Cranbrook
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cranbrook 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	4 7% 3 8% 6% 6% 6% 5% 7% 9% 39%	111973
	Night	3 8% 3 9% 6% 6% 6% 5% 7% 8% 39%	114917
Transparency	Day	40% 11% 17% 20% 10% 2	111730
	Night	41% 12% 18% 19% 8% 2	114805
Seeing	Day	11% 18% 30% 26% 12% 2	15364
	Night	8% 11% 28% 35% 16% 2	24645

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	1 9% 7% 19% 10% 10% 8% 6% 8% 8% 15%	3207
	11th to 20th	Day	2 9% 4 12% 7% 6% 7% 5% 7% 9% 31%	3616
	21st to end	Day	1 7% 4 10% 6% 7% 8% 6% 8% 12% 32%	4004
February	1st to 10th	Day	3 7% 3 14% 8% 7% 8% 6% 9% 10% 26%	1560
	11th to 20th	Day	2 8% 4 12% 9% 7% 6% 6% 9% 10% 28%	1688
	21st to end	Day	1 8% 4 10% 8% 8% 8% 7% 9% 9% 27%	1570
March	1st to 10th	Day	1 9% 3 9% 6% 6% 7% 6% 9% 10% 33%	3138
	11th to 20th	Day	1 6% 2 6% 5% 5% 6% 6% 8% 11% 43%	3357
	21st to end	Day	4 11% 4 10% 8% 7% 8% 6% 9% 9% 24%	4005
April	1st to 10th	Day	5% 8% 4 10% 7% 7% 6% 6% 8% 9% 29%	2280
	11th to 20th	Day	4 11% 4 10% 7% 5% 6% 5% 8% 8% 32%	2407
	21st to end	Day	6% 11% 4 9% 7% 6% 5% 5% 7% 8% 32%	2470
May	1st to 10th	Day	8% 13% 5% 10% 7% 6% 6% 4 6% 6% 29%	4313
	11th to 20th	Day	6% 7% 3 7% 6% 6% 7% 6% 8% 10% 33%	4490
	21st to end	Day	10% 9% 3 7% 6% 6% 7% 6% 8% 9% 28%	5161
June	1st to 10th	Day	11% 9% 3 8% 7% 7% 6% 4 6% 8% 31%	2850
	11th to 20th	Day	9% 8% 3 8% 7% 8% 7% 6% 8% 9% 27%	2850
	21st to end	Day	5% 4 2 6% 6% 6% 6% 5% 7% 9% 44%	2850
July	1st to 10th	Day	2 2 1 4 4 4 4 4 6% 7% 60%	4740
	11th to 20th	Day	3 1 1 2 3 2 4 3 5% 7% 70%	4627
	21st to end	Day	1 2 1 3 3 3 4 4 6% 8% 64%	4790
August	1st to 10th	Day	3 3 1 3 3 4 5% 5% 7% 10% 55%	4401
	11th to 20th	Day	5% 3 1 3 3 4 4 5% 7% 8% 58%	4118
	21st to end	Day	3 3 1 3 3 4 6% 5% 7% 10% 54%	4469
September	1st to 10th	Day	4 4 2 5% 3 4 5% 5% 6% 7% 55%	2280
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 4 6% 8% 49%	2125
	21st to end	Day	6% 4 2 5% 5% 5% 6% 4 8% 9% 47%	2090
October	1st to 10th	Day	3 4 2 5% 4 4 5% 5% 7% 10% 52%	3445
	11th to 20th	Day	2 4 2 6% 5% 5% 7% 6% 7% 11% 46%	2842
	21st to end	Day	5% 8% 4 7% 5% 6% 6% 5% 8% 8% 39%	3101
November	1st to 10th	Day	5% 11% 4 12% 9% 7% 7% 4 7% 9% 27%	1764
	11th to 20th	Day	2 8% 4 10% 6% 6% 8% 7% 8% 9% 32%	1535
	21st to end	Day	2 10% 3 11% 8% 7% 7% 6% 8% 11% 29%	1400
December	1st to 10th	Day	2 7% 3 12% 9% 9% 9% 7% 9% 10% 25%	2290
	11th to 20th	Day	2 10% 7% 17% 9% 8% 8% 7% 7% 7% 18%	2642
	21st to end	Day	2 10% 6% 17% 10% 9% 8% 5% 6% 7% 20%	3498
January	1st to 10th	Night	1 15% 8% 15% 8% 8% 7% 5% 8% 8% 17%	6766
	11th to 20th	Night	2 9% 4 11% 7% 8% 8% 6% 8% 8% 30%	6285
	21st to end	Night	2 10% 4 10% 6% 7% 8% 6% 9% 10% 28%	6952
February	1st to 10th	Night	3 12% 5% 11% 8% 6% 6% 5% 8% 9% 28%	2730
	11th to 20th	Night	2 9% 4 9% 6% 6% 8% 6% 8% 8% 34%	2377
	21st to end	Night	2 9% 4 10% 6% 7% 6% 5% 9% 9% 33%	1884
March	1st to 10th	Night	2 5% 3 9% 6% 5% 6% 5% 9% 10% 41%	3736
	11th to 20th	Night	2 6% 3 5% 5% 5% 6% 6% 7% 8% 47%	3621
	21st to end	Night	6% 11% 4 9% 7% 6% 6% 5% 8% 9% 30%	3647
April	1st to 10th	Night	4 7% 3 7% 5% 5% 5% 4 7% 11% 42%	1900
	11th to 20th	Night	4 9% 4 7% 6% 5% 6% 5% 7% 10% 38%	1724
	21st to end	Night	5% 9% 3 7% 5% 6% 5% 4 7% 9% 40%	1710
May	1st to 10th	Night	9% 12% 5% 9% 6% 5% 6% 4 5% 7% 33%	2648
	11th to 20th	Night	4 5% 2 7% 5% 5% 5% 5% 8% 10% 45%	2401
	21st to end	Night	8% 10% 3 8% 6% 4 6% 5% 6% 9% 35%	2414
June	1st to 10th	Night	8% 4 2 7% 6% 5% 4 4 7% 10% 41%	1330
	11th to 20th	Night	8% 6% 2 8% 4 6% 7% 6% 7% 9% 37%	1330
	21st to end	Night	3 3 2 5% 3 4 5% 5% 7% 8% 54%	1330
July	1st to 10th	Night	2 3 1 4 3 3 4 4 6% 8% 63%	2220
	11th to 20th	Night	1 2 1 3 3 3 3 2 4 6% 73%	2316
	21st to end	Night	1 2 1 2 2 2 2 2 4 6% 76%	2762
August	1st to 10th	Night	4 3 1 3 3 2 4 3 5% 9% 64%	2529
	11th to 20th	Night	2 2 2 4 3 3 4 3 7% 10% 60%	2716
	21st to end	Night	3 2 1 3 3 3 4 4 7% 10% 60%	3196
September	1st to 10th	Night	4 3 1 3 2 3 4 3 5% 7% 63%	1900
	11th to 20th	Night	5% 5% 2 5% 3 3 4 3 6% 7% 57%	2054
	21st to end	Night	4 4 2 5% 3 4 4 5% 6% 8% 55%	2090
October	1st to 10th	Night	3 4 1 4 2 3 4 3 6% 8% 61%	3532
	11th to 20th	Night	2 4 1 5% 4 5% 5% 4 6% 9% 55%	4086
	21st to end	Night	2 6% 2 6% 4 5% 5% 5% 9% 9% 47%	4457
November	1st to 10th	Night	4 9% 3 9% 6% 5% 7% 5% 8% 11% 33%	2535
	11th to 20th	Night	2 9% 5% 9% 7% 6% 7% 5% 8% 10% 32%	2841
	21st to end	Night	2 14% 5% 12% 7% 6% 7% 6% 8% 8% 27%	3000
December	1st to 10th	Night	2 7% 3 11% 9% 8% 10% 6% 8% 10% 28%	4890
	11th to 20th	Night	1 10% 6% 20% 11% 8% 8% 5% 6% 7% 17%	5550
	21st to end	Night	1 11% 6% 16% 10% 9% 8% 6% 7% 7% 19%	7458

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	63% 13% 11% 10% 3	3207
	11th to 20th	Day	45% 14% 18% 16% 4 3	3640
	21st to end	Day	49% 13% 21% 11% 6%	4004
February	1st to 10th	Day	52% 15% 16% 10% 6% 1	1560
	11th to 20th	Day	50% 16% 18% 12% 5%	1697
	21st to end	Day	49% 16% 17% 13% 5%	1570
March	1st to 10th	Day	46% 10% 18% 19% 8%	3138
	11th to 20th	Day	35% 10% 34% 17% 3	3358
	21st to end	Day	57% 12% 17% 11% 3	4005
April	1st to 10th	Day	52% 11% 19% 15% 3	2280
	11th to 20th	Day	48% 11% 17% 20% 4	2407
	21st to end	Day	51% 10% 16% 18% 4 1	2470
May	1st to 10th	Day	55% 8% 14% 18% 5%	4313
	11th to 20th	Day	41% 12% 16% 25% 6%	4493
	21st to end	Day	50% 11% 18% 15% 5%	5162
June	1st to 10th	Day	52% 8% 15% 16% 7% 1	2850
	11th to 20th	Day	51% 10% 16% 15% 6% 1	2850
	21st to end	Day	34% 10% 18% 23% 12% 2	2850
July	1st to 10th	Day	21% 8% 12% 25% 26% 8%	4666
	11th to 20th	Day	16% 5% 8% 18% 39% 14%	4546
	21st to end	Day	17% 7% 9% 25% 26% 16%	4639
August	1st to 10th	Day	21% 8% 19% 27% 20% 5%	4384
	11th to 20th	Day	22% 8% 23% 32% 14% 2	4159
	21st to end	Day	23% 9% 21% 33% 13% 1	4468
September	1st to 10th	Day	28% 6% 15% 31% 17% 3	2280
	11th to 20th	Day	33% 8% 17% 29% 12% 1	2125
	21st to end	Day	34% 9% 24% 23% 10% 1	2090
October	1st to 10th	Day	27% 10% 22% 36% 5%	3445
	11th to 20th	Day	30% 13% 24% 27% 6%	2842
	21st to end	Day	38% 13% 21% 19% 8%	3101
November	1st to 10th	Day	55% 15% 16% 12% 3	1764
	11th to 20th	Day	48% 17% 19% 11% 5%	1535
	21st to end	Day	49% 20% 20% 8% 3	1400
December	1st to 10th	Day	49% 20% 18% 12% 1	2290
	11th to 20th	Day	63% 11% 15% 8% 2	2644
	21st to end	Day	59% 15% 14% 8% 3 1	3498
January	1st to 10th	Night	63% 11% 13% 11% 2	6766
	11th to 20th	Night	47% 15% 13% 15% 8% 3	6323
	21st to end	Night	51% 13% 20% 12% 5%	6952
February	1st to 10th	Night	51% 14% 16% 12% 7% 1	2730
	11th to 20th	Night	44% 15% 19% 15% 6%	2378
	21st to end	Night	44% 13% 20% 16% 6% 1	1884
March	1st to 10th	Night	38% 12% 24% 19% 7%	3736
	11th to 20th	Night	34% 10% 34% 18% 3	3621
	21st to end	Night	50% 12% 21% 14% 3	3647
April	1st to 10th	Night	36% 14% 25% 18% 7%	1900
	11th to 20th	Night	41% 9% 20% 24% 6%	1724
	21st to end	Night	40% 10% 19% 24% 6% 1	1710
May	1st to 10th	Night	50% 9% 15% 20% 6%	2648
	11th to 20th	Night	32% 10% 18% 30% 10%	2401
	21st to end	Night	45% 9% 20% 19% 7%	2414
June	1st to 10th	Night	40% 9% 22% 20% 8% 2	1330
	11th to 20th	Night	41% 10% 19% 21% 8% 2	1330
	21st to end	Night	25% 9% 20% 28% 16% 2	1330
July	1st to 10th	Night	17% 7% 16% 25% 22% 12%	2185
	11th to 20th	Night	14% 5% 9% 17% 34% 20%	2274
	21st to end	Night	11% 4 9% 29% 28% 18%	2674
August	1st to 10th	Night	18% 5% 20% 26% 25% 6%	2521
	11th to 20th	Night	19% 7% 25% 30% 17% 2	2741
	21st to end	Night	17% 8% 22% 32% 19% 2	3196
September	1st to 10th	Night	21% 7% 14% 35% 18% 5%	1900
	11th to 20th	Night	27% 8% 19% 30% 14% 2	2054
	21st to end	Night	27% 9% 23% 26% 13% 1	2090
October	1st to 10th	Night	21% 7% 21% 39% 11%	3532
	11th to 20th	Night	27% 11% 19% 35% 9%	4086
	21st to end	Night	31% 13% 25% 27% 4	4457
November	1st to 10th	Night	46% 17% 19% 14% 4 1	2535
	11th to 20th	Night	46% 15% 20% 13% 6%	2841
	21st to end	Night	53% 16% 16% 12% 3	3000
December	1st to 10th	Night	44% 20% 19% 14% 3	4890
	11th to 20th	Night	65% 13% 12% 8% 1	5547
	21st to end	Night	60% 14% 14% 9% 3	7458

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	7% 23% 31% 28% 9% 4	200
	11th to 20th	Day	6% 13% 39% 26% 16% 2	200
	21st to end	Day	4 11% 46% 26% 9% 4	220
February	1st to 10th	Day	9% 19% 48% 17% 6%	190
	11th to 20th	Day	9% 17% 35% 27% 11%	318
	21st to end	Day	12% 19% 34% 27% 7% 1	382
March	1st to 10th	Day	12% 31% 29% 18% 10% 1	460
	11th to 20th	Day	10% 25% 26% 27% 10% 2	460
	21st to end	Day	14% 23% 35% 19% 7% 2	506
April	1st to 10th	Day	15% 28% 30% 21% 5% 1	460
	11th to 20th	Day	14% 28% 26% 23% 7% 2	452
	21st to end	Day	16% 25% 29% 20% 8% 2	460
May	1st to 10th	Day	13% 22% 32% 23% 8% 2	463
	11th to 20th	Day	16% 14% 29% 25% 14% 2	549
	21st to end	Day	18% 14% 24% 27% 15% 2	734
June	1st to 10th	Day	18% 19% 25% 26% 10% 1	680
	11th to 20th	Day	15% 21% 26% 24% 12% 1	680
	21st to end	Day	9% 11% 30% 32% 15% 3	680
July	1st to 10th	Day	6% 10% 27% 33% 21% 2	671
	11th to 20th	Day	8% 15% 28% 30% 16% 2	607
	21st to end	Day	5% 16% 34% 30% 13% 2	531
August	1st to 10th	Day	6% 11% 33% 30% 17% 3	487
	11th to 20th	Day	5% 12% 29% 30% 19% 5%	477
	21st to end	Day	9% 17% 29% 29% 12% 4	539
September	1st to 10th	Day	8% 17% 31% 26% 14% 5%	490
	11th to 20th	Day	10% 18% 27% 27% 15% 4	490
	21st to end	Day	15% 23% 25% 26% 11% 1	490
October	1st to 10th	Day	12% 18% 30% 28% 11% 1	485
	11th to 20th	Day	12% 19% 29% 23% 14% 4	300
	21st to end	Day	19% 17% 36% 19% 8% 1	330
November	1st to 10th	Day	16% 16% 29% 31% 7%	300
	11th to 20th	Day	14% 21% 34% 22% 7% 1	253
	21st to end	Day	9% 17% 32% 29% 13% 2	200
December	1st to 10th	Day	9% 14% 32% 28% 12% 6%	200
	11th to 20th	Day	7% 21% 34% 25% 13% 2	200
	21st to end	Day	9% 10% 38% 32% 9% 1	220
January	1st to 10th	Night	9% 18% 31% 31% 9% 2	1000
	11th to 20th	Night	6% 11% 37% 33% 11% 2	803
	21st to end	Night	6% 10% 35% 38% 10% 2	880
February	1st to 10th	Night	8% 14% 33% 34% 11% 1	780
	11th to 20th	Night	6% 13% 29% 39% 12% 1	744
	21st to end	Night	8% 11% 29% 34% 15% 2	628
March	1st to 10th	Night	7% 13% 31% 34% 14% 1	760
	11th to 20th	Night	8% 16% 28% 32% 15% 2	760
	21st to end	Night	9% 12% 30% 34% 13% 2	836
April	1st to 10th	Night	8% 13% 27% 33% 18% 1	760
	11th to 20th	Night	8% 15% 32% 32% 13% 1	596
	21st to end	Night	10% 12% 34% 30% 15%	570
May	1st to 10th	Night	8% 12% 27% 34% 18% 1	449
	11th to 20th	Night	9% 10% 23% 31% 26% 2	374
	21st to end	Night	14% 10% 19% 31% 24% 2	411
June	1st to 10th	Night	13% 8% 23% 35% 19% 2	380
	11th to 20th	Night	12% 7% 29% 34% 18% 1	380
	21st to end	Night	4 8% 22% 40% 25% 2	380
July	1st to 10th	Night	3 4 17% 46% 29% 2	370
	11th to 20th	Night	2 4 23% 48% 21% 2	368
	21st to end	Night	2 4 22% 43% 28% 1	416
August	1st to 10th	Night	6% 4 14% 40% 33% 4	378
	11th to 20th	Night	3 4 19% 39% 31% 4	505
	21st to end	Night	5% 6% 21% 42% 24% 2	651
September	1st to 10th	Night	6% 4 23% 38% 25% 4	760
	11th to 20th	Night	6% 7% 19% 34% 30% 4	760
	21st to end	Night	8% 9% 23% 39% 19% 3	760
October	1st to 10th	Night	9% 9% 26% 36% 19% 1	760
	11th to 20th	Night	6% 12% 26% 35% 19% 2	760
	21st to end	Night	9% 12% 31% 35% 11% 2	836
November	1st to 10th	Night	10% 12% 33% 37% 7% 1	780
	11th to 20th	Night	8% 20% 31% 30% 10% 1	950
	21st to end	Night	8% 17% 27% 35% 11% 1	1000
December	1st to 10th	Night	9% 10% 32% 34% 14% 1	1000
	11th to 20th	Night	8% 11% 35% 36% 10%	1000
	21st to end	Night	8% 13% 35% 34% 10%	1100

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