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 2024-02-20.

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	15% 9% 4 8% 6% 4 5% 5% 6% 7% 32%	78322
	Night	16% 9% 4 7% 6% 4 5% 5% 5% 7% 33%	80844
Transparency	Day	49% 8% 12% 20% 10% 1	78036
	Night	49% 7% 12% 20% 10% 1	80664
Seeing	Day	17% 9% 27% 28% 15% 5%	13415
	Night	19% 8% 29% 30% 13% 2	22971

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	17% 11% 4 8% 7% 3 5% 4 4 6% 30%	2517
	11th to 20th	Day	11% 15% 7% 14% 11% 4 5% 5% 4 5% 18%	2726
	21st to end	Day	20% 18% 6% 11% 7% 4 5% 4 4 5% 17%	3124
February	1st to 10th	Day	15% 14% 5% 11% 8% 4 7% 4 5% 5% 22%	1710
	11th to 20th	Day	12% 13% 8% 11% 7% 3 5% 4 4 7% 25%	1654
	21st to end	Day	18% 13% 5% 10% 7% 3 4 4 3 5% 27%	1380
March	1st to 10th	Day	14% 11% 4 9% 8% 4 5% 4 5% 6% 30%	1977
	11th to 20th	Day	13% 11% 5% 8% 7% 4 5% 5% 5% 5% 32%	1980
	21st to end	Day	19% 10% 4 6% 6% 4 4 5% 5% 5% 33%	2178
April	1st to 10th	Day	17% 11% 5% 8% 6% 3 4 5% 6% 6% 29%	2131
	11th to 20th	Day	18% 9% 3 6% 6% 3 4 4 5% 6% 35%	2195
	21st to end	Day	20% 8% 3 7% 6% 4 5% 4 5% 6% 33%	2340
May	1st to 10th	Day	16% 8% 4 8% 6% 3 5% 5% 5% 8% 33%	2340
	11th to 20th	Day	15% 7% 3 6% 5% 3 4 5% 5% 7% 40%	2473
	21st to end	Day	11% 5% 2 7% 5% 3 5% 5% 7% 8% 43%	2725
June	1st to 10th	Day	14% 6% 3 5% 7% 3 5% 6% 7% 8% 37%	2520
	11th to 20th	Day	13% 5% 2 5% 5% 3 6% 6% 7% 10% 39%	2520
	21st to end	Day	14% 5% 2 5% 6% 4 5% 5% 6% 8% 39%	2520
July	1st to 10th	Day	8% 4 2 5% 5% 4 6% 5% 7% 9% 45%	2520
	11th to 20th	Day	11% 5% 3 6% 6% 4 5% 6% 7% 9% 40%	2520
	21st to end	Day	9% 5% 3 7% 7% 4 6% 6% 7% 10% 34%	2587
August	1st to 10th	Day	9% 6% 2 6% 6% 4 5% 6% 7% 11% 39%	2327
	11th to 20th	Day	8% 6% 3 6% 7% 4 5% 7% 7% 9% 39%	2303
	21st to end	Day	11% 6% 2 5% 6% 3 6% 5% 6% 10% 39%	2476
September	1st to 10th	Day	13% 5% 2 6% 6% 3 5% 6% 6% 7% 41%	2160
	11th to 20th	Day	12% 5% 3 5% 5% 2 5% 6% 7% 8% 43%	2063
	21st to end	Day	13% 5% 3 7% 6% 5% 6% 5% 6% 7% 38%	1938
October	1st to 10th	Day	19% 7% 3 6% 5% 3 5% 5% 5% 7% 34%	1800
	11th to 20th	Day	19% 9% 3 7% 6% 4 5% 5% 5% 6% 30%	1800
	21st to end	Day	23% 12% 4 7% 7% 4 4 5% 6% 5% 22%	1980
November	1st to 10th	Day	14% 9% 4 9% 6% 3 5% 5% 6% 8% 32%	1600
	11th to 20th	Day	15% 10% 5% 8% 6% 4 6% 7% 6% 8% 25%	1520
	21st to end	Day	18% 12% 5% 8% 7% 4 6% 6% 5% 8% 21%	1520
December	1st to 10th	Day	17% 14% 6% 11% 7% 5% 5% 6% 6% 7% 17%	1520
	11th to 20th	Day	19% 12% 4 9% 7% 4 5% 6% 7% 6% 23%	1906
	21st to end	Day	27% 15% 6% 12% 8% 4 5% 3 4 4 12%	2772
January	1st to 10th	Night	17% 13% 5% 9% 7% 4 5% 5% 5% 7% 23%	4429
	11th to 20th	Night	15% 11% 5% 11% 10% 6% 7% 5% 5% 6% 19%	4216
	21st to end	Night	20% 12% 6% 8% 9% 5% 5% 5% 5% 5% 20%	4532
February	1st to 10th	Night	13% 14% 5% 9% 8% 4 5% 5% 6% 6% 25%	2470
	11th to 20th	Night	14% 11% 5% 9% 7% 4 5% 6% 5% 6% 27%	2397
	21st to end	Night	14% 12% 4 8% 7% 4 5% 5% 5% 7% 29%	1874
March	1st to 10th	Night	13% 7% 4 8% 7% 4 6% 5% 5% 5% 36%	1970
	11th to 20th	Night	16% 11% 4 7% 6% 3 4 4 4 6% 34%	1979
	21st to end	Night	15% 9% 3 6% 5% 4 4 4 4 6% 40%	2178
April	1st to 10th	Night	18% 9% 4 6% 5% 3 5% 4 6% 6% 32%	1828
	11th to 20th	Night	15% 8% 4 5% 6% 4 5% 5% 4 6% 38%	1728
	21st to end	Night	16% 8% 2 7% 6% 4 4 5% 5% 9% 35%	1620
May	1st to 10th	Night	16% 5% 3 6% 5% 3 4 6% 5% 8% 39%	1620
	11th to 20th	Night	14% 4 2 5% 4 3 5% 4 5% 8% 46%	1419
	21st to end	Night	10% 4 2 3 4 3 4 4 6% 8% 52%	1550
June	1st to 10th	Night	13% 4 3 5% 5% 4 5% 4 6% 9% 43%	1440
	11th to 20th	Night	9% 5% 3 6% 5% 3 5% 5% 5% 10% 43%	1440
	21st to end	Night	13% 5% 2 5% 4 3 5% 5% 6% 7% 45%	1440
July	1st to 10th	Night	5% 4 2 4 4 3 5% 4 5% 9% 56%	1440
	11th to 20th	Night	9% 4 2 4 4 3 5% 4 7% 8% 50%	1440
	21st to end	Night	7% 4 2 4 5% 3 4 5% 7% 9% 50%	1727
August	1st to 10th	Night	7% 4 3 5% 6% 3 4 5% 6% 8% 50%	1611
	11th to 20th	Night	7% 4 2 5% 4 3 4 5% 6% 9% 51%	1598
	21st to end	Night	10% 5% 2 5% 5% 3 5% 4 6% 7% 48%	1862
September	1st to 10th	Night	9% 5% 3 5% 5% 4 4 6% 6% 7% 46%	1800
	11th to 20th	Night	11% 3 2 4 4 2 3 4 5% 7% 54%	1894
	21st to end	Night	14% 6% 2 5% 5% 3 4 4 6% 6% 44%	2020
October	1st to 10th	Night	16% 6% 3 6% 7% 4 4 5% 5% 7% 38%	2160
	11th to 20th	Night	19% 8% 3 6% 6% 4 5% 5% 6% 7% 32%	2160
	21st to end	Night	21% 9% 4 6% 6% 3 5% 5% 6% 7% 28%	2376
November	1st to 10th	Night	18% 7% 3 5% 5% 3 4 5% 6% 6% 38%	2474
	11th to 20th	Night	17% 9% 4 7% 6% 3 5% 6% 6% 7% 30%	2660
	21st to end	Night	19% 10% 4 7% 7% 4 5% 5% 6% 8% 26%	2660
December	1st to 10th	Night	19% 13% 5% 9% 6% 4 5% 4 6% 7% 22%	2660
	11th to 20th	Night	15% 17% 5% 9% 7% 4 5% 6% 5% 7% 22%	3288
	21st to end	Night	30% 13% 4 8% 7% 4 6% 5% 5% 5% 13%	4884

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	52% 10% 19% 11% 7% 1	2518
	11th to 20th	Day	69% 6% 11% 10% 4	2726
	21st to end	Day	68% 7% 10% 10% 4	3124
February	1st to 10th	Day	62% 6% 11% 16% 3 1	1710
	11th to 20th	Day	57% 7% 11% 17% 7% 1	1663
	21st to end	Day	58% 7% 12% 16% 6% 1	1380
March	1st to 10th	Day	52% 7% 14% 16% 10% 1	1980
	11th to 20th	Day	52% 7% 9% 17% 12% 2	1980
	21st to end	Day	51% 6% 9% 17% 14% 2	2178
April	1st to 10th	Day	54% 6% 11% 17% 11% 1	2131
	11th to 20th	Day	47% 7% 10% 21% 15% 1	2195
	21st to end	Day	50% 7% 11% 18% 12% 3	2340
May	1st to 10th	Day	49% 6% 10% 20% 15% 1	2340
	11th to 20th	Day	42% 7% 10% 25% 14% 2	2477
	21st to end	Day	37% 8% 15% 24% 14% 2	2725
June	1st to 10th	Day	41% 9% 12% 21% 15% 1	2520
	11th to 20th	Day	38% 7% 13% 24% 16% 2	2520
	21st to end	Day	38% 9% 13% 25% 14% 1	2520
July	1st to 10th	Day	33% 9% 11% 25% 18% 3	2450
	11th to 20th	Day	36% 9% 14% 29% 12% 1	2445
	21st to end	Day	40% 11% 13% 22% 13% 1	2442
August	1st to 10th	Day	36% 10% 15% 24% 14% 1	2314
	11th to 20th	Day	36% 10% 15% 26% 12% 1	2303
	21st to end	Day	37% 9% 13% 28% 12% 1	2476
September	1st to 10th	Day	38% 9% 14% 27% 11% 1	2160
	11th to 20th	Day	35% 8% 14% 28% 15% 1	2063
	21st to end	Day	42% 9% 13% 26% 9% 1	1938
October	1st to 10th	Day	48% 6% 15% 19% 11% 1	1800
	11th to 20th	Day	53% 8% 14% 19% 7% 1	1800
	21st to end	Day	59% 7% 12% 16% 6% 1	1980
November	1st to 10th	Day	47% 8% 14% 21% 9% 1	1600
	11th to 20th	Day	53% 10% 14% 15% 8% 1	1520
	21st to end	Day	58% 7% 16% 16% 3	1520
December	1st to 10th	Day	64% 7% 13% 12% 3	1520
	11th to 20th	Day	58% 11% 14% 11% 6%	1906
	21st to end	Day	72% 9% 8% 9% 2	2772
January	1st to 10th	Night	58% 8% 14% 14% 4 1	4429
	11th to 20th	Night	64% 11% 11% 10% 3 1	4216
	21st to end	Night	65% 8% 12% 11% 4 1	4532
February	1st to 10th	Night	57% 7% 12% 19% 4 1	2470
	11th to 20th	Night	56% 7% 10% 18% 8% 1	2399
	21st to end	Night	54% 5% 13% 19% 8%	1874
March	1st to 10th	Night	50% 6% 12% 21% 10% 2	1980
	11th to 20th	Night	52% 4 9% 21% 12% 2	1980
	21st to end	Night	47% 4 8% 22% 15% 3	2178
April	1st to 10th	Night	49% 6% 10% 20% 14% 1	1828
	11th to 20th	Night	47% 6% 8% 21% 16% 2	1728
	21st to end	Night	46% 6% 11% 20% 15% 2	1620
May	1st to 10th	Night	43% 6% 10% 22% 16% 3	1620
	11th to 20th	Night	37% 6% 10% 25% 17% 4	1420
	21st to end	Night	30% 7% 15% 25% 18% 5%	1551
June	1st to 10th	Night	38% 6% 12% 25% 17% 2	1440
	11th to 20th	Night	37% 6% 12% 22% 22% 2	1440
	21st to end	Night	36% 7% 12% 25% 18% 1	1440
July	1st to 10th	Night	26% 6% 12% 28% 24% 4	1400
	11th to 20th	Night	30% 7% 12% 30% 18% 2	1392
	21st to end	Night	29% 9% 15% 30% 16% 3	1631
August	1st to 10th	Night	31% 7% 18% 29% 14% 2	1600
	11th to 20th	Night	28% 8% 15% 34% 14% 2	1598
	21st to end	Night	33% 7% 13% 30% 16% 2	1862
September	1st to 10th	Night	34% 8% 16% 26% 15% 1	1800
	11th to 20th	Night	29% 6% 12% 35% 17% 1	1894
	21st to end	Night	39% 7% 13% 28% 12%	2020
October	1st to 10th	Night	44% 8% 11% 21% 15% 1	2160
	11th to 20th	Night	49% 7% 13% 23% 7%	2160
	21st to end	Night	54% 6% 11% 17% 10% 1	2376
November	1st to 10th	Night	44% 7% 12% 24% 12% 1	2474
	11th to 20th	Night	51% 7% 14% 18% 10% 1	2660
	21st to end	Night	55% 8% 18% 15% 4	2660
December	1st to 10th	Night	60% 8% 12% 15% 5%	2660
	11th to 20th	Night	60% 8% 13% 15% 4	3288
	21st to end	Night	70% 8% 11% 9% 2	4884

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% 15% 33% 22% 9% 1	180
	11th to 20th	Day	20% 20% 35% 14% 9% 1	240
	21st to end	Day	23% 14% 35% 24% 4 1	297
February	1st to 10th	Day	20% 16% 42% 18% 4	270
	11th to 20th	Day	20% 18% 40% 19% 3	267
	21st to end	Day	26% 25% 26% 22% 1	220
March	1st to 10th	Day	19% 12% 39% 23% 7%	420
	11th to 20th	Day	19% 14% 35% 25% 7% 1	420
	21st to end	Day	25% 10% 34% 23% 8% 1	462
April	1st to 10th	Day	18% 12% 35% 26% 8%	420
	11th to 20th	Day	23% 11% 30% 22% 11% 4	412
	21st to end	Day	23% 8% 31% 26% 10% 2	420
May	1st to 10th	Day	22% 7% 26% 28% 12% 5%	423
	11th to 20th	Day	15% 6% 22% 33% 18% 5%	447
	21st to end	Day	13% 4 20% 33% 22% 8%	485
June	1st to 10th	Day	13% 5% 24% 40% 15% 3	450
	11th to 20th	Day	14% 7% 22% 32% 22% 3	450
	21st to end	Day	16% 4 18% 36% 21% 6%	450
July	1st to 10th	Day	9% 3 16% 35% 29% 9%	445
	11th to 20th	Day	13% 2 17% 34% 24% 10%	444
	21st to end	Day	8% 4 19% 39% 22% 8%	488
August	1st to 10th	Day	10% 3 15% 34% 24% 13%	446
	11th to 20th	Day	9% 3 16% 31% 30% 12%	439
	21st to end	Day	12% 5% 17% 32% 25% 9%	495
September	1st to 10th	Day	13% 4 21% 30% 22% 11%	450
	11th to 20th	Day	11% 4 17% 32% 21% 15%	450
	21st to end	Day	12% 6% 26% 31% 15% 9%	450
October	1st to 10th	Day	22% 8% 23% 25% 15% 6%	450
	11th to 20th	Day	20% 10% 36% 24% 9% 2	450
	21st to end	Day	27% 10% 32% 23% 7%	495
November	1st to 10th	Day	18% 16% 27% 26% 13% 1	262
	11th to 20th	Day	18% 19% 32% 24% 6% 2	180
	21st to end	Day	24% 13% 38% 21% 4	180
December	1st to 10th	Day	22% 13% 40% 18% 6% 1	180
	11th to 20th	Day	23% 19% 34% 18% 6% 1	180
	21st to end	Day	27% 15% 41% 16% 2	198
January	1st to 10th	Night	24% 11% 36% 22% 7% 1	760
	11th to 20th	Night	26% 13% 31% 25% 5%	760
	21st to end	Night	18% 9% 37% 28% 8%	836
February	1st to 10th	Night	20% 13% 39% 25% 3	760
	11th to 20th	Night	21% 12% 36% 25% 5%	742
	21st to end	Night	20% 12% 39% 23% 5%	592
March	1st to 10th	Night	18% 11% 36% 28% 7%	720
	11th to 20th	Night	21% 12% 34% 24% 7% 1	720
	21st to end	Night	20% 8% 31% 30% 10% 1	792
April	1st to 10th	Night	23% 10% 30% 31% 7%	720
	11th to 20th	Night	17% 10% 32% 29% 10% 2	664
	21st to end	Night	20% 6% 29% 33% 11%	540
May	1st to 10th	Night	18% 4 26% 33% 17% 2	540
	11th to 20th	Night	16% 3 27% 37% 18% 1	358
	21st to end	Night	10% 3 22% 41% 23% 1	389
June	1st to 10th	Night	14% 2 22% 42% 18% 1	360
	11th to 20th	Night	13% 3 25% 38% 21% 1	360
	21st to end	Night	14% 4 19% 43% 20% 1	360
July	1st to 10th	Night	6% 3 17% 44% 29% 2	350
	11th to 20th	Night	12% 3 18% 39% 24% 3	348
	21st to end	Night	7% 1 17% 47% 25% 3	547
August	1st to 10th	Night	10% 1 13% 40% 32% 4	537
	11th to 20th	Night	6% 2 15% 39% 34% 4	534
	21st to end	Night	11% 3 20% 36% 27% 3	674
September	1st to 10th	Night	11% 4 20% 36% 23% 6%	720
	11th to 20th	Night	13% 4 19% 31% 28% 6%	720
	21st to end	Night	16% 5% 20% 32% 22% 4	720
October	1st to 10th	Night	19% 6% 24% 28% 21% 2	720
	11th to 20th	Night	22% 7% 30% 30% 10% 2	720
	21st to end	Night	27% 7% 28% 31% 6% 1	792
November	1st to 10th	Night	21% 10% 25% 31% 12% 1	740
	11th to 20th	Night	20% 11% 33% 26% 8% 2	760
	21st to end	Night	24% 10% 32% 27% 7% 1	760
December	1st to 10th	Night	28% 10% 35% 21% 5% 1	760
	11th to 20th	Night	21% 12% 39% 23% 5%	760
	21st to end	Night	31% 9% 34% 22% 3	836

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