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 2025-06-10.

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 8% 5% 4 4 3 5% 6% 32%	124446
	Night	21% 10% 3 8% 5% 4 4 3 4 6% 31%	122515
Transparency	Day	54% 4 13% 18% 9% 2	124206
	Night	55% 4 13% 18% 8% 1	122398
Seeing	Day	25% 10% 20% 26% 15% 4	15688
	Night	28% 8% 13% 26% 21% 3	24143

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	35% 15% 3 9% 6% 6% 4 3 5% 5% 8%	3639
	11th to 20th	Day	18% 12% 3 8% 5% 4 5% 4 5% 7% 30%	3616
	21st to end	Day	28% 9% 3 7% 4 3 3 3 4 4 31%	4168
February	1st to 10th	Day	29% 13% 4 8% 6% 4 4 3 5% 6% 18%	1800
	11th to 20th	Day	31% 13% 4 9% 5% 4 3 3 4 5% 19%	1755
	21st to end	Day	29% 17% 4 8% 5% 4 4 3 3 4 19%	1560
March	1st to 10th	Day	17% 14% 4 10% 6% 6% 6% 5% 7% 7% 19%	4640
	11th to 20th	Day	19% 16% 4 9% 6% 5% 4 3 3 4 26%	5000
	21st to end	Day	23% 14% 4 11% 7% 6% 6% 4 6% 5% 12%	5659
April	1st to 10th	Day	27% 14% 4 8% 6% 6% 5% 4 6% 6% 15%	2400
	11th to 20th	Day	22% 13% 3 8% 6% 4 5% 3 5% 6% 25%	2382
	21st to end	Day	21% 14% 4 9% 6% 5% 4 3 5% 5% 23%	2630
May	1st to 10th	Day	16% 8% 3 6% 5% 4 5% 4 6% 6% 38%	6320
	11th to 20th	Day	16% 11% 4 10% 7% 6% 5% 4 5% 7% 25%	6277
	21st to end	Day	20% 9% 3 7% 5% 5% 5% 4 5% 8% 28%	6903
June	1st to 10th	Day	23% 10% 2 8% 5% 5% 4 4 6% 6% 28%	2789
	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	17% 8% 2 5% 4 3 3 3 5% 9% 40%	3180
	11th to 20th	Day	25% 9% 3 7% 4 4 4 3 5% 8% 27%	3161
	21st to end	Day	27% 10% 3 8% 5% 5% 4 4 5% 6% 23%	3269
November	1st to 10th	Day	29% 11% 4 8% 6% 5% 5% 4 5% 5% 17%	1629
	11th to 20th	Day	30% 11% 4 7% 5% 5% 6% 3 6% 5% 17%	1600
	21st to end	Day	28% 13% 3 7% 4 4 4 3 6% 6% 21%	1600
December	1st to 10th	Day	21% 10% 2 6% 4 5% 5% 4 5% 6% 32%	2620
	11th to 20th	Day	31% 13% 4 10% 6% 5% 4 3 4 8% 13%	3025
	21st to end	Day	32% 14% 4 10% 6% 5% 5% 3 4 5% 13%	4004
January	1st to 10th	Night	34% 15% 6% 10% 6% 4 4 3 3 4 10%	6306
	11th to 20th	Night	19% 12% 5% 9% 4 4 4 2 5% 8% 29%	6282
	21st to end	Night	27% 9% 3 7% 5% 4 3 3 3 5% 31%	6793
February	1st to 10th	Night	29% 13% 4 10% 6% 5% 4 4 3 6% 18%	2600
	11th to 20th	Night	28% 13% 4 8% 5% 4 4 3 3 5% 23%	2532
	21st to end	Night	27% 13% 3 7% 4 4 5% 3 5% 5% 22%	2069
March	1st to 10th	Night	18% 15% 4 9% 5% 5% 5% 5% 6% 7% 21%	5249
	11th to 20th	Night	21% 12% 3 8% 5% 4 4 3 5% 6% 30%	4999
	21st to end	Night	29% 15% 4 10% 6% 5% 5% 4 5% 5% 13%	5315
April	1st to 10th	Night	23% 13% 3 8% 6% 5% 5% 5% 5% 7% 20%	2000
	11th to 20th	Night	20% 11% 4 7% 5% 5% 4 4 5% 6% 30%	1967
	21st to end	Night	21% 11% 4 8% 6% 5% 4 4 5% 7% 26%	1770
May	1st to 10th	Night	18% 9% 2 7% 4 4 4 3 4 5% 41%	3640
	11th to 20th	Night	18% 10% 3 9% 5% 5% 5% 4 5% 7% 31%	3616
	21st to end	Night	15% 12% 2 8% 5% 5% 5% 4 6% 8% 30%	3972
June	1st to 10th	Night	17% 8% 2 7% 5% 5% 5% 3 5% 8% 35%	1595
	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	15% 7% 2 7% 4 4 4 3 4 8% 42%	3800
	11th to 20th	Night	25% 8% 3 7% 4 4 4 3 5% 6% 32%	3777
	21st to end	Night	23% 11% 3 9% 5% 4 4 3 5% 6% 26%	4293
November	1st to 10th	Night	32% 9% 3 7% 5% 5% 5% 4 4 5% 22%	2667
	11th to 20th	Night	30% 11% 4 8% 6% 5% 4 3 4 4 21%	2800
	21st to end	Night	26% 12% 4 8% 5% 5% 4 4 5% 5% 22%	2800
December	1st to 10th	Night	20% 10% 3 7% 5% 4 4 4 5% 6% 31%	4560
	11th to 20th	Night	32% 13% 5% 10% 5% 5% 4 3 4 5% 14%	5180
	21st to end	Night	30% 15% 4 11% 6% 5% 4 3 4 5% 15%	6952

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	78% 3 9% 8% 2 1	3639
	11th to 20th	Day	53% 5% 15% 19% 7% 1	3640
	21st to end	Day	58% 2 7% 18% 14%	4168
February	1st to 10th	Day	69% 4 11% 10% 5%	1800
	11th to 20th	Day	70% 3 9% 11% 6% 1	1764
	21st to end	Day	71% 4 10% 11% 4	1560
March	1st to 10th	Day	62% 4 14% 14% 6%	4640
	11th to 20th	Day	64% 3 10% 21% 2	5000
	21st to end	Day	72% 4 13% 9% 1	5659
April	1st to 10th	Day	70% 5% 14% 9% 1	2400
	11th to 20th	Day	61% 4 13% 14% 7% 1	2382
	21st to end	Day	65% 4 13% 13% 3 1	2630
May	1st to 10th	Day	48% 3 15% 25% 9%	6320
	11th to 20th	Day	59% 5% 18% 15% 2	6280
	21st to end	Day	56% 4 23% 15% 2	6904
June	1st to 10th	Day	58% 4 17% 15% 5%	2789
	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	41% 4 13% 31% 10% 1	3180
	11th to 20th	Day	56% 4 13% 18% 8% 1	3161
	21st to end	Day	62% 4 12% 12% 9% 1	3269
November	1st to 10th	Day	69% 5% 10% 12% 4 1	1629
	11th to 20th	Day	69% 5% 10% 9% 6% 2	1600
	21st to end	Day	65% 5% 7% 13% 9% 1	1600
December	1st to 10th	Day	52% 4 9% 20% 13% 1	2620
	11th to 20th	Day	70% 4 12% 11% 3	3028
	21st to end	Day	75% 3 10% 9% 2	4004
January	1st to 10th	Night	79% 3 8% 7% 3 1	6306
	11th to 20th	Night	54% 5% 13% 22% 6% 1	6320
	21st to end	Night	57% 3 9% 20% 11% 1	6793
February	1st to 10th	Night	69% 4 11% 9% 6% 1	2600
	11th to 20th	Night	65% 4 11% 11% 7% 2	2534
	21st to end	Night	66% 5% 11% 14% 5%	2069
March	1st to 10th	Night	61% 4 12% 16% 7%	5249
	11th to 20th	Night	57% 4 14% 22% 2 1	5000
	21st to end	Night	72% 5% 13% 10%	5315
April	1st to 10th	Night	64% 8% 16% 11% 2	2000
	11th to 20th	Night	57% 5% 14% 15% 8% 1	1967
	21st to end	Night	61% 5% 16% 14% 4 1	1770
May	1st to 10th	Night	47% 4 13% 27% 8%	3637
	11th to 20th	Night	55% 6% 21% 14% 3 1	3616
	21st to end	Night	55% 8% 22% 14% 1	3972
June	1st to 10th	Night	51% 4 19% 21% 4	1595
	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	41% 5% 18% 30% 6% 1	3800
	11th to 20th	Night	53% 4 15% 21% 7% 1	3777
	21st to end	Night	58% 4 12% 16% 9%	4293
November	1st to 10th	Night	66% 4 11% 13% 4	2667
	11th to 20th	Night	67% 5% 11% 11% 5% 2	2800
	21st to end	Night	65% 4 9% 12% 9% 1	2800
December	1st to 10th	Night	52% 5% 11% 22% 9% 1	4560
	11th to 20th	Night	73% 4 11% 9% 3	5176
	21st to end	Night	73% 4 11% 10% 3	6952

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% 13% 16% 16% 14% 6%	200
	11th to 20th	Day	31% 14% 16% 22% 14% 4	200
	21st to end	Day	36% 10% 18% 17% 13% 5%	293
February	1st to 10th	Day	41% 10% 17% 19% 11% 2	400
	11th to 20th	Day	45% 12% 17% 15% 8% 2	395
	21st to end	Day	38% 15% 21% 18% 8% 1	369
March	1st to 10th	Day	30% 15% 26% 21% 6% 1	500
	11th to 20th	Day	31% 14% 18% 22% 13% 3	500
	21st to end	Day	33% 15% 25% 18% 8% 1	550
April	1st to 10th	Day	34% 14% 21% 20% 9% 2	500
	11th to 20th	Day	26% 18% 25% 18% 8% 4	492
	21st to end	Day	27% 14% 24% 23% 9% 3	500
May	1st to 10th	Day	21% 12% 27% 24% 14% 3	503
	11th to 20th	Day	30% 9% 22% 23% 12% 5%	528
	21st to end	Day	29% 9% 21% 24% 14% 3	573
June	1st to 10th	Day	25% 9% 21% 27% 13% 4	527
	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% 16% 16% 17% 14% 2	232
	11th to 20th	Day	39% 14% 13% 13% 13% 9%	200
	21st to end	Day	31% 12% 17% 17% 14% 10%	200
December	1st to 10th	Day	38% 12% 10% 13% 16% 12%	200
	11th to 20th	Day	40% 15% 18% 14% 13% 2	200
	21st to end	Day	41% 12% 15% 16% 15% 1	220
January	1st to 10th	Night	43% 11% 13% 19% 13% 2	800
	11th to 20th	Night	35% 10% 11% 25% 17% 3	800
	21st to end	Night	32% 9% 14% 25% 17% 4	880
February	1st to 10th	Night	37% 9% 14% 25% 13% 2	800
	11th to 20th	Night	36% 10% 11% 27% 15% 2	784
	21st to end	Night	36% 11% 15% 26% 11% 2	660
March	1st to 10th	Night	30% 12% 16% 26% 14% 2	800
	11th to 20th	Night	32% 12% 16% 24% 14% 2	800
	21st to end	Night	32% 13% 17% 25% 10% 2	880
April	1st to 10th	Night	31% 8% 18% 25% 16% 2	800
	11th to 20th	Night	26% 11% 18% 27% 16% 2	779
	21st to end	Night	28% 10% 11% 34% 14% 3	570
May	1st to 10th	Night	21% 9% 14% 35% 18% 3	400
	11th to 20th	Night	24% 7% 11% 27% 27% 3	394
	21st to end	Night	23% 7% 11% 27% 27% 4	433
June	1st to 10th	Night	19% 8% 13% 29% 27% 3	399
	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% 9% 17% 26% 11% 1	780
	11th to 20th	Night	38% 10% 15% 21% 13% 3	800
	21st to end	Night	32% 11% 16% 22% 15% 4	800
December	1st to 10th	Night	30% 9% 11% 28% 17% 4	800
	11th to 20th	Night	39% 9% 14% 24% 11% 2	800
	21st to end	Night	42% 12% 17% 18% 10% 1	880

Page generated at 2025-06-07 07:28:15. Version ac07

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