4 - The basics of hydrodynamics Flashcards
At the surface, how does pressure interact with columns over warm and cold air?
Higher pressure under a column of cold air & lower pressure under a column of warm air
Why does warm air rise over cold air?
Cold air is denser therefore warm air rises over cold air as the molecules are further apart
Describe the pressure gradient force.
It creates a wind blowing from high to low pressure that will reduce the pressure gradient - stronger pressure gradients = stronger winds.
This dominates small scale circulation & acts both horizontally and vertically - rising air in low pressure cools & water vapour condenses leading to clouds and precipitation whilst descending air in high pressure is dry resulting in clear skies
Describe the process of the land sea breeze.
As air over land is warmer, cold air from the sea breeze causes the warm land air to rise, creating a low pressure system over the land
The rising warm air cools and then moves towards the sea due to the pressure gradient between the land and sea
The air then sinks over the sea, creating high pressure here as cool air descends
This becomes the cold air from the sea breeze
If the vertical column of air from the land is heated enough, this can create vertical instability, and therefore, thunderstorm clouds can form
This creates a sea breeze with a return flow above once the warm air from the land rises
At night this is reversed as water retains more heat and is warmer than land, therefore land breeze occurs
What are the two effects of rotation?
Centripetal force - keeps objects on Earth’s surface - & Coriolos force - deflects objects that are moving relative to Earth’s surface
Describe the Coriolis force.
It leads to deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere - it is only important for objects travelling considerable distances e.g. flight paths.
It is always perpendicular to velocity, and can not perform work, therefore making it an apparent force.
Describe the Geostrophic Wind.
Wind begins to blow perpendicular to the isobars under the influence of the pressure gradient force
As the movement begins, teh Coriolis force begins to influence the moving air causing it to deflect to the right of its path(on the Northern Hemisphere
This deflection continues until the pressure gradient force and the Coriolis force are opposite and in balance with each other
How does the geostrophic wind relate to the pressure gradient and the Coriolis force?
The geostrophic wind is the equilibrium between the pressure gradient and the Coriolis force.
Describe the effects of friction in the atmosphere.
It is responsible for turbulent motion and is most noticeable at the surface.
What is the effect of friction on the geostrophic wind?
It reduces the geostrophic wind, leading to the slowing of the Coriolis force and the deflection of winds and limiting the lifetime of low and high pressure systems as the winds move in spirals
Pressure gradient force is now balanced by sum of frictional force and Coriolis force
Why are hurricanes arranged in spiral structures?
These spiral structures are formed by rotating geostrophic winds under the influence of pressure, Coriolis force and friction.
How does latent heat assist in the formation of hurricanes?
The heat and moisture from latent heat release from the warm water is the source of energy for hurricanes.
Why can hurricanes not travel long distances over land?
Friction & reduced water vapour supply does not allow the storms to travel long distances over land
How do mid-latitude cyclones form?
Wave like structures(fronts) form between cold and warm air masses
Waves intensify and become unstable – this is called cyclogenesis – to form cyclones
These are preferentially formed in mid-latitude storm tracks of the North Atlantic and North Pacific
Name some differences between hurricanes and mid-latitude cyclones.
Hurricanes have warm cores and sinking air in the hurricane’s eye whereas mid-latitude storms have cold core and rising air in the centre of the storm.
Hurricanes get their energy from warm water and latent heat of condensation whereas mid-latitude storms get their energy from horizontal temperature gradients.
Hurricanes weaken with height(strongest winds at surface) whereas mid-latitude storms intensify with height(strongest winds at higher altitudes).
