Clouds!

1. Clouds!

2. Fog!

3. condensation • Remember: condensation – the change from vapor to liquid • How does condensation happen in the atmosphere? • Warm air can hold more water vapor than cold air • So, on a sunny spring day, the air is not saturated with water vapor; however, at night, it cools and the capacity to hold water vapor decreases • So, the air becomes saturated. If the air cools past the point of saturation, condensation occurs

4. The water vapor can condense into droplets, forming clouds or fog • If condensation occurs on a surface like grass, we get dew (if air temp is below freezing, we get frost) • Dew point – the temperature at which saturation occurs and condensation begins

5. Fog formation • Fog forms when a cold surface cools the warmer moist air above it • As water vapor condenses in the air, tiny droplets fill the air and form fog. • Each droplet is centered around a condensation nucleus (tiny particle)

6. Different types of fog: • Radiation fog – forms when the night sky is clear and the ground loses heat rapidly through radiation • As the ground cools, light winds mix the cooled bottom air with the warmer air a short distance above • The whole layer of air cools to its dew point • Resulting fog at ground level is colder than the layer of air above it – called a temperature inversion

7. Radiation fog • Common in humid valleys near rivers or lakes • Most frequent in late fall and winter • Thickest in early morning - “burned away” by later morning sunshine

8. Advection Fog • Forms when warm, moist air blows over a cool surface • In northern US and southern Canada, form when warm, moist southerly winds blow over snow-covered ground • Summer fogs in coastal California form when warm ocean air moves over cold coastal waters • Winter fogs form along Gulf Coast when cold Mississippi River waters chill the warm gulf air

9. What causes fog? • Fog can be considered a cloud at ground level. The processes forming it, however, are usually different from those that form clouds. • Like clouds, fog is made up of condensed water droplets which are the result of the air being cooled to the point where it can no longer hold all of the water vapor it contains. (dew point) • For clouds, that cooling is almost always the result of rising air, which cools from expansion. But for fog, which occurs next to the ground, there are usually other reasons for this cooling. • For instance, rain can cool and moisten the air near the surface until fog forms.

10. Or, as is more often the case, infrared cooling of a cloud-free, humid air mass at night can lead to fog formation - this is called "radiation fog". Radiation fog is most common in the fall, when nights get longer, air masses begin to cool, and land and water surfaces that have warmed up during the summer are still evaporating a lot of water into the atmosphere. • Finally, a warm moist air mass blowing over a cold surface (usually snow or ice, or over a cold ocean surface) can also cause fog to form-this is called "advection fog".

11. How do clouds form? • Clouds (and fog) form when the air cools to its dew point. • Dew point – the temperature at which the air is saturated with water vapor and condensation begins • Clouds can form at any altitude in the troposphere

12. At temperatures above freezing, clouds are made of water droplets • Below freezing, clouds are mixtures of snow crystals and super-cooled water (water that has cooled below 0○C without freezing) • At -20○C, clouds are made up mostly of snow and ice crystals

13. Types of Clouds • 4 main types of clouds: • Low clouds • Middle clouds • High clouds • Clouds of vertical development

16. Classification • Clouds are classified according to : • Height or altitude – low, middle or high • Shape • If air movement is mainly horizontal, clouds form in layers – stratiform clouds • If air movement is mainly vertical, clouds grow upward in great puffs – cumuliform clouds

17. Cloud heights are measured as distance above the ground, not above sea level. • Cloud names are formed from one or more of the same 5 words or parts

18. Classification According to Height • High clouds – above 7000 m • Cirrus, cirrostratus, cirrocumulus • Middle clouds – 2000 – 7000 m • Altostratus, altocumulus • Low clouds – below 2000 m • Stratus, nimbostratus, stratocumulus

19. Clouds of Vertical Development • Cumulus and cumulonimbus can span more than one altitude level because they grow vertically • Cumulonimbus clouds - dark rain clouds

20. Strato and stratus– describe clouds that form in layers • Stratus clouds – layered, low clouds • Cumulus and cumulo - describe clouds that grow upward • Cumulus clouds – fluffy clouds with flat bases • Cirrus and cirro– describe feathery clcouds • Cirrus clouds are high, feathery ice clouds

21. Alto – describes clouds between 2000 and 7000 m. • Nimbo and nimbus refer to dark rain clouds