Transcript Precipitation

Precipitation
Western Canada Precipitation
• There are three basic types of precipitation
with which you need to be familiar
• Orographic or ‘Mountain’ precipitation
• Convectional (associated with open areas)
precipitation
• Frontal (where two air masses meet) and is
generally either:
– A Warm Front – warm air is moving forward, or
– A Cold Front – the cold air is advancing
All Precipitation
• All precipitation requires:
– A water source
– A lifting force to gain elevation
– Condensation
– Saturation – leading to
– Precipitation
• In each of the types that we will look at these
steps can be applied with necessary
modifications
Western Canada – Prevailing Winds
Western Canada – Prevailing Winds
• The part of the preceding diagram that we are
interested in is the on-flow of wind from the
West – these are the Prevailing Westerlies
• They come across the Pacific Ocean and pick up
moisture on the way either because of their
– Warm air mass, or
– The area from which they come (the Pineapple
Express in the Fall
– Particles of dust or dried salt from the ocean spray
become the nucleus around which droplets can form
Orographic Precipitation
• As the Prevailing Westerlies approach the
coast, they are pushed across the lowlands
towards the mountain barrier
• On reaching the mountain barrier they are
forced upwards – pressure continues from the
air mass coming behind
• Warm air can hold more moisture than cold
air – think of a bathroom during a hot shower
– no fan
• As the air is forced up, it cools – why?
• _____________________________________
Orographic Precipitation
• As the air cools, it can no longer hold as much
moisture. What happens? Think of holding a
cool plate above steam coming from a kettle.
• _____________________________________
• What physical observation can we make that
demonstrates to us that something has
changed?
• _____________________________________
Orographic Precipitation
• If clouds form, does that mean that there will
be precipitation? ________
• If condensation continues until saturation
takes place, precipitation will occur. Think of a
sponge on a countertop underneath a
dripping tap.
• Remember that the air mass is still being
pushed by the prevailing westerlies.
Orographic Precipitation
• As the air is pushed over the top of the
mountains we have a mass of cold air – cold
air is heavier than warm air – the air mass
begins to sink
• As the air sinks, it starts to warm up
• As it warms up, the air mass can hold more
water content
• The precipitation stops
• This is the Rainshadow Effect
Orographic Precipitation
Convectional Precipitation
• Examine the diagram on the next slide
• Explain the process by analogy to the forces
involved in orographic precipitation
Convectional Precipitation
Convectional Precipitation
• Convectional precipitation is characterized by
• Location – flatter, open spaces (prairies,
plateaus)
• More active weather
– Thunderstorms
• Thunderstorms are commonly associated with
Convectional Precip but also occur in Orographic Precip.
– Shorter duration followed by clearing - wind
– Dissipation of clouds without precipitation
Frontal Precipitation
• Cold Front
– Moves more quickly
– Weather is more vigorous (heavy rain, wind, etc)
– Characterized by a steep cold air mass front
• Warm Front
– A long sloping warm air mass front
– Weather drags on and on (November grey days)
Cold Front
Warm Front