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 2024-01-13.

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% 12% 5% 9% 8% 4 6% 5% 6% 7% 27%	74549
	Night	12% 14% 5% 9% 8% 4 6% 5% 6% 7% 24%	75169
Transparency	Day	57% 10% 12% 14% 6% 1	74232
	Night	59% 9% 12% 13% 6% 1	75004
Seeing	Day	18% 15% 27% 24% 12% 3	14747
	Night	20% 12% 23% 28% 16% 2	23186

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	14% 22% 7% 13% 11% 5% 6% 4 5% 5% 8%	1258
	11th to 20th	Day	14% 15% 7% 11% 10% 4 6% 6% 7% 8% 12%	1296
	21st to end	Day	11% 19% 7% 12% 8% 5% 7% 5% 5% 6% 15%	1584
February	1st to 10th	Day	13% 22% 6% 9% 10% 5% 6% 4 4 4 17%	1447
	11th to 20th	Day	10% 16% 7% 11% 8% 5% 5% 6% 5% 7% 20%	1575
	21st to end	Day	6% 14% 8% 9% 9% 5% 7% 6% 7% 7% 21%	1417
March	1st to 10th	Day	7% 14% 7% 12% 9% 5% 6% 6% 7% 7% 21%	1800
	11th to 20th	Day	9% 16% 6% 9% 8% 5% 6% 4 5% 7% 23%	1894
	21st to end	Day	8% 16% 5% 10% 9% 4 7% 6% 6% 7% 21%	2337
April	1st to 10th	Day	7% 17% 8% 10% 10% 5% 7% 6% 7% 6% 17%	2160
	11th to 20th	Day	9% 14% 6% 10% 8% 4 7% 6% 6% 7% 24%	2175
	21st to end	Day	10% 13% 4 9% 8% 5% 6% 6% 7% 7% 23%	2520
May	1st to 10th	Day	13% 14% 5% 7% 8% 4 5% 5% 5% 6% 28%	2520
	11th to 20th	Day	11% 11% 4 9% 7% 3 6% 7% 6% 8% 27%	2477
	21st to end	Day	17% 12% 4 8% 7% 5% 6% 4 4 7% 25%	2889
June	1st to 10th	Day	18% 10% 4 8% 8% 4 6% 5% 6% 7% 24%	2880
	11th to 20th	Day	21% 11% 5% 9% 9% 6% 6% 5% 5% 6% 16%	2880
	21st to end	Day	12% 8% 4 9% 9% 4 6% 5% 6% 8% 29%	2880
July	1st to 10th	Day	10% 7% 3 7% 7% 4 6% 6% 6% 8% 36%	2880
	11th to 20th	Day	7% 6% 3 6% 7% 5% 6% 6% 7% 8% 39%	2702
	21st to end	Day	8% 4 2 4 5% 3 6% 5% 6% 8% 47%	2766
August	1st to 10th	Day	8% 6% 2 5% 6% 4 5% 5% 7% 8% 43%	2507
	11th to 20th	Day	8% 4 2 5% 5% 4 5% 6% 6% 9% 47%	2475
	21st to end	Day	11% 7% 3 6% 7% 4 6% 6% 7% 8% 35%	2473
September	1st to 10th	Day	11% 6% 3 7% 7% 4 4 4 5% 7% 43%	2160
	11th to 20th	Day	13% 8% 3 6% 6% 4 5% 5% 5% 8% 37%	2160
	21st to end	Day	16% 10% 4 8% 6% 4 5% 5% 6% 7% 29%	1935
October	1st to 10th	Day	15% 12% 4 7% 8% 4 5% 5% 5% 6% 30%	1800
	11th to 20th	Day	16% 12% 4 6% 8% 4 5% 5% 5% 8% 28%	1738
	21st to end	Day	17% 15% 6% 9% 6% 5% 5% 4 4 7% 23%	1782
November	1st to 10th	Day	17% 18% 6% 10% 8% 7% 6% 5% 4 5% 16%	1511
	11th to 20th	Day	15% 14% 6% 9% 8% 4 6% 6% 6% 7% 18%	1497
	21st to end	Day	15% 18% 7% 10% 9% 6% 6% 4 6% 5% 14%	1330
December	1st to 10th	Day	12% 16% 7% 12% 11% 5% 8% 6% 6% 4 14%	1312
	11th to 20th	Day	14% 21% 9% 14% 9% 4 7% 5% 4 5% 8%	1442
	21st to end	Day	11% 17% 6% 11% 9% 5% 6% 5% 6% 7% 18%	2090
January	1st to 10th	Night	16% 21% 7% 11% 7% 4 6% 5% 5% 5% 11%	2922
	11th to 20th	Night	13% 18% 7% 13% 10% 4 6% 5% 6% 6% 10%	2708
	21st to end	Night	10% 20% 8% 12% 10% 5% 6% 5% 5% 6% 13%	2772
February	1st to 10th	Night	12% 21% 6% 12% 8% 4 6% 4 5% 5% 17%	2511
	11th to 20th	Night	11% 17% 7% 11% 8% 4 6% 5% 6% 6% 20%	2272
	21st to end	Night	7% 15% 6% 10% 9% 5% 7% 7% 6% 8% 21%	1838
March	1st to 10th	Night	7% 16% 7% 11% 9% 4 5% 5% 5% 7% 23%	2160
	11th to 20th	Night	6% 14% 6% 9% 8% 4 6% 7% 7% 7% 26%	2063
	21st to end	Night	6% 14% 6% 10% 8% 5% 6% 6% 6% 8% 25%	2018
April	1st to 10th	Night	7% 16% 5% 9% 8% 4 6% 5% 5% 9% 26%	1800
	11th to 20th	Night	7% 12% 5% 8% 8% 5% 7% 7% 6% 8% 27%	1731
	21st to end	Night	7% 10% 4 9% 8% 5% 6% 6% 7% 7% 30%	1440
May	1st to 10th	Night	11% 11% 4 8% 7% 3 6% 5% 5% 5% 33%	1440
	11th to 20th	Night	9% 7% 4 8% 7% 5% 6% 6% 8% 8% 32%	1416
	21st to end	Night	17% 10% 4 7% 6% 4 6% 5% 6% 8% 27%	1380
June	1st to 10th	Night	17% 8% 4 8% 7% 4 5% 4 6% 8% 30%	1080
	11th to 20th	Night	19% 9% 5% 10% 8% 4 6% 5% 6% 7% 22%	1080
	21st to end	Night	9% 8% 4 7% 9% 5% 5% 6% 6% 8% 33%	1080
July	1st to 10th	Night	12% 8% 3 7% 5% 4 5% 6% 7% 8% 37%	1080
	11th to 20th	Night	9% 5% 3 5% 6% 4 6% 6% 7% 9% 41%	1255
	21st to end	Night	8% 5% 2 4 5% 2 5% 6% 5% 9% 48%	1548
August	1st to 10th	Night	7% 4 1 6% 6% 3 5% 6% 6% 8% 48%	1432
	11th to 20th	Night	8% 5% 2 4 4 3 4 5% 6% 8% 51%	1424
	21st to end	Night	9% 7% 3 7% 6% 5% 6% 5% 7% 8% 39%	1880
September	1st to 10th	Night	10% 6% 3 6% 5% 4 5% 5% 6% 7% 44%	1800
	11th to 20th	Night	15% 8% 2 5% 5% 4 4 4 6% 8% 40%	1800
	21st to end	Night	12% 10% 3 6% 7% 3 4 5% 5% 7% 37%	2024
October	1st to 10th	Night	15% 10% 3 6% 5% 3 5% 4 5% 7% 36%	2160
	11th to 20th	Night	15% 12% 5% 7% 8% 4 5% 5% 6% 6% 26%	2222
	21st to end	Night	15% 14% 5% 9% 7% 4 5% 4 7% 6% 22%	2574
November	1st to 10th	Night	19% 17% 6% 9% 8% 4 5% 6% 5% 6% 15%	2565
	11th to 20th	Night	16% 17% 7% 10% 7% 4 6% 5% 5% 6% 17%	2664
	21st to end	Night	15% 17% 8% 11% 9% 5% 7% 4 6% 6% 13%	2850
December	1st to 10th	Night	12% 16% 7% 11% 10% 5% 6% 5% 5% 7% 16%	2864
	11th to 20th	Night	14% 22% 9% 14% 10% 5% 5% 4 4 4 9%	3750
	21st to end	Night	9% 19% 6% 12% 10% 5% 7% 5% 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	79% 8% 6% 5% 1 1	1258
	11th to 20th	Day	70% 9% 11% 5% 3 2	1296
	21st to end	Day	70% 8% 9% 8% 4 1	1584
February	1st to 10th	Day	69% 6% 10% 10% 4 1	1447
	11th to 20th	Day	64% 9% 13% 9% 5% 1	1584
	21st to end	Day	61% 11% 12% 9% 6% 1	1417
March	1st to 10th	Day	63% 11% 11% 8% 7% 1	1800
	11th to 20th	Day	62% 8% 11% 14% 4	1894
	21st to end	Day	65% 11% 12% 9% 3	2337
April	1st to 10th	Day	67% 10% 10% 11% 2	2160
	11th to 20th	Day	60% 9% 13% 13% 4 1	2175
	21st to end	Day	57% 11% 12% 16% 4	2520
May	1st to 10th	Day	56% 10% 14% 14% 7%	2520
	11th to 20th	Day	53% 11% 14% 16% 6%	2480
	21st to end	Day	59% 9% 13% 13% 5% 1	2890
June	1st to 10th	Day	61% 10% 10% 10% 7% 3	2880
	11th to 20th	Day	68% 11% 9% 10% 2	2880
	21st to end	Day	53% 11% 14% 13% 7% 1	2880
July	1st to 10th	Day	46% 11% 13% 17% 11% 1	2801
	11th to 20th	Day	42% 11% 13% 21% 11% 1	2620
	21st to end	Day	32% 9% 16% 28% 13% 2	2612
August	1st to 10th	Day	37% 11% 15% 25% 11% 1	2490
	11th to 20th	Day	32% 11% 18% 26% 12% 1	2475
	21st to end	Day	45% 10% 16% 22% 6% 1	2473
September	1st to 10th	Day	40% 9% 18% 24% 8% 1	2160
	11th to 20th	Day	46% 10% 12% 23% 9% 1	2160
	21st to end	Day	52% 10% 14% 15% 8% 1	1935
October	1st to 10th	Day	54% 9% 15% 19% 3	1800
	11th to 20th	Day	56% 11% 15% 14% 3	1738
	21st to end	Day	61% 9% 13% 12% 5%	1782
November	1st to 10th	Day	70% 8% 9% 9% 4	1511
	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	75% 8% 7% 6% 3 1	2922
	11th to 20th	Night	72% 8% 10% 6% 2 2	2708
	21st to end	Night	70% 8% 9% 9% 3 1	2772
February	1st to 10th	Night	68% 7% 10% 9% 6% 1	2511
	11th to 20th	Night	64% 8% 10% 13% 5% 1	2274
	21st to end	Night	58% 10% 12% 13% 5% 1	1838
March	1st to 10th	Night	61% 9% 11% 10% 7% 1	2160
	11th to 20th	Night	56% 10% 11% 17% 5%	2064
	21st to end	Night	58% 12% 13% 13% 5%	2018
April	1st to 10th	Night	57% 10% 13% 14% 6%	1800
	11th to 20th	Night	55% 9% 12% 14% 9%	1731
	21st to end	Night	52% 11% 10% 20% 7%	1440
May	1st to 10th	Night	52% 10% 13% 19% 7%	1440
	11th to 20th	Night	49% 11% 14% 19% 7%	1416
	21st to end	Night	56% 9% 13% 17% 5% 1	1380
June	1st to 10th	Night	52% 10% 11% 17% 8% 2	1080
	11th to 20th	Night	62% 12% 11% 11% 4	1080
	21st to end	Night	49% 11% 15% 16% 8% 1	1080
July	1st to 10th	Night	44% 10% 12% 17% 13% 3	1050
	11th to 20th	Night	39% 11% 17% 21% 11% 1	1214
	21st to end	Night	31% 9% 16% 29% 13% 2	1462
August	1st to 10th	Night	33% 9% 16% 27% 14% 1	1424
	11th to 20th	Night	31% 10% 15% 29% 13% 3	1424
	21st to end	Night	41% 9% 19% 24% 6% 1	1880
September	1st to 10th	Night	39% 9% 15% 25% 10% 2	1800
	11th to 20th	Night	43% 8% 13% 23% 11% 3	1800
	21st to end	Night	46% 10% 16% 15% 12% 1	2024
October	1st to 10th	Night	48% 7% 15% 25% 5%	2160
	11th to 20th	Night	57% 9% 13% 17% 4	2222
	21st to end	Night	60% 8% 15% 12% 5%	2574
November	1st to 10th	Night	68% 10% 9% 9% 3 1	2565
	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	19% 18% 33% 20% 9% 2	164
	21st to end	Day	24% 14% 35% 19% 7%	176
February	1st to 10th	Day	23% 27% 32% 15% 3 1	167
	11th to 20th	Day	21% 20% 28% 26% 4 1	335
	21st to end	Day	11% 21% 34% 24% 8% 2	318
March	1st to 10th	Day	18% 22% 32% 19% 9% 1	420
	11th to 20th	Day	20% 22% 27% 20% 9%	420
	21st to end	Day	21% 23% 26% 16% 10% 3	462
April	1st to 10th	Day	22% 21% 29% 20% 7% 2	420
	11th to 20th	Day	19% 22% 24% 21% 10% 4	412
	21st to end	Day	17% 17% 29% 21% 12% 3	420
May	1st to 10th	Day	17% 13% 29% 26% 11% 2	423
	11th to 20th	Day	14% 14% 25% 27% 17% 3	448
	21st to end	Day	22% 10% 22% 25% 16% 4	651
June	1st to 10th	Day	21% 17% 28% 24% 8% 1	810
	11th to 20th	Day	24% 14% 27% 23% 10% 2	810
	21st to end	Day	14% 11% 26% 28% 18% 3	810
July	1st to 10th	Day	15% 11% 23% 29% 18% 3	796
	11th to 20th	Day	10% 14% 28% 29% 16% 2	619
	21st to end	Day	8% 8% 25% 36% 18% 5%	512
August	1st to 10th	Day	11% 8% 23% 31% 21% 6%	447
	11th to 20th	Day	11% 8% 29% 30% 18% 5%	437
	21st to end	Day	13% 12% 27% 29% 13% 5%	495
September	1st to 10th	Day	19% 13% 31% 22% 11% 4	450
	11th to 20th	Day	17% 14% 27% 27% 14% 2	450
	21st to end	Day	22% 15% 27% 24% 10% 2	450
October	1st to 10th	Day	22% 14% 27% 21% 12% 3	450
	11th to 20th	Day	25% 18% 23% 22% 10% 2	388
	21st to end	Day	28% 17% 25% 19% 11% 1	297
November	1st to 10th	Day	29% 20% 27% 16% 6% 2	192
	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	22% 16% 26% 26% 10%	800
	21st to end	Night	22% 13% 27% 26% 11% 1	792
February	1st to 10th	Night	24% 16% 24% 23% 11% 1	720
	11th to 20th	Night	21% 13% 22% 34% 9%	704
	21st to end	Night	15% 10% 24% 30% 18% 3	592
March	1st to 10th	Night	18% 15% 24% 28% 13% 1	720
	11th to 20th	Night	15% 16% 27% 28% 15% 1	720
	21st to end	Night	14% 15% 23% 27% 18% 3	792
April	1st to 10th	Night	17% 10% 22% 29% 19% 3	720
	11th to 20th	Night	14% 11% 25% 32% 17% 1	663
	21st to end	Night	14% 12% 25% 30% 17% 3	360
May	1st to 10th	Night	16% 8% 20% 34% 20% 2	360
	11th to 20th	Night	11% 8% 21% 37% 21% 2	354
	21st to end	Night	21% 8% 16% 31% 21% 3	389
June	1st to 10th	Night	22% 9% 22% 32% 15% 1	360
	11th to 20th	Night	22% 9% 19% 31% 17% 2	360
	21st to end	Night	14% 9% 19% 28% 27% 3	360
July	1st to 10th	Night	15% 4 18% 35% 26% 2	350
	11th to 20th	Night	11% 6% 19% 36% 25% 3	348
	21st to end	Night	9% 5% 15% 34% 31% 5%	394
August	1st to 10th	Night	9% 4 11% 29% 41% 6%	358
	11th to 20th	Night	8% 3 13% 38% 33% 4	356
	21st to end	Night	13% 7% 17% 34% 27% 3	692
September	1st to 10th	Night	14% 6% 21% 29% 26% 3	720
	11th to 20th	Night	18% 7% 15% 28% 28% 5%	720
	21st to end	Night	20% 10% 18% 31% 18% 3	720
October	1st to 10th	Night	22% 9% 16% 30% 20% 3	720
	11th to 20th	Night	24% 12% 22% 25% 16% 1	720
	21st to end	Night	27% 11% 27% 23% 11% 1	792
November	1st to 10th	Night	31% 13% 26% 22% 8% 1	740
	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 2024-01-09 22:25:03. Version ac07

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