Markham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Markham clear sky chart from 2005-11-06 to 2026-08-15.

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	14% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	144143
	Night	14% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	137722
Transparency	Day	48% 5% 12% 22% 12% 1	147736
	Night	48% 5% 12% 22% 13% 1	142216
Seeing	Day	18% 10% 28% 27% 13% 5%	16659
	Night	19% 8% 29% 30% 13% 1	25996

By Month

Percentage Cloud Cover NE (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	16% 10% 4 11% 6% 5% 5% 4 5% 7% 26%	4661
	11th to 20th	Day	9% 17% 8% 18% 9% 7% 5% 4 5% 4 14%	5048
	21st to end	Day	13% 14% 5% 14% 7% 6% 6% 4 5% 5% 19%	5780
February	1st to 10th	Day	15% 15% 5% 12% 6% 6% 5% 4 5% 5% 23%	1890
	11th to 20th	Day	12% 14% 7% 12% 7% 4 5% 3 5% 6% 24%	1843
	21st to end	Day	16% 14% 4 11% 6% 4 5% 3 4 6% 26%	1614
March	1st to 10th	Day	18% 10% 4 9% 5% 4 5% 3 5% 5% 32%	5042
	11th to 20th	Day	11% 12% 4 10% 5% 5% 6% 4 6% 7% 30%	5090
	21st to end	Day	18% 12% 3 8% 5% 5% 4 3 5% 5% 33%	5586
April	1st to 10th	Day	17% 12% 4 9% 5% 4 4 4 6% 6% 29%	2487
	11th to 20th	Day	19% 10% 3 8% 5% 5% 4 3 5% 6% 34%	2567
	21st to end	Day	19% 8% 2 8% 6% 5% 4 3 5% 6% 33%	2730
May	1st to 10th	Day	19% 9% 3 8% 5% 4 5% 4 6% 7% 31%	7642
	11th to 20th	Day	14% 8% 3 7% 5% 4 5% 4 6% 8% 36%	8216
	21st to end	Day	17% 5% 2 5% 4 4 4 3 6% 8% 43%	9060
June	1st to 10th	Day	13% 7% 2 6% 6% 5% 6% 4 6% 8% 37%	2940
	11th to 20th	Day	12% 5% 2 7% 4 4 6% 5% 7% 10% 38%	2940
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 7% 8% 39%	2940
July	1st to 10th	Day	9% 6% 3 7% 6% 6% 6% 5% 7% 10% 37%	8280
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 10% 49%	8280
	21st to end	Day	8% 5% 2 7% 5% 5% 5% 5% 7% 8% 44%	8463
August	1st to 10th	Day	16% 7% 3 6% 4 4 4 4 6% 9% 37%	4364
	11th to 20th	Day	10% 7% 3 8% 5% 5% 5% 5% 7% 10% 36%	4226
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 4 6% 10% 41%	4487
September	1st to 10th	Day	13% 5% 2 7% 5% 4 5% 4 6% 7% 43%	2400
	11th to 20th	Day	10% 5% 3 5% 4 4 5% 4 6% 8% 46%	2291
	21st to end	Day	14% 6% 3 9% 5% 5% 5% 4 6% 7% 37%	2152
October	1st to 10th	Day	18% 7% 3 7% 5% 4 5% 3 5% 7% 38%	2000
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 6% 6% 31%	2000
	21st to end	Day	22% 11% 4 8% 6% 5% 5% 5% 6% 6% 23%	2200
November	1st to 10th	Day	14% 10% 3 10% 5% 5% 5% 4 6% 8% 31%	1770
	11th to 20th	Day	14% 11% 4 9% 6% 5% 6% 6% 6% 8% 25%	1680
	21st to end	Day	17% 13% 5% 10% 6% 6% 5% 5% 5% 7% 20%	1680
December	1st to 10th	Day	17% 16% 5% 11% 7% 6% 5% 5% 5% 7% 15%	2680
	11th to 20th	Day	19% 16% 4 9% 6% 5% 4 3 6% 5% 22%	3066
	21st to end	Day	25% 15% 6% 14% 6% 5% 4 3 4 4 14%	4048
January	1st to 10th	Night	14% 11% 4 10% 6% 5% 5% 4 6% 8% 26%	8234
	11th to 20th	Night	11% 14% 6% 14% 9% 8% 7% 4 6% 6% 17%	7819
	21st to end	Night	12% 12% 5% 11% 7% 6% 5% 5% 6% 7% 24%	8401
February	1st to 10th	Night	12% 15% 4 10% 7% 5% 5% 4 6% 5% 27%	2730
	11th to 20th	Night	14% 12% 5% 10% 6% 5% 5% 4 6% 6% 26%	2663
	21st to end	Night	13% 13% 4 10% 6% 5% 5% 4 6% 6% 28%	2189
March	1st to 10th	Night	15% 10% 4 9% 6% 5% 5% 4 5% 6% 32%	5018
	11th to 20th	Night	10% 8% 3 7% 5% 5% 6% 5% 7% 8% 36%	5089
	21st to end	Night	20% 8% 3 6% 4 4 5% 3 4 6% 37%	5588
April	1st to 10th	Night	18% 10% 3 7% 5% 5% 5% 4 6% 7% 31%	2132
	11th to 20th	Night	15% 9% 3 6% 5% 5% 5% 4 4 6% 38%	2019
	21st to end	Night	15% 7% 2 7% 5% 5% 4 4 6% 8% 37%	1890
May	1st to 10th	Night	20% 7% 3 7% 4 4 4 4 5% 7% 36%	5257
	11th to 20th	Night	14% 7% 2 8% 4 5% 5% 4 5% 7% 38%	4643
	21st to end	Night	17% 5% 2 5% 3 4 3 3 5% 7% 46%	5114
June	1st to 10th	Night	12% 5% 3 6% 4 4 5% 3 6% 9% 43%	1680
	11th to 20th	Night	8% 5% 2 7% 5% 4 5% 4 6% 10% 43%	1680
	21st to end	Night	12% 6% 2 6% 4 5% 5% 4 6% 7% 44%	1680
July	1st to 10th	Night	6% 5% 2 6% 4 4 5% 4 6% 10% 47%	4680
	11th to 20th	Night	7% 4 1 5% 4 4 4 3 7% 9% 52%	4680
	21st to end	Night	9% 5% 2 5% 3 3 4 4 6% 8% 51%	5610
August	1st to 10th	Night	9% 5% 2 6% 5% 5% 4 4 5% 7% 47%	3005
	11th to 20th	Night	11% 5% 2 5% 3 4 3 3 5% 8% 50%	2911
	21st to end	Night	9% 6% 2 6% 4 5% 4 4 6% 8% 47%	3378
September	1st to 10th	Night	9% 5% 2 6% 4 4 5% 5% 6% 8% 48%	2000
	11th to 20th	Night	10% 4 2 4 4 3 3 3 5% 7% 56%	2105
	21st to end	Night	15% 7% 3 6% 4 4 4 3 6% 6% 43%	2246
October	1st to 10th	Night	15% 6% 3 7% 5% 4 4 3 5% 7% 41%	2400
	11th to 20th	Night	18% 8% 3 7% 5% 5% 4 3 6% 7% 34%	2400
	21st to end	Night	20% 8% 3 7% 5% 5% 4 4 6% 7% 31%	2640
November	1st to 10th	Night	16% 8% 3 7% 4 4 4 3 6% 7% 37%	2743
	11th to 20th	Night	16% 10% 3 8% 5% 5% 5% 5% 6% 7% 30%	2940
	21st to end	Night	19% 11% 4 8% 6% 6% 5% 4 6% 8% 24%	2940
December	1st to 10th	Night	16% 15% 5% 10% 6% 6% 6% 4 7% 7% 19%	4720
	11th to 20th	Night	14% 17% 5% 10% 6% 5% 5% 4 5% 6% 23%	5348
	21st to end	Night	24% 13% 5% 12% 6% 5% 5% 3 5% 5% 15%	7150

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	56% 5% 12% 15% 11%	4662
	11th to 20th	Day	71% 3 7% 11% 6% 1	5082
	21st to end	Day	64% 4 10% 12% 10%	5780
February	1st to 10th	Day	61% 6% 10% 17% 5% 1	1890
	11th to 20th	Day	59% 6% 10% 17% 6% 1	1852
	21st to end	Day	57% 7% 12% 17% 6% 1	1614
March	1st to 10th	Day	53% 5% 14% 14% 14% 1	5045
	11th to 20th	Day	50% 6% 16% 18% 8% 2	5090
	21st to end	Day	51% 5% 8% 20% 14% 1	5586
April	1st to 10th	Day	54% 5% 10% 17% 11% 1	2487
	11th to 20th	Day	49% 6% 9% 21% 13% 1	2567
	21st to end	Day	51% 6% 10% 19% 12% 3	2730
May	1st to 10th	Day	52% 4 12% 17% 11% 3	7651
	11th to 20th	Day	46% 7% 11% 19% 15% 1	8220
	21st to end	Day	40% 5% 10% 29% 14% 2	9061
June	1st to 10th	Day	41% 8% 11% 22% 17% 1	2940
	11th to 20th	Day	39% 6% 13% 24% 16% 2	2940
	21st to end	Day	40% 8% 12% 25% 14% 1	2940
July	1st to 10th	Day	42% 3 14% 27% 13% 1	8210
	11th to 20th	Day	30% 3 11% 38% 17% 1	8205
	21st to end	Day	37% 4 12% 30% 16%	8276
August	1st to 10th	Day	43% 6% 14% 21% 15% 1	4351
	11th to 20th	Day	41% 6% 15% 27% 11%	4265
	21st to end	Day	37% 5% 11% 36% 10% 1	4487
September	1st to 10th	Day	37% 8% 14% 29% 11% 1	2400
	11th to 20th	Day	33% 7% 14% 30% 15%	2291
	21st to end	Day	43% 8% 12% 26% 10%	2152
October	1st to 10th	Day	38% 4 14% 28% 15% 1	3260
	11th to 20th	Day	40% 4 15% 25% 15%	3240
	21st to end	Day	53% 5% 13% 21% 7%	3538
November	1st to 10th	Day	48% 8% 13% 21% 9% 1	1770
	11th to 20th	Day	54% 9% 13% 15% 8% 1	1680
	21st to end	Day	59% 7% 15% 16% 3	1680
December	1st to 10th	Day	67% 4 15% 10% 4	2680
	11th to 20th	Day	61% 7% 12% 10% 10%	3066
	21st to end	Day	72% 6% 6% 9% 6%	4048
January	1st to 10th	Night	56% 5% 13% 21% 6%	8234
	11th to 20th	Night	66% 6% 8% 12% 7% 1	7860
	21st to end	Night	58% 4 10% 14% 13% 1	8401
February	1st to 10th	Night	56% 6% 12% 19% 6% 1	2730
	11th to 20th	Night	57% 6% 10% 18% 8%	2665
	21st to end	Night	54% 5% 13% 19% 9%	2189
March	1st to 10th	Night	53% 4 10% 18% 13% 2	5028
	11th to 20th	Night	43% 5% 17% 23% 10% 2	5090
	21st to end	Night	47% 4 8% 21% 17% 2	5588
April	1st to 10th	Night	51% 6% 10% 19% 13% 1	2132
	11th to 20th	Night	48% 6% 8% 22% 15% 2	2019
	21st to end	Night	45% 6% 11% 21% 15% 2	1890
May	1st to 10th	Night	50% 3 10% 17% 15% 5%	5264
	11th to 20th	Night	45% 6% 13% 20% 14% 2	4644
	21st to end	Night	40% 4 11% 27% 17% 2	5115
June	1st to 10th	Night	38% 6% 11% 25% 18% 2	1680
	11th to 20th	Night	36% 5% 13% 23% 22% 2	1680
	21st to end	Night	38% 6% 12% 25% 18% 1	1680
July	1st to 10th	Night	35% 2 16% 28% 17% 2	4640
	11th to 20th	Night	30% 3 11% 34% 21% 1	4632
	21st to end	Night	32% 4 12% 29% 22% 1	5482
August	1st to 10th	Night	36% 4 14% 28% 15% 3	2994
	11th to 20th	Night	32% 5% 15% 34% 13% 1	2938
	21st to end	Night	36% 4 12% 34% 13% 1	3378
September	1st to 10th	Night	33% 7% 15% 29% 15% 1	2000
	11th to 20th	Night	28% 5% 12% 36% 18%	2105
	21st to end	Night	41% 6% 12% 27% 12% 1	2246
October	1st to 10th	Night	34% 4 8% 28% 25% 1	3920
	11th to 20th	Night	43% 4 14% 27% 11%	3899
	21st to end	Night	41% 4 15% 28% 11% 1	4252
November	1st to 10th	Night	44% 6% 12% 24% 12% 1	2743
	11th to 20th	Night	51% 6% 13% 18% 10% 1	2940
	21st to end	Night	56% 7% 17% 16% 4	2940
December	1st to 10th	Night	62% 4 14% 15% 5%	4720
	11th to 20th	Night	59% 6% 10% 13% 11% 1	5348
	21st to end	Night	70% 6% 9% 10% 5%	7150

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	20% 17% 34% 20% 8% 1	220
	11th to 20th	Day	21% 20% 36% 13% 8% 1	294
	21st to end	Day	23% 15% 35% 22% 4 1	363
February	1st to 10th	Day	22% 16% 38% 19% 3 1	330
	11th to 20th	Day	21% 20% 38% 18% 3	330
	21st to end	Day	27% 21% 28% 21% 2	299
March	1st to 10th	Day	21% 13% 38% 22% 6%	540
	11th to 20th	Day	20% 16% 34% 23% 6% 1	540
	21st to end	Day	26% 12% 33% 21% 7% 1	594
April	1st to 10th	Day	23% 14% 33% 23% 6%	540
	11th to 20th	Day	25% 13% 30% 20% 9% 3	532
	21st to end	Day	23% 10% 32% 24% 9% 2	540
May	1st to 10th	Day	22% 10% 28% 26% 11% 4	543
	11th to 20th	Day	16% 7% 25% 30% 16% 5%	567
	21st to end	Day	15% 6% 24% 29% 20% 6%	617
June	1st to 10th	Day	13% 6% 28% 36% 14% 3	570
	11th to 20th	Day	14% 8% 27% 30% 18% 3	570
	21st to end	Day	17% 5% 23% 33% 18% 5%	570
July	1st to 10th	Day	10% 4 18% 35% 25% 7%	565
	11th to 20th	Day	12% 4 22% 32% 22% 9%	564
	21st to end	Day	8% 4 23% 38% 20% 8%	620
August	1st to 10th	Day	13% 3 18% 33% 22% 11%	566
	11th to 20th	Day	10% 4 19% 29% 27% 11%	535
	21st to end	Day	12% 5% 19% 32% 24% 9%	583
September	1st to 10th	Day	13% 5% 22% 28% 21% 12%	530
	11th to 20th	Day	10% 3 15% 32% 24% 16%	530
	21st to end	Day	15% 5% 26% 31% 15% 8%	530
October	1st to 10th	Day	20% 8% 25% 26% 15% 6%	530
	11th to 20th	Day	19% 10% 37% 22% 9% 2	530
	21st to end	Day	26% 11% 33% 23% 7% 1	583
November	1st to 10th	Day	21% 15% 29% 24% 11% 1	312
	11th to 20th	Day	19% 21% 33% 20% 5% 1	220
	21st to end	Day	25% 17% 37% 17% 4	220
December	1st to 10th	Day	25% 13% 37% 19% 5%	220
	11th to 20th	Day	25% 19% 32% 18% 5%	220
	21st to end	Day	28% 16% 38% 15% 2	242
January	1st to 10th	Night	23% 12% 37% 22% 6%	840
	11th to 20th	Night	24% 15% 33% 24% 4	840
	21st to end	Night	18% 11% 38% 26% 7%	924
February	1st to 10th	Night	20% 13% 38% 26% 3	840
	11th to 20th	Night	22% 12% 36% 25% 4	823
	21st to end	Night	20% 12% 39% 24% 5%	692
March	1st to 10th	Night	19% 12% 35% 28% 6%	840
	11th to 20th	Night	21% 13% 34% 25% 6% 1	840
	21st to end	Night	21% 10% 32% 28% 9% 1	924
April	1st to 10th	Night	24% 10% 32% 29% 6%	840
	11th to 20th	Night	19% 10% 33% 28% 9% 1	775
	21st to end	Night	18% 6% 32% 33% 11%	630
May	1st to 10th	Night	19% 5% 27% 33% 15% 2	630
	11th to 20th	Night	16% 3 27% 37% 17%	418
	21st to end	Night	12% 4 24% 39% 20% 1	455
June	1st to 10th	Night	15% 3 22% 42% 17% 1	420
	11th to 20th	Night	12% 4 26% 38% 19% 1	420
	21st to end	Night	13% 4 20% 43% 19% 1	420
July	1st to 10th	Night	7% 2 18% 44% 26% 2	410
	11th to 20th	Night	11% 3 17% 42% 23% 3	408
	21st to end	Night	8% 1 17% 47% 24% 3	640
August	1st to 10th	Night	10% 1 14% 41% 31% 4	627
	11th to 20th	Night	7% 2 16% 38% 33% 4	606
	21st to end	Night	11% 3 20% 37% 26% 3	750
September	1st to 10th	Night	11% 5% 20% 36% 23% 6%	800
	11th to 20th	Night	12% 3 17% 31% 31% 6%	800
	21st to end	Night	18% 4 19% 33% 22% 4	800
October	1st to 10th	Night	18% 6% 24% 28% 21% 3	800
	11th to 20th	Night	21% 7% 30% 30% 10% 1	800
	21st to end	Night	25% 7% 29% 33% 6%	880
November	1st to 10th	Night	21% 10% 26% 30% 11% 1	820
	11th to 20th	Night	20% 11% 34% 26% 8% 1	840
	21st to end	Night	25% 10% 33% 25% 7%	840
December	1st to 10th	Night	26% 11% 35% 22% 5% 1	840
	11th to 20th	Night	21% 13% 39% 22% 4	840
	21st to end	Night	31% 9% 35% 21% 3	924

Page generated at 2026-08-12 03:37:17. Version ac07

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