Brandon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brandon clear sky chart from 2005-11-06 to 2026-07-16.

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	10% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	142293
	Night	8% 9% 4 9% 6% 5% 5% 4 6% 8% 35%	139237
Transparency	Day	44% 7% 15% 25% 9% 1	142006
	Night	45% 7% 14% 24% 10% 1	139109
Seeing	Day	14% 9% 30% 28% 15% 4	18612
	Night	14% 7% 31% 33% 13% 1	28539

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	4 13% 6% 15% 9% 7% 7% 5% 7% 6% 21%	4125
	11th to 20th	Day	3 9% 5% 16% 10% 6% 9% 5% 8% 8% 20%	4554
	21st to end	Day	5% 10% 4 11% 6% 4 5% 4 7% 10% 33%	5188
February	1st to 10th	Day	9% 13% 5% 13% 8% 6% 6% 5% 8% 7% 21%	1680
	11th to 20th	Day	6% 10% 4 10% 8% 7% 7% 5% 7% 6% 29%	1870
	21st to end	Day	6% 11% 5% 11% 7% 7% 8% 5% 6% 8% 26%	1730
March	1st to 10th	Day	7% 13% 4 12% 7% 6% 5% 5% 7% 11% 23%	4600
	11th to 20th	Day	8% 14% 5% 12% 8% 6% 6% 4 6% 7% 24%	4953
	21st to end	Day	9% 12% 4 13% 8% 6% 5% 4 5% 6% 27%	5765
April	1st to 10th	Day	10% 10% 4 7% 5% 4 4 4 7% 9% 36%	2521
	11th to 20th	Day	13% 13% 4 9% 5% 5% 5% 3 6% 7% 31%	2702
	21st to end	Day	14% 9% 3 8% 6% 4 5% 4 5% 7% 35%	2730
May	1st to 10th	Day	12% 8% 2 6% 4 3 3 3 5% 7% 45%	7932
	11th to 20th	Day	21% 9% 3 7% 5% 5% 4 4 5% 6% 31%	8601
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 7% 46%	9683
June	1st to 10th	Day	13% 7% 2 6% 5% 5% 5% 4 7% 8% 37%	3150
	11th to 20th	Day	14% 7% 2 8% 5% 6% 5% 4 7% 8% 33%	3150
	21st to end	Day	12% 5% 2 6% 6% 6% 5% 5% 7% 9% 37%	3150
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 10% 41%	6950
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 4 7% 9% 44%	6876
	21st to end	Day	7% 4 2 6% 4 4 5% 5% 7% 10% 46%	6926
August	1st to 10th	Day	9% 6% 2 6% 5% 4 5% 4 6% 8% 44%	4552
	11th to 20th	Day	9% 5% 2 4 4 4 5% 4 7% 9% 47%	4216
	21st to end	Day	9% 5% 3 5% 4 4 4 4 6% 9% 49%	4675
September	1st to 10th	Day	9% 6% 2 6% 4 3 4 4 5% 7% 51%	2424
	11th to 20th	Day	15% 6% 3 6% 4 4 4 4 5% 7% 42%	2210
	21st to end	Day	13% 7% 2 5% 4 4 4 3 5% 5% 47%	2200
October	1st to 10th	Day	11% 8% 3 7% 4 4 3 3 4 6% 47%	3493
	11th to 20th	Day	8% 6% 2 7% 5% 5% 4 3 5% 9% 47%	2954
	21st to end	Day	12% 12% 4 8% 5% 4 4 3 5% 7% 37%	3190
November	1st to 10th	Day	18% 12% 5% 9% 5% 5% 5% 3 5% 5% 29%	1820
	11th to 20th	Day	9% 16% 5% 9% 5% 5% 5% 3 5% 7% 30%	1609
	21st to end	Day	9% 14% 4 11% 7% 5% 5% 3 4 7% 31%	1470
December	1st to 10th	Day	8% 12% 7% 16% 9% 9% 7% 5% 6% 5% 16%	2360
	11th to 20th	Day	8% 12% 5% 11% 7% 7% 7% 6% 6% 8% 23%	2709
	21st to end	Day	5% 9% 4 16% 8% 6% 7% 4 6% 7% 29%	3575
January	1st to 10th	Night	5% 12% 6% 16% 9% 8% 6% 4 6% 7% 22%	8799
	11th to 20th	Night	5% 10% 5% 13% 8% 7% 9% 7% 7% 9% 20%	8356
	21st to end	Night	5% 11% 6% 13% 5% 5% 6% 5% 7% 9% 28%	9026
February	1st to 10th	Night	8% 13% 5% 12% 7% 6% 5% 4 6% 7% 27%	2940
	11th to 20th	Night	6% 9% 3 8% 6% 6% 5% 5% 6% 8% 36%	2653
	21st to end	Night	6% 9% 4 10% 6% 5% 6% 5% 6% 8% 34%	2076
March	1st to 10th	Night	7% 11% 4 11% 6% 6% 6% 6% 7% 9% 27%	5494
	11th to 20th	Night	7% 13% 4 11% 5% 5% 4 4 5% 7% 34%	5217
	21st to end	Night	9% 12% 5% 14% 7% 6% 4 3 5% 6% 28%	5414
April	1st to 10th	Night	6% 9% 3 7% 5% 5% 5% 5% 6% 8% 40%	2098
	11th to 20th	Night	14% 12% 3 7% 5% 4 4 4 6% 8% 32%	1869
	21st to end	Night	13% 11% 2 6% 6% 5% 4 4 5% 7% 37%	1890
May	1st to 10th	Night	12% 9% 3 7% 4 5% 4 3 5% 6% 42%	5014
	11th to 20th	Night	15% 7% 3 6% 4 4 5% 5% 7% 9% 35%	4290
	21st to end	Night	12% 7% 2 6% 4 3 4 3 4 5% 49%	4523
June	1st to 10th	Night	11% 5% 3 5% 4 5% 5% 4 7% 9% 42%	1470
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 6% 9% 41%	1470
	21st to end	Night	11% 5% 2 6% 4 4 4 4 6% 10% 43%	1470
July	1st to 10th	Night	6% 4 1 4 3 4 4 4 9% 9% 50%	3250
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 6% 8% 51%	3215
	21st to end	Night	6% 4 2 6% 4 4 4 4 6% 8% 52%	3936
August	1st to 10th	Night	7% 4 2 6% 3 3 4 4 6% 8% 54%	2608
	11th to 20th	Night	6% 3 2 6% 5% 4 5% 4 6% 10% 49%	2841
	21st to end	Night	9% 3 2 5% 4 3 4 4 5% 9% 52%	3234
September	1st to 10th	Night	8% 5% 2 4 3 3 3 3 6% 7% 54%	1974
	11th to 20th	Night	13% 6% 2 5% 3 3 4 4 6% 7% 47%	2190
	21st to end	Night	13% 7% 3 6% 4 3 4 3 4 7% 46%	2200
October	1st to 10th	Night	11% 7% 2 6% 4 4 3 3 5% 7% 48%	3653
	11th to 20th	Night	11% 6% 2 5% 4 5% 5% 4 5% 8% 45%	4189
	21st to end	Night	8% 8% 3 7% 4 5% 5% 4 6% 7% 42%	4610
November	1st to 10th	Night	12% 12% 3 9% 5% 4 5% 4 5% 8% 33%	2697
	11th to 20th	Night	10% 13% 4 11% 6% 5% 4 4 6% 7% 30%	3008
	21st to end	Night	9% 12% 4 11% 7% 4 4 4 5% 6% 32%	3150
December	1st to 10th	Night	7% 13% 4 14% 9% 8% 7% 5% 7% 7% 20%	5050
	11th to 20th	Night	9% 14% 5% 11% 6% 6% 6% 5% 6% 8% 24%	5718
	21st to end	Night	5% 9% 3 12% 9% 7% 8% 5% 6% 7% 28%	7645

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	59% 6% 10% 18% 7%	4125
	11th to 20th	Day	57% 7% 10% 18% 8%	4578
	21st to end	Day	44% 6% 14% 28% 7%	5188
February	1st to 10th	Day	57% 7% 13% 16% 7%	1680
	11th to 20th	Day	50% 8% 11% 20% 10%	1879
	21st to end	Day	53% 7% 14% 18% 7%	1730
March	1st to 10th	Day	51% 8% 17% 17% 6%	4600
	11th to 20th	Day	55% 9% 11% 18% 6%	4955
	21st to end	Day	54% 7% 11% 20% 7%	5765
April	1st to 10th	Day	43% 6% 16% 24% 10% 1	2521
	11th to 20th	Day	52% 6% 12% 18% 11% 1	2702
	21st to end	Day	47% 7% 14% 23% 9% 1	2730
May	1st to 10th	Day	38% 5% 13% 30% 12% 2	7932
	11th to 20th	Day	52% 6% 9% 22% 9% 1	8605
	21st to end	Day	36% 6% 9% 25% 24% 1	9683
June	1st to 10th	Day	42% 9% 16% 22% 11% 1	3150
	11th to 20th	Day	47% 8% 16% 20% 9% 1	3150
	21st to end	Day	41% 10% 17% 25% 6% 1	3150
July	1st to 10th	Day	34% 9% 27% 25% 4	6876
	11th to 20th	Day	35% 7% 19% 34% 6%	6795
	21st to end	Day	30% 8% 25% 29% 8%	6732
August	1st to 10th	Day	37% 7% 17% 28% 11%	4536
	11th to 20th	Day	31% 7% 22% 34% 6%	4255
	21st to end	Day	32% 7% 19% 33% 8%	4674
September	1st to 10th	Day	33% 6% 15% 34% 10% 2	2424
	11th to 20th	Day	41% 7% 16% 26% 9% 1	2210
	21st to end	Day	38% 6% 12% 32% 12%	2200
October	1st to 10th	Day	39% 4 12% 37% 8%	3493
	11th to 20th	Day	36% 5% 12% 37% 10%	2954
	21st to end	Day	48% 5% 11% 29% 8%	3190
November	1st to 10th	Day	57% 6% 12% 18% 7% 1	1820
	11th to 20th	Day	53% 7% 17% 16% 7% 1	1609
	21st to end	Day	54% 6% 15% 20% 5%	1470
December	1st to 10th	Day	67% 8% 12% 9% 3	2360
	11th to 20th	Day	55% 8% 17% 16% 4	2710
	21st to end	Day	51% 7% 12% 19% 10% 1	3575
January	1st to 10th	Night	58% 7% 11% 18% 6%	8800
	11th to 20th	Night	57% 7% 10% 20% 7%	8394
	21st to end	Night	50% 6% 13% 24% 7%	9026
February	1st to 10th	Night	54% 6% 12% 17% 9% 1	2940
	11th to 20th	Night	42% 9% 13% 22% 14%	2654
	21st to end	Night	45% 6% 14% 24% 11%	2076
March	1st to 10th	Night	49% 8% 16% 19% 8%	5494
	11th to 20th	Night	46% 7% 13% 21% 12% 1	5217
	21st to end	Night	55% 7% 10% 23% 6%	5414
April	1st to 10th	Night	40% 7% 11% 24% 16% 2	2098
	11th to 20th	Night	49% 6% 12% 20% 12% 1	1869
	21st to end	Night	46% 5% 14% 21% 12% 2	1890
May	1st to 10th	Night	41% 5% 10% 26% 18% 1	5014
	11th to 20th	Night	43% 6% 12% 28% 9% 3	4290
	21st to end	Night	37% 5% 9% 22% 25% 1	4526
June	1st to 10th	Night	36% 8% 18% 24% 12% 2	1470
	11th to 20th	Night	37% 8% 18% 23% 12% 1	1470
	21st to end	Night	35% 9% 19% 29% 7% 1	1470
July	1st to 10th	Night	27% 9% 26% 30% 7%	3215
	11th to 20th	Night	29% 7% 17% 39% 7%	3173
	21st to end	Night	28% 7% 23% 30% 11% 1	3825
August	1st to 10th	Night	27% 6% 18% 32% 15% 2	2600
	11th to 20th	Night	29% 7% 20% 35% 8% 1	2868
	21st to end	Night	29% 7% 19% 34% 10% 1	3234
September	1st to 10th	Night	28% 5% 15% 34% 16% 2	1974
	11th to 20th	Night	35% 8% 17% 26% 13% 1	2190
	21st to end	Night	38% 5% 13% 31% 12% 2	2200
October	1st to 10th	Night	36% 5% 13% 29% 15% 2	3653
	11th to 20th	Night	37% 4 12% 35% 12% 1	4189
	21st to end	Night	40% 6% 13% 33% 8%	4610
November	1st to 10th	Night	50% 6% 14% 21% 7% 2	2697
	11th to 20th	Night	52% 8% 15% 17% 6% 1	3008
	21st to end	Night	51% 8% 16% 19% 6%	3150
December	1st to 10th	Night	60% 8% 14% 14% 4	5050
	11th to 20th	Night	56% 6% 15% 17% 5%	5716
	21st to end	Night	51% 8% 12% 18% 10% 1	7645

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	10% 7% 37% 29% 15% 2	330
	11th to 20th	Day	7% 8% 38% 33% 12% 1	330
	21st to end	Day	15% 9% 32% 29% 12% 2	363
February	1st to 10th	Day	16% 11% 39% 26% 7% 2	330
	11th to 20th	Day	9% 6% 35% 35% 12% 3	330
	21st to end	Day	4 8% 37% 33% 15% 2	273
March	1st to 10th	Day	12% 10% 32% 31% 14% 1	330
	11th to 20th	Day	12% 11% 30% 29% 15% 3	330
	21st to end	Day	16% 10% 33% 25% 13% 3	440
April	1st to 10th	Day	14% 12% 34% 29% 9% 2	541
	11th to 20th	Day	17% 11% 33% 24% 12% 2	740
	21st to end	Day	15% 9% 32% 29% 13% 2	750
May	1st to 10th	Day	16% 10% 27% 29% 16% 2	753
	11th to 20th	Day	21% 8% 28% 26% 13% 4	775
	21st to end	Day	18% 9% 25% 28% 18% 3	844
June	1st to 10th	Day	15% 13% 30% 24% 13% 6%	780
	11th to 20th	Day	17% 10% 33% 25% 12% 3	780
	21st to end	Day	15% 9% 27% 30% 16% 3	780
July	1st to 10th	Day	8% 5% 25% 32% 20% 9%	771
	11th to 20th	Day	10% 7% 31% 28% 19% 6%	744
	21st to end	Day	6% 7% 24% 32% 24% 7%	794
August	1st to 10th	Day	10% 7% 25% 32% 19% 7%	726
	11th to 20th	Day	12% 6% 23% 30% 21% 8%	715
	21st to end	Day	11% 8% 24% 33% 19% 5%	803
September	1st to 10th	Day	9% 10% 34% 27% 15% 5%	554
	11th to 20th	Day	21% 9% 27% 31% 11% 2	340
	21st to end	Day	19% 10% 28% 28% 12% 3	330
October	1st to 10th	Day	22% 12% 25% 25% 14% 3	330
	11th to 20th	Day	17% 10% 35% 26% 10% 2	330
	21st to end	Day	20% 10% 37% 21% 9% 3	363
November	1st to 10th	Day	23% 11% 31% 22% 12% 2	330
	11th to 20th	Day	17% 10% 34% 29% 8% 1	330
	21st to end	Day	15% 11% 36% 27% 10% 1	330
December	1st to 10th	Day	13% 11% 37% 24% 13% 3	330
	11th to 20th	Day	15% 12% 39% 28% 5% 1	330
	21st to end	Day	11% 6% 36% 30% 15% 2	363
January	1st to 10th	Night	13% 8% 33% 33% 11% 1	1050
	11th to 20th	Night	16% 8% 36% 31% 9% 1	1050
	21st to end	Night	13% 9% 38% 30% 9%	1155
February	1st to 10th	Night	14% 10% 34% 30% 11% 2	1050
	11th to 20th	Night	10% 6% 35% 34% 11% 3	998
	21st to end	Night	9% 7% 31% 36% 15% 2	692
March	1st to 10th	Night	15% 7% 30% 33% 12% 3	840
	11th to 20th	Night	10% 6% 37% 34% 12% 2	692
	21st to end	Night	15% 6% 29% 34% 13% 2	693
April	1st to 10th	Night	10% 6% 34% 37% 12% 1	630
	11th to 20th	Night	18% 6% 32% 33% 9% 1	623
	21st to end	Night	17% 7% 32% 32% 11% 1	630
May	1st to 10th	Night	19% 7% 28% 33% 13%	630
	11th to 20th	Night	15% 9% 29% 33% 13%	621
	21st to end	Night	18% 7% 25% 36% 14%	683
June	1st to 10th	Night	14% 6% 33% 31% 14% 1	630
	11th to 20th	Night	15% 4 35% 32% 13% 1	630
	21st to end	Night	13% 5% 27% 39% 17% 1	630
July	1st to 10th	Night	7% 4 23% 41% 24% 1	615
	11th to 20th	Night	7% 7% 26% 40% 20% 1	597
	21st to end	Night	6% 3 22% 47% 19% 2	657
August	1st to 10th	Night	8% 2 24% 44% 21% 1	597
	11th to 20th	Night	9% 3 22% 39% 23% 4	594
	21st to end	Night	12% 4 24% 41% 18% 1	660
September	1st to 10th	Night	10% 4 29% 34% 21% 3	600
	11th to 20th	Night	14% 8% 27% 38% 13%	600
	21st to end	Night	15% 9% 27% 36% 13% 1	600
October	1st to 10th	Night	16% 8% 33% 31% 11% 1	637
	11th to 20th	Night	18% 9% 30% 31% 11% 1	975
	21st to end	Night	17% 9% 35% 31% 9% 1	1100
November	1st to 10th	Night	16% 10% 35% 29% 9% 1	1025
	11th to 20th	Night	16% 9% 34% 29% 10% 2	1050
	21st to end	Night	16% 10% 37% 28% 8% 1	1050
December	1st to 10th	Night	11% 7% 36% 32% 12% 2	1050
	11th to 20th	Night	17% 11% 32% 28% 9% 2	1050
	21st to end	Night	15% 6% 31% 33% 12% 2	1155

Page generated at 2026-07-13 02:14:51. Version ac07

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