Grey Highland Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grey Highland Skies Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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	13% 12% 5% 11% 6% 5% 5% 4 5% 6% 28%	114318
	Night	12% 12% 5% 12% 6% 5% 5% 4 5% 6% 28%	114080
Transparency	Day	55% 5% 11% 18% 10% 1	117939
	Night	54% 5% 12% 18% 10% 1	118623
Seeing	Day	21% 22% 31% 17% 7% 2	15467
	Night	20% 22% 31% 19% 7% 1	24479

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	12% 13% 7% 20% 10% 6% 7% 4 5% 4 12%	3414
	11th to 20th	Day	8% 18% 8% 23% 11% 10% 5% 4 4 3 5%	3962
	21st to end	Day	12% 18% 8% 19% 8% 7% 6% 4 4 4 11%	4466
February	1st to 10th	Day	11% 16% 7% 16% 12% 7% 5% 3 4 4 14%	1800
	11th to 20th	Day	10% 13% 7% 17% 11% 6% 5% 5% 6% 5% 15%	1753
	21st to end	Day	14% 17% 6% 11% 8% 7% 6% 4 5% 4 19%	1560
March	1st to 10th	Day	15% 13% 5% 13% 8% 5% 4 3 4 5% 26%	4888
	11th to 20th	Day	9% 14% 7% 12% 6% 4 5% 4 5% 7% 27%	4980
	21st to end	Day	15% 15% 6% 10% 5% 4 3 3 4 5% 29%	5465
April	1st to 10th	Day	15% 16% 5% 10% 5% 4 4 3 4 7% 26%	2333
	11th to 20th	Day	16% 13% 4 7% 4 4 4 4 5% 6% 34%	2509
	21st to end	Day	19% 12% 4 8% 4 4 4 3 5% 6% 32%	2535
May	1st to 10th	Day	16% 10% 3 7% 5% 4 4 3 5% 8% 34%	4108
	11th to 20th	Day	16% 9% 4 8% 5% 5% 5% 4 6% 7% 31%	4324
	21st to end	Day	12% 6% 2 6% 4 4 4 3 5% 8% 44%	4837
June	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 5% 6% 8% 34%	2660
	11th to 20th	Day	11% 6% 3 7% 5% 4 5% 5% 6% 9% 39%	2832
	21st to end	Day	14% 7% 2 7% 4 6% 4 4 7% 8% 36%	2850
July	1st to 10th	Day	11% 6% 3 9% 6% 6% 6% 5% 7% 8% 33%	4760
	11th to 20th	Day	10% 6% 2 6% 5% 6% 6% 5% 7% 8% 39%	4491
	21st to end	Day	8% 6% 3 7% 5% 6% 5% 4 6% 9% 40%	4466
August	1st to 10th	Day	14% 7% 3 7% 6% 5% 6% 4 6% 8% 34%	4104
	11th to 20th	Day	11% 8% 3 8% 5% 6% 6% 5% 8% 9% 29%	4044
	21st to end	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 9% 38%	4418
September	1st to 10th	Day	15% 8% 3 7% 4 5% 5% 4 5% 7% 37%	2280
	11th to 20th	Day	12% 7% 2 7% 4 4 5% 4 5% 8% 42%	2123
	21st to end	Day	15% 9% 4 8% 6% 5% 4 4 6% 7% 32%	2081
October	1st to 10th	Day	17% 10% 4 9% 5% 4 5% 3 5% 7% 31%	1900
	11th to 20th	Day	17% 16% 4 9% 6% 5% 5% 3 6% 6% 24%	1900
	21st to end	Day	23% 20% 5% 9% 4 4 5% 3 4 4 19%	2081
November	1st to 10th	Day	13% 18% 6% 10% 6% 5% 4 3 5% 5% 26%	1677
	11th to 20th	Day	12% 18% 6% 11% 7% 6% 5% 4 6% 7% 19%	1600
	21st to end	Day	14% 19% 7% 16% 6% 6% 6% 5% 5% 5% 13%	1600
December	1st to 10th	Day	14% 23% 10% 18% 7% 5% 4 3 4 4 7%	2600
	11th to 20th	Day	17% 21% 7% 14% 6% 4 4 3 4 4 14%	2986
	21st to end	Day	20% 20% 7% 18% 8% 6% 4 2 3 3 9%	3931
January	1st to 10th	Night	12% 13% 7% 18% 10% 7% 6% 4 5% 7% 12%	6365
	11th to 20th	Night	10% 12% 7% 22% 14% 10% 6% 4 4 3 7%	5907
	21st to end	Night	12% 12% 7% 17% 10% 7% 6% 4 6% 6% 13%	6490
February	1st to 10th	Night	11% 15% 7% 14% 9% 7% 6% 5% 6% 5% 16%	2600
	11th to 20th	Night	10% 12% 6% 16% 9% 7% 6% 4 6% 5% 19%	2533
	21st to end	Night	11% 12% 5% 13% 8% 6% 6% 4 6% 5% 23%	2065
March	1st to 10th	Night	12% 11% 5% 12% 6% 6% 6% 5% 6% 7% 25%	4952
	11th to 20th	Night	7% 10% 6% 10% 6% 6% 5% 4 6% 7% 32%	4979
	21st to end	Night	16% 10% 4 10% 5% 5% 4 4 5% 5% 32%	5467
April	1st to 10th	Night	17% 13% 4 8% 7% 4 4 4 5% 6% 28%	2066
	11th to 20th	Night	15% 10% 3 8% 5% 5% 4 3 4 7% 36%	1857
	21st to end	Night	13% 12% 4 10% 5% 3 5% 4 5% 6% 32%	1761
May	1st to 10th	Night	15% 9% 2 6% 4 4 4 4 5% 7% 41%	2833
	11th to 20th	Night	15% 7% 2 6% 5% 4 5% 3 6% 7% 40%	2566
	21st to end	Night	9% 6% 2 6% 3 4 4 4 5% 7% 51%	2738
June	1st to 10th	Night	13% 7% 2 7% 4 5% 5% 4 6% 8% 39%	1520
	11th to 20th	Night	9% 6% 2 6% 3 6% 5% 5% 7% 10% 42%	1343
	21st to end	Night	11% 7% 2 7% 4 5% 3 3 5% 9% 44%	1330
July	1st to 10th	Night	6% 7% 3 6% 4 5% 5% 4 5% 8% 49%	2210
	11th to 20th	Night	10% 5% 2 6% 4 3 3 3 5% 7% 53%	2463
	21st to end	Night	6% 5% 2 6% 5% 4 5% 4 7% 8% 48%	3087
August	1st to 10th	Night	10% 6% 2 7% 4 4 4 4 5% 7% 48%	2825
	11th to 20th	Night	12% 6% 1 6% 4 3 5% 4 5% 8% 46%	2785
	21st to end	Night	10% 6% 2 6% 4 4 5% 3 6% 7% 47%	3206
September	1st to 10th	Night	9% 7% 2 6% 3 4 4 3 7% 8% 46%	1900
	11th to 20th	Night	11% 5% 2 4 3 4 3 3 6% 7% 52%	2055
	21st to end	Night	14% 9% 3 8% 4 4 4 3 5% 7% 38%	2099
October	1st to 10th	Night	16% 8% 3 7% 5% 5% 4 3 6% 8% 36%	2280
	11th to 20th	Night	16% 14% 4 8% 4 4 5% 4 5% 7% 30%	2280
	21st to end	Night	18% 15% 4 9% 6% 5% 5% 4 5% 7% 22%	2513
November	1st to 10th	Night	14% 14% 4 9% 6% 5% 6% 4 5% 6% 28%	2621
	11th to 20th	Night	14% 16% 5% 10% 5% 6% 5% 4 6% 7% 22%	2800
	21st to end	Night	15% 16% 7% 12% 5% 6% 6% 4 5% 7% 17%	2800
December	1st to 10th	Night	13% 17% 8% 19% 7% 7% 6% 3 5% 5% 10%	4580
	11th to 20th	Night	11% 20% 8% 18% 8% 6% 5% 3 4 4 13%	5208
	21st to end	Night	18% 20% 9% 19% 7% 6% 4 3 3 4 7%	6996

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	71% 6% 9% 11% 3	3415
	11th to 20th	Day	80% 4 7% 6% 3	3996
	21st to end	Day	74% 5% 9% 8% 3	4466
February	1st to 10th	Day	72% 5% 8% 11% 3 1	1800
	11th to 20th	Day	68% 5% 10% 11% 5% 1	1762
	21st to end	Day	65% 5% 10% 14% 6% 1	1560
March	1st to 10th	Day	60% 5% 12% 13% 10%	4891
	11th to 20th	Day	56% 3 12% 20% 6% 2	4980
	21st to end	Day	57% 3 9% 17% 11% 2	5465
April	1st to 10th	Day	58% 5% 8% 18% 10% 1	2333
	11th to 20th	Day	49% 5% 8% 22% 14% 1	2509
	21st to end	Day	53% 5% 10% 18% 11% 2	2535
May	1st to 10th	Day	47% 6% 11% 21% 15% 1	4120
	11th to 20th	Day	47% 11% 13% 20% 8% 1	4328
	21st to end	Day	37% 7% 12% 21% 19% 4	4837
June	1st to 10th	Day	44% 7% 13% 21% 14% 1	2660
	11th to 20th	Day	39% 7% 12% 25% 16% 1	2832
	21st to end	Day	43% 6% 13% 24% 13% 1	2850
July	1st to 10th	Day	47% 5% 13% 19% 15% 1	4686
	11th to 20th	Day	38% 6% 12% 32% 11% 1	4413
	21st to end	Day	39% 6% 13% 23% 18% 1	4324
August	1st to 10th	Day	45% 6% 13% 23% 12% 1	4091
	11th to 20th	Day	47% 6% 15% 24% 8%	4083
	21st to end	Day	40% 6% 12% 31% 10% 1	4418
September	1st to 10th	Day	45% 6% 13% 24% 10% 2	2280
	11th to 20th	Day	39% 7% 13% 27% 14%	2123
	21st to end	Day	49% 7% 12% 23% 8% 1	2081
October	1st to 10th	Day	48% 5% 11% 24% 12% 1	3160
	11th to 20th	Day	52% 4 11% 20% 13% 1	3140
	21st to end	Day	64% 3 11% 16% 5%	3407
November	1st to 10th	Day	59% 5% 11% 17% 7%	1677
	11th to 20th	Day	64% 5% 11% 14% 6% 1	1600
	21st to end	Day	70% 6% 11% 10% 2	1600
December	1st to 10th	Day	80% 3 9% 6% 2	2600
	11th to 20th	Day	73% 5% 7% 10% 5%	2986
	21st to end	Day	80% 6% 5% 6% 4	3931
January	1st to 10th	Night	69% 7% 11% 10% 4	6365
	11th to 20th	Night	77% 4 9% 7% 2	5948
	21st to end	Night	67% 6% 11% 8% 8% 1	6490
February	1st to 10th	Night	65% 5% 12% 13% 3	2600
	11th to 20th	Night	64% 5% 12% 14% 5%	2535
	21st to end	Night	60% 6% 13% 14% 8%	2065
March	1st to 10th	Night	54% 7% 13% 16% 9% 2	4962
	11th to 20th	Night	49% 4 17% 20% 8% 2	4980
	21st to end	Night	52% 4 11% 17% 14% 2	5467
April	1st to 10th	Night	55% 5% 9% 18% 13% 1	2066
	11th to 20th	Night	48% 5% 8% 21% 16% 2	1857
	21st to end	Night	51% 5% 11% 16% 16% 2	1761
May	1st to 10th	Night	42% 7% 12% 20% 15% 5%	2840
	11th to 20th	Night	40% 9% 13% 25% 11% 2	2567
	21st to end	Night	30% 7% 13% 25% 22% 2	2739
June	1st to 10th	Night	41% 6% 12% 22% 18% 2	1520
	11th to 20th	Night	34% 7% 13% 26% 19% 1	1343
	21st to end	Night	37% 6% 14% 27% 15% 1	1330
July	1st to 10th	Night	33% 4 16% 25% 19% 4	2175
	11th to 20th	Night	30% 4 14% 35% 17% 1	2418
	21st to end	Night	33% 5% 15% 30% 15% 1	2988
August	1st to 10th	Night	36% 4 14% 28% 17% 1	2814
	11th to 20th	Night	35% 5% 17% 28% 14% 1	2812
	21st to end	Night	36% 4 13% 32% 13% 1	3206
September	1st to 10th	Night	33% 8% 15% 30% 13% 1	1900
	11th to 20th	Night	30% 5% 14% 31% 18% 1	2055
	21st to end	Night	45% 6% 13% 25% 10%	2099
October	1st to 10th	Night	39% 5% 9% 26% 21% 1	3800
	11th to 20th	Night	48% 4 12% 20% 14% 2	3779
	21st to end	Night	51% 5% 15% 22% 6%	4137
November	1st to 10th	Night	53% 6% 14% 18% 8% 1	2621
	11th to 20th	Night	58% 6% 14% 14% 6% 1	2800
	21st to end	Night	64% 8% 14% 11% 2 1	2800
December	1st to 10th	Night	73% 4 14% 7% 2	4580
	11th to 20th	Night	72% 4 8% 11% 5%	5208
	21st to end	Night	80% 5% 5% 7% 2	6996

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	24% 32% 30% 14% 2 1	200
	11th to 20th	Day	24% 36% 30% 9% 2	287
	21st to end	Day	26% 31% 32% 10% 1	330
February	1st to 10th	Day	30% 34% 29% 4 2	300
	11th to 20th	Day	27% 35% 33% 4 1	300
	21st to end	Day	35% 35% 27% 4	249
March	1st to 10th	Day	26% 29% 31% 11% 3	480
	11th to 20th	Day	26% 26% 33% 11% 3 1	500
	21st to end	Day	32% 23% 29% 12% 3	550
April	1st to 10th	Day	28% 26% 33% 9% 4	500
	11th to 20th	Day	33% 23% 23% 12% 6% 3	492
	21st to end	Day	28% 24% 30% 12% 4 2	480
May	1st to 10th	Day	21% 25% 29% 17% 6% 1	463
	11th to 20th	Day	18% 18% 33% 21% 7% 3	487
	21st to end	Day	17% 17% 32% 22% 9% 3	529
June	1st to 10th	Day	17% 20% 32% 23% 7% 1	490
	11th to 20th	Day	14% 18% 34% 24% 8% 2	662
	21st to end	Day	18% 17% 36% 19% 7% 3	680
July	1st to 10th	Day	15% 12% 32% 27% 11% 3	671
	11th to 20th	Day	16% 14% 32% 23% 10% 6%	524
	21st to end	Day	11% 10% 41% 23% 10% 4	532
August	1st to 10th	Day	12% 13% 31% 27% 12% 6%	486
	11th to 20th	Day	12% 10% 30% 26% 16% 6%	479
	21st to end	Day	15% 13% 30% 26% 12% 4	539
September	1st to 10th	Day	17% 16% 28% 24% 10% 5%	490
	11th to 20th	Day	11% 12% 27% 25% 18% 7%	490
	21st to end	Day	18% 19% 27% 20% 13% 3	490
October	1st to 10th	Day	23% 21% 26% 16% 11% 3	490
	11th to 20th	Day	22% 29% 32% 13% 3	490
	21st to end	Day	32% 28% 28% 10% 2	530
November	1st to 10th	Day	26% 32% 26% 12% 3	257
	11th to 20th	Day	26% 31% 32% 9% 3 1	200
	21st to end	Day	30% 27% 37% 8%	200
December	1st to 10th	Day	26% 37% 29% 9%	200
	11th to 20th	Day	32% 31% 28% 9% 2 1	200
	21st to end	Day	26% 35% 29% 7% 2	220
January	1st to 10th	Night	20% 35% 31% 11% 2	816
	11th to 20th	Night	25% 29% 30% 14% 2 1	800
	21st to end	Night	17% 26% 36% 16% 4	880
February	1st to 10th	Night	21% 36% 30% 12% 1	800
	11th to 20th	Night	22% 30% 33% 12% 2	783
	21st to end	Night	23% 32% 32% 11% 3	660
March	1st to 10th	Night	22% 29% 33% 14% 3	800
	11th to 20th	Night	22% 28% 32% 15% 3	800
	21st to end	Night	22% 23% 32% 19% 4	880
April	1st to 10th	Night	25% 24% 34% 15% 2	800
	11th to 20th	Night	22% 23% 29% 18% 7% 1	673
	21st to end	Night	22% 20% 31% 19% 7%	587
May	1st to 10th	Night	16% 16% 31% 26% 9% 1	570
	11th to 20th	Night	18% 12% 34% 26% 10%	419
	21st to end	Night	11% 15% 32% 32% 11%	411
June	1st to 10th	Night	17% 12% 37% 28% 6% 1	380
	11th to 20th	Night	13% 13% 27% 36% 11%	380
	21st to end	Night	15% 12% 32% 27% 13% 1	380
July	1st to 10th	Night	9% 6% 34% 33% 18%	370
	11th to 20th	Night	9% 11% 34% 28% 16% 1	369
	21st to end	Night	10% 8% 33% 33% 15% 1	624
August	1st to 10th	Night	10% 7% 31% 32% 19% 1	567
	11th to 20th	Night	7% 9% 26% 35% 22% 2	564
	21st to end	Night	14% 9% 31% 27% 17% 2	670
September	1st to 10th	Night	13% 13% 28% 28% 14% 4	760
	11th to 20th	Night	13% 11% 28% 27% 16% 4	760
	21st to end	Night	18% 14% 24% 27% 14% 3	760
October	1st to 10th	Night	20% 16% 33% 17% 12% 2	760
	11th to 20th	Night	26% 22% 28% 17% 6% 1	760
	21st to end	Night	26% 26% 31% 14% 3	836
November	1st to 10th	Night	25% 23% 31% 16% 5%	780
	11th to 20th	Night	22% 32% 27% 14% 5% 1	800
	21st to end	Night	26% 28% 30% 14% 2	800
December	1st to 10th	Night	26% 30% 32% 10% 3	800
	11th to 20th	Night	22% 33% 33% 10% 2	800
	21st to end	Night	28% 32% 27% 12% 1	880

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