Atmospheric Pressure

What Is Weather? (continued) Humid air (air containing more water vapour) has lower pressure than dry air.  the more H 2 O vapour in the atmosphere, the lighter the air is  H 2 O displaces an = volume of dry air  H 2 O is not as heavy as O 2 and N 2 gases  humid air exerts less pressure than (cold) dry air

What Is Weather? (continued)

Specific humidity = the total amount of water vapour in the air Dew point = the temperature at which no more water vapour can be held by air. Relative humidity = a comparison of the amount of water vapour in the air and the amount the air could hold if it were saturated.  “45% relative humidity” means that the air is holding 45% of the water vapour it could before reaching its dew point.

Convection in the Atmosphere Wind is the movement of air from high pressure to low pressure. An air mass is a large body of air with similar temperature and humidity. Air masses take on the conditions of the weather below. Air masses can be as large as an entire province, or even larger! Global wind patterns are influenced by convection, conduction & radiation.

Convection in the Atmosphere

Low pressure systems form when an air mass warms. This usually occurs over warm water or warm land. air mass warms & rises  particles gain KE  air mass becomes less dense  lower atmospheric pressure as air rises, it cools & water vapour condenses…lows usually bring wet weather in northern hemisphere, winds blow CCW around the center of the low.

Prevailing Winds Prevailing winds are winds that are typical for a location. Ex. Winds in BC usually blow in from the ocean. Precipitation falls as air is forced up the mountain slopes. Air gets drier as it moves inland, continuing to drop precipitation. Dry air rushes down the far side of the mountains into the prairies.

Prevailing Winds The prevailing winds off BC’s coast, crossing into Alberta.

The Coriolis Effect The Coriolis Effect is a change in the direction of moving… air, water, or objects …due to Earth’s rotation.

The Coriolis Effect Winds move from high pressure to low pressure. In a simple model (i.e. for a small, non-rotating Earth), air would warm in the tropics, and rise. Cooler air from the N would rush in below to fill the empty spot. The warm air at higher altitudes would move N to replace the cooler air. This occurs at several latitudes as we move N.

The Coriolis Effect As earth rotates, winds are “bent” / deflected CW (in the northern hemisphere).  A the equator moves much more quickly than a pole. Wind systems develop:  the trade winds  the prevailing westerlies  the polar easterlies Wind systems of the world

Jet Streams, Local Winds and Fronts Strong winds occur in areas between high and low pressure systems. The boundaries between the global wind systems thus have very strong winds. In the stratosphere, there are bands of fast- moving air called jet streams.  Jet streams often look like streams of water.  The polar jet stream can move at 185 km/h for 1000s of miles.  Planes flying E across Canada “ride” the jet stream, and avoid it flying W.

Jet Streams, Local Winds and Fronts Local winds arise and are influenced by local geography.  In BC, sea breezes (local winds that are caused by different rates of cooling & heating of land & water) blow inland (onshore breeze) when the land warms in the morning, and outward (offshore breeze) when the land cools in the evening.

Onshore Breeze – in the morning During the day, land heats up faster than water. -land radiates heat & warms the air at its surface -warm air rises -replaced by cool air drawn in from over the water

Offshore Breeze – at night During the night, land cools down faster than water. -warm air over water rises -draws in cool air from over the land

Fronts A front is a boundary between 2 different air masses. may be several 100 km wide and 1000s of kms long! an approaching front = a change in weather (the extent of which depends on the degree of difference in temp. & pressure between air masses)  Cold, dense air forces warm, moist air to rise, so fronts usually bring precipitation.

Fronts

Extreme Weather Air masses often have very large amounts of thermal energy. Extreme weather can arise under certain conditions as this energy is released. Thunderstorms occur when warm air rises, water vapour condenses (which releases even more thermal energy), building the thunderhead even higher.  Static energy can build up and be released as lightning, which heats air to °C or more!  sea breezes in the tropics and advancing, energetic cold fronts can cause thunderstorms.

Extreme Weather Tornadoes form when large thunderstorms meet fast, high- altitude, horizontal winds.  A “funnel” of rotating air may form, which can extend all the way to, and touch the ground (the storm is now a tornado), with winds of up to 400 km/h.

Extreme Weather The tropics, with their intense heat, can often have severe weather.  large masses of warm, moist air rise quickly, and cool air rushes in  moisture in air condenses, releasing more thermal energy; cyclone rotates faster  Coriolis effect forces air to rotate CCW in the northern hemisphere, CW in the south.  hurricanes = tropical cyclones = typhoons

Extreme Weather – Funnel Clouds