Chetwynd
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chetwynd clear sky chart from 2005-11-06 to 2025-02-06.

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	8% 10% 4 9% 6% 6% 6% 5% 7% 7% 33%	111445
	Night	7% 11% 4 10% 6% 6% 6% 4 7% 8% 33%	115432
Transparency	Day	46% 7% 16% 21% 9% 1	111175
	Night	47% 7% 16% 21% 8% 1	115349
Seeing	Day	18% 17% 29% 23% 11% 3	16900
	Night	16% 16% 28% 26% 13% 1	24938

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	3 10% 6% 13% 9% 9% 8% 4 6% 7% 25%	2719
	11th to 20th	Day	5% 13% 4 11% 6% 8% 8% 5% 9% 7% 23%	2989
	21st to end	Day	6% 11% 4 10% 6% 6% 5% 4 7% 8% 33%	3498
February	1st to 10th	Day	6% 11% 5% 10% 8% 8% 8% 4 6% 5% 29%	1502
	11th to 20th	Day	3 15% 6% 11% 6% 6% 5% 5% 6% 7% 31%	1617
	21st to end	Day	3 12% 5% 14% 9% 7% 7% 5% 6% 5% 27%	1413
March	1st to 10th	Day	2 9% 4 13% 10% 8% 6% 5% 7% 8% 27%	3122
	11th to 20th	Day	9% 13% 4 9% 5% 4 5% 4 6% 8% 34%	3419
	21st to end	Day	3 13% 5% 14% 7% 5% 5% 4 5% 6% 32%	4037
April	1st to 10th	Day	5% 11% 5% 12% 8% 7% 6% 4 7% 7% 28%	2280
	11th to 20th	Day	8% 15% 5% 11% 7% 6% 5% 4 6% 6% 27%	2474
	21st to end	Day	11% 13% 5% 9% 5% 5% 5% 4 7% 6% 29%	2660
May	1st to 10th	Day	6% 8% 3 10% 6% 6% 7% 5% 7% 8% 33%	4420
	11th to 20th	Day	8% 9% 4 10% 7% 7% 7% 5% 7% 7% 28%	4829
	21st to end	Day	13% 10% 4 9% 7% 7% 6% 5% 7% 7% 24%	5509
June	1st to 10th	Day	12% 8% 3 8% 7% 6% 6% 5% 7% 8% 30%	3040
	11th to 20th	Day	16% 10% 4 9% 7% 5% 6% 4 7% 7% 26%	3040
	21st to end	Day	9% 7% 3 7% 6% 7% 6% 5% 7% 8% 35%	3040
July	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 5% 7% 48%	5060
	11th to 20th	Day	8% 4 2 5% 4 4 5% 4 6% 8% 49%	5060
	21st to end	Day	8% 6% 2 7% 5% 5% 6% 6% 8% 10% 37%	5179
August	1st to 10th	Day	9% 5% 2 5% 5% 4 5% 3 5% 8% 48%	4546
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 7% 41%	4330
	21st to end	Day	11% 6% 3 7% 6% 5% 5% 5% 8% 8% 35%	4596
September	1st to 10th	Day	13% 6% 3 6% 4 5% 5% 4 6% 7% 41%	2280
	11th to 20th	Day	12% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	2280
	21st to end	Day	10% 9% 4 8% 6% 5% 6% 5% 7% 9% 31%	2016
October	1st to 10th	Day	11% 12% 3 9% 6% 7% 7% 4 6% 7% 29%	3180
	11th to 20th	Day	9% 11% 4 7% 5% 5% 5% 4 7% 9% 34%	3034
	21st to end	Day	9% 12% 4 8% 4 5% 5% 4 7% 8% 35%	2853
November	1st to 10th	Day	11% 16% 5% 10% 5% 4 5% 3 6% 6% 29%	1568
	11th to 20th	Day	6% 16% 6% 10% 8% 5% 4 4 5% 6% 28%	1425
	21st to end	Day	6% 15% 4 11% 9% 4 5% 4 6% 7% 29%	1216
December	1st to 10th	Day	7% 18% 6% 11% 7% 6% 6% 6% 6% 5% 24%	1960
	11th to 20th	Day	6% 17% 5% 10% 6% 7% 10% 6% 6% 7% 21%	2262
	21st to end	Day	4 9% 5% 8% 5% 4 5% 5% 7% 8% 41%	2992
January	1st to 10th	Night	3 12% 6% 15% 8% 6% 6% 4 6% 7% 27%	7226
	11th to 20th	Night	4 10% 4 12% 8% 7% 7% 6% 11% 8% 22%	6951
	21st to end	Night	6% 12% 5% 9% 5% 5% 5% 4 7% 8% 33%	7458
February	1st to 10th	Night	6% 15% 5% 11% 7% 7% 6% 4 6% 7% 27%	2787
	11th to 20th	Night	5% 13% 5% 11% 8% 5% 6% 5% 6% 8% 28%	2446
	21st to end	Night	3 13% 6% 12% 8% 6% 7% 5% 6% 7% 29%	2041
March	1st to 10th	Night	2 10% 4 13% 10% 8% 7% 4 7% 8% 28%	3752
	11th to 20th	Night	7% 12% 4 10% 7% 5% 6% 4 8% 8% 30%	3556
	21st to end	Night	4 11% 5% 13% 5% 4 4 4 6% 6% 39%	3616
April	1st to 10th	Night	3 10% 4 8% 7% 6% 6% 5% 7% 9% 34%	1900
	11th to 20th	Night	8% 13% 5% 7% 6% 5% 4 4 6% 7% 34%	1652
	21st to end	Night	11% 14% 3 8% 5% 5% 5% 3 6% 8% 31%	1520
May	1st to 10th	Night	6% 7% 4 8% 6% 5% 6% 6% 8% 8% 34%	2540
	11th to 20th	Night	12% 9% 2 5% 4 4 5% 5% 8% 9% 36%	2064
	21st to end	Night	12% 10% 4 9% 6% 6% 7% 6% 6% 8% 26%	2066
June	1st to 10th	Night	12% 11% 3 9% 7% 4 5% 4 5% 6% 34%	1140
	11th to 20th	Night	14% 10% 4 9% 6% 7% 5% 5% 7% 8% 25%	1140
	21st to end	Night	10% 6% 2 8% 5% 6% 3 5% 8% 11% 35%	1140
July	1st to 10th	Night	9% 5% 3 5% 5% 4 4 3 6% 9% 48%	1900
	11th to 20th	Night	6% 5% 2 5% 4 4 6% 4 7% 8% 49%	1900
	21st to end	Night	9% 5% 2 6% 6% 6% 6% 4 7% 9% 40%	2371
August	1st to 10th	Night	8% 5% 2 5% 3 4 4 4 6% 7% 53%	2370
	11th to 20th	Night	11% 4 2 5% 4 4 5% 4 7% 8% 46%	2497
	21st to end	Night	9% 7% 3 8% 5% 5% 5% 4 7% 9% 39%	3065
September	1st to 10th	Night	13% 7% 2 7% 5% 5% 4 3 6% 7% 41%	1900
	11th to 20th	Night	10% 7% 2 6% 4 4 6% 4 6% 8% 42%	1900
	21st to end	Night	10% 10% 3 7% 5% 5% 5% 4 6% 7% 37%	2161
October	1st to 10th	Night	11% 12% 4 8% 5% 5% 5% 4 6% 7% 33%	3800
	11th to 20th	Night	7% 9% 3 7% 5% 6% 6% 5% 7% 9% 37%	3899
	21st to end	Night	11% 14% 6% 9% 5% 4 4 4 5% 6% 31%	4697
November	1st to 10th	Night	10% 16% 4 8% 6% 5% 5% 3 5% 7% 30%	2730
	11th to 20th	Night	6% 15% 5% 13% 6% 5% 5% 3 5% 7% 30%	2953
	21st to end	Night	5% 14% 5% 13% 7% 6% 5% 4 6% 7% 28%	3180
December	1st to 10th	Night	6% 14% 5% 9% 7% 6% 5% 4 6% 7% 31%	5220
	11th to 20th	Night	8% 14% 5% 12% 7% 6% 7% 4 5% 6% 25%	5930
	21st to end	Night	3 10% 4 9% 6% 6% 6% 5% 7% 8% 36%	7964

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	55% 7% 14% 15% 7% 1	2719
	11th to 20th	Day	55% 8% 15% 13% 9% 1	3010
	21st to end	Day	46% 7% 23% 15% 10%	3498
February	1st to 10th	Day	53% 7% 13% 14% 12% 1	1502
	11th to 20th	Day	49% 10% 14% 20% 7% 1	1626
	21st to end	Day	56% 7% 14% 16% 6% 1	1413
March	1st to 10th	Day	50% 5% 14% 20% 11%	3122
	11th to 20th	Day	47% 5% 16% 24% 8%	3419
	21st to end	Day	49% 7% 14% 21% 8%	4037
April	1st to 10th	Day	51% 8% 13% 19% 9% 1	2280
	11th to 20th	Day	56% 6% 11% 19% 9% 1	2474
	21st to end	Day	52% 7% 14% 20% 6% 1	2660
May	1st to 10th	Day	43% 8% 15% 26% 8% 1	4420
	11th to 20th	Day	46% 11% 12% 20% 10% 1	4833
	21st to end	Day	52% 9% 11% 17% 10% 1	5510
June	1st to 10th	Day	47% 8% 14% 21% 9% 1	3040
	11th to 20th	Day	55% 7% 13% 18% 7%	3040
	21st to end	Day	42% 8% 17% 23% 9% 1	3040
July	1st to 10th	Day	33% 6% 13% 22% 23% 2	4981
	11th to 20th	Day	32% 6% 17% 30% 14% 1	4973
	21st to end	Day	37% 8% 21% 27% 6%	5015
August	1st to 10th	Day	34% 6% 18% 30% 12%	4528
	11th to 20th	Day	40% 6% 16% 23% 15% 1	4372
	21st to end	Day	42% 8% 27% 19% 3	4595
September	1st to 10th	Day	40% 7% 24% 24% 5% 1	2280
	11th to 20th	Day	44% 7% 18% 25% 6%	2280
	21st to end	Day	46% 9% 19% 19% 6% 1	2016
October	1st to 10th	Day	53% 7% 18% 17% 5%	3180
	11th to 20th	Day	44% 7% 20% 25% 4	3034
	21st to end	Day	45% 7% 25% 19% 5%	2853
November	1st to 10th	Day	53% 8% 13% 21% 5% 1	1568
	11th to 20th	Day	56% 7% 14% 17% 5%	1425
	21st to end	Day	51% 7% 17% 17% 7% 1	1216
December	1st to 10th	Day	61% 7% 14% 12% 6% 1	1960
	11th to 20th	Day	60% 8% 15% 13% 4	2264
	21st to end	Day	38% 6% 19% 27% 8% 1	2992
January	1st to 10th	Night	54% 7% 13% 17% 8% 1	7226
	11th to 20th	Night	52% 8% 14% 16% 10% 1	6992
	21st to end	Night	46% 7% 21% 14% 10%	7458
February	1st to 10th	Night	56% 5% 14% 15% 10%	2787
	11th to 20th	Night	50% 8% 16% 18% 7% 1	2448
	21st to end	Night	53% 7% 13% 19% 8% 1	2041
March	1st to 10th	Night	51% 7% 13% 17% 11% 1	3752
	11th to 20th	Night	48% 8% 12% 22% 9% 1	3557
	21st to end	Night	44% 6% 15% 24% 11%	3616
April	1st to 10th	Night	41% 9% 17% 23% 10% 1	1900
	11th to 20th	Night	48% 6% 12% 24% 10%	1652
	21st to end	Night	50% 6% 11% 25% 7% 1	1520
May	1st to 10th	Night	40% 8% 15% 23% 12% 2	2540
	11th to 20th	Night	37% 9% 15% 27% 11% 1	2064
	21st to end	Night	49% 9% 14% 17% 9% 1	2066
June	1st to 10th	Night	48% 6% 11% 23% 11%	1140
	11th to 20th	Night	55% 7% 11% 20% 8% 1	1140
	21st to end	Night	39% 7% 19% 24% 10% 1	1140
July	1st to 10th	Night	33% 5% 13% 29% 18% 1	1870
	11th to 20th	Night	31% 6% 19% 25% 19% 1	1864
	21st to end	Night	37% 8% 21% 25% 8%	2296
August	1st to 10th	Night	28% 7% 19% 32% 14%	2363
	11th to 20th	Night	34% 7% 16% 27% 17% 1	2521
	21st to end	Night	40% 7% 25% 22% 5%	3065
September	1st to 10th	Night	41% 6% 19% 27% 6% 1	1900
	11th to 20th	Night	38% 8% 21% 28% 5%	1900
	21st to end	Night	42% 10% 17% 22% 8% 1	2161
October	1st to 10th	Night	48% 7% 19% 19% 6%	3800
	11th to 20th	Night	40% 8% 20% 26% 6%	3899
	21st to end	Night	53% 6% 16% 20% 4	4697
November	1st to 10th	Night	52% 8% 14% 20% 5% 1	2730
	11th to 20th	Night	54% 7% 14% 19% 6%	2953
	21st to end	Night	53% 7% 16% 18% 5% 1	3180
December	1st to 10th	Night	51% 7% 18% 18% 5%	5220
	11th to 20th	Night	58% 7% 15% 15% 5%	5927
	21st to end	Night	43% 7% 20% 23% 6% 1	7964

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	16% 17% 29% 27% 8% 4	200
	11th to 20th	Day	19% 18% 28% 23% 11% 3	200
	21st to end	Day	23% 19% 30% 19% 8% 1	220
February	1st to 10th	Day	16% 18% 32% 20% 11% 2	190
	11th to 20th	Day	16% 18% 34% 21% 10% 1	317
	21st to end	Day	13% 20% 37% 20% 8% 1	307
March	1st to 10th	Day	15% 20% 28% 22% 12% 2	454
	11th to 20th	Day	14% 21% 34% 21% 8% 2	460
	21st to end	Day	20% 16% 28% 22% 13% 2	506
April	1st to 10th	Day	20% 18% 30% 22% 8% 3	460
	11th to 20th	Day	20% 17% 31% 19% 12% 2	452
	21st to end	Day	21% 20% 30% 19% 8% 2	460
May	1st to 10th	Day	19% 19% 30% 21% 9% 2	463
	11th to 20th	Day	17% 14% 26% 27% 12% 3	743
	21st to end	Day	18% 14% 25% 25% 15% 4	940
June	1st to 10th	Day	17% 19% 30% 22% 12% 1	870
	11th to 20th	Day	21% 15% 26% 24% 12% 2	870
	21st to end	Day	13% 14% 25% 25% 18% 5%	870
July	1st to 10th	Day	15% 12% 26% 29% 16% 2	856
	11th to 20th	Day	15% 12% 32% 22% 16% 3	852
	21st to end	Day	12% 13% 35% 27% 9% 4	767
August	1st to 10th	Day	14% 12% 24% 28% 18% 4	581
	11th to 20th	Day	14% 20% 34% 18% 10% 3	477
	21st to end	Day	18% 21% 29% 20% 8% 3	539
September	1st to 10th	Day	21% 15% 33% 18% 10% 4	490
	11th to 20th	Day	20% 15% 29% 23% 9% 4	490
	21st to end	Day	19% 27% 30% 17% 6%	490
October	1st to 10th	Day	29% 17% 27% 19% 6% 2	490
	11th to 20th	Day	24% 18% 28% 21% 7% 1	414
	21st to end	Day	23% 15% 31% 21% 9% 2	252
November	1st to 10th	Day	24% 26% 26% 16% 7% 2	200
	11th to 20th	Day	25% 22% 33% 15% 5% 1	200
	21st to end	Day	23% 22% 28% 20% 8% 1	200
December	1st to 10th	Day	16% 22% 29% 26% 7% 2	200
	11th to 20th	Day	17% 23% 26% 20% 13% 2	200
	21st to end	Day	16% 15% 23% 30% 13% 4	220
January	1st to 10th	Night	13% 18% 32% 25% 11% 2	1100
	11th to 20th	Night	17% 20% 30% 23% 8% 1	995
	21st to end	Night	19% 24% 29% 21% 6% 1	990
February	1st to 10th	Night	18% 17% 22% 26% 15% 2	807
	11th to 20th	Night	14% 17% 30% 27% 11% 1	744
	21st to end	Night	10% 16% 31% 27% 15% 1	628
March	1st to 10th	Night	11% 17% 30% 27% 13% 2	760
	11th to 20th	Night	11% 18% 30% 28% 12% 1	760
	21st to end	Night	13% 13% 25% 26% 20% 2	836
April	1st to 10th	Night	10% 15% 28% 28% 18% 2	760
	11th to 20th	Night	16% 14% 24% 27% 17% 2	524
	21st to end	Night	18% 13% 27% 25% 16% 1	380
May	1st to 10th	Night	15% 12% 27% 27% 17% 3	380
	11th to 20th	Night	17% 9% 22% 25% 26% 2	374
	21st to end	Night	17% 11% 19% 27% 23% 2	411
June	1st to 10th	Night	16% 9% 28% 28% 17% 2	380
	11th to 20th	Night	20% 8% 25% 26% 19% 3	380
	21st to end	Night	13% 8% 24% 30% 22% 3	380
July	1st to 10th	Night	13% 6% 28% 36% 16% 1	370
	11th to 20th	Night	14% 7% 29% 29% 20% 1	368
	21st to end	Night	9% 10% 26% 33% 19% 3	416
August	1st to 10th	Night	12% 5% 20% 28% 29% 5%	378
	11th to 20th	Night	9% 13% 26% 30% 20% 2	376
	21st to end	Night	13% 8% 29% 31% 16% 3	572
September	1st to 10th	Night	16% 10% 29% 29% 14% 1	760
	11th to 20th	Night	16% 13% 24% 26% 19% 3	760
	21st to end	Night	19% 21% 26% 24% 8% 1	760
October	1st to 10th	Night	22% 15% 25% 27% 10% 1	760
	11th to 20th	Night	18% 17% 31% 26% 8% 1	760
	21st to end	Night	23% 18% 28% 23% 7%	836
November	1st to 10th	Night	21% 18% 26% 24% 8% 2	780
	11th to 20th	Night	18% 23% 32% 24% 4	953
	21st to end	Night	19% 21% 31% 22% 7%	1090
December	1st to 10th	Night	16% 20% 35% 22% 6% 1	1100
	11th to 20th	Night	16% 19% 30% 25% 9% 1	1100
	21st to end	Night	14% 18% 28% 28% 11% 2	1210

Page generated at 2025-02-03 01:05:52. Version ac07

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