Graham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Graham clear sky chart from 2005-11-06 to 2024-11-03.

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	19% 10% 3 7% 5% 4 4 3 5% 6% 33%	102231
	Night	22% 10% 3 7% 5% 4 4 3 4 5% 32%	96845
Transparency	Day	53% 4 13% 18% 9% 2	101991
	Night	56% 4 13% 17% 8% 2	96731
Seeing	Day	25% 10% 20% 26% 15% 4	15010
	Night	28% 8% 13% 26% 21% 4	23359

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	36% 16% 3 9% 5% 5% 4 3 4 4 11%	2539
	11th to 20th	Day	26% 13% 3 8% 4 3 4 4 4 6% 25%	2516
	21st to end	Day	38% 13% 4 9% 5% 4 4 3 4 4 12%	2902
February	1st to 10th	Day	31% 13% 4 8% 6% 4 4 3 4 6% 18%	1710
	11th to 20th	Day	31% 13% 4 9% 5% 4 3 3 4 5% 18%	1665
	21st to end	Day	29% 17% 4 8% 5% 5% 5% 3 3 4 19%	1484
March	1st to 10th	Day	15% 14% 4 10% 7% 6% 6% 4 6% 6% 21%	3221
	11th to 20th	Day	19% 12% 3 7% 4 4 4 3 3 4 36%	3490
	21st to end	Day	22% 16% 4 11% 7% 6% 5% 4 5% 5% 15%	3946
April	1st to 10th	Day	27% 14% 4 8% 5% 6% 4 4 5% 6% 16%	2280
	11th to 20th	Day	23% 14% 3 8% 6% 4 5% 4 4 7% 23%	2261
	21st to end	Day	22% 14% 4 10% 6% 5% 4 3 5% 5% 21%	2498
May	1st to 10th	Day	20% 10% 3 6% 4 4 4 3 5% 6% 35%	4420
	11th to 20th	Day	14% 9% 3 7% 5% 5% 5% 3 5% 8% 34%	4377
	21st to end	Day	26% 11% 4 9% 5% 6% 5% 4 5% 6% 18%	4813
June	1st to 10th	Day	24% 10% 2 8% 5% 5% 4 4 6% 6% 26%	2660
	11th to 20th	Day	21% 11% 3 9% 7% 6% 5% 4 5% 7% 21%	2820
	21st to end	Day	12% 7% 3 8% 6% 5% 4 3 6% 7% 39%	2850
July	1st to 10th	Day	7% 4 2 4 3 3 3 3 4 5% 63%	4740
	11th to 20th	Day	6% 4 1 4 3 2 2 2 3 5% 68%	4423
	21st to end	Day	11% 4 2 5% 3 3 3 3 5% 6% 56%	4790
August	1st to 10th	Day	5% 3 1 4 3 3 3 3 5% 7% 63%	4288
	11th to 20th	Day	8% 6% 3 7% 5% 5% 4 4 6% 8% 44%	4026
	21st to end	Day	15% 8% 2 6% 4 4 4 3 4 7% 43%	4314
September	1st to 10th	Day	10% 6% 2 6% 4 4 4 3 5% 7% 51%	2280
	11th to 20th	Day	14% 7% 3 7% 5% 4 5% 4 6% 8% 38%	2280
	21st to end	Day	17% 9% 3 6% 4 4 4 3 5% 7% 39%	2009
October	1st to 10th	Day	20% 10% 4 7% 5% 4 4 3 5% 6% 33%	1900
	11th to 20th	Day	23% 10% 3 7% 4 4 3 3 5% 8% 30%	1900
	21st to end	Day	29% 10% 3 7% 5% 4 4 4 4 5% 25%	1986
November	1st to 10th	Day	30% 11% 4 9% 6% 5% 5% 4 5% 5% 17%	1564
	11th to 20th	Day	30% 11% 4 7% 5% 5% 5% 3 6% 5% 18%	1520
	21st to end	Day	29% 13% 3 7% 4 4 4 3 6% 5% 21%	1520
December	1st to 10th	Day	26% 13% 3 7% 5% 5% 4 3 4 5% 26%	1520
	11th to 20th	Day	30% 13% 4 8% 6% 4 4 4 5% 8% 15%	1925
	21st to end	Day	28% 13% 4 10% 6% 5% 5% 3 4 5% 16%	2794
January	1st to 10th	Night	35% 16% 5% 9% 5% 5% 4 3 3 4 11%	4406
	11th to 20th	Night	27% 12% 4 9% 4 4 4 2 5% 6% 23%	4382
	21st to end	Night	36% 11% 4 8% 5% 4 4 3 4 5% 14%	4755
February	1st to 10th	Night	30% 12% 4 9% 6% 4 4 4 3 6% 18%	2470
	11th to 20th	Night	28% 13% 4 8% 5% 4 4 3 3 5% 23%	2402
	21st to end	Night	26% 14% 3 7% 5% 4 5% 3 5% 5% 22%	1969
March	1st to 10th	Night	17% 16% 5% 10% 5% 5% 5% 5% 6% 6% 19%	3648
	11th to 20th	Night	19% 10% 3 6% 5% 3 3 3 4 5% 39%	3489
	21st to end	Night	27% 17% 4 10% 6% 5% 5% 4 4 4 14%	3706
April	1st to 10th	Night	24% 13% 3 9% 6% 4 5% 5% 5% 7% 20%	1900
	11th to 20th	Night	20% 11% 4 7% 5% 5% 4 4 6% 6% 29%	1868
	21st to end	Night	22% 11% 4 8% 6% 5% 4 4 5% 7% 24%	1682
May	1st to 10th	Night	21% 10% 3 8% 4 3 3 2 4 4 37%	2540
	11th to 20th	Night	14% 8% 3 8% 4 5% 4 4 5% 7% 39%	2516
	21st to end	Night	19% 14% 3 9% 5% 5% 5% 4 5% 6% 26%	2762
June	1st to 10th	Night	18% 8% 3 7% 5% 4 5% 3 5% 8% 33%	1520
	11th to 20th	Night	17% 10% 4 9% 5% 5% 3 4 5% 7% 30%	1354
	21st to end	Night	10% 7% 2 7% 5% 4 4 3 6% 5% 47%	1330
July	1st to 10th	Night	5% 3 1 3 3 3 3 3 4 5% 67%	2220
	11th to 20th	Night	4 3 1 3 3 2 2 2 3 4 73%	2533
	21st to end	Night	7% 4 1 3 2 2 3 3 4 5% 66%	2762
August	1st to 10th	Night	4 3 1 3 3 2 2 2 3 6% 71%	2643
	11th to 20th	Night	10% 7% 2 6% 3 3 3 2 5% 7% 51%	2812
	21st to end	Night	16% 6% 2 5% 4 3 3 3 4 6% 49%	3360
September	1st to 10th	Night	9% 3 2 4 3 4 4 3 4 6% 57%	1900
	11th to 20th	Night	16% 8% 3 6% 4 3 3 3 4 5% 44%	1900
	21st to end	Night	15% 6% 2 6% 5% 5% 4 2 4 6% 44%	2167
October	1st to 10th	Night	18% 8% 3 7% 5% 4 3 3 4 6% 39%	2280
	11th to 20th	Night	24% 9% 3 7% 4 4 4 3 5% 5% 33%	2280
	21st to end	Night	25% 9% 3 7% 4 4 4 3 5% 6% 31%	2611
November	1st to 10th	Night	32% 9% 3 7% 5% 4 4 4 4 5% 22%	2556
	11th to 20th	Night	30% 10% 4 8% 5% 5% 4 3 4 4 22%	2660
	21st to end	Night	26% 12% 4 8% 5% 5% 4 3 5% 4 23%	2660
December	1st to 10th	Night	27% 11% 4 8% 6% 4 4 3 4 5% 25%	2660
	11th to 20th	Night	29% 12% 4 8% 5% 5% 4 4 5% 5% 19%	3280
	21st to end	Night	28% 14% 4 11% 6% 5% 4 3 4 5% 16%	4862

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	79% 2 7% 8% 3 1	2539
	11th to 20th	Day	62% 4 15% 13% 4 2	2540
	21st to end	Day	75% 3 7% 10% 4	2902
February	1st to 10th	Day	69% 4 11% 10% 5%	1710
	11th to 20th	Day	70% 4 9% 12% 5% 1	1674
	21st to end	Day	71% 4 10% 10% 4	1484
March	1st to 10th	Day	63% 3 12% 17% 5%	3221
	11th to 20th	Day	55% 2 11% 28% 2 1	3490
	21st to end	Day	73% 3 12% 11% 1	3946
April	1st to 10th	Day	71% 5% 14% 9% 1	2280
	11th to 20th	Day	63% 4 12% 13% 7% 1	2261
	21st to end	Day	67% 4 13% 12% 3 1	2498
May	1st to 10th	Day	54% 3 13% 21% 10%	4420
	11th to 20th	Day	51% 4 23% 18% 4	4380
	21st to end	Day	69% 4 17% 9% 1	4814
June	1st to 10th	Day	60% 4 17% 13% 5%	2660
	11th to 20th	Day	65% 4 16% 12% 3	2820
	21st to end	Day	45% 5% 20% 20% 8% 1	2850
July	1st to 10th	Day	26% 5% 9% 21% 20% 19%	4666
	11th to 20th	Day	21% 3 7% 26% 37% 6%	4347
	21st to end	Day	29% 3 12% 35% 18% 3	4639
August	1st to 10th	Day	22% 4 11% 38% 22% 3	4271
	11th to 20th	Day	38% 8% 18% 26% 8% 2	4065
	21st to end	Day	43% 4 13% 25% 15% 1	4313
September	1st to 10th	Day	34% 4 16% 28% 14% 5%	2280
	11th to 20th	Day	45% 6% 15% 18% 14% 3	2280
	21st to end	Day	46% 4 17% 20% 12% 2	2009
October	1st to 10th	Day	51% 5% 11% 21% 10% 1	1900
	11th to 20th	Day	55% 3 15% 19% 6% 1	1900
	21st to end	Day	62% 3 10% 14% 10% 1	1986
November	1st to 10th	Day	69% 5% 10% 12% 4 1	1564
	11th to 20th	Day	68% 4 10% 9% 6% 2	1520
	21st to end	Day	65% 5% 6% 13% 9% 1	1520
December	1st to 10th	Day	62% 5% 10% 13% 8% 2	1520
	11th to 20th	Day	67% 4 12% 12% 4 1	1928
	21st to end	Day	72% 3 11% 10% 3	2794
January	1st to 10th	Night	80% 3 7% 6% 3 1	4406
	11th to 20th	Night	63% 4 11% 17% 4 1	4420
	21st to end	Night	73% 3 10% 9% 5% 1	4755
February	1st to 10th	Night	69% 4 11% 9% 5% 1	2470
	11th to 20th	Night	65% 4 11% 11% 7% 1	2404
	21st to end	Night	66% 5% 11% 13% 5% 1	1969
March	1st to 10th	Night	65% 4 10% 16% 5%	3648
	11th to 20th	Night	49% 3 18% 26% 3 1	3490
	21st to end	Night	74% 4 12% 9%	3706
April	1st to 10th	Night	65% 7% 16% 10% 2	1900
	11th to 20th	Night	58% 6% 14% 14% 8% 1	1868
	21st to end	Night	63% 5% 15% 13% 4 1	1682
May	1st to 10th	Night	52% 5% 11% 22% 9%	2540
	11th to 20th	Night	48% 4 24% 19% 4 1	2516
	21st to end	Night	64% 6% 18% 10% 2	2762
June	1st to 10th	Night	53% 4 19% 20% 4	1520
	11th to 20th	Night	56% 6% 20% 15% 3	1354
	21st to end	Night	40% 4 24% 22% 9% 2	1330
July	1st to 10th	Night	23% 6% 11% 23% 16% 22%	2185
	11th to 20th	Night	19% 2 8% 27% 39% 5%	2486
	21st to end	Night	23% 5% 13% 35% 20% 4	2674
August	1st to 10th	Night	18% 3 11% 42% 23% 2	2635
	11th to 20th	Night	35% 6% 25% 23% 8% 3	2839
	21st to end	Night	39% 4 15% 30% 11% 1	3360
September	1st to 10th	Night	30% 5% 17% 28% 16% 4	1900
	11th to 20th	Night	44% 5% 13% 21% 13% 4	1900
	21st to end	Night	42% 5% 16% 22% 13% 3	2167
October	1st to 10th	Night	48% 4 13% 25% 9% 1	2280
	11th to 20th	Night	53% 4 14% 21% 8% 1	2280
	21st to end	Night	55% 4 13% 17% 11%	2611
November	1st to 10th	Night	66% 4 12% 13% 4 1	2556
	11th to 20th	Night	66% 5% 11% 11% 5% 2	2660
	21st to end	Night	65% 4 8% 12% 10% 1	2660
December	1st to 10th	Night	63% 4 10% 14% 7% 2	2660
	11th to 20th	Night	66% 4 14% 10% 5% 1	3276
	21st to end	Night	72% 3 10% 11% 3	4862

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	37% 14% 14% 14% 14% 6%	180
	11th to 20th	Day	33% 16% 15% 19% 13% 3	180
	21st to end	Day	39% 11% 18% 17% 11% 4	267
February	1st to 10th	Day	42% 9% 17% 19% 11% 2	370
	11th to 20th	Day	44% 12% 17% 16% 8% 2	365
	21st to end	Day	38% 16% 21% 17% 8% 1	341
March	1st to 10th	Day	31% 16% 25% 21% 6% 1	460
	11th to 20th	Day	31% 13% 18% 22% 14% 3	460
	21st to end	Day	33% 15% 25% 18% 8% 1	506
April	1st to 10th	Day	35% 14% 20% 19% 10% 2	460
	11th to 20th	Day	27% 19% 25% 17% 7% 4	452
	21st to end	Day	29% 15% 24% 21% 9% 2	460
May	1st to 10th	Day	22% 12% 27% 23% 13% 3	463
	11th to 20th	Day	29% 8% 22% 23% 12% 5%	488
	21st to end	Day	30% 9% 20% 24% 14% 3	529
June	1st to 10th	Day	27% 9% 22% 26% 13% 4	490
	11th to 20th	Day	25% 12% 26% 25% 10% 3	650
	21st to end	Day	14% 9% 21% 34% 19% 3	680
July	1st to 10th	Day	13% 7% 19% 37% 21% 3	671
	11th to 20th	Day	10% 6% 20% 38% 21% 5%	486
	21st to end	Day	10% 3 18% 38% 26% 5%	531
August	1st to 10th	Day	11% 4 15% 40% 22% 8%	487
	11th to 20th	Day	8% 4 14% 41% 24% 9%	477
	21st to end	Day	14% 9% 22% 32% 17% 7%	539
September	1st to 10th	Day	14% 6% 21% 36% 17% 5%	490
	11th to 20th	Day	17% 9% 20% 29% 22% 3	490
	21st to end	Day	19% 9% 22% 30% 17% 3	490
October	1st to 10th	Day	26% 9% 20% 22% 18% 5%	490
	11th to 20th	Day	30% 10% 17% 20% 17% 6%	490
	21st to end	Day	33% 12% 19% 20% 11% 6%	435
November	1st to 10th	Day	35% 17% 15% 18% 13% 1	215
	11th to 20th	Day	39% 11% 12% 13% 14% 10%	180
	21st to end	Day	32% 13% 17% 14% 13% 11%	180
December	1st to 10th	Day	39% 13% 11% 13% 13% 11%	180
	11th to 20th	Day	39% 16% 17% 13% 14% 2	180
	21st to end	Day	38% 12% 16% 17% 16% 2	198
January	1st to 10th	Night	44% 11% 12% 18% 13% 2	760
	11th to 20th	Night	37% 10% 11% 23% 17% 3	760
	21st to end	Night	33% 9% 13% 25% 16% 4	836
February	1st to 10th	Night	37% 8% 13% 25% 14% 2	760
	11th to 20th	Night	35% 10% 12% 26% 16% 2	744
	21st to end	Night	36% 11% 15% 25% 12% 2	628
March	1st to 10th	Night	31% 12% 15% 25% 14% 3	760
	11th to 20th	Night	31% 12% 15% 25% 15% 2	760
	21st to end	Night	32% 13% 17% 25% 11% 2	836
April	1st to 10th	Night	32% 9% 18% 24% 16% 2	760
	11th to 20th	Night	26% 11% 18% 26% 16% 2	740
	21st to end	Night	28% 10% 11% 33% 13% 3	542
May	1st to 10th	Night	22% 9% 14% 34% 19% 3	380
	11th to 20th	Night	23% 6% 11% 28% 28% 3	374
	21st to end	Night	24% 8% 11% 27% 27% 4	411
June	1st to 10th	Night	20% 8% 13% 29% 27% 3	380
	11th to 20th	Night	22% 8% 16% 32% 22% 1	380
	21st to end	Night	13% 8% 13% 32% 32% 3	380
July	1st to 10th	Night	11% 3 8% 33% 39% 6%	370
	11th to 20th	Night	10% 3 8% 37% 39% 3	368
	21st to end	Night	5% 3 8% 31% 48% 4	416
August	1st to 10th	Night	11% 3 6% 25% 45% 10%	440
	11th to 20th	Night	10% 2 6% 25% 47% 10%	564
	21st to end	Night	13% 5% 9% 31% 36% 5%	750
September	1st to 10th	Night	11% 4 9% 34% 34% 7%	760
	11th to 20th	Night	18% 5% 10% 24% 35% 9%	760
	21st to end	Night	20% 6% 12% 29% 29% 4	760
October	1st to 10th	Night	22% 6% 10% 29% 27% 5%	760
	11th to 20th	Night	27% 8% 12% 26% 21% 6%	760
	21st to end	Night	31% 7% 17% 25% 17% 4	836
November	1st to 10th	Night	36% 10% 17% 26% 10% 1	748
	11th to 20th	Night	37% 10% 14% 22% 14% 3	760
	21st to end	Night	33% 10% 16% 21% 15% 4	760
December	1st to 10th	Night	31% 9% 12% 28% 16% 3	760
	11th to 20th	Night	39% 9% 14% 24% 12% 2	760
	21st to end	Night	42% 11% 16% 19% 11% 1	836

Page generated at 2024-10-31 00:39:25. Version ac07

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