Spirit River Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spirit River Observatory clear sky chart from 2005-11-06 to 2025-01-22.

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	9% 9% 3 8% 6% 5% 5% 5% 7% 8% 35%	108799
	Night	8% 11% 4 9% 6% 6% 6% 5% 7% 8% 32%	109126
Transparency	Day	45% 7% 16% 23% 8% 1	108519
	Night	49% 7% 14% 21% 8% 1	109052
Seeing	Day	14% 8% 20% 29% 21% 7%	16811
	Night	14% 7% 23% 34% 19% 3	24987

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	5% 13% 8% 16% 9% 9% 7% 4 5% 7% 18%	1959
	11th to 20th	Day	6% 12% 4 11% 7% 10% 8% 5% 7% 7% 23%	1942
	21st to end	Day	8% 12% 4 8% 5% 5% 7% 6% 7% 11% 29%	2541
February	1st to 10th	Day	7% 14% 5% 10% 7% 6% 5% 4 9% 8% 25%	1520
	11th to 20th	Day	5% 16% 4 10% 8% 6% 4 5% 7% 9% 26%	1523
	21st to end	Day	6% 13% 4 11% 7% 8% 5% 5% 8% 6% 28%	1479
March	1st to 10th	Day	3 8% 4 12% 10% 9% 6% 5% 8% 9% 28%	3138
	11th to 20th	Day	5% 10% 4 10% 7% 6% 7% 5% 6% 10% 31%	3270
	21st to end	Day	4 12% 4 13% 6% 5% 5% 4 6% 7% 32%	4165
April	1st to 10th	Day	4 11% 4 10% 8% 7% 7% 5% 6% 6% 32%	2280
	11th to 20th	Day	9% 12% 4 8% 5% 5% 5% 5% 7% 10% 32%	2458
	21st to end	Day	11% 9% 4 8% 5% 6% 6% 5% 7% 8% 31%	2660
May	1st to 10th	Day	7% 10% 3 7% 5% 5% 6% 5% 7% 8% 37%	4478
	11th to 20th	Day	8% 9% 4 8% 6% 6% 6% 5% 7% 8% 33%	4776
	21st to end	Day	12% 8% 3 8% 6% 5% 6% 5% 7% 9% 30%	5509
June	1st to 10th	Day	13% 8% 2 7% 6% 6% 5% 5% 7% 8% 35%	3040
	11th to 20th	Day	17% 9% 3 9% 6% 6% 5% 5% 6% 8% 26%	3040
	21st to end	Day	10% 6% 2 8% 6% 5% 6% 5% 7% 9% 35%	3040
July	1st to 10th	Day	11% 5% 2 5% 4 3 5% 4 6% 8% 48%	5060
	11th to 20th	Day	7% 4 2 5% 5% 5% 5% 4 7% 7% 50%	5060
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 40%	5377
August	1st to 10th	Day	9% 6% 2 5% 5% 4 5% 4 6% 8% 47%	4401
	11th to 20th	Day	10% 6% 3 7% 5% 4 5% 4 7% 8% 40%	4330
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	4650
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 7% 8% 42%	2280
	11th to 20th	Day	11% 7% 2 6% 5% 4 5% 4 7% 9% 40%	2252
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 8% 9% 40%	2072
October	1st to 10th	Day	12% 10% 3 8% 5% 5% 4 4 6% 8% 34%	3180
	11th to 20th	Day	9% 9% 3 8% 5% 5% 6% 5% 7% 8% 35%	2884
	21st to end	Day	9% 11% 4 10% 5% 4 4 4 6% 7% 34%	3015
November	1st to 10th	Day	13% 14% 5% 9% 6% 6% 4 3 4 6% 30%	1475
	11th to 20th	Day	8% 15% 4 10% 8% 6% 5% 4 6% 7% 27%	1400
	21st to end	Day	6% 15% 4 10% 8% 5% 4 4 6% 8% 29%	1331
December	1st to 10th	Day	12% 19% 4 9% 6% 5% 6% 5% 5% 7% 22%	1960
	11th to 20th	Day	9% 13% 4 11% 8% 8% 6% 5% 6% 8% 21%	2262
	21st to end	Day	3 7% 3 9% 6% 6% 7% 5% 8% 9% 36%	2992
January	1st to 10th	Night	4 15% 7% 17% 8% 6% 5% 5% 5% 7% 22%	5206
	11th to 20th	Night	5% 11% 4 10% 7% 6% 7% 7% 8% 9% 26%	5176
	21st to end	Night	10% 15% 4 9% 5% 6% 6% 4 6% 8% 26%	5164
February	1st to 10th	Night	7% 14% 5% 10% 8% 7% 5% 5% 6% 7% 26%	2660
	11th to 20th	Night	6% 14% 5% 12% 7% 5% 5% 5% 6% 8% 28%	2539
	21st to end	Night	4 12% 4 10% 7% 7% 7% 5% 6% 7% 30%	1974
March	1st to 10th	Night	3 9% 5% 14% 9% 6% 6% 5% 6% 8% 30%	3736
	11th to 20th	Night	4 11% 4 10% 6% 6% 5% 5% 7% 10% 30%	3706
	21st to end	Night	5% 9% 3 11% 6% 5% 5% 5% 7% 8% 36%	3488
April	1st to 10th	Night	5% 11% 3 9% 8% 6% 6% 4 8% 7% 32%	1900
	11th to 20th	Night	10% 12% 2 8% 6% 5% 6% 5% 6% 7% 32%	1669
	21st to end	Night	12% 10% 4 9% 6% 5% 5% 5% 7% 7% 31%	1520
May	1st to 10th	Night	6% 10% 4 9% 5% 4 5% 4 9% 10% 34%	2481
	11th to 20th	Night	9% 8% 3 7% 6% 6% 6% 5% 7% 8% 35%	2116
	21st to end	Night	13% 9% 4 9% 6% 6% 6% 5% 7% 10% 27%	2066
June	1st to 10th	Night	13% 8% 2 8% 7% 5% 6% 5% 6% 7% 34%	1140
	11th to 20th	Night	17% 9% 3 7% 5% 5% 6% 5% 8% 7% 29%	1140
	21st to end	Night	11% 6% 3 7% 6% 7% 5% 5% 6% 7% 38%	1140
July	1st to 10th	Night	9% 5% 2 6% 4 3 4 4 7% 8% 47%	1900
	11th to 20th	Night	7% 4 2 5% 5% 5% 7% 5% 7% 9% 43%	1900
	21st to end	Night	9% 6% 2 6% 4 5% 5% 6% 8% 11% 36%	2172
August	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 6% 8% 48%	2529
	11th to 20th	Night	10% 5% 2 7% 4 5% 5% 4 7% 8% 44%	2497
	21st to end	Night	9% 5% 2 7% 5% 6% 6% 5% 8% 8% 39%	3011
September	1st to 10th	Night	13% 6% 2 7% 5% 5% 4 4 7% 8% 40%	1900
	11th to 20th	Night	10% 6% 3 6% 4 5% 5% 4 6% 7% 44%	1928
	21st to end	Night	9% 8% 2 7% 5% 5% 6% 4 6% 8% 40%	2104
October	1st to 10th	Night	10% 11% 4 10% 5% 5% 5% 3 5% 6% 33%	3800
	11th to 20th	Night	7% 8% 3 6% 6% 5% 6% 4 6% 8% 40%	4045
	21st to end	Night	9% 10% 3 8% 6% 5% 6% 4 7% 7% 33%	4537
November	1st to 10th	Night	13% 15% 5% 9% 6% 6% 4 3 6% 7% 28%	2803
	11th to 20th	Night	7% 16% 5% 11% 7% 6% 5% 4 6% 7% 28%	3000
	21st to end	Night	7% 15% 5% 9% 8% 6% 5% 4 6% 7% 27%	3065
December	1st to 10th	Night	9% 13% 4 10% 6% 7% 7% 5% 6% 6% 26%	5220
	11th to 20th	Night	9% 16% 5% 12% 7% 7% 7% 5% 6% 7% 19%	5930
	21st to end	Night	4 10% 4 10% 7% 7% 6% 4 7% 7% 34%	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	65% 9% 9% 13% 4	1959
	11th to 20th	Day	58% 5% 9% 11% 15% 2	1960
	21st to end	Day	46% 15% 19% 13% 6% 1	2541
February	1st to 10th	Day	54% 7% 13% 14% 11% 1	1520
	11th to 20th	Day	54% 9% 14% 17% 7% 1	1531
	21st to end	Day	54% 7% 12% 19% 8%	1479
March	1st to 10th	Day	51% 7% 14% 22% 7%	3138
	11th to 20th	Day	45% 10% 14% 21% 9%	3270
	21st to end	Day	48% 8% 16% 22% 7%	4165
April	1st to 10th	Day	49% 8% 13% 20% 9%	2280
	11th to 20th	Day	46% 7% 15% 22% 10% 1	2458
	21st to end	Day	47% 6% 15% 24% 7% 1	2660
May	1st to 10th	Day	42% 6% 14% 24% 13% 1	4478
	11th to 20th	Day	44% 6% 13% 25% 11% 1	4780
	21st to end	Day	46% 8% 14% 23% 8% 1	5510
June	1st to 10th	Day	45% 6% 15% 24% 9% 1	3040
	11th to 20th	Day	55% 7% 13% 19% 6%	3040
	21st to end	Day	42% 8% 18% 25% 8%	3040
July	1st to 10th	Day	33% 6% 16% 26% 18% 1	4981
	11th to 20th	Day	32% 7% 16% 30% 13% 2	4973
	21st to end	Day	39% 7% 21% 26% 6% 1	5206
August	1st to 10th	Day	35% 8% 18% 34% 6%	4384
	11th to 20th	Day	39% 7% 18% 23% 12%	4372
	21st to end	Day	43% 6% 23% 24% 4	4649
September	1st to 10th	Day	39% 6% 25% 23% 6% 1	2280
	11th to 20th	Day	40% 6% 22% 26% 6%	2252
	21st to end	Day	39% 7% 20% 24% 10% 1	2072
October	1st to 10th	Day	47% 5% 19% 23% 5% 1	3180
	11th to 20th	Day	44% 6% 18% 27% 5%	2884
	21st to end	Day	49% 5% 22% 22% 4	3015
November	1st to 10th	Day	56% 5% 12% 23% 4	1475
	11th to 20th	Day	57% 8% 14% 16% 5%	1400
	21st to end	Day	55% 8% 11% 18% 6% 1	1331
December	1st to 10th	Day	63% 8% 12% 9% 7% 1	1960
	11th to 20th	Day	61% 7% 15% 13% 4	2264
	21st to end	Day	42% 9% 14% 24% 9% 1	2992
January	1st to 10th	Night	61% 9% 9% 15% 5% 1	5206
	11th to 20th	Night	52% 6% 11% 12% 16% 2	5220
	21st to end	Night	53% 11% 17% 13% 6%	5164
February	1st to 10th	Night	56% 6% 13% 16% 10%	2660
	11th to 20th	Night	55% 6% 13% 18% 8% 1	2541
	21st to end	Night	51% 5% 15% 21% 8% 1	1974
March	1st to 10th	Night	51% 6% 12% 22% 9% 1	3736
	11th to 20th	Night	44% 9% 18% 20% 8%	3707
	21st to end	Night	42% 8% 15% 26% 10%	3488
April	1st to 10th	Night	47% 7% 10% 21% 14% 1	1900
	11th to 20th	Night	50% 5% 12% 19% 12% 2	1669
	21st to end	Night	50% 7% 9% 24% 9% 2	1520
May	1st to 10th	Night	41% 7% 10% 25% 15% 1	2481
	11th to 20th	Night	42% 8% 11% 24% 13% 2	2116
	21st to end	Night	47% 11% 11% 22% 8% 1	2066
June	1st to 10th	Night	48% 7% 9% 24% 11% 1	1140
	11th to 20th	Night	50% 7% 14% 22% 7%	1140
	21st to end	Night	44% 8% 14% 23% 12%	1140
July	1st to 10th	Night	33% 6% 14% 28% 18% 1	1870
	11th to 20th	Night	36% 7% 15% 23% 17% 1	1864
	21st to end	Night	38% 7% 23% 23% 7%	2104
August	1st to 10th	Night	34% 6% 18% 37% 5%	2521
	11th to 20th	Night	36% 7% 15% 27% 14% 1	2521
	21st to end	Night	40% 7% 26% 22% 5% 1	3011
September	1st to 10th	Night	40% 7% 20% 25% 7% 1	1900
	11th to 20th	Night	39% 7% 17% 30% 7%	1928
	21st to end	Night	40% 8% 15% 24% 12% 1	2104
October	1st to 10th	Night	51% 6% 15% 19% 9% 1	3800
	11th to 20th	Night	42% 5% 15% 30% 8%	4045
	21st to end	Night	48% 5% 17% 24% 6%	4537
November	1st to 10th	Night	56% 6% 12% 21% 5%	2803
	11th to 20th	Night	56% 8% 13% 18% 5%	3000
	21st to end	Night	56% 7% 12% 18% 5% 1	3065
December	1st to 10th	Night	58% 7% 11% 16% 6%	5220
	11th to 20th	Night	64% 8% 12% 11% 5%	5927
	21st to end	Night	48% 7% 14% 24% 6% 2	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	12% 10% 17% 36% 19% 8%	200
	11th to 20th	Day	13% 12% 22% 32% 17% 6%	200
	21st to end	Day	14% 9% 25% 30% 19% 4	200
February	1st to 10th	Day	15% 8% 22% 33% 18% 4	180
	11th to 20th	Day	11% 9% 20% 28% 26% 6%	223
	21st to end	Day	9% 6% 20% 36% 25% 4	339
March	1st to 10th	Day	9% 6% 19% 34% 24% 8%	460
	11th to 20th	Day	10% 6% 18% 36% 23% 8%	460
	21st to end	Day	15% 7% 19% 28% 23% 8%	506
April	1st to 10th	Day	9% 12% 26% 31% 14% 9%	460
	11th to 20th	Day	15% 7% 19% 30% 21% 8%	452
	21st to end	Day	15% 9% 21% 27% 21% 7%	460
May	1st to 10th	Day	12% 9% 22% 34% 17% 5%	495
	11th to 20th	Day	12% 6% 21% 27% 27% 7%	717
	21st to end	Day	14% 6% 18% 29% 24% 9%	940
June	1st to 10th	Day	15% 8% 24% 30% 18% 5%	870
	11th to 20th	Day	20% 7% 18% 27% 23% 6%	870
	21st to end	Day	11% 5% 18% 27% 26% 13%	870
July	1st to 10th	Day	14% 4 16% 30% 26% 9%	856
	11th to 20th	Day	11% 4 19% 30% 24% 12%	852
	21st to end	Day	13% 6% 17% 31% 25% 8%	886
August	1st to 10th	Day	16% 7% 14% 26% 24% 13%	487
	11th to 20th	Day	10% 10% 27% 25% 21% 8%	477
	21st to end	Day	16% 10% 23% 24% 18% 9%	539
September	1st to 10th	Day	18% 7% 24% 25% 18% 8%	490
	11th to 20th	Day	16% 8% 22% 27% 21% 7%	490
	21st to end	Day	13% 14% 30% 23% 16% 4	490
October	1st to 10th	Day	17% 12% 22% 27% 17% 4	490
	11th to 20th	Day	18% 13% 21% 29% 16% 4	329
	21st to end	Day	17% 10% 24% 27% 17% 6%	303
November	1st to 10th	Day	19% 9% 18% 32% 18% 5%	200
	11th to 20th	Day	15% 8% 23% 36% 15% 4	200
	21st to end	Day	12% 13% 22% 34% 16% 4	200
December	1st to 10th	Day	11% 15% 22% 34% 15% 5%	200
	11th to 20th	Day	14% 11% 21% 37% 15% 4	200
	21st to end	Day	10% 5% 21% 33% 25% 6%	220
January	1st to 10th	Night	13% 8% 20% 36% 19% 3	1100
	11th to 20th	Night	14% 11% 23% 34% 16% 2	1100
	21st to end	Night	20% 13% 24% 29% 12% 1	929
February	1st to 10th	Night	14% 7% 20% 35% 18% 6%	760
	11th to 20th	Night	12% 8% 24% 34% 17% 4	744
	21st to end	Night	8% 7% 21% 37% 23% 4	628
March	1st to 10th	Night	9% 7% 26% 32% 22% 5%	760
	11th to 20th	Night	13% 5% 21% 41% 18% 2	760
	21st to end	Night	16% 5% 18% 34% 25% 3	836
April	1st to 10th	Night	9% 7% 23% 36% 23% 3	760
	11th to 20th	Night	15% 6% 21% 34% 23% 2	541
	21st to end	Night	15% 7% 22% 31% 21% 3	380
May	1st to 10th	Night	12% 8% 24% 37% 17% 2	380
	11th to 20th	Night	11% 6% 23% 33% 25% 2	374
	21st to end	Night	17% 4 16% 35% 26% 3	411
June	1st to 10th	Night	16% 6% 23% 33% 22% 1	380
	11th to 20th	Night	20% 5% 19% 31% 23% 3	380
	21st to end	Night	13% 4 17% 34% 29% 3	380
July	1st to 10th	Night	12% 5% 18% 35% 28% 2	370
	11th to 20th	Night	13% 2 18% 36% 25% 6%	368
	21st to end	Night	8% 4 18% 37% 28% 4	416
August	1st to 10th	Night	9% 2 17% 33% 33% 6%	378
	11th to 20th	Night	8% 5% 26% 36% 22% 3	376
	21st to end	Night	12% 5% 24% 38% 20% 2	544
September	1st to 10th	Night	16% 5% 20% 38% 20% 2	760
	11th to 20th	Night	12% 8% 19% 36% 22% 3	760
	21st to end	Night	14% 7% 31% 30% 16% 2	760
October	1st to 10th	Night	16% 9% 25% 33% 14% 2	760
	11th to 20th	Night	16% 8% 26% 34% 13% 2	760
	21st to end	Night	19% 10% 28% 30% 11% 2	836
November	1st to 10th	Night	20% 7% 25% 29% 16% 3	853
	11th to 20th	Night	18% 8% 26% 35% 12% 1	1000
	21st to end	Night	16% 10% 24% 36% 12% 2	1033
December	1st to 10th	Night	13% 10% 27% 35% 13% 2	1100
	11th to 20th	Night	15% 11% 22% 33% 16% 3	1100
	21st to end	Night	12% 6% 22% 35% 20% 3	1210

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