[Grav!ty]

So if Dew Point is -5 degrees C and there's frost on the grass, that means the temperature must have dropped to at least -5 C right?

If that's the case I always wonder how the weather reports can post a low of say 4 degrees C. I mean for the dew to have formed the temp must have dropped to -5 C and for it to have frozen to form frost the temp must have been at least below 0 degrees C hey?

[kanaloa]

I'm not sure about that. I'd need to pull out the old weather books, but I think frost can form above the ground and "fall" to the grass itself. There's also some weird affect where the surface temps of things like metal, grass, etc can be colder than the surrounding air - so the frost forms on them.

Brain's a little fuzzy, but I'm fairly positive you can have frost without the temperature reaching the dew point level.

[kanaloa]

Actually, Wiki does a darn nice job of explaining what I was failing to say, LOL.

Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air.

If a solid surface is chilled below the dew point of the surrounding air and the surface itself is colder than freezing, frost will form on the surface.

In general, for frost to form the deposition surface must be colder than the surrounding air. For instance frost may be observed around cracks in cold wooden sidewalks when moist air escapes from the ground below. Other objects on which frost tends to form are those with low specific heat or high thermal emissivity, such as blackened metals; hence the accumulation of frost on the heads of rusty nails. The apparently erratic occurrence of frost in adjacent localities is due partly to differences of elevation, the lower areas becoming colder on calm nights. It is also affected by differences in absorptivity and specific heat of the ground which in the absence of wind greatly influences the temperature attained by the superincumbent air.

Because cold air denser than warm air, in calm weather cold air pools at ground level. This is known as surface temperature inversion. It explains why frost is more common and extensive in low-lying areas. Areas where frost forms due to cold air trapped against the ground or against a solid barrier such as a wall are known as "frost pockets".

[Grav!ty]

Thanks John. So it looks like it means that the surface temp of say the grass will have cooled to the dew point (-5 C) and that would cause the dew to freeze as it touched down on the grass.

mmm...that could also explain why an outdoor thermometer reads quite a bit lower than the forecast temp I suppose

[kanaloa]

Not even sure the temps would need to get that low. If they are literally at freezing, even if the air isn't at the dew point or fully saturated, there is definitely still enough moisture in the air to "stick" to the object and freeze. I think of it like a cold can on a table in a warmer room. If it was cold enough, it'd freeze.