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 2024-02-05.

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	5% 7% 4 7% 7% 4 7% 7% 7% 9% 37%	77781
	Night	3 8% 4 8% 7% 4 6% 6% 7% 8% 37%	81019
Transparency	Day	45% 11% 16% 18% 8% 2	77497
	Night	44% 12% 17% 18% 8% 1	80882
Seeing	Day	11% 18% 30% 26% 12% 2	14058
	Night	8% 11% 28% 35% 16% 2	23362

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 10% 9% 18% 10% 6% 9% 7% 8% 8% 13%	2233
	11th to 20th	Day	3 12% 5% 10% 8% 5% 6% 7% 7% 9% 29%	2516
	21st to end	Day	1 10% 6% 11% 10% 6% 8% 9% 9% 11% 20%	2794
February	1st to 10th	Day	3 7% 3 12% 10% 6% 7% 9% 8% 10% 26%	1472
	11th to 20th	Day	2 7% 4 10% 11% 5% 7% 7% 8% 11% 28%	1598
	21st to end	Day	2 7% 5% 9% 10% 5% 8% 9% 9% 8% 28%	1480
March	1st to 10th	Day	2 7% 5% 8% 9% 5% 7% 8% 9% 10% 30%	1800
	11th to 20th	Day	3 8% 4 8% 8% 5% 7% 7% 7% 11% 31%	1899
	21st to end	Day	5% 9% 4 8% 9% 5% 8% 8% 8% 9% 26%	2281
April	1st to 10th	Day	5% 7% 5% 9% 9% 6% 7% 7% 8% 9% 28%	2160
	11th to 20th	Day	5% 10% 5% 9% 8% 4 6% 6% 7% 8% 32%	2278
	21st to end	Day	7% 10% 5% 8% 8% 5% 6% 6% 7% 8% 33%	2340
May	1st to 10th	Day	7% 10% 4 6% 7% 4 6% 6% 7% 8% 35%	2452
	11th to 20th	Day	9% 7% 3 6% 6% 4 7% 7% 7% 9% 36%	2534
	21st to end	Day	15% 9% 4 6% 7% 4 6% 6% 7% 8% 29%	2917
June	1st to 10th	Day	12% 8% 4 7% 7% 5% 7% 5% 6% 8% 31%	2700
	11th to 20th	Day	10% 6% 4 7% 7% 6% 8% 7% 8% 10% 26%	2700
	21st to end	Day	6% 3 1 5% 7% 4 7% 6% 8% 9% 44%	2700
July	1st to 10th	Day	3 3 1 4 6% 4 5% 5% 6% 9% 53%	2700
	11th to 20th	Day	5% 2 1 3 5% 3 5% 6% 6% 8% 57%	2643
	21st to end	Day	1 2 1 3 3 2 4 4 6% 8% 65%	2741
August	1st to 10th	Day	3 2 1 3 4 4 5% 6% 7% 10% 54%	2507
	11th to 20th	Day	3 2 1 3 4 2 4 5% 7% 8% 60%	2353
	21st to end	Day	4 3 1 3 5% 3 6% 6% 7% 9% 54%	2548
September	1st to 10th	Day	4 4 2 4 4 3 6% 6% 7% 8% 54%	2160
	11th to 20th	Day	6% 5% 2 4 5% 3 4 5% 7% 8% 49%	2014
	21st to end	Day	6% 4 2 5% 6% 3 6% 6% 8% 10% 46%	1980
October	1st to 10th	Day	6% 5% 2 6% 6% 4 6% 6% 7% 10% 42%	1962
	11th to 20th	Day	3 5% 3 5% 7% 3 7% 7% 6% 9% 44%	1620
	21st to end	Day	5% 8% 4 7% 7% 3 6% 7% 7% 9% 37%	1782
November	1st to 10th	Day	5% 11% 4 10% 9% 5% 7% 6% 7% 9% 27%	1674
	11th to 20th	Day	2 6% 5% 9% 8% 3 9% 8% 8% 9% 34%	1459
	21st to end	Day	1 8% 4 10% 8% 5% 7% 7% 7% 11% 31%	1330
December	1st to 10th	Day	3 10% 5% 12% 9% 5% 8% 8% 8% 9% 22%	1330
	11th to 20th	Day	3 10% 8% 14% 12% 6% 8% 9% 6% 7% 17%	1682
	21st to end	Day	2 9% 6% 14% 12% 8% 10% 6% 6% 7% 19%	2442
January	1st to 10th	Night	2 16% 10% 15% 10% 6% 8% 7% 7% 7% 14%	4726
	11th to 20th	Night	3 11% 5% 10% 8% 5% 9% 7% 7% 7% 28%	4385
	21st to end	Night	3 13% 7% 12% 9% 5% 7% 7% 7% 9% 21%	4862
February	1st to 10th	Night	3 11% 5% 9% 9% 5% 7% 7% 7% 9% 28%	2582
	11th to 20th	Night	2 8% 4 8% 7% 4 8% 7% 8% 9% 34%	2247
	21st to end	Night	2 8% 5% 9% 7% 5% 6% 6% 8% 9% 34%	1776
March	1st to 10th	Night	3 6% 3 8% 9% 3 7% 7% 8% 10% 36%	2160
	11th to 20th	Night	2 7% 3 5% 6% 4 7% 8% 9% 10% 39%	2059
	21st to end	Night	4 6% 3 6% 7% 4 6% 7% 8% 11% 38%	2074
April	1st to 10th	Night	4 6% 3 6% 6% 3 6% 5% 7% 11% 43%	1800
	11th to 20th	Night	4 8% 5% 6% 7% 3 6% 6% 7% 10% 39%	1633
	21st to end	Night	6% 8% 3 6% 5% 4 6% 5% 6% 9% 41%	1620
May	1st to 10th	Night	4 8% 3 6% 6% 4 7% 6% 7% 7% 41%	1506
	11th to 20th	Night	6% 6% 2 7% 6% 3 4 5% 6% 9% 46%	1357
	21st to end	Night	11% 9% 4 6% 6% 3 6% 5% 5% 9% 37%	1358
June	1st to 10th	Night	8% 4 2 7% 7% 4 5% 6% 7% 10% 41%	1260
	11th to 20th	Night	9% 6% 2 6% 5% 4 7% 7% 7% 9% 38%	1260
	21st to end	Night	3 3 2 4 4 3 5% 6% 7% 9% 54%	1260
July	1st to 10th	Night	2 2 1 3 3 2 4 6% 6% 9% 62%	1260
	11th to 20th	Night	2 2 1 3 3 3 3 5% 6% 9% 65%	1314
	21st to end	Night	1 1 1 2 2 2 4 3 5% 6% 73%	1576
August	1st to 10th	Night	4 3 1 3 3 2 3 4 5% 8% 64%	1432
	11th to 20th	Night	3 2 2 3 2 2 3 3 6% 9% 66%	1546
	21st to end	Night	3 3 1 3 3 2 3 5% 5% 8% 63%	1804
September	1st to 10th	Night	4 4 1 3 3 2 4 4 5% 7% 63%	1800
	11th to 20th	Night	5% 4 2 4 4 3 4 5% 5% 7% 57%	1945
	21st to end	Night	4 4 2 4 4 2 5% 5% 6% 9% 55%	1980
October	1st to 10th	Night	5% 6% 3 5% 5% 3 6% 6% 6% 7% 49%	1996
	11th to 20th	Night	3 5% 2 5% 5% 3 5% 7% 7% 10% 48%	2340
	21st to end	Night	4 8% 3 7% 6% 4 4 6% 8% 10% 40%	2574
November	1st to 10th	Night	4 9% 4 8% 6% 4 7% 6% 8% 11% 33%	2405
	11th to 20th	Night	2 8% 6% 7% 8% 4 7% 7% 8% 10% 33%	2698
	21st to end	Night	2 10% 7% 10% 8% 4 7% 8% 8% 8% 28%	2850
December	1st to 10th	Night	3 10% 6% 13% 10% 6% 7% 6% 7% 7% 26%	2850
	11th to 20th	Night	2 12% 7% 18% 11% 6% 8% 7% 7% 7% 16%	3510
	21st to end	Night	2 10% 7% 15% 12% 7% 8% 7% 6% 7% 19%	5214

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	66% 13% 12% 7% 2 1	2233
	11th to 20th	Day	52% 11% 17% 11% 4 5%	2540
	21st to end	Day	64% 14% 14% 6% 2	2794
February	1st to 10th	Day	52% 15% 16% 10% 5% 1	1472
	11th to 20th	Day	50% 16% 18% 12% 3	1607
	21st to end	Day	49% 16% 16% 13% 5%	1480
March	1st to 10th	Day	50% 13% 19% 14% 4	1800
	11th to 20th	Day	49% 12% 18% 17% 4	1900
	21st to end	Day	56% 13% 18% 10% 3	2281
April	1st to 10th	Day	53% 11% 18% 15% 3	2160
	11th to 20th	Day	49% 11% 18% 18% 4	2278
	21st to end	Day	51% 9% 17% 18% 4 1	2340
May	1st to 10th	Day	47% 9% 19% 17% 7% 1	2452
	11th to 20th	Day	45% 11% 18% 20% 6%	2537
	21st to end	Day	54% 10% 17% 15% 4	2918
June	1st to 10th	Day	53% 8% 15% 16% 6% 1	2700
	11th to 20th	Day	53% 11% 16% 15% 5% 1	2700
	21st to end	Day	35% 10% 18% 23% 12% 2	2700
July	1st to 10th	Day	28% 8% 15% 28% 17% 4	2626
	11th to 20th	Day	26% 8% 12% 23% 24% 7%	2562
	21st to end	Day	18% 7% 10% 29% 28% 7%	2589
August	1st to 10th	Day	23% 8% 19% 26% 18% 6%	2490
	11th to 20th	Day	21% 7% 18% 32% 19% 3	2353
	21st to end	Day	27% 9% 16% 30% 16% 2	2548
September	1st to 10th	Day	29% 6% 15% 30% 16% 3	2160
	11th to 20th	Day	33% 8% 17% 29% 12% 2	2014
	21st to end	Day	35% 9% 23% 22% 9% 1	1980
October	1st to 10th	Day	40% 10% 18% 26% 6%	1962
	11th to 20th	Day	36% 13% 22% 22% 6%	1620
	21st to end	Day	45% 12% 20% 18% 6%	1782
November	1st to 10th	Day	55% 14% 15% 12% 3	1674
	11th to 20th	Day	47% 17% 20% 11% 5%	1459
	21st to end	Day	49% 20% 20% 8% 3	1330
December	1st to 10th	Day	58% 14% 14% 12% 3	1330
	11th to 20th	Day	67% 11% 12% 8% 2	1684
	21st to end	Day	62% 15% 11% 7% 4 1	2442
January	1st to 10th	Night	70% 10% 10% 7% 2	4726
	11th to 20th	Night	53% 12% 11% 14% 6% 4	4423
	21st to end	Night	63% 13% 14% 7% 3	4862
February	1st to 10th	Night	51% 15% 16% 12% 6% 1	2582
	11th to 20th	Night	45% 15% 20% 16% 4	2248
	21st to end	Night	43% 13% 20% 17% 7% 1	1776
March	1st to 10th	Night	43% 15% 22% 15% 6%	2160
	11th to 20th	Night	41% 13% 20% 20% 6% 1	2059
	21st to end	Night	41% 13% 25% 16% 5%	2074
April	1st to 10th	Night	35% 14% 24% 19% 7%	1800
	11th to 20th	Night	41% 9% 20% 23% 6%	1633
	21st to end	Night	40% 10% 19% 25% 6% 1	1620
May	1st to 10th	Night	41% 10% 18% 20% 10%	1506
	11th to 20th	Night	36% 9% 21% 24% 9% 1	1357
	21st to end	Night	46% 8% 19% 21% 5%	1358
June	1st to 10th	Night	40% 10% 21% 20% 7% 2	1260
	11th to 20th	Night	41% 10% 20% 20% 7% 1	1260
	21st to end	Night	25% 9% 21% 29% 15% 2	1260
July	1st to 10th	Night	18% 8% 20% 30% 18% 6%	1225
	11th to 20th	Night	18% 7% 15% 26% 26% 8%	1272
	21st to end	Night	13% 5% 11% 33% 28% 9%	1488
August	1st to 10th	Night	20% 6% 16% 29% 21% 8%	1424
	11th to 20th	Night	16% 7% 15% 37% 23% 3	1546
	21st to end	Night	19% 7% 15% 34% 23% 3	1804
September	1st to 10th	Night	22% 7% 14% 34% 18% 5%	1800
	11th to 20th	Night	27% 8% 18% 30% 14% 3	1945
	21st to end	Night	28% 9% 23% 26% 13% 2	1980
October	1st to 10th	Night	34% 9% 19% 30% 8%	1996
	11th to 20th	Night	33% 11% 23% 25% 8% 1	2340
	21st to end	Night	40% 13% 22% 20% 4	2574
November	1st to 10th	Night	46% 17% 19% 14% 4 1	2405
	11th to 20th	Night	46% 15% 20% 13% 6%	2698
	21st to end	Night	52% 16% 16% 13% 3	2850
December	1st to 10th	Night	58% 11% 15% 12% 3	2850
	11th to 20th	Night	68% 13% 12% 5% 1	3507
	21st to end	Night	61% 14% 13% 8% 4	5214

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% 24% 29% 27% 9% 3	180
	11th to 20th	Day	6% 14% 39% 23% 17% 2	180
	21st to end	Day	4 12% 45% 25% 10% 4	198
February	1st to 10th	Day	10% 21% 48% 18% 4	168
	11th to 20th	Day	10% 18% 36% 25% 10%	288
	21st to end	Day	9% 18% 37% 28% 8% 1	346
March	1st to 10th	Day	12% 31% 30% 16% 10% 1	420
	11th to 20th	Day	11% 26% 26% 25% 10% 2	420
	21st to end	Day	15% 23% 35% 18% 7% 2	462
April	1st to 10th	Day	13% 27% 31% 21% 6% 1	420
	11th to 20th	Day	15% 29% 26% 22% 7% 2	412
	21st to end	Day	16% 25% 28% 21% 9% 2	420
May	1st to 10th	Day	13% 21% 34% 23% 7% 2	423
	11th to 20th	Day	17% 14% 29% 25% 14% 2	505
	21st to end	Day	19% 13% 23% 27% 16% 2	679
June	1st to 10th	Day	18% 18% 24% 27% 11% 2	630
	11th to 20th	Day	15% 21% 26% 24% 12% 1	630
	21st to end	Day	8% 11% 29% 32% 16% 3	630
July	1st to 10th	Day	6% 10% 27% 33% 21% 2	621
	11th to 20th	Day	9% 16% 29% 29% 15% 2	561
	21st to end	Day	4 16% 34% 30% 14% 2	487
August	1st to 10th	Day	5% 12% 34% 30% 17% 3	447
	11th to 20th	Day	5% 12% 30% 29% 19% 5%	437
	21st to end	Day	7% 17% 31% 29% 12% 4	495
September	1st to 10th	Day	9% 18% 33% 25% 12% 3	450
	11th to 20th	Day	10% 18% 27% 27% 14% 4	450
	21st to end	Day	15% 23% 26% 25% 11% 1	450
October	1st to 10th	Day	12% 17% 30% 29% 11% 1	446
	11th to 20th	Day	12% 19% 29% 22% 14% 3	270
	21st to end	Day	20% 16% 37% 19% 7% 1	297
November	1st to 10th	Day	17% 18% 30% 29% 6%	270
	11th to 20th	Day	13% 19% 36% 23% 7% 1	228
	21st to end	Day	7% 19% 32% 27% 13% 2	180
December	1st to 10th	Day	10% 16% 33% 24% 12% 5%	180
	11th to 20th	Day	8% 21% 34% 24% 11% 2	180
	21st to end	Day	9% 10% 39% 32% 9% 1	198
January	1st to 10th	Night	9% 19% 30% 31% 9% 2	950
	11th to 20th	Night	6% 12% 37% 32% 11% 2	763
	21st to end	Night	6% 10% 34% 37% 10% 2	836
February	1st to 10th	Night	7% 14% 34% 34% 11% 1	738
	11th to 20th	Night	6% 13% 29% 38% 12% 1	704
	21st to end	Night	7% 10% 30% 35% 16% 3	592
March	1st to 10th	Night	7% 13% 32% 33% 15% 1	720
	11th to 20th	Night	8% 16% 29% 31% 14% 2	720
	21st to end	Night	8% 12% 31% 34% 13% 2	792
April	1st to 10th	Night	7% 12% 27% 34% 19% 1	720
	11th to 20th	Night	8% 15% 32% 31% 13% 1	565
	21st to end	Night	10% 11% 33% 30% 16%	540
May	1st to 10th	Night	8% 12% 27% 34% 19% 1	426
	11th to 20th	Night	9% 9% 23% 31% 26% 2	354
	21st to end	Night	14% 9% 19% 32% 24% 2	389
June	1st to 10th	Night	12% 8% 23% 35% 19% 3	360
	11th to 20th	Night	12% 6% 28% 35% 18% 1	360
	21st to end	Night	3 8% 21% 40% 25% 2	360
July	1st to 10th	Night	3 4 17% 45% 29% 2	350
	11th to 20th	Night	2 4 23% 47% 21% 2	348
	21st to end	Night	2 4 21% 43% 29% 2	394
August	1st to 10th	Night	5% 4 14% 39% 34% 4	358
	11th to 20th	Night	3 3 20% 39% 31% 4	478
	21st to end	Night	4 6% 19% 43% 25% 2	616
September	1st to 10th	Night	6% 4 24% 39% 23% 4	720
	11th to 20th	Night	6% 6% 19% 34% 30% 4	720
	21st to end	Night	8% 8% 22% 39% 20% 3	720
October	1st to 10th	Night	9% 8% 26% 36% 20% 1	720
	11th to 20th	Night	6% 11% 26% 35% 19% 2	720
	21st to end	Night	10% 12% 30% 35% 11% 2	792
November	1st to 10th	Night	11% 12% 33% 37% 7% 1	740
	11th to 20th	Night	8% 19% 31% 30% 11%	902
	21st to end	Night	8% 17% 27% 35% 12% 1	950
December	1st to 10th	Night	10% 10% 32% 34% 13% 1	950
	11th to 20th	Night	9% 10% 35% 36% 10%	950
	21st to end	Night	8% 13% 33% 35% 10%	1045

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