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Posted
  • Location: Canmore, Canada [4296ft] & North Kent [350ft]
  • Location: Canmore, Canada [4296ft] & North Kent [350ft]

    Can anyone tell me what the temp difference would be for 74 metres? I know there is a formula for this but cant remember it

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    Posted
  • Location: Guess!
  • Location: Guess!
    Can anyone tell me what the temp difference would be for 74 metres? I know there is a formula for this but cant remember it

    HI CC,

    If it's at the DALR, it would be about 0.74C, given dry (unsaturated air - basically, not foggy, or cloudy). If you were half way up a mountain, in cloud, above the cloud base, it would be between 0.22C and 0.52C, as it would be at the SALR. The rate of cooling here depends on the amount of water the air contains.

    SALR - Saturated Adiabatic Lapse Rate (variable, between about 0.3C and 0.7C ish)

    DALR - Dry adiabatic Lapse Rate (constant about 1C per 100m, or 3C per 1000 ft)

    Paul

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    Posted
  • Location: Aviemore
  • Location: Aviemore

    The general rule of thumb rule is generally 0.65c per 100 metres. Obviously this doesn't take into account inversions etc etc, but is often a reasonable way to work things out.

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    Posted
  • Location: Canmore, Canada [4296ft] & North Kent [350ft]
  • Location: Canmore, Canada [4296ft] & North Kent [350ft]

    Thanks for that guys...Banff has been given a snowfall warning, and it is 79 metres higher than me, so I am assuming at a difference of about 0.5oC, we should get some here too

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    Posted
  • Location: Rossland BC Canada
  • Location: Rossland BC Canada

    Rich, it certainly looks like quite a snowfall event as low as the valley floor and even into the higher parts of Calgary, probably not starting suddenly but a gradual transition from a cold rain to heavy wet snow. Could see 5-10 inches in places and 10-20 inches on the slopes, a lot of which will then melt gradually over the weekend. Thursday night into Friday looks like being the snowiest part of this event. The fronts will make progress well south through BC into WA and ID but further west the snow will be confined to the alpine elevations as it will remain 8-10 C in the valleys. Rather early but not that unusual for September as you know.

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    Posted
  • Location: Canmore, Canada [4296ft] & North Kent [350ft]
  • Location: Canmore, Canada [4296ft] & North Kent [350ft]
    Rich, it certainly looks like quite a snowfall event as low as the valley floor and even into the higher parts of Calgary, probably not starting suddenly but a gradual transition from a cold rain to heavy wet snow. Could see 5-10 inches in places and 10-20 inches on the slopes, a lot of which will then melt gradually over the weekend. Thursday night into Friday looks like being the snowiest part of this event. The fronts will make progress well south through BC into WA and ID but further west the snow will be confined to the alpine elevations as it will remain 8-10 C in the valleys. Rather early but not that unusual for September as you know.

    Hi Roger

    Its started, with moderate snow and about an inch here in Canmore in certain places, but most of the roads and pavements are just wet)...I personally didnt expect anything until Thursday night. Lake Louise through to JMasper could easily have a foot of snow IMO

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    Posted
  • Location: Oxford / Reading University/ Oklahoma University
  • Location: Oxford / Reading University/ Oklahoma University

    The dry adiabatic lapse rate is given mathematically by:

    dT/dz = - g/cp

    where dT/dz is the rate of change of temperature with height,

    g = acceleration due to gravity = 9.8m/s/s

    cp = specific heat at constant pressure. This is the amount of energy required to create a unit temperature change of an air parcel when it is kept at constant pressure = 1004 J/kg/K

    = 0.0098 K/m or 9.8K/km

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    correct, and now for the SALR just to keep things straight??

    jh

    Hi Roger

    Its started, with moderate snow and about an inch here in Canmore in certain places, but most of the roads and pavements are just wet)...I personally didnt expect anything until Thursday night. Lake Louise through to JMasper could easily have a foot of snow IMO

    both look like the wet stuff we get here

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    Posted
  • Location: Oxford / Reading University/ Oklahoma University
  • Location: Oxford / Reading University/ Oklahoma University

    The SALR is a little bit more complicated, because unlike dry air parcels, you have to take into consideration the amount of heat absorbed or released by the moist air parcel when moisture evaporates/condenses into it. It is given by the following formula:

    dT/dz = -g/cp - (L/cp)*(dr/dz)

    where L = latent heat of vaporisation = 2,500,000 J/Kg

    and dr/dz is the change in the mixing ratio of the air parcel with height - ie the amount of water vapour per unit mass that evaporates from or condenses in the air parcel.

    Hence the SALR is not constant, but depends on the change in moisture content of the air parcel.

    Consider an ascending DRY air parcel. As it ascends, it cools at the DALR and its relative humidity approaches 100%, causing it to become saturated. Further ascent causes condensation and the release of latent heat which offests the rate of cooling. Its temperature now falls at a lesser rate called the SALR. In this situation, moisture is lost, hence the term dr is negative in the equation, making the whole of the second term positive and resulting in a smaller dT/dz. As you go higher, the air parcel becomes colder and is less able to 'hold' moisture. Consequently, dr/dz becomes very small and the SALR approximates to the DALR.

    Now consider a descending moist air parcel. As it warms, moisture evaporates (dr is positive) and this evaporation cools the air parcel, offsetting its rate of warming.

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