Fog Flashcards
Explain advection fog and how it is formed
Caused by warm air moving over a cold sea surface. The cold sea cools the warm, moist air above it to below the dew point, causing condensation
Is advection fog easy or difficult to get rid of?
Difficult. It requires a change in air mass or wind speed/direction
Explain frontal fog and how it is formed
Caused by two very different air masses meeting. Cold air meets warm, moist air.
The warm air rises above the cold air because it is less dense.
When the moisture from the warm air falls as rain into the cold air below, it evaporates and becomes fog
Explain radiation fog and how it is formed
It forms over land when it has been a warm day and the earth is heated up. At night, the land cools quickly by radiation.
This then causes the air above the land to cool to below the dew point, causing condensation (fog)
Is radiation fog easy or difficult to get rid of?
Easy. It only ever reaches a few miles offshore and will burn off quickly after the sun rises
What is the dew point?
The dew point is the temperature at which air is saturated with water vapour
What happens when the air temperature falls below the dew point temperature?
The liquid in the air will begin to condense on particles in the air, causing airborne suspension of the liquid (fog)
How can the dew point temperature be measured?
By comparing the wet and dry bulb temperatures. The Mariner’s Handbook contains a table for calculating the dew point temperature using the dry bulb temperature versus the wet bulb depression.
Will the dry bulb usually read higher or lower than the wet bulb?
Higher
Explain how a Mason’s Hygrometer is set up
It should be housed in a Stevenson Screen.
A Mason’s Hygrometer has two thermometers - a dry bulb and a wet bulb. The wet bulb is connected by a muslin wick to a small reservoir of distilled water.
What is the term for the difference between the dry and wet bulb?
Wet bulb depression
What does it mean when the wet bulb depression is zero?
It means that the wet and dry bulb temperatures are the same, and therefore the air is at 100% saturation.
If the air temperature falls any lower, fog will form.
How can the formation of fog be predicted?
By plotting the sea surface temperature against the dewpoint temperature (obtained from the table in the Mariner’s Handbook using the data read from the Mason’s Hygrometer)
How is the sea surface temperature measured?
By dipping a bucket into the sea and measuring the temperature with a thermometer
