
	21st to end	Night	19% 9% 39% 29% 3	693
April	1st to 10th	Night	20% 11% 38% 28% 3	630
	11th to 20th	Night	24% 12% 36% 24% 4	623
	21st to end	Night	22% 12% 35% 22% 8%	630
May	1st to 10th	Night	20% 11% 36% 28% 5%	630
	11th to 20th	Night	17% 7% 31% 34% 10%	609
	21st to end	Night	14% 7% 33% 34% 12%	650
June	1st to 10th	Night	7% 3 23% 45% 21% 1	600
	11th to 20th	Night	7% 3 30% 47% 13% 1	600
	21st to end	Night	7% 4 16% 45% 25% 4	600
July	1st to 10th	Night	10% 3 11% 42% 31% 3	585
	11th to 20th	Night	3 1 12% 51% 30% 2	582
	21st to end	Night	5% 3 9% 50% 31% 3	657
August	1st to 10th	Night	1 1 8% 52% 35% 3	597
	11th to 20th	Night	4 2 7% 50% 34% 4	594
	21st to end	Night	7% 1 5% 38% 43% 6%	660
September	1st to 10th	Night	11% 2 8% 36% 37% 5%	600
	11th to 20th	Night	11% 2 7% 41% 37% 3	600
	21st to end	Night	10% 2 12% 37% 36% 4	600
October	1st to 10th	Night	8% 1 11% 45% 31% 4	600
	11th to 20th	Night	13% 4 21% 37% 23% 2	721
	21st to end	Night	16% 8% 23% 32% 20% 2	880
November	1st to 10th	Night	16% 7% 19% 38% 19% 1	820
	11th to 20th	Night	14% 7% 26% 37% 14% 1	956
	21st to end	Night	21% 10% 28% 30% 11% 1	1050
December	1st to 10th	Night	20% 8% 30% 31% 10% 1	1050
	11th to 20th	Night	26% 9% 26% 31% 7%	1050
	21st to end	Night	21% 8% 30% 33% 8%	1155

Page generated at 2026-05-14 00:58:56. Version ac07

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