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 2024-10-25.

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% 4 10% 6% 5% 5% 4 6% 7% 29%	101933
	Night	13% 11% 5% 11% 6% 5% 5% 4 5% 6% 30%	96736
Transparency	Day	53% 6% 11% 19% 10% 1	101694
	Night	53% 6% 12% 18% 10% 1	96595
Seeing	Day	20% 22% 31% 17% 7% 2	14927
	Night	19% 22% 31% 19% 7% 1	23760

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	14% 17% 9% 16% 8% 5% 6% 3 5% 4 14%	2386
	11th to 20th	Day	9% 19% 9% 22% 10% 8% 4 4 4 3 6%	2790
	21st to end	Day	16% 21% 8% 15% 8% 5% 5% 3 4 4 11%	3124
February	1st to 10th	Day	11% 17% 7% 15% 13% 7% 5% 4 4 4 14%	1710
	11th to 20th	Day	10% 13% 7% 17% 10% 6% 5% 5% 6% 5% 16%	1663
	21st to end	Day	14% 17% 6% 11% 7% 6% 6% 4 5% 4 20%	1484
March	1st to 10th	Day	14% 16% 5% 12% 7% 5% 4 3 4 5% 26%	3402
	11th to 20th	Day	9% 16% 9% 14% 6% 4 5% 4 4 5% 25%	3480
	21st to end	Day	14% 13% 5% 9% 5% 4 4 4 5% 5% 32%	3815
April	1st to 10th	Day	15% 16% 5% 10% 5% 5% 4 3 4 7% 26%	2216
	11th to 20th	Day	17% 13% 3 6% 4 4 4 4 5% 6% 34%	2382
	21st to end	Day	19% 12% 4 8% 4 4 4 3 5% 6% 32%	2470
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% 8% 4 4 5% 3 4 5% 19%	2019
November	1st to 10th	Day	12% 18% 6% 10% 6% 5% 4 4 5% 5% 26%	1592
	11th to 20th	Day	12% 17% 6% 11% 7% 7% 5% 4 6% 7% 19%	1520
	21st to end	Day	15% 18% 7% 15% 6% 5% 6% 5% 5% 5% 13%	1520
December	1st to 10th	Day	13% 22% 8% 20% 8% 5% 5% 3 4 4 9%	1520
	11th to 20th	Day	16% 18% 7% 16% 7% 5% 5% 4 5% 5% 13%	1906
	21st to end	Day	20% 22% 8% 20% 6% 5% 3 2 3 3 7%	2756
January	1st to 10th	Night	14% 15% 8% 17% 7% 5% 5% 4 5% 7% 13%	4445
	11th to 20th	Night	12% 12% 7% 22% 12% 11% 6% 3 4 3 7%	4154
	21st to end	Night	17% 14% 7% 16% 9% 7% 5% 4 5% 5% 10%	4532
February	1st to 10th	Night	11% 15% 6% 14% 9% 7% 6% 4 6% 5% 16%	2470
	11th to 20th	Night	10% 12% 6% 16% 8% 7% 6% 4 6% 6% 20%	2403
	21st to end	Night	11% 12% 5% 13% 8% 6% 6% 4 6% 5% 23%	1965
March	1st to 10th	Night	13% 12% 6% 10% 6% 5% 5% 4 5% 7% 28%	3438
	11th to 20th	Night	9% 12% 7% 10% 6% 4 4 4 5% 6% 33%	3479
	21st to end	Night	13% 9% 4 9% 5% 5% 5% 4 5% 5% 36%	3817
April	1st to 10th	Night	17% 13% 4 7% 7% 4 4 3 5% 7% 29%	1963
	11th to 20th	Night	15% 10% 3 7% 5% 5% 4 3 4 7% 37%	1763
	21st to end	Night	13% 12% 4 10% 5% 3 5% 4 5% 6% 32%	1710
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% 4 4 5% 7% 22%	2439
November	1st to 10th	Night	14% 14% 4 8% 6% 5% 6% 4 5% 5% 29%	2486
	11th to 20th	Night	13% 16% 5% 10% 5% 6% 5% 4 6% 7% 22%	2660
	21st to end	Night	15% 16% 6% 12% 6% 6% 6% 4 5% 8% 17%	2660
December	1st to 10th	Night	15% 17% 8% 16% 6% 6% 5% 4 6% 5% 12%	2660
	11th to 20th	Night	11% 19% 8% 17% 7% 6% 5% 4 4 4 13%	3288
	21st to end	Night	21% 21% 9% 19% 6% 5% 4 3 4 3 7%	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	70% 7% 11% 10% 1	2387
	11th to 20th	Day	81% 5% 7% 4 3	2790
	21st to end	Day	75% 7% 7% 7% 3	3124
February	1st to 10th	Day	72% 5% 8% 11% 3 1	1710
	11th to 20th	Day	67% 5% 10% 12% 5% 1	1672
	21st to end	Day	65% 5% 10% 13% 6% 1	1484
March	1st to 10th	Day	60% 6% 11% 15% 8%	3405
	11th to 20th	Day	61% 3 9% 19% 6% 1	3480
	21st to end	Day	52% 4 11% 18% 12% 2	3815
April	1st to 10th	Day	58% 5% 9% 17% 10% 1	2216
	11th to 20th	Day	49% 6% 8% 21% 15% 1	2382
	21st to end	Day	53% 5% 10% 18% 12% 2	2470
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	52% 7% 11% 17% 12% 1	1900
	11th to 20th	Day	60% 5% 11% 15% 7% 1	1900
	21st to end	Day	68% 4 11% 11% 6%	2019
November	1st to 10th	Day	59% 5% 11% 17% 7%	1592
	11th to 20th	Day	63% 6% 12% 13% 6% 1	1520
	21st to end	Day	69% 7% 12% 10% 2	1520
December	1st to 10th	Day	76% 5% 10% 8% 2	1520
	11th to 20th	Day	73% 8% 8% 10% 2	1906
	21st to end	Day	81% 6% 6% 6% 1	2756
January	1st to 10th	Night	69% 8% 12% 9% 2	4445
	11th to 20th	Night	79% 5% 9% 4 3	4154
	21st to end	Night	72% 8% 11% 6% 3 1	4532
February	1st to 10th	Night	66% 5% 12% 13% 3	2470
	11th to 20th	Night	63% 5% 12% 14% 5%	2405
	21st to end	Night	59% 6% 13% 14% 7% 1	1965
March	1st to 10th	Night	53% 8% 12% 20% 7% 1	3448
	11th to 20th	Night	50% 5% 14% 21% 8% 2	3480
	21st to end	Night	47% 5% 11% 21% 14% 2	3817
April	1st to 10th	Night	55% 5% 9% 18% 13% 1	1963
	11th to 20th	Night	47% 5% 9% 21% 16% 2	1763
	21st to end	Night	51% 5% 11% 15% 16% 2	1710
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	46% 7% 10% 20% 15% 1	2280
	11th to 20th	Night	53% 6% 14% 19% 8% 1	2280
	21st to end	Night	60% 7% 11% 14% 7%	2439
November	1st to 10th	Night	53% 6% 14% 18% 8% 1	2486
	11th to 20th	Night	57% 7% 14% 14% 6% 2	2660
	21st to end	Night	64% 8% 15% 11% 2 1	2660
December	1st to 10th	Night	71% 6% 13% 8% 2	2660
	11th to 20th	Night	71% 7% 11% 10% 2	3288
	21st to end	Night	81% 6% 6% 6% 1	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	22% 31% 29% 15% 2 1	180
	11th to 20th	Day	22% 36% 29% 10% 2	258
	21st to end	Day	24% 29% 35% 11% 1	297
February	1st to 10th	Day	27% 36% 30% 4 3	270
	11th to 20th	Day	23% 37% 36% 4 1	270
	21st to end	Day	34% 36% 26% 4	225
March	1st to 10th	Day	24% 29% 32% 11% 3	441
	11th to 20th	Day	24% 26% 35% 12% 3 1	460
	21st to end	Day	30% 24% 30% 13% 3	506
April	1st to 10th	Day	26% 26% 34% 10% 4	460
	11th to 20th	Day	33% 23% 24% 11% 6% 3	452
	21st to end	Day	28% 24% 30% 12% 4 2	460
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	31% 27% 28% 11% 2	506
November	1st to 10th	Day	24% 34% 26% 13% 3	232
	11th to 20th	Day	25% 29% 33% 9% 3 1	180
	21st to end	Day	28% 27% 36% 8%	180
December	1st to 10th	Day	24% 38% 30% 8%	180
	11th to 20th	Day	27% 34% 29% 8% 2 1	180
	21st to end	Day	25% 39% 29% 7% 1	198
January	1st to 10th	Night	21% 34% 30% 12% 2	776
	11th to 20th	Night	25% 29% 30% 14% 2 1	760
	21st to end	Night	16% 25% 37% 17% 4	836
February	1st to 10th	Night	20% 36% 31% 12% 1	760
	11th to 20th	Night	20% 30% 34% 13% 2	743
	21st to end	Night	23% 32% 32% 11% 3	628
March	1st to 10th	Night	21% 28% 34% 15% 3	760
	11th to 20th	Night	21% 28% 32% 14% 3	760
	21st to end	Night	20% 23% 33% 19% 5%	836
April	1st to 10th	Night	25% 24% 35% 15% 2	760
	11th to 20th	Night	22% 23% 29% 19% 7% 1	639
	21st to end	Night	22% 21% 31% 19% 8%	570
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% 25% 31% 14% 3	812
November	1st to 10th	Night	24% 23% 31% 16% 5%	740
	11th to 20th	Night	21% 33% 27% 13% 5% 1	760
	21st to end	Night	25% 29% 30% 14% 2	760
December	1st to 10th	Night	25% 30% 32% 9% 3	760
	11th to 20th	Night	21% 33% 33% 10% 2	760
	21st to end	Night	28% 33% 27% 12% 1	836

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