Weather, Winds, and Fronts

Air masses, Jet streams, Humidity and Dew Points.

Wind Formation

• Wind is movement of air from high to low pressure• These differences in pressure are caused by the

Earth’s tilt, orbit, and orientation to sun.• Heated air has low density and pressure, so it is

pushed up by colder, denser air.• Hot air comes from the sun, cold air from the Poles.• All these movements create wind formation, but

they are the only explanation for wind.

Coriolis Effect

• The rotation of the Earth creates the Coriolis effect

• It deflects all free moving objects ( air, water, etc) to the right on the North side of the Equator

• It deflects all free moving objects to the left on the South side of the Equator

• So, air moving south in the Northern Hemisphere will turn westward

Coriolis Effect

Types of Winds

• Doldrums – air appears motionless. This is the windless zone at the equator

• Tradewinds – winds that blow toward the southwest in the northern hemisphere and toward the northwest in the southern hemisphere. These are steady winds between the equator and 30 Degrees Latitude north or south

Types of winds - Surface

• Prevailing Westerlies – these blow in opposite directions of the tradewinds. Between 30 – 60 degrees latitude north or south. These are responsible from much of the weather across North America (U.S. and Canada)

• Polar Easterlies – Northeast to southwest near the North Pole and from the southeast to northwest near the South Pole

Types of winds - Surface

Types of Winds – High Altitude

• Winds also exist at high altitudes• They are narrow belts of STRONG (FAST) winds

that blow near the top of the troposphere (Earth’s closest atmospheric layer), at about 10 Km

• They are called Jet Streams and they blow from west to east.

• Their speeds average between 97 and 185 km/h• They have a major effect on our weather

Jet Streams

• Sailors (who operate at Earth’s surface) rely on tradewinds, Westerlies, and Easterlies. They try to avoid Doldrums

• Just as sailors rely on these surface winds, jet aircraft rely on jet streams (high altitude winds) to save time and fuel when traveling East

• When flying west, planes try to avoid the jet streams by flying at different altitudes

Weather

• Weather refers to the present state of the atmosphere and describes current conditions

• Weather affects our day to day lives. We usually ask “What will the weather be like today?” first thing in the morning

• Weather affects: what you wear to school, how you get to and from school (or if you go to school) and what you do after school.

• So far, we’ve learned that air pressure, temperature, and winds affect weather

Humidity and Dew Points

• Amount of moisture in the air also affects weather

• Remember the gases in our Atmosphere? Nitrogen, Oxygen, and other (plus water vapor). The amount of water vapor in the air is called humidity!

• At warmer temps, the air can hold more water vapor. At cooler temps, it can hold less.

Humidity and Dew Points

• The actual amount of water vapor held in the atmosphere compared to the maxium amount it can hold at a given temperature is called Relative Humidity.

• When air contains as much water vapor (or moisture) at the given temp, it is saturated.

• If you hear a weather forecaster say that relative humidity is 50%, that means that the air contains 50% of the water needed for the air to be saturated.

Humidity and Dew Points

• Saturated air is at 100% relative humidity• Any additional water vapor will condense back to

a liquid or solid (freeze), depending on the temperature.

• When relative humidity is at 100% saturated and additional water vapor undergoes condensation, this is called the Dew Point

• Dew Point changes with the amount of moisture in the air.

Humidity and Dew Point

• Dew is seen as water droplets on grass in the early morning

• If you pour a glass of ice water and wait, you will see droplets form on the surface of the class. This happens because the cold glass cooled the air next to it, turning the air into water droplets. This is similar to dew on the grass.

Air mass and weather fronts

• An air mass is a large body of air that has the same properties as the surface over which it develops

• Ex. Air masses that form over land are dry compared to air masses that form over water. Air masses that form in the tropics are warmer than those that form in colder regions.

• When you witness a change in weather form day to day, it is due to the movement in air masses.

Air Masses and Weather Fronts

• Remember, warm and cool air masses are associated with high and low pressures. Warm air has low pressure and cool air has high pressure.

• The boundary between two different air masses (or pressures) is called a Front.

• Storms and precipitation occur at these fronts.• Like wind, air in fronts moves from high pressure to

low pressure. As low pressure air rises, it cools. At certain elevations, the air reaches it’s dew point, and water vapor condenses, forming clouds.

Air Masses and Weather Fronts

• At fronts, as cold air masses move under warm air, winds begin.

• Most changes in weather occur at one of four types of fronts.

• Weather fronts: Warm, Cold, Occluded, and Stationary

• Different weather occurs as a result of the meeting of these weather fronts

Warm and Cold fronts

Warm Fronts

• A warm air mass approaches a cold air mass and slowly moves upward over the cold air

• This is a slow process and some rain or light thunderstorms may occur as a result

Cold Fronts

• Cold fronts – a cold air mass pushes into a warm air mass, forcing it upward forcefully (twice as fast as when warm air masses glide easily over cold air masses in a warm front)

• These cold fronts produce violent weather – thunderstorms, lighting, heavy precipitation, and sometimes hail

Stationary Front

Stationary Front

• Pressure differences cause a warm or cold front to stop moving.

• A stationary front may remain in place for several days

• Weather conditions include light wind and precipitation across the entire frontal region

• Weather can be different on opposite sides of the front

Occluded Front

Occluded Front

• Two cool air masses merge and force warm air between them to rise.

• Weather includes strong winds and heavy precipitation