Weather Forecasting

1. Weather Forecasting Predicting Changes in our Atmosphere

2. What Causes Changes in the Weather ? • Air Masses / Air Movement • Fronts • Pressure Systems

3. Weather Forecasting Because much of modern society depends on weather conditions, knowing current weather patterns and methods used to determine the future state of the atmosphere is important. This attempt to predict the future weather is called weather forecasting. Forecasting is based on probability of occurrence.

4. Weather Maps Station models are used to record the atmospheric variables recorded at different locations. These variables are: amount of cloud cover barometric pressure temperature dewpoint wind direction wind speed

7. Using Isolines Isolines are lines connecting points of equal value. Weather maps use different types of isolines: Isobars show barometric pressure. Isotherms show temperatures.

9. Synoptic Weather Maps Synoptic weather mapsprovide a synopsis, or summary of weather conditions all over the United States at the indicated hour.

12. What is an Air Mass? Air mass = a large body of air in the troposphere having similar pressure, wind and temperature characteristics throughout.

13. Air Mass Characteristics Air masses have characteristics that are determined by the part of the Earth’s surface that the air mass forms over. These areas are called source regions. Examples: Forming over land = dry air mass Forming over water = moist air mass Forming over equator = warm air mass Forming over the poles = cold air mass.

15. Source Regions

19. Air masses move and swirl over the surface of the Earth in different directions and at different speeds and often bump into one another. Rain, thunderstorms, snow, tornadoes and all weather-related events can result when air masses meet.

21. What is a Weather Front? • Front = the boundary formed as two air masses meet and collide.

22. Types of Fronts • WarmFront= forms when a warm air mass slides over a departing cold air mass. As warm air rises the temperature drops and rain or precipitation often forms. • ColdFront = forms as a cold air mass overtakes and moves underneath a warm air mass. The warm air is forced upward rapidly and a cold front forms. Cold fronts usually bring violent weather.

23. Types of Fronts (cont.) • Stationary Front = occurs when a cold air mass meets a warm air mass and neither air mass moves. Precipitation sometimes forms between these two fronts. • Occluded Front = forms when two cold air masses meet and force a warm air mass between them to rise completely off the ground. The weather in an occluded front is difficult to predict because there are three air masses involved.

29. What is a Pressure System? • Pressure = as particles move and collide they exert pressure; • Differences in air pressure have a great affect on the weather. • When particles are more densely packed together they exert high pressure, • High air pressure usually means clear, fair weather. • When they are less densely packed they exert less (low) pressure. • Low air pressure usually means clouds and rainy weather.

31. High Pressure System • Cold air is dense and it sinks. • As the air sinks it warms up. • Warm air holds more water vapor. • The total amount of water vapor remains the same. • As the air warms the relative humidity decreases as droplets in the clouds evaporate. • A high pressure system usually means fair weather as the moisture in the air is evaporated so few clouds form.

33. Low Pressure System • Warm air has low density and it rises (forced upwards by surrounding denser air) • As the air rises it cools • As the air cools the relative humidity increases – eventually reaching the dew point • At the dew point condensation takes place and clouds form • Low pressure systems usually form along fronts where warm air and cold air meet • Low pressure systems cause most of the weather in the US. • A low pressure system leads to precipitation in the form of rain, snow, hail, sleet.

36. Moving Fronts • When we witness a change in the weather from day to day it is due to the movement of air masses. It is the movement and collision of air masses that causes the weather to change.

38. What is Severe Weather? • Severe weather are events that fall outside of the ordinary weather patterns. • Severe weather can include the following: • Thunderstorms • Lightning • Tornadoes • Hurricanes • Floods • Droughts • Blizzards • Fog • Ice Storms

39. Thunderstorms • Thunderstorms are formed by rapid upward movement of warm moist air. • Thunderstorms can occur within a warm moist air mass. • Thunderstorms most often occur at cold fronts. • Thunderstorms account for the greatest number of weather-related fatalities in the US due to the flooding, lightning and tornadoes that can be associated with them.

40. How Thunderstorms Form • As warm moist air is forced upward it cools and the water vapor condenses forming cumulus clouds. (These clouds can be very tall – up to 10km high) • As the drips fall they collide and combine to form larger drops • The falling drops create a downward movement of air that spreads out as it hits the surface causing winds • If the winds accompanying a thunderstorm are greater than 54mph then forcasters classify it as a severe thunderstorm (these usually contain hail).

43. Lightning

45. Hurricanes • Hurricanes are the largest storms that occur on Earth. • Hurricanes are very large, swirling, low pressure systems that from over tropical waters (water temperature must be at least 80 degrees F). • Hurricanes can last for days due to the constant supply of energy from the warm waters • To be classified as a hurricane there must be sustained winds of greater than 74mph. • Greatest winds recorded are 236mph; Category 5 “Mitch” in Nov 1998 • 1900 Galveston TX was most deadly hurricane ever with 8000 deaths.

46. Eye of the Hurricane The eye of a hurricane is the relatively calm area in the middle of the storm. The eye has the lowest pressure of the storm and the eye wall is where the strongest winds occur. A storm surge is a dome of water that can raise the water level higher than high tide where the storm lands.

48. How Hurricanes Form • Hurricanes form over warm tropical oceans where two opposing winds meet and begin to swirl • A low pressure forms in the middle of the swirling winds and begins rotating • Warm moist air is forced upwards in the center • The dropping pressure in the center pulls more air toward the center creating increasing winds and lower pressure. This cycle of increasing strength continues as long as the storm remains over warm water. • Hurricanes weaken as they hit land because there is no longer a supply of energy (from the warm water) available.

50. Life Cycle of a Hurricane • Life-cycle of a hurricane = form first as a low pressure system, then grow into a tropical depression (<31mph winds), then grow into a tropical storm (32-74mph winds), then grow into a hurricane (>74mph winds). • Out of the 10 tropical storms per year average, 6 will develop into hurricanes and 2 will strike the US.