Clearwater
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clearwater clear sky chart from 2005-11-06 to 2024-11-10.

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	10% 12% 4 9% 6% 6% 5% 4 6% 7% 30%	105571
	Night	11% 15% 5% 11% 6% 6% 5% 4 6% 6% 26%	102141
Transparency	Day	52% 9% 13% 17% 7% 1	105317
	Night	57% 8% 12% 15% 6% 1	102041
Seeing	Day	18% 15% 27% 25% 13% 3	15953
	Night	20% 12% 23% 28% 16% 2	24151

By Month

Percentage Cloud Cover NW (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% 24% 8% 19% 9% 7% 4 2 4 4 7%	2096
	11th to 20th	Day	13% 16% 6% 11% 6% 4 5% 6% 6% 10% 18%	2235
	21st to end	Day	11% 27% 7% 13% 6% 6% 5% 4 4 5% 12%	2794
February	1st to 10th	Day	12% 24% 5% 11% 9% 6% 5% 3 4 4 16%	1527
	11th to 20th	Day	9% 17% 7% 11% 7% 6% 4 5% 5% 7% 22%	1665
	21st to end	Day	6% 17% 6% 11% 9% 6% 6% 5% 7% 7% 20%	1503
March	1st to 10th	Day	4 14% 6% 15% 10% 7% 6% 4 6% 7% 21%	3138
	11th to 20th	Day	5% 14% 4 10% 6% 6% 6% 4 6% 7% 32%	3352
	21st to end	Day	5% 14% 5% 10% 8% 7% 6% 5% 7% 7% 26%	4100
April	1st to 10th	Day	6% 19% 6% 12% 9% 7% 7% 5% 7% 6% 17%	2280
	11th to 20th	Day	8% 16% 5% 10% 7% 7% 6% 4 6% 7% 24%	2298
	21st to end	Day	9% 15% 4 10% 7% 6% 6% 5% 8% 7% 22%	2660
May	1st to 10th	Day	10% 12% 4 8% 7% 5% 5% 4 6% 7% 32%	4420
	11th to 20th	Day	11% 12% 4 10% 7% 6% 6% 5% 6% 7% 25%	4377
	21st to end	Day	17% 17% 5% 9% 6% 6% 6% 4 5% 6% 19%	5133
June	1st to 10th	Day	17% 11% 3 10% 7% 6% 5% 5% 6% 6% 24%	3040
	11th to 20th	Day	20% 12% 4 10% 8% 7% 6% 4 6% 6% 16%	3040
	21st to end	Day	11% 9% 4 10% 8% 6% 5% 4 6% 8% 28%	3040
July	1st to 10th	Day	6% 5% 2 5% 4 4 4 4 6% 7% 53%	5060
	11th to 20th	Day	4 4 1 4 3 4 4 3 5% 7% 60%	4742
	21st to end	Day	9% 5% 2 5% 4 5% 5% 4 8% 9% 45%	4829
August	1st to 10th	Day	5% 4 1 5% 4 4 5% 4 6% 9% 52%	4401
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 6% 8% 11% 42%	4330
	21st to end	Day	12% 6% 2 6% 5% 4 5% 4 6% 8% 42%	4329
September	1st to 10th	Day	11% 6% 3 8% 5% 5% 4 4 5% 7% 44%	2280
	11th to 20th	Day	13% 8% 3 8% 5% 4 5% 4 5% 8% 37%	2280
	21st to end	Day	16% 10% 4 9% 6% 5% 5% 4 7% 6% 29%	2042
October	1st to 10th	Day	14% 11% 3 8% 6% 4 5% 5% 6% 8% 29%	3180
	11th to 20th	Day	20% 13% 4 8% 5% 4 4 3 4 6% 29%	3044
	21st to end	Day	11% 12% 4 10% 6% 7% 7% 5% 5% 7% 26%	3101
November	1st to 10th	Day	16% 19% 5% 11% 8% 8% 5% 3 4 4 16%	1584
	11th to 20th	Day	15% 16% 5% 11% 7% 6% 6% 5% 6% 7% 18%	1497
	21st to end	Day	14% 20% 5% 12% 8% 8% 5% 4 6% 5% 14%	1330
December	1st to 10th	Day	12% 17% 6% 15% 9% 7% 7% 4 6% 4 14%	1312
	11th to 20th	Day	14% 22% 7% 15% 8% 6% 5% 4 4 5% 8%	1442
	21st to end	Day	10% 18% 6% 13% 8% 6% 6% 4 6% 7% 17%	2090
January	1st to 10th	Night	13% 26% 8% 18% 6% 5% 4 3 4 4 8%	4877
	11th to 20th	Night	10% 17% 6% 13% 7% 6% 6% 4 7% 6% 17%	4663
	21st to end	Night	15% 28% 7% 12% 7% 5% 4 3 4 4 9%	4862
February	1st to 10th	Night	11% 23% 6% 14% 7% 5% 5% 4 5% 5% 16%	2651
	11th to 20th	Night	10% 17% 7% 12% 6% 5% 6% 4 5% 6% 21%	2402
	21st to end	Night	7% 17% 6% 11% 8% 6% 6% 5% 6% 8% 20%	1950
March	1st to 10th	Night	5% 16% 6% 12% 6% 6% 5% 4 7% 8% 24%	3736
	11th to 20th	Night	4 13% 4 9% 6% 5% 5% 6% 8% 8% 31%	3625
	21st to end	Night	5% 14% 5% 11% 7% 7% 5% 5% 6% 8% 28%	3552
April	1st to 10th	Night	7% 18% 5% 10% 7% 6% 5% 4 5% 8% 25%	1900
	11th to 20th	Night	6% 13% 4 9% 7% 7% 6% 5% 7% 8% 27%	1828
	21st to end	Night	8% 12% 4 10% 7% 7% 5% 4 7% 7% 29%	1520
May	1st to 10th	Night	10% 11% 3 9% 6% 5% 6% 4 6% 5% 35%	2540
	11th to 20th	Night	13% 9% 3 8% 6% 6% 5% 5% 8% 8% 31%	2516
	21st to end	Night	19% 16% 4 10% 5% 5% 5% 3 5% 6% 23%	2436
June	1st to 10th	Night	17% 9% 4 8% 6% 5% 4 4 7% 8% 30%	1140
	11th to 20th	Night	18% 10% 4 12% 6% 5% 6% 4 7% 7% 21%	1140
	21st to end	Night	9% 9% 3 8% 8% 7% 5% 5% 6% 7% 33%	1140
July	1st to 10th	Night	8% 6% 2 6% 3 3 3 3 5% 6% 56%	1900
	11th to 20th	Night	6% 3 1 4 3 4 3 3 5% 7% 61%	2215
	21st to end	Night	9% 6% 3 7% 5% 5% 5% 5% 6% 8% 42%	2720
August	1st to 10th	Night	4 3 1 6% 4 4 5% 4 7% 9% 52%	2529
	11th to 20th	Night	6% 5% 2 7% 5% 6% 6% 6% 8% 10% 39%	2497
	21st to end	Night	11% 8% 3 7% 5% 6% 5% 4 6% 6% 38%	3328
September	1st to 10th	Night	9% 6% 3 7% 4 5% 5% 4 5% 7% 46%	1900
	11th to 20th	Night	14% 8% 2 5% 4 5% 4 3 6% 8% 40%	1900
	21st to end	Night	12% 10% 3 7% 5% 5% 4 4 5% 7% 36%	2137
October	1st to 10th	Night	14% 13% 3 9% 5% 5% 6% 4 6% 7% 29%	3800
	11th to 20th	Night	13% 12% 4 9% 6% 5% 4 5% 6% 7% 28%	3890
	21st to end	Night	12% 13% 4 11% 6% 5% 5% 4 6% 6% 27%	4457
November	1st to 10th	Night	18% 18% 5% 11% 7% 5% 5% 4 5% 6% 15%	2696
	11th to 20th	Night	15% 19% 6% 11% 6% 6% 5% 4 5% 6% 17%	2664
	21st to end	Night	14% 20% 6% 12% 8% 7% 5% 3 6% 6% 13%	2850
December	1st to 10th	Night	12% 17% 6% 13% 8% 7% 6% 4 5% 6% 16%	2864
	11th to 20th	Night	14% 23% 8% 15% 9% 6% 5% 3 4 4 9%	3750
	21st to end	Night	8% 20% 5% 14% 8% 7% 6% 4 6% 6% 16%	5566

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	81% 10% 5% 4 1	2096
	11th to 20th	Day	60% 8% 12% 8% 5% 7%	2256
	21st to end	Day	77% 9% 7% 5% 2	2794
February	1st to 10th	Day	70% 6% 10% 10% 4 1	1527
	11th to 20th	Day	62% 8% 13% 9% 6% 2	1674
	21st to end	Day	62% 11% 12% 9% 5% 1	1503
March	1st to 10th	Day	59% 12% 10% 11% 8% 1	3138
	11th to 20th	Day	49% 9% 16% 22% 3	3352
	21st to end	Day	54% 12% 14% 12% 7%	4100
April	1st to 10th	Day	67% 10% 10% 11% 2	2280
	11th to 20th	Day	59% 10% 13% 13% 5% 1	2298
	21st to end	Day	57% 11% 12% 15% 5%	2660
May	1st to 10th	Day	47% 11% 13% 22% 7%	4420
	11th to 20th	Day	51% 12% 13% 19% 4	4380
	21st to end	Day	62% 11% 12% 12% 3	5134
June	1st to 10th	Day	60% 10% 10% 10% 8% 3	3040
	11th to 20th	Day	68% 10% 9% 11% 3	3040
	21st to end	Day	54% 11% 13% 14% 8% 1	3040
July	1st to 10th	Day	31% 6% 9% 22% 24% 8%	4981
	11th to 20th	Day	26% 6% 8% 30% 24% 5%	4660
	21st to end	Day	35% 5% 17% 24% 13% 5%	4676
August	1st to 10th	Day	29% 7% 10% 38% 16%	4384
	11th to 20th	Day	35% 7% 24% 27% 7% 1	4372
	21st to end	Day	42% 7% 24% 23% 4	4328
September	1st to 10th	Day	39% 9% 17% 25% 9% 1	2280
	11th to 20th	Day	46% 9% 12% 23% 9% 1	2280
	21st to end	Day	53% 10% 14% 14% 8% 1	2042
October	1st to 10th	Day	52% 7% 18% 18% 4	3180
	11th to 20th	Day	59% 7% 19% 14% 2	3044
	21st to end	Day	57% 6% 21% 13% 3	3101
November	1st to 10th	Day	70% 8% 9% 9% 3	1584
	11th to 20th	Day	66% 13% 9% 7% 3 1	1497
	21st to end	Day	71% 9% 8% 6% 5% 1	1330
December	1st to 10th	Day	71% 9% 8% 10% 3	1312
	11th to 20th	Day	79% 7% 6% 6% 2	1444
	21st to end	Day	66% 13% 10% 8% 2	2090
January	1st to 10th	Night	79% 8% 6% 5% 2	4877
	11th to 20th	Night	62% 7% 10% 10% 5% 5%	4704
	21st to end	Night	80% 6% 6% 6% 2	4862
February	1st to 10th	Night	69% 7% 9% 8% 6% 1	2651
	11th to 20th	Night	62% 8% 10% 13% 6% 2	2404
	21st to end	Night	59% 10% 12% 13% 5% 1	1950
March	1st to 10th	Night	55% 10% 11% 13% 10% 1	3736
	11th to 20th	Night	44% 11% 17% 24% 4	3626
	21st to end	Night	51% 13% 13% 14% 8% 1	3552
April	1st to 10th	Night	57% 10% 13% 13% 6%	1900
	11th to 20th	Night	54% 10% 11% 15% 9%	1828
	21st to end	Night	52% 11% 10% 19% 8%	1520
May	1st to 10th	Night	48% 11% 9% 23% 9%	2540
	11th to 20th	Night	46% 11% 14% 22% 8%	2516
	21st to end	Night	61% 10% 12% 15% 3	2436
June	1st to 10th	Night	52% 10% 11% 17% 8% 2	1140
	11th to 20th	Night	61% 11% 11% 12% 5%	1140
	21st to end	Night	49% 11% 15% 16% 9% 1	1140
July	1st to 10th	Night	31% 6% 8% 20% 24% 12%	1870
	11th to 20th	Night	24% 6% 10% 29% 28% 3	2174
	21st to end	Night	39% 5% 11% 25% 15% 5%	2634
August	1st to 10th	Night	28% 5% 10% 40% 16% 1	2521
	11th to 20th	Night	39% 6% 22% 24% 7% 2	2521
	21st to end	Night	46% 5% 29% 16% 4 1	3328
September	1st to 10th	Night	38% 9% 15% 25% 11% 2	1900
	11th to 20th	Night	43% 7% 14% 23% 11% 3	1900
	21st to end	Night	46% 10% 16% 15% 12% 1	2137
October	1st to 10th	Night	55% 4 17% 21% 3	3800
	11th to 20th	Night	56% 6% 15% 20% 3	3890
	21st to end	Night	56% 5% 18% 17% 3	4457
November	1st to 10th	Night	68% 9% 10% 9% 3 1	2696
	11th to 20th	Night	68% 7% 10% 7% 5% 2	2664
	21st to end	Night	70% 9% 9% 6% 4 1	2850
December	1st to 10th	Night	67% 9% 11% 10% 4	2864
	11th to 20th	Night	80% 6% 7% 5% 2	3747
	21st to end	Night	69% 14% 8% 6% 3	5566

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% 16% 33% 19% 8% 3	180
	11th to 20th	Day	17% 18% 35% 21% 8% 2	180
	21st to end	Day	23% 17% 35% 18% 7%	198
February	1st to 10th	Day	20% 28% 31% 16% 5% 1	187
	11th to 20th	Day	20% 19% 28% 26% 6% 1	365
	21st to end	Day	13% 22% 33% 23% 7% 2	350
March	1st to 10th	Day	18% 21% 31% 20% 9% 1	460
	11th to 20th	Day	19% 23% 26% 22% 10% 1	460
	21st to end	Day	20% 22% 27% 17% 11% 3	506
April	1st to 10th	Day	22% 20% 29% 20% 7% 2	460
	11th to 20th	Day	18% 21% 25% 22% 10% 4	452
	21st to end	Day	17% 18% 29% 21% 12% 3	460
May	1st to 10th	Day	17% 14% 29% 27% 12% 2	463
	11th to 20th	Day	14% 15% 26% 26% 16% 3	488
	21st to end	Day	22% 12% 23% 24% 15% 4	706
June	1st to 10th	Day	22% 18% 28% 24% 7% 1	870
	11th to 20th	Day	23% 15% 27% 23% 11% 2	870
	21st to end	Day	14% 11% 26% 28% 17% 3	870
July	1st to 10th	Day	15% 11% 23% 30% 18% 3	856
	11th to 20th	Day	10% 13% 27% 30% 18% 3	669
	21st to end	Day	9% 8% 26% 35% 18% 5%	557
August	1st to 10th	Day	10% 7% 23% 31% 22% 7%	487
	11th to 20th	Day	10% 8% 27% 29% 20% 6%	477
	21st to end	Day	14% 12% 26% 30% 14% 5%	539
September	1st to 10th	Day	18% 13% 29% 23% 13% 4	490
	11th to 20th	Day	16% 14% 26% 27% 15% 2	490
	21st to end	Day	23% 16% 27% 24% 9% 2	490
October	1st to 10th	Day	23% 14% 28% 21% 11% 3	490
	11th to 20th	Day	26% 17% 23% 22% 10% 2	424
	21st to end	Day	26% 16% 26% 21% 11% 1	330
November	1st to 10th	Day	28% 22% 27% 16% 5% 2	211
	11th to 20th	Day	26% 15% 28% 22% 9% 1	180
	21st to end	Day	25% 18% 28% 17% 8% 4	180
December	1st to 10th	Day	21% 12% 21% 26% 18% 3	180
	11th to 20th	Day	21% 18% 29% 18% 13% 1	180
	21st to end	Day	16% 17% 34% 24% 9% 1	198
January	1st to 10th	Night	29% 15% 28% 20% 7% 1	936
	11th to 20th	Night	21% 15% 27% 27% 9%	839
	21st to end	Night	22% 14% 27% 25% 10% 1	836
February	1st to 10th	Night	24% 17% 24% 24% 11% 1	760
	11th to 20th	Night	20% 13% 22% 34% 10%	744
	21st to end	Night	16% 11% 24% 29% 18% 2	628
March	1st to 10th	Night	18% 14% 24% 29% 13% 1	760
	11th to 20th	Night	14% 15% 26% 28% 15% 1	760
	21st to end	Night	14% 14% 23% 28% 18% 3	836
April	1st to 10th	Night	17% 11% 23% 28% 18% 3	760
	11th to 20th	Night	13% 12% 26% 32% 16% 1	700
	21st to end	Night	14% 12% 25% 30% 17% 2	380
May	1st to 10th	Night	16% 9% 20% 34% 20% 2	380
	11th to 20th	Night	11% 9% 22% 36% 21% 2	374
	21st to end	Night	21% 9% 17% 30% 20% 3	411
June	1st to 10th	Night	21% 10% 22% 32% 14% 1	380
	11th to 20th	Night	21% 9% 20% 32% 17% 2	380
	21st to end	Night	14% 9% 20% 28% 26% 3	380
July	1st to 10th	Night	14% 4 18% 36% 26% 2	370
	11th to 20th	Night	11% 5% 18% 37% 26% 3	368
	21st to end	Night	10% 5% 15% 35% 30% 5%	416
August	1st to 10th	Night	8% 3 11% 31% 41% 6%	378
	11th to 20th	Night	9% 3 13% 38% 34% 4	376
	21st to end	Night	14% 7% 17% 34% 26% 3	731
September	1st to 10th	Night	14% 6% 21% 31% 26% 3	760
	11th to 20th	Night	18% 7% 15% 28% 28% 5%	760
	21st to end	Night	21% 10% 20% 30% 17% 2	760
October	1st to 10th	Night	23% 9% 17% 30% 19% 2	760
	11th to 20th	Night	23% 12% 23% 25% 16% 1	760
	21st to end	Night	26% 12% 27% 23% 11% 1	836
November	1st to 10th	Night	30% 13% 27% 22% 7% 1	778
	11th to 20th	Night	29% 15% 22% 24% 8% 1	764
	21st to end	Night	24% 15% 28% 24% 7% 1	950
December	1st to 10th	Night	21% 12% 29% 27% 10% 1	956
	11th to 20th	Night	25% 14% 28% 26% 8%	1040
	21st to end	Night	21% 15% 32% 26% 6%	1144

Page generated at 2024-11-07 03:55:49. Version ac07

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